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reduced graphene oxide-methoxylether polyethylene glycol functionalised silica for scavenging of estrogen: Adsorption performance and mechanism
Journal Pre-proof Covalently linked graphene oxide/reduced graphene oxide-methoxylether polyethylene glycol functionalised silica for scavenging of estrogen: Adsorption performance and mechanism Samson O. Akpotu, Isiaka A. Lawal, Brenda Moodley, Augustine E. Ofomaja PII:
S0045-6535(19)32970-4
DOI:
https://doi.org/10.1016/j.chemosphere.2019.125729
Reference:
CHEM 125729
To appear in:
ECSN
Received Date: 21 October 2019 Revised Date:
17 December 2019
Accepted Date: 21 December 2019
Please cite this article as: Akpotu, S.O., Lawal, I.A., Moodley, B., Ofomaja, A.E., Covalently linked graphene oxide/reduced graphene oxide-methoxylether polyethylene glycol functionalised silica for scavenging of estrogen: Adsorption performance and mechanism, Chemosphere (2020), doi: https:// doi.org/10.1016/j.chemosphere.2019.125729. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Covalently linked graphene oxide/reduced graphene oxide-methoxylether polyethylene glycol
2
functionalized silica for scavenging of estrogen: Adsorption performance and mechanism
3
Samson O. Akpotu*,‡, Isiaka A. Lawal‡, Brenda Moodley† and Augustine E. Ofomaja‡
4
‡Wastewater Treatment Research Laboratory, Faculty of Applied and Computer Sciences, Department of
5
Chemistry, Vaal University of Technology, Vanderbijlpark 1911, South Africa.
6
†School of Chemistry and Physics, University of Kwazulu-Natal, Durban 4000, South Africa.
7
*Corresponding author: [email protected]
8
Abstract
9
Water pollution by pharmaceuticals is a global issue and its remediation is important. To overcome
10
this, we synthesised super hydrophobic nanoporous 3-dimensional ordered nanomaterials with multi-
11
functional binding chemistry for highly efficient adsorption of estrogen (17β-estradiol). Graphene
12
oxide (GO) was synthesised via Tours method and methoxylether polyethylene glycol (mPEG) was
13
covalently introduced onto GO surface via facile amidation mild process to give GO-mPEG. GO-
14
mPEG was anchored on nanoporous SBA-15 and homogenously reduced in-situ to SBA-rGO-mPEG.
15
XRD analysis confirmed successful synthesis of SBA-15 and cross-linked GO/rGO-mPEG on SBA-15
16
surface. Image analysis revealed the architecture of SBA-15 as porous 3-dimensional silica network
17
and presence of interwoven/crosslinked thin-films of GO-mPEG on SBA-15 surface. EDX
18
mapping/elemental analysis showed expected elements were present. FTIR and textural analysis
19
revealed the presence of different functional groups and high surface area as well as porosity,
20
respectively. Optimal molar ratio experiments showed that 0.5SBA-rGO-mPEG had the highest
21
sorption capacity. The relatively large surface area, 3-dimensional nanoprous silica structure and
22
excess of polyamide/amido-carbonic functional groups on nanocomposites were suited for adsorption
23
of 17β-estradiol. Equilibrium time was 30 min and effect of pH on adsorption was negligible.
24
Sorption kinetic process of SBA-rGO-mPEG suited the pseudo-second-order model and equilibrium 1
25
data fitted both Freundlich and Langmuir models. Qm values of 57.1, 78.5, 102.6 and 192.3 mg/g was
26
recorded for SBA-GO, 0.1SBA-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG,
27
respectively. H-bond, hydrophobic and π−π interactions were the sorption mechanism of SBA-rGO-
28
mPEG after detailed analysis of data. Adsorbents was regenerated/re-used after 4 cycles with high
29
remediation from environmental/real water samples.
30
Keywords: 17β-estradiol; adsorption; polyamide carbonic groups; reduced graphene oxide,
31
methoxylether polyethylene glycol; hydrophobicity; wastewater treatment
32 33
1. INTRODUCTION
34
Pharmaceuticals (endocrine disrupting chemicals (EDCs)) are present in the environment (Huerta-
35
Fontela et al., 2011). Estrogen, a class of EDC have caused endocrine disrupting function and
36
negatively impact human and aquatic health. EDCs and its metabolites pathway into the environment
37
are via domestic sewage treatment plants, seeps into lakes/rivers, ultimately polluting water bodies
38
(Okoro et al., 2017; Mnguni et al., 2018). 17 β-estradiol an EDCs have been found to be present in
39
drinking, surface, ground and wastewater (Huerta-Fontela et al., 2011). At concentration < 1 ng L-1,
40
studies by Tabata et al. (2001), Diamanti-Kandarakis et al. (2009), Han et al. (2015), have shown EDC
41
causes reproductive disorder, feminisation of aquatic species, malformation and cancer. 17 β-estradiol
42
(Figure Supplemental Information (S1)) is a steroid hormone produced from cholesterol via
43
androstenedione in body fat deposits, brain and ovaries (Mnguni et al., 2018). Medically, it is most
44
commonly used as an oral contraceptive pill. It possesses the most disruptive endocrine activity which
45
is 1000-10000 times the impact of nonylphenol (Jiang et al., 2017a). Hansen et al. (1998), found that
46
blood of males exposed to 5 ng L-1 of 17 β-estradiol for 2 weeks resulted in the inducement of a
47
specific female hormone. In South Africa, 0.24-0.35 ng L-1 of 17 β-estradiol was found in Vaal
48
catchment river (Mnguni et al., 2018). Conventional water treatment methods are ineffective for its 2
49
removal. Techniques such as photodegradation (Zhang et al., 2007), catalysis (Shappell et al., 2008),
50
photolysis (Rose et al., 2014), biodegradation (Li et al., 2018) and adsorption (Dong et al., 2018) have
51
been applied for the remediation of 17 β-estradiol. Adsorption is preferred because of ease of use, low
52
cost and high efficiency (Akpotu and Moodley, 2018c). 17 β-estradiol, has been adsorbed with
53
carbonaceous materials and their modified counterpart such as resin (Zhang and Zhou, 2005) biochar
54
(Dong et al., 2018) carbon nanotubes, activated carbon and graphene (Jiang et al., 2017b).
55
Nonetheless, these adsorbents are plagued with slow uptake rate, low sorption capacity and
56
interference from co-existing macromolecules. A cost efficient and rapid adsorbent for 17 β-estradiol
57
is required. To overcome these challenges, development of chemically versatile adsorbents from
58
natural /waste materials such as graphite, clay, zeolites with green production and sustainability is
59
deemed as a positive for the planet.
60
Graphene (G) and graphene-based nanomaterials are exceptional materials for adsorption due to large
61
theoretical surface area (SA) (~2630 m2/g) and a variety of easily modifiable oxygen and carbon
62
functional groups on their surface. However, graphene oxide (GO) and reduced GO (rGO) do not
63
actualise their sorption potential because G/rGO are aggregated in wastewater which reduces SA and
64
severely limit sorption ability (Akpotu and Moodley, 2018a). Aggregation during adsorption is due to
65
strong interplanar interaction of graphene sheet. GO is hydrophilic and its separation from adsorbate is
66
difficult which hinders adsorptive capacity. Consequently, GO and rGO modification is essential for
67
recovery and real-world application in wastewater remediation. Separation/aggregation challenges in
68
GO/rGO are remedied by anchoring on a support (polymer, mesoporous silica (MS)) which provides
69
an expanded structure. The structure in solution due to stearic hindrance restricts aggregation, ensuring
70
adsorbents achieve full sorption potential.
71
MS are chemically inert, possess large SA, high pore volume, extensive silanol network which permits
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surface modification with organic species and applied as support for GO/rGO (Akpotu and Moodley,
73
2018e). Akpotu and Moodley (2018b) reported an increased sorption capacity of pharmaceuticals 3
74
when MS was organically modified. Gao et al. (2019), demonstrated that hydrophobic N-propyl
75
modified MCM-41 was effective in estrogen adsorption. Similarly, MS modification with GO/rGO has
76
been shown to increase sorption capacity toward pharmaceuticals in wastewater (Akpotu and
77
Moodley, 2018e). Polyethyleneimine (PEI), a hydrophilic polymer has been used to expand GO
78
structure and applied in adsorption of organic molecules. Reduced PEI interacts with organics through
79
hydrogen bonding and hydrophobic interaction (Geng et al., 2019). A polymer with similarities to PEI
80
is poly(ethylene glycol) (mPEG), which is a biological inert reagent with low toxicity, high
81
biocompatibility, excellent solubility in water and other solvents (Zalipsky, 1995; Xu et al., 2014).
82
mPEG modified nanomaterials has been widely used in drug delivery (Liu et al., 2016). Consequently,
83
mPEG is suited for water treatment because it does not leach or act as secondary pollutant. It is
84
therefore our opinion that mPEG functionalised nanomaterials would potentially be an efficient
85
adsorbent of organic pollutant from wastewater through an expansion of adsorbent structure and
86
functional groups manipulation. mPEG structure has an abundance of hyper branched network of C, H
87
and O with several routes to surface functionalisation/modification of the -O, -OH and C groups. The
88
presence of an extensive network of surface -O can be exploited through thermal/chemical reduction
89
resulting in a more hydrophobic molecule. Thus, mPEG is potentially an efficient adsorbent modifier
90
for the removal of water soluble/insoluble organic pollutants. This can be inferred from a study by Xu
91
et al. (2014) where mPEG was employed as a carrier for the delivery of hydrophobic anti-cancer drug
92
with high loading capacity. Liu et al. (2008), functionalised GO with mPEG which was effective in
93
uptake and delivery of aromatic anti-cancer drugs. GO and mPEG reaction is promoted by the
94
presence of hydroxyl and carboxylic acid groups interaction with GO planar structure resulting in an
95
expanded nucleated material highly ordered crystal structure (β or phase) (Lee et al., 2019). Effective
96
sorption of 17 β-estradiol requires a material with optimised surface functional groups, fast separation,
97
high SA, high sorption, easily separated and regeneratable.
4
98
In addressing these limitations, this article reports a facile approach for the development of multi-
99
functional groups adsorbents with a hybrid base of multi-carboxylic hyperbranched rGO-mPEG
100
modified mesoporous silica SBA-15 for efficient remediation of 17 β-estradiol. The ideation is based
101
on the covalent linkage of C and O from mPEG to GO via a simple polymerisation reaction. The
102
merits of the individual components of the hybrid adsorbents are (i) GO has a variety of oxygen
103
functional groups on its surface which are useful for adsorption, functionalisation and can be reduced
104
to rGO (ii) SBA-15 is an ordered material with large SA and pore volume which aids adsorbate
105
separation from adsorbent, also provide a platform for grafting and/or encapsulation of modifiers (iii)
106
mPEG is a large repeating hyperbranched polymer with an extensive network of carboxylic groups
107
that may significantly affect adsorption in its pristine condition or be a more effective medium towards
108
the removal of hydrophobic molecules when reduced. mPEG molecules are imprinted onto GO by a
109
ring opening polymerisation reaction ensuring an expanded structure and then grafted/encapsulated on
110
SBA-15. Subsequent in-situ chemical reduction of hybrid adsorbent (-OH groups) produces new
111
amine and carboxylic functionalisation ensuring improved hydrophobicity. This material has
112
advantages such as use of environmentally benign chemical and synthesis ease, thus making it cost
113
effective and easily scalable. To the best of the authors knowledge modification of silica with mPEG
114
and graphene oxide and subsequent in-situ reduction of the adsorbent materials and its application in
115
the adsorption of 17 β-estradiol from real water samples (river) and simulated water samples has not
116
been previously carried out. Synthesised materials were extensively characterised and applied for
117
batch sorption studies of 17 β-estradiol from simulated and real wastewater.
118
2. EXPERIMENTAL SECTION
119
2.1 Materials and Method. Pluronic P123 (average Mn ~5800, EO20PO70EO20), HCl (32%),
120
tetraethylorthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), methoxy polyethylene glycol
121
(mPEG),
122
(EDC.HCl), N-hydrosuccinimide (NHS), 17 β-estradiol, glutaraldehyde, and graphite powder were
hydrazine
hydrate,
N-(3-(dimethylamino)propyl)-N’-ethylcarbodiimide
5
hydrochloride
123
obtained from Aldrich and used without further purification. All reagents were used without further
124
purification.
125
2.2. Synthesis of SBA-15, NH2-SBA-15, GO, GO-mPEG, SBA-GO and XSBA-(r)GO-mPEG
126
SBA-15 was prepared by modifying the method of Chen et al. (2012). About 4 g of Pluronic P123
127
surfactant was dissolved in 30 mL of double-distilled deionised water and 120 mL of 2 mol L -1 HCl
128
under stirring. About 8.5 g of TEOS was added to the mixture at a temperature of 35-40 °C. The gel
129
was stirred continuously for 24 h and kept in a sealed bottle at 100 °C for 48 h. Afterwards, the
130
product was filtered and washed with double-distilled water. Thereafter, the material was dried in an
131
oven at 100 °C for 24 h and calcined at 550 °C for 6 h at a ramp rate of 2 °C min -1. This was labelled
132
as SBA-15.
133
Amine functionalised SBA-15 was synthesised with the method of Akpotu and Moodley (Akpotu and
134
Moodley, 2018e). About 1 g of SBA-15 was added to 20 mL of ethanol whilst stirring. Also, 1 mL of
135
APTES was added to the mixture and stirred for 24 h. Thereafter, the product was filtered and rinsed
136
with doubled-distilled water. The product was oven dried at 100 °C for 24 h and labelled as NH2-SBA-
137
15.
138
GO was synthesised using Tours’ method (Marcano et al., 2010; Akpotu and Moodley, 2018d),. A
139
specific amount of GO was dispersed in double-distilled deionised water placed in an ultrasonic bath
140
for 2 h. Thereafter, EDC.HCl and NHS were added as crosslinkers. Furthermore, certain amount
141
(ratio) of mPEG was added to the highly dispersed GO solution. The resulting hydrogel was stirred for
142
24 h at room temperature. The product was rinsed in a bid to get rid of any residual GO and mPEG
143
and subsequently freeze dried. mPEG molar ratio was varied and 0.10 mPEG, 0.25 mPEG and 0.5
144
mPEG and was used as a precursor for 0.10 GO-mPEG, 0.25 GO-mPEG 30 and 0.5 GO-mPEG.
145
mPEG ratio is denoted as X in the adsorbents.
6
146
XSBA-GO-mPEG was synthesised by stirring (1.0 g) NH2-SBA-15 with GO-mPEG at room
147
temperature. The pH was adjusted to 7 with the addition of NH4OH. Thereafter, 2 mL of
148
glutaraldehyde was added as a crosslinker. The product was filtered and washed with water and oven
149
dried for 6 h at 50 °C. XSBA-rGO-mPEG was synthesised with a similar procedure as XSBA-GO-
150
mPEG; however, the synthesis temperature was 80 °C. The mixture was stirred for 8 h and
151
subsequently filtered and washed with double-distilled water to a neutral pH (scheme 1). The product
152
was freeze dried and stored until further use.
7
153 154
Scheme 1. Schematic illustration of the synthesis of SBA-15-reduced graphene oxide-methyl ether polyethylene glycol
155
(SBA-rGO-mPEG). A = GO, B = mPEG, C = GO-mPEG, D = SBA-rGO-mPEG
156
2.3. Characterisation. Fourier-transform infrared (FTIR) spectra analysis of the samples were
157
recorded with a PerkinElmer Spectrum 100 spectrometer fitted with a universal ATR accessory, in the
158
range 4000−400 cm−1. Low and wide-angle X-ray diffraction (XRD) profiles of adsorbents were 8
159
recorded in the 2θ range of 1-5° and 10−90°, respectively with a Bruker D8 Advance instrument, using
160
a Cu Kα radiation source. The surface area, pore sizes and diameters of the adsorbents were
161
determined on a Micromeritics Tristar II 3020 instrument, and the surface area was calculated by BET
162
method. Prior to sample analysis, samples were degassed at 100 °C and under pressure for 12 h to
163
eliminate any physically adsorbed moisture. Morphological structural analyses of the samples were
164
carried out on a high-resolution transmission electron microscopy (HRTEM, JEOL 2100) and field
165
emission scanning electron microscopy (FESEM). The FESEM was used in energy dispersive X-ray
166
(EDX) analysis and elemental mapping. FESEM samples were gold coated on copper grids, while
167
samples for HRTEM analysis were kept in alcohol and sonicated for 30 min. Zeta potential of samples
168
were measured with a Malvern Zetasizer Nanoseries NanoZS instrument using a dip cell. Zeta
169
potential readings were obtained by adjusting the pH of the electrolyte (10 mM NaCl background
170
solution) with suitable quantities of 0.01 mol−3 HNO3 or NaOH. Elemental analyser of the samples
171
was determined with Thermo Scientific CHNS/O analyser. Thermogravimetric analysis of the
172
adsorbents were obtained by weighing a 5 mg sample mass in an aluminium pan (PerkinElmer) at a
173
temperature range of 50-900 °C and at a ramp rate of 5 °C min-1 in a nitrogen environment.
174
2.4. Analytical method. The 17 β-estradiol was quantified at a wavelength of 292 nm with a
175
Shimadzu Prominence UFLCxr (Shimadzu corporation, Japan) with two LC-20AD XR pump. An
176
Agilent C-18 Eclispe column (4.6 x 150 mm, 5 μm particle size) with a mobile flow rate of 1.0 mL
177
min⁻ 1 in isocratic conditions was used and an injection volume of 10 μL. The mobile phase was a
178
mixture of methanol/water (70:30; v/v). pH of the water used was adjusted to 2.3 with 1 mol L⁻ 1
179
phosphoric acid.
180
2.5. Adsorption studies. Adsorption experiments were performed in batch mode to investigate the
181
effects of pH, dose and contact time of 17 β-estradiol concentrations on SBA-15, SBA-GO, XSBA-15-
182
rGO-mPEG. Effect of dose was carried out by varying the amount of adsorbent from 10-60 mg in 20
183
mL, 15 mg L−1 of 17 β-estradiol solution. Effect of pH was carried out by adjusting the pH of 17 β9
184
estradiol (20 mL, 15 mg L−1) with 0.01 M NaOH/HCl from 2 – 10. This was carried out in a 50 mL
185
glass bottle with stopper containing the adsorbent materials. Blank sample analysis was carried out
186
under the same conditions as the samples without the adsorbents. For kinetic studies, experiments
187
were carried out by determining the amount of 17 β-estradiol adsorbed at optimum pH form different
188
solution concentration (50 – 200 mg L-1) while adding 150 mg of the adsorbents. Periodically, liquid
189
samples (2-1440 min) were withdrawn intermittently and concentration determined.
190
The concentration of 17 β-estradiol was determined and the amount adsorbed per unit of the adsorbent
191
mass, at a specific time, t (qt mg/L) was determined with the following equation 1:
192
(
)
(1)
193
V is solution volume (L), adsorbent mass is W (g) Co and Ce are initial and final 17 β-estradiol
194
concentration at a given time t (mg/L), V is solution volume (L). The percentage removal is calculated
195
with equation 2:
196
(2)
197 198
The sorption rate process was determined by fitting the experimental data obtained into pseudo first
199
order (PFO) (Lin and Wang, 2009), pseudo second order (PSO) (Ho, 2003), and intraparticle diffusion
200
models (Weber and Morris, 1963) with equation (SI, Kinetics).
201
Evaluation of equilibrium data from the sorption process of the adsorbents on 17 β-estradiol was
202
carried out by applying Langmuir (1918), Freundlich (1906), Dubinin-Radushkevich (DR) (Dubinin,
203
1947) and Temkin (Mane et al., 2007), isotherm models (SI, Isotherm).
204
Thermodynamics experiments was carried out by placing 20 mg of adsorbents in the 17 β-estradiol
205
solution which was agitated in a thermostatic shaker at temperatures of 298, 308 and 318 K for a 24 h
206
period. Thereafter, the solutions were centrifuged at 6000 rpm for 2 min and filtered with a 0.45 µm 10
207
cellulose acetate filter and the filtrate concentration was quantified with HPLC. Change in enthalpy
208
(ΔH°), Gibbs energy (ΔG°) and entropy (ΔS°) were calculated with data obtained from the
209
thermodynamics studies (SI, Thermodynamics).
210
2.6. Regeneration studies. To regenerate XSBA-rGO-mPEG, the adsorbents were preloaded with 17
211
β-estradiol, at neutral pH and ambient temperature. The preloaded adsorbents were washed with acidic
212
ethanol (adjusted with 0.1 M HCl) as eluent on a shaker for 180 min and dried under vacuum at 60 °C
213
to remove the bound 17 β-estradiol from the 0.5SBA-15-rGO-mPEG. The solution was filtered, and
214
filtrate concentration was determined with HPLC. Adsorption-desorption process was repeated 4
215
times.
216
2.7. Real water application study. In a bid to determine the efficiency of adsorption of the 0.5SBA-
217
15-rGO-mPEG on real water, river water was collected from Umgeni River (Durban, South Africa,
218
29° 37’16” S, 30° 40’ 46” E) in an amber bottle with an approximate pH of 6, filtered with a 0.45 µm
219
filter. A 10 mL aliquot of filtered real water sample was spiked with 0.150 mL (1000 mg/L) 17 β-
220
estradiol solution resulting in a 15 mg/L concentration. About 40 mg of the adsorbents were added to
221
the samples and shaken for 120 min at room temperature. The suspension obtained was filtered and the
222
concentration of the 17 β-estradiol left was determined with HPLC.
223
3. RESULTS AND DISCUSSION
224
3.1. Adsorbents characterisation.
225
FTIR analysis was carried out to validate functional groups attachment on SBA-15 surface as depicted
226
in Figure 1a. Absorption peak around 3200 cm-1 for all samples except SBA-15 and GO is attributed to
227
N-H stretching vibration (Tizaoui et al., 2017). However, the peak was relatively subdued by -OH
228
stretching vibration which arises from the interlayer water molecules present (Yap et al., 2018). GO
229
contains many oxygen functional groups on its basal plane which is attributed to oxidation. These
230
oxygen functionalities provides reactive sites for covalent modification by esterification or ring 11
231
opening reaction of the carboxyl and epoxy groups (Xu et al., 2014). For GO the vibration at 1729
232
cm-1 was assigned as -C=O stretch from the -COOH functional groups, and the peak at 1634 cm-1
233
emerged from the graphite framework vibration (-C=C-, -C-C-) (Akpotu and Moodley, 2018e). The
234
absorption vibrations at 1389 and 1068 cm-1 was assigned as stretching vibrations of -C-OH- and -C-O
235
groups, respectively. After the amidation reaction between GO and mPEG, the peak at 1732 cm-1
236
present in GO disappeared and a new peak appeared at 1643 cm-1 which signified the formation of
237
new amide functional groups (-NH-CO-). Prominent peaks at 2870 cm-1 and 1115 cm-1 showed the
238
presence of mPEG chains on the GO sheets surface. The peak at 2870 cm-1 is the symmetric stretch
239
mode of the -CH2 moiety on the carbon chain of the GO-mPEG and was found on the profile of
240
XSBA-15-rGO-mPEG. SBA-15, SBA-GO and XSBA-15-rGO-mPEG had peaks typical of siliceous
241
materials. This indicated that SBA-15 was successfully modified with GO and rGO-mPEG. The
242
absorption bands at 1642 and 1420 cm-1 on SBA-GO can be attributed to -C=O stretching vibrations.
243
The vibration present at 1026 cm-1 in XSBA-15-rGO-mPEG was attributed to Si-O-Si/Si-O-C
244
asymmetric vibration of which the latter vibration was covalently linked. Also, there was the
245
disappearance of the -C=O which was present in both GO and SBA-GO and a replacement with a new
246
amide peaks present at around and 1555 cm-1 and 1643 cm-1 due to the functionalisation reaction. The
247
peak at 1555 cm-1 is an amide group stretching vibration from the vinyl group which was successfully
248
introduced to the rGO-mPEG sheet due to the thermo-chemical reduction of SBA-GO-mPEG with a
249
reducing agent. The -N-C- bond enables the XSBA-rGO-mPEG to act as H-bond acceptor, also, the -
250
NH- bond allows the adsorbent to function as H-bond donor (Tizaoui et al., 2017). The presence of
251
rGO-mPEG in the interlayer of the silica sheet resulted in a symmetric vibration peak seen at 1370 cm -
252
1
253
surface of the SBA-15. The peaks at 430 and 790 cm-1 was due to Si-O-Si/silica-hydroxyl bending and
254
symmetric vibrations in the C-Si lattice, respectively. The obtained results demonstrated that the
. The absorption vibrations at 1132 and 2867 cm-1 was due to the presence of rGO-mPEG on the
12
successful grafting of rGO and mPEG in the XSBA-rGO-mPEG material through a ring opening
256
polymerisation.
(a)
(b)
(100)
SBA-15
(c)
Transmittance/a.u
Intensity/a.u
SBA-GO 0.1SBA-rGO-mPEG
0.25SBA-rGO-mPEG
0.5SBA-rGO-mPEG
OH stretch
CH2
stretch
rGO peak
Intensity/a.u
SBA SBA-GO 0.10SBA-rGO-mPEG 0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
GO
Intensity/a.u
255
-GO
10
20
30
40
2 Theta/
50
60
SBA-15
GO peak
SBA-GO 0.1SBA-rGO-mPEG 0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
(110) (200)
C=O NHCO
Si-O-Si
2000
1500
1
500
2
80 70 60 50
30 20
4
5
600
20
30
400
40
50
60
70
(e) 0.10SBA-rGO-mPEG
SBA-15 SBA-GO
200 0 0.0
0.2
0.4
0.6
0.8
1.0 o
GO 0.1SBA-rGO-mPEG 0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
10 0.0
10
2 Theta/
0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
500
SBA-GO
GO
1000 1500 2000 2500 3000
Raman shift/cm-1
0
257
6
(d)
800
Relative Pressure/ P/P
40
3
2 Theta/
o
Wavenumbers/cm-1
1000
Intensity/a.u
2500
Intensity/a.u
3000
Quantity adsorbed/ P/P
3500
Quantity adsorbed/ cm3/g
4000
0.2
0.4
0.6
0.8
1.0
500
1000
1500
2000
2500
3000
Raman shift/cm-1
Relative Pressure/P/Po
258
Figure 1. (a) FTIR, (b) Low angle XRD, (c) Wide angle XRD, (d) N 2 adsorption desorption isotherm, (e) Raman for GO,
259
SBA-15, SBA-GO, 0.1SBA-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG.
260 261
X-ray diffraction profiles for SBA-15, GO, SBA-GO, and XSBA-15-rGO-mPEG are presented
262
(Figures 1b and c). Small angle XRD profile of SBA-15 obtained between 0-5(θ) was defined by three
263
characteristic diffraction peaks (2θ = 0.84° (100), 1.5° (110) and 1.8° (200)) and the corresponding d
264
spacings are 10.5, 6.0 and 5.3 nm, respectively, which revealed its hexagonal p6mm 2D well-ordered
265
hexagonal pore structure (Chen et al., 2012). The 3 peaks are indexed as 100, 110 and 200 have a
266
specific d spacing ratio of 1:1/√3:1/2. After modification with GO, there was a very slight change in
267
diffraction peak/intensity of SBA-GO which could be attributed to the presence of amorphous GO on 13
80
268
the internal and external surface of the SBA-15. In contrast, modification with rGO-mPEG, caused a
269
reduction in peak intensities but no significant shift in diffraction peaks between the 0.5-10°. The
270
reduction in the prevailing peak at 2θ = 0.8° (100) could be attributed to chemical reduction and the
271
introduction of new carbon functionalities from the bulky mPEG molecule. The main diffraction peaks
272
of pristine SBA-15, SBA-GO, and 0.1SBA-15-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-
273
mPEG are 0.84, 0.83, 0.85, 0.83 and 0.82, respectively. There was a shift to lower angles as compared
274
to pristine SBA as GO/rGO-mPEG was loaded and this could be attributed to contraction of the silica
275
framework. This was also observed by Lu et al. (2009). For GO, the intense diffraction (001) peak at
276
2θ = 9.76° with an interplanar basal spacing (8.71 Å) confirms the introduction of oxygen containing
277
functional groups and complete oxidation of graphite with peak which is present at 25.6° (Akpotu and
278
Moodley, 2018b). The decreased interlayer spacing (d001) of the reduced materials (XSBA-rGO-
279
mPEG) to 7.81 Å and the disappearance of the GO peak (9.76°) signified the partial reduction of GO
280
and the chemical functionalisation from the bulky mPEG molecules. This created a larger spacing
281
between the rGO-mPEG and the silica sheets. The wide angle XRD patterns of SBA-15, SBA-GO and
282
XSBA-rGO-mPEG had a broad amorphous siliceous peak between 2(Ɵ) 15-35°. SBA-GO had a peak
283
at 2(Ɵ) = 26.1° which is attributed to the partial transformation of the GO containing functional groups
284
on its surface as was also observed by Li et al. (2015). Whereas, XSBA-rGO-mPEG had a peak at
285
2(Ɵ) =25.1 which is attributed to the reduction of graphite as was also observed by Luo et al. (2013).
286
In addition, the disappearance of the 2θ = 9.76° peak is also evidence of the reduction of GO.
287
The elemental analyses (EA) of the adsorbents are presented in Table 1. For this calculation, it should
288
be noted that the percentage of silica was not considered, and this assumption was necessary to
289
calculate the ratio of O/C, N/C and H/C. SBA-GO had a higher H/C ratio but a lower O/C as compared
290
to XSBA-rGO-mPEG samples which implied that SBA-GO was more hydrophilic and less
291
hydrophobic. Interestingly, it was observed that as the mPEG ratio increased, the H/C decreased which
292
implied that the materials became more hydrophobic. Amide groups occurrence was verified by EA, 14
293
because prior to amidation, the N content in SBA-15 was negligible (< 0.05%) but after amidation it
294
increased to 1.92-2.03%. EA studies corroborated the FTIR (peaks around 1132 and 2867 cm-1) which
295
indicated the successful covalent grafting of the mPEG chains onto the surface of the reduced GO
296
sheets. For XSBA-rGO-mPEG materials, this proved that rGO-mPEG was successfully embedded in
297
the SBA-15 structure. An increase in mPEG ratio of the composites resulted to lower N/C ratio with
298
increased hydrophobicity. This corroborates the results obtained from the FTIR analysis.
299 300
Table 1. Elemental composition and textural characteristics of adsorbents H%
N%
O%
H/C
O/C
N/C
SA/m2/ g
Sample
C%
Pore volume/cm3/ g
GO SBA-15 SBA-GO 0.1SBA-rGOmPEG 0.25SBArGO-mPEG 0.5SBA-rGOmPEG
37.6 8.02 30.3
2.23 1.75 2.93 4.82
0.58 0.05 2.2 2.03
59.5 10.92 86.9 62.9
0.71 4.38 1.91
1.75 8.13 1.56
0.015 0.27 0.07
39 625 278 39
0.02 1.09 0.62 0.31
Average pore diamete r (nm) 38.2 7.9 6.54 4.6
31.9
5.47
1.95
60.9
2.06
1.43
0.061
29
0.3
4.3
32.5
5.63
1.92
60
2.07
1.38
0.059
23
0.3
4.1
301 302
N2 adsorption/desorption isotherms measurement were carried out and BET method was used to
303
calculate surface area and other textural characteristics of the adsorbents (Figure 1d) with detailed
304
analysis presented (Table 1). All synthesised materials had type IV isotherm as defined by IUPAC
305
(Kruk and Jaroniec, 2001). It was observed that increased rGO-mPEG ratio grafted onto silica resulted
306
in an inverse marked reduction of SA and mesoporous volume due to pore filling as compared to
307
pristine SBA-15. GO had a H3 hysteresis loop that do not level off at relative pressures close to
308
saturated vapour pressure which is characterised by loose aggregates of platelike particles forming
309
narrow slit like pores of irregular shape and broad size distribution. SBA-15, SBA-GO had H1
310
hysteresis loop which was between 0.5
311
mesopores. This loop is typical of a highly ordered cage-like cylindrical mesoporous material with
312
high pore size uniformity and interconnecting mesopores network (Mirzaie et al., 2017). XSBA-rGO-
313
mPEG had H3 hysteresis loop which is characterised by high porosity and interconnection. XSBA-
314
rGO-mPEG had lower point of inflection on the P/Po which confirms that rGO-mPEG has been
315
successfully grafted into the pores of SBA-15 as was also observed by Boukoussa et al. (2018). The
316
increased surface area of the XSBA-rGO-mPEG as compared to GO can be attributed to the spacing of
317
the GO sheet in the pores of SBA-15. The pore sizes of the modified materials were approximately 15
318
times larger than GO. The effective interlayer separation of rGO sheets resulted in SA enhancement
319
with more active sorption sites and larger pore volume which improves the adsorbents sorption
320
capabilities.
321
Raman spectra (Figure 1e) of GO, SBA-GO and XSBA-rGO-mPEG show 2 major bands at
322
approximately1334 and 1598 cm-1 which corresponds to the D and G bands, respectively. The G band
323
is the in-plane vibration of the sp2 C transitioning to sp3 hybridized C which could be attributed to the
324
destruction of the sp2 graphitic structure or the covalent attachment of functional groups. In contrast,
325
the D band represents the defects in the graphitic structure. There was a slight shift in the G band
326
values of GO as compared to SBA-GO and XSBA-rGO-mPEG which indicates functionalisation
327
(Kabiri et al., 2015). The ID/IG intensity ratio is a measure of the degree of functionalisation and an
328
evidence of carbon structural defects. XSBA-rGO-mPEG had a higher ID/IG of 1.08 which can be
329
attributed to the presence of the disordered partially reduced GO-mPEG as compared to SBA-GO and
330
GO of 1.02 and 0.95, respectively. The results obtained further reaffirmed the functionalisation of
331
SBA-15 with rGO-mPEG. The results of the Raman, TGA, XRD, EA and FTIR characterisation
332
provides irrefutable evidence of the synthesis of XSBA-rGO-mPEG through the formation of new
333
amine, amide and -CH2 group from the rGO-mPEG on the SBA.
334
FESEM and HRTEM was used to characterise the microstructure and morphological differences
335
between SBA-15, GO and XSBA-rGO-mPEG (Figure 2). The SEM image of SBA-15 appeared as 16
336
vertical stacks of lengthy interconnected tubular channel-like constrictions. GO (Figure 2a) appeared
337
as a transparent film with a rough wrinkled surface and very agglomerated. SBA-GO had a similar
338
appearance as SBA-15, however, uniform GO sheets can be seen on its surface. XSBA-rGO-mPEG
339
(Figures 2d-f) composed of SBA-15 which were grown and reduced in-situ in GO-mPEG. They
340
appeared to be highly crosslinked with a 3D porous architecture on the reduced GO sheets.
(a)
(c)
(b)
GO
(d)
rGO-mPEG
(e)
rGO-mPEG
(f)
(g)
(h)
(k)
(l)
rGO-mPEG
GO
(i)
(j)
rGO-mPEG
rGO-mPEG
rGO-mPEG
341 342
Figure 2. SEM micrographs of (a) GO (b) SBA-15 (c) SBA-GO (d) 0.1SBA-rGO-mPEG (e) 0.25SBA-rGO-mPEG (f)
343
0.5SBA-rGO-mPEG; HRTEM micrographs of (g) GO (h) SBA-15 (i) SBA-GO (j) 0.1SBA-rGO-mPEG (k) 0.25SBA-rGO-
344
mPEG (l) 0.5SBA-rGO-mPEG
345 346
HRTEM images (Figures 2g-l) showed the structure of SBA-15 to have an ordered 2-dimensional
347
(p6mm) hexagonal symmetry. The identical and uniform mesoporous channels which bares similarity
348
with a honeycomb is clearly seen. GO appeared as a smooth transparent surface with large pores. In
17
349
SBA-GO, the transparent GO film is visible as it appears well layered over the mesoporous channels
350
of the SBA-15. A similar observation was made for XSBA-rGO-mPEG, as the layered structure of the
351
reduced GO-mPEG can be seen over the mesopores.
352
Figure 3 shows the EDX profile and elemental mapping for 0.5SBA-rGO-mPEG. All the elements (C
353
44.7%, Si 11.7%, O 43.5%) were present in different percentages and uniformly distributed.
354
(a)
(b)
(c)
355 356
Figure 3. EDX image and mapping of 0.5SBA-rGO-mPEG
357
3.2. ADSORPTION STUDIES
358
Preliminary studies were carried out to determine the sorption potential of 17β-estradiol on SBA-15,
359
SBA-GO, 0.1SBA-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG at pH 4. It was
360
observed that adsorption with SBA-15, was negligible, SBA-GO had ~13.4 %, 0.1SBA-rGO-mPEG
361
had ~30.2 %, 0.25SBA-rGO-mPEG had ~35.4 % and 0.5SBA-rGO-mPEG exhibited excellent
18
362
sorption capacity over 60%. Thus, SBA-15, SBA-GO and 0.5SBA-rGO-mPEG adsorption properties
363
were further investigated.
364 365 366
3.2.1. Influence of Solution pH.
367
The impact of solution pH towards 17β-estradiol removal by the sorbent materials was studied over a
368
pH range of 2-10 and an initial 17β-estradiol concentration of 15 mg/L (Figure 4). Our proposed
369
mechanism for the removal of the 17β-estradiol are (i) hydrophobic and π-π interactions (ii) hydrogen
370
bonding (iii) physical trapping by the amine-amide groups on the XSBA-rGO-mPEG adsorbents.
371
Removal efficiencies of 15.2, 39.5, 44, 65 % were observed for SBA-GO, 0.1SBA-rGO-mPEG,
372
0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG, respectively. A direct relationship was observed
373
between adsorbent uptake and an increase in the rGO-mPEG concentration. This could be due to the
374
increasing non-polar nature of the adsorbents and also the lower oxygen content. Oxygen containing
375
functional groups could reduce the access to hydrophobic organic compounds, invariably reducing the
376
available number of adsorption sites due to water cluster formation on their surface (Jiang et al.,
377
2017b). This is because the water flux of the XSBA-rGO-mPEG is improved due to the reduction of
378
the rGO-mPEG sheets on the SBA-15. Thus, resulting in narrow planar channels and reduced
379
hydrophilic properties of the rGO-mPEG sheets with decreased water permeability. This phenomenon
380
partly explains the inverse relationship between the reducing oxygen content (Table 1) of the
381
adsorbents and increase in 17β-estradiol removal as the ratio of rGO-mPEG increased in XSBA-rGO-
382
mPEG. This observation is further supported by the negative zeta potential value of XSBA-rGO-
383
mPEG across the pH range of 2-10. Zeta-potential (Figure 4) values was used to provide further
384
insight into the effect of pH on the adsorption capacity of the adsorbents. The study showed that the
385
sorbent materials were effective for the removal of 17β-estradiol from solution with the highest 19
386
removal efficiency of approximately 60% at pH 2 for 0.5SBA-rGO-mPEG. This observation was
387
supported by the very negative zeta potential value of ~ -40 mV obtained for 0.5SBA-rGO-mPEG. At
388
a pH of 2, 17β-estradiol exists as neutral molecules in solution, therefore its high sorption efficiency
389
via hydrophobic interaction. Increased adsorption can be attributed to the electron rich surface of the
390
adsorbents which promotes the formation of H-bond between the adsorbent and the adsorbate.
391
Across the pH range, the differences in removal efficiency for all adsorbents was negligible with only
392
a slight dip occurring at neutral to basic pH. This is ascribed to the surface charge present on XSBA-
393
rGO-mPEG and the different molecular species of 17β-estradiol in solution at varying pH. This
394
behaviour could also be linked to the ionisation of 17β-estradiol in aqueous solution because 17β-
395
estradiol molecules are weak Lewis acid and strongly pH dependent during ionisation. Deprotonation
396
of 17β-estradiol molecules in solution starts at about pH 8 and at pH 9.2, it is apparent. Hydroxyl ions
397
are strong base and are proficient in proton extraction from the phenolic -OH moieties of 17β-estradiol
398
molecules from solution which results in deprotonation. Deprotonation increases the 17β-estradiol
399
solubility in aqueous solution and causes dissociation of H-bond between the protons on 17β-estradiol
400
and the adsorbents functional groups (Jiang et al., 2017a). For all adsorbents the zeta potential values
401
were negative range with very little impact on adsorption. Therefore, this implies that surface charge
402
does not play a significant role in adsorption. As a further confirmation that electrostatic interaction
403
did not significantly contribute to adsorption, the pKa of 17β-estradiol is 10.46 which signifies that its
404
impact on adsorption was minimal in the pH range of 2-7 because it was present as neutral molecules.
405
As the pH of the solution approached 9, most of the 17β-estradiol molecules have been deprotonated
406
which resulted in a slight dip in percentage removal and adsorbent hydrophobicity. Furthermore, an
407
increase in solution alkalinity resulted in several functional groups such as amide, hydroxyl and
408
carboxyl groups present on the XSBA-rGO-mPEG surface tends to be slightly deprotonated which
409
results in the adsorbents surface to be more negatively charged. An increase in anionic 17β-estradiol
410
molecules inhibits further sorption and uptake due to charge similarities/electronic repulsion between 20
411
the adsorbate and adsorbent. The adsorbent material was found to be significantly effective across a
412
wide pH range. Therefore, this further supports the potential application of this adsorbent as efficient
413
adsorbent of 17β-estradiol from real water sample via hydrophobic and π−π interactions.
-25 -30
0 -5
10
-10 -15
5
-20
0 30 -10 20 -20 10
-25
-35 0
2
4
6
8
2
10
-30
-30 4
6
8
0
10
2
4
pH
pH 50
70
10
(d) -10
20
-20
10
-30
0
Removal efficiency/%
30
Zeta Potential/ mV
0
4
414
6
8
(e)
10
-50
50
-40
40
-30
30
-20
20
-10
10
0
0
-40 2
8
-60
60
40
6
pH
10 2
10
Zeta Potential/ mV
-40
4
6
8
10
pH
pH
415
Figure 4. Effect of pH removal efficiency and zeta potential of 17β-estradiol carried out at different pH on (a) SBA-15 (b)
416
SBA-GO (c) 0.1SBA-rGO-mPEG (d) 0.25SBA-rGO-mPEG (e) 0.5SBA-rGO-mPEG. Conditions: 15 mg/L of 17β-
417
estradiol, T = 25 °C, n = 2.
418
3.3. Kinetics studies. Contact time effect of the adsorption of 17β-estradiol onto SBA-GO and
419
XSBA-rGO-mPEG results is presented (Figure 5). For all adsorbents, initially, there was rapid
420
removal of 17β-estradiol as the interaction period increased between the adsorbate and adsorbents. In
421
the next phase, the increment was gradual until it became negligible. At the initiation stage (2-30 min)
422
of the reaction, the availability of active sorption sites on the adsorbents resulted in the rapid removal
423
of 17β-estradiol. As the reaction progressed and contact time increased, sorption sites becomes
424
saturated and uptake became slower. Equilibrium was reached around the 30 min mark, thus, depicting
21
Zeta Potential/mV
-20
15
(c)
40
5
Zeta Potential/mV
Removal Efficiency/%
-15
10
10
(b)
-10
Removal Efficiency/%
Zeta Potential/mV
20
(a)
-5
Removal Efficiency/%
0
425
fast sorption rate by the adsorbents. XSBA-rGO-mPEG had higher sorption capacities as compared to
426
SBA-GO with 0.5SBA-rGO-mPEG having the highest efficiency which is attributed to its highly
427
hydrophobic nature (Table 2).
(b)
50
(a)
40
200 mg/L
20
150 mg/L
qt/ mg/g
qt/ mg/min
40 30
200 mg/L
30 150 mg/L 20 100 mg/L
100 mg/L 10
50 mg/L
50 mg/L
0 0
400
800
75 mg/L
10
75 mg/L
0 1200
0
1600
400
800
1200
1600
Time/min
Time/min 180
120
(d)
(c) 160
120
200 mg/L
80
150 mg/L
60
100 mg/L
100 mg/L
100 80
75 mg/L
60
40 75 mg/L
40
20
50 mg/L
20
50 mg/L
25 mg/L 0
0 0
428
200 mg/L
140
qt/ mg/g
qt/ mg/g
100
400
800
1200
0
1600
400
800
1200
1600
Time/min
Time/min
429
Figure 5. Effect of time on the adsorption of 17β-estradiol (a) SBA-GO (b) 0.1SBA-rGO-mPEG (c) 0.25SBA-rGO-mPEG
430
(d) 0.5SBA-rGO-mPEG. Conditions: T= 25 °C, time = 1440 min, concentration = 25-200 mg/L, dose = 150 mg, n=2.
431 432
In a bid to determine the adsorption dynamics, rate, mechanism and transport of 17β-estradiol onto the
433
various adsorbents, kinetics models such as PFO, PSO and IPD models were fitted against the
434
experimental data obtained from the time experiments.
435
The kinetics parameters for the models applied in the sorption studies of 17β-estradiol onto all the
436
adsorbents are presented in Table 2. The suitability of the model that describes the data was selected 22
437
based on the closeness of R2 value of the non-linear regression analysis to unity. The experimental
438
data obtained was for the sorption of 17β-estradiol onto SBA-GO, XSBA-rGO-mPEG best suited the
439
PSO model. Furthermore, qe values obtained from the PSO model was much closer to the
440
experimental values. PSO model is routinely applied to describe the adsorption of pollutants from
441
aqueous system in wastewater treatment process which occurs through chemisorption viz-a-viz the
442
number of sites accessible for the exchange process at solution-solid interface. It assumes that there is
443
bimolecular interaction between the adsorbent and the adsorbate molecule in solution which causes
444
adsorption. This process involves the exchange and sharing of electrons with the highly hydrophobic
445
17β-estradiol molecules and the amido-carbonic of the rGO-mPEG groups of the XSBA-rGO-mPEG
446
adsorbents. This implied that the adsorption rate was predominantly controlled through chemical
447
sorption and the number of active sites on the adsorbents determined the adsorption capacity (Akpotu
448
and Moodley, 2018a). An increase in the amount of rGO-mPEG in XSBA-rGO-mPEG resulted in the
449
creation of more active site on the adsorbents, consequently increasing the uptake of 17β-estradiol
450
from solution (Table 2). The rate constant value k2 of 0.5SBA-rGO-mPEG was 2.0761 g/mg.min and
451
higher than that of the other sorbent materials.
452
An increase in boundary thickness (Kl) of the rGO-mPEG adsorbents was observed as the
453
concentration of rGO-mPEG in SBA increases (Table 2). Increased thickness in l is likely to have
454
significantly impacted the adsorption of 17β-estradiol on the sorbent materials. Thus, implying the
455
adsorption of 17β-estradiol on the sorbent materials proceed through a multi-phase adsorption process.
456
This process may involve simultaneous chemical interaction such as hydrophobic, π−π and hydrogen
457
bonding between the adsorbate and the adsorbents.
458
The rate controlling step of the sorption process was determined using the IPD model. This model
459
assumes adsorption occurs through 4 processes; (a) diffusion of the 17β-estradiol molecules from the
460
bulk solution onto the surface of the adsorbents, (b) adsorbate diffusion through the boundary layer to
461
the surface of the adsorbents, (c) sorption of adsorbate onto the adsorbent active sites and, (d) initial 23
462
fast adsorption of adsorbate followed by slow intraparticle diffusion to the internal adsorbent surface.
463
The rate controlling step is normally the slowest step of the sorption process and this may stage can be
464
attributed to external mechanisms or intraparticle diffusion. An examination of the qt vs t1/2 plots
465
shows that it did not pass the origin. Hence, this affirms that IPD was not the only rate controlling step
466
(Sen et al., 2012). This further confirms that the sorption of 17β-estradiol onto SBA-GO and XSBA-
467
rGO-mPEG could be said to be a multistep adsorption process and of a heterogeneous nature.
468
Table 2. Kinetics Parameters for the Adsorption of 17β-estradiol on SBA-GO and XSBA-rGO-
469
mPEG adsorbents
Model
Parameters
SBA-GO
0.1SBA-rGO-
0.25SBA-rGO-
0.5SBA-rGO-
mPEG
mPEG
mPEG
Experimental
qexp/ mg/g
5.5800
8.5629
17.7000
25.4230
Pseudo-first-order
k1/ min-1
0.0067
0.0078
0.0079
0.0099
qeq/mg/g
0.6182
1.3897
2.2867
2.6325
R2
0.6020
0.8188
0.8990
0.9417
k2/ g/mg/min
0.6658
1.1626
1.7779
2.0761
qeq/mg/g
5.6200
8.6500
17.9000
25.7060
R2
0.9999
0.9999
0.9999
0.9999
Ki/ mg/g/min-0.5
0.0971
0.1523
0.5014
0.6610
C/ mg/g
3.0628
4.3616
7.6630
11.8250
R2
0.4044
0.6015
0.5846
0.5588
Pseudo-second-order
Intraparticle diffusion
470 471 24
472
3.4. Adsorption Isotherms. Equilibrium isotherms of 2 parameters (Langmuir, Freundlich, Temkin
473
and Dubinin-Radushkevich) were applied to study the adsorption interaction, mechanism and evaluate
474
the equilibrium between 17β-estradiol and the surface of the adsorbents. The suitability of the models
475
that best describes equilibrium data was obtained from the regression analysis that was closest to unity
476
(1). Based on Giles system for isotherm classification, the isotherms are classed as L type based on
477
shape (Figure S3) (Giles et al., 1960). This suggests that adsorbed solutes were vertically oriented and
478
there was strong interaction between the adsorbate molecules and the adsorption sites of the sorbent
479
materials (Okoli et al., 2014; Akpotu and Moodley, 2018b; Lawal and Moodley, 2018). The isotherm
480
parameters for the models that fits the equilibrium data for the adsorbents are presented (Table 3).
481
Langmuir isotherm was the most suited for the equilibrium data obtained for the adsorbents. This
482
model assumes that the adsorption of 17β-estradiol occurred on a monolayer surface with similar
483
energies without interaction between adsorbed entities. Langmuir model is usually suited to the
484
occurrence of adsorption on diverse sites with similar energies (Yap et al., 2018). Based on isotherm
485
and experimental data obtained, the interaction between the 17β-estradiol and the XSBA-rGO-mPEG
486
was principally through H-bonding, π−π and hydrophobic interactions because of the higher qm values
487
obtained. These can be explained by the hydrophobic nature of the XSBA-rGO-mPEG and the
488
corresponding chemical structure of 17β-estradiol which possesses phenolic moiety in position C3. The
489
17β-estradiol hydroxyl moieties have high H-bond donor activity towards XSBA-rGO-mPEG
490
molecules. The phenolic moiety in 17β-estradiol could also act as H-bond donor and acceptor to the
491
XSBA-rGO-mPEG (Duax et al., 1976; Fevig et al., 1988). The highest qm value was obtained for
492
0.5SBA-rGO-mPEG (192.3 mg/g). Non-dimensional separation factor (RL) (SI, isotherm) which is
493
obtained from the Langmuir isotherm is used to calculate the favourability factor (Hall et al., 1966).
494
RL values obtained was (0.012-0.092) which shows that the adsorption was favourable.
495 496 25
497
Table 3. Isotherm Parameters for the adsorption of 17β-estradiol onto SBA-GO and XSBA-
498
rGO-mPEG
Isotherm
Parameter
SBA-GO
0.1SBA-rGO-mPEG
0.25SBA-rGO-mPEG
0.5SBA-rGO-mPEG
Langmuir
qm (mg/g)
57.10
75.80
102.60
192.30
b (L/mg)
0.3538
0.3568
1.6333
3.0888
R2
0.985
0.982
0.982
0.980
Kf(mg/g/(mg/L)1/n)
3.02
4.28
6.18
9.35
N
1.26
1.36
1.37
1.64
R2
0.996
0.984
0.995
0.993
E (kJ/mol)
3.53
7.45
2.36
7.07
qD (mg/g)
0.96
2.09
2.65
3.01
B (mol/kJ2)
1.0X10-6
8.0X10-6
9.0X10-6
6.0X10-6
R2
0.9143
0.9956
0.9826
0.8628
B
36.7
71.9
149.7
193.6
b (j/mol)
12.8
16.5
34.4
67.5
A
34
37.7
52.8
137.7
R2
0.9558
0.849
0.9321
0.9521
Freundlich
DubininRadushkevich
Temkin
499 500
Langmuir maximum capacities (qm) obtained from this study was favourable when compared to that
501
obtained from similar adsorbents in other studies (Table 4). A comparison of the Langmuir parameters
502
to Freundlich shows consistency in value. This is further affirmed by the R 2 value obtained from the 26
503
Freundlich isotherm (Table 2). Freundlich model assumes that 17β-estradiol were absorbed onto a
504
non-uniform surface into multilayers of the adsorbents and adsorption involves several mechanisms.
505
For Freundlich model, metals are adsorbed on a non-uniform surface into multilayers, and the
506
adsorption capacity rises infinitely with increasing concentration. Highest KF value was obtained for
507
0.5SBA-rGO-mPEG which suggests that it had higher sorption capacity and affinity for 17β-estradiol
508
in solution. The KF values also corresponds with the qm value obtained from the Langmuir isotherm.
509
N>1 greater value shows that the adsorption of 17β-estradiol on the adsorbents was favourable.
510
D-R model was used to determine if the adsorption was a physical or chemical process (Dubinin,
511
1947). The E values (Table 3) for 17β-estradiol on the adsorbents was between the range of 2.36-7.45.
512
These values were less than E < 8.00 kJ mol−1 which indicated that the adsorption process was physical
513
(Akpotu and Moodley, 2016).
514 515 516 517 518 519 520 521 522
27
523
Table 4. Comparison of Langmuir (qm) values for the sorption of 17β-estradiol onto different
524
adsorbents
525
Adsorbent
SBET (cm3 g-1)
Isotherm model
β -CD-PLGraphene SiO2-GO
105.5
qm (mg.g-1)
pH
Reference
Langmuir 85.5
Equil. Kinetic Time model (min) 480 PSO
4.2-5.1
Jiang et al. (2017a)
132
Langmuir 101.48-141.89
480
PSO
9
Jiang et al. (2018)
Al2O3-GO
132
Langmuir 89.51-141.69
480
PSO
9
Jiang et al. (2018)
GO
92
Langmuir 149.4
480
PSO
7
Jiang et al. (2016)
GO
15.4
Langmuir 96.2
NA
NA
NA
Sun et al. (2017)
Magnetic GO1
247.4
Langmuir 52.9
720
PSO
3
Bai et al. (2017)
Magnetic GO2
251.6
Langmuir 106.38
720
PSO
3
Bai et al. (2017)
SBA-GO
278
Langmuir 57.1
30
PSO
4
This study
0.1SBA-rGOmPEG 0.25SBA-rGOmPEG 0.5SBA-rGOmpEG
39
Langmuir 75.8
30
PSO
4
This study
29
Langmuir 102.6
30
PSO
4
This study
23
Langmuir 192.3
30
PSO
4
This study
NA-not applicable, βCD-beta cyclodextrin
526 527
3.5. Effect of temperature and thermodynamics studies. Change in adsorbate temperature has been
528
shown to influence adsorption capacity of adsorbents (Khan et al., 2012). The resultant change in
529
sorption capacity of adsorbents can be attributed to changes in; rate of diffusion of the adsorbed
530
molecules; variation in viscosity of adsorbate; changes in adsorptive forces which plays a critical role
531
in adsorption and a variation in adsorbent textural properties (Srivastava and Rupainwar, 2009; Khan
532
et al., 2012). A temperature range of 298-318 K was considered for the adsorbents (Figure S4).
533
A slight increase in sorption capacity was noticed for SBA-GO and XSBA-rGO-mPEG when
534
adsorbate temperature was increased. Increase adsorbate temperature results in increased diffusion rate
535
across external boundary layer, decrease in solution viscosity, increased activation of sorbent active
536
sites, increased pore volume and adsorbent textural properties (Srivastava and Rupainwar, 2009; Khan
537
et al., 2012). The above-mentioned factors resulted in increased uptake of the 17β-estradiol molecules
28
538
onto the adsorbents active sites due to increased mobility of the adsorbate molecules. Thus,
539
demonstrating that adsorption was endothermic and shows the sorbents to be efficient for the
540
remediation point source pollution. Furthermore, the adsorbents showed effectiveness and
541
functionality across a range of low and high temperatures. Thermodynamics parameters such as
542
changes in mean free energy (ΔG°), entropy (ΔS°) and enthalpy (∆H°) were obtained for the
543
adsorption of 17β-estradiol onto SBA-GO, XSBA-rGO-mPEG adsorbents (Table 5). Negative ΔG°
544
values obtained established the spontaneity and thermodynamic favourability of the adsorption process
545
which increased with increase in adsorbate temperature. The adsorption of 17β-estradiol was favoured
546
at higher temperature. The positive ∆H° and ΔS° values obtained for the adsorbents, indicates the
547
adsorption process was endothermic and had increased randomness/disorderliness between the
548
solid/solution interface, respectively. This shows the adsorption process was entropy driven. 17β-
549
estradiol ions interaction with the adsorbents could be inferred from adsorption heat (∆H°). ∆H° value
550
between obtained for adsorbate-adsorbent interaction between 2-20 KJ is termed as physisorption, in
551
contrast values between 80-200 KJ is chemisorption. This implies that SBA-GO interaction was
552
physisorption, 0.1SBA-rGO-mPEG was partly phyisorption and chemisorption because the 30.7 KJ
553
value obtained was between physisorption and chemisorption. However, 0.25SBA-rGO-mPEG and
554
0.5SBA-rGO-mPEG lean towards to chemisorption as they had higher mPEG concentration. The
555
thermodynamic parameters show the adsorption process was spontaneous and was effective for the
556
removal of 17β-estradiol (organic pollutants) from wastewater.
557 558 559
29
560
Table 5. Thermodynamic parameters for the adsorption of 17β-estradiol on SBA-GO and XSBA-rGO-
561
mPEG
Adsorbents SBA-GO
0.1SBA-rGO-mPEG
0.25SBA-rGO-mPEG
0.5SBA-rGO-mPEG
T/K 298.15
ΔH°/kJ/mol 2.37
ΔS°/kJ/mol 0.05
ΔG°/kJ/mol -13.2
308.15
-13.7
318.15
-14.2
298.15
30.7
0.19
-25.9
308.15
-27.8
318.15
-29.7
298.15
161.5
0.59
-30.2
308.15
-31.8
318.15
-36.8
298.15
165.1
0.67
-36.4
308.15
-43.2
318.15
-49.9
562
563
3.6. Adsorption mechanism. An understanding of the sorption mechanism of 17β-estradiol onto
564
SBA-GO and XSBA-rGO-mPEG adsorbents is essential for future studies. Previous studies on
565
adsorption of estrogen onto silica, GO, rGO, organo-silica and silica-graphene composites are through
566
adsorption mechanisms such as; H-bond, coordinate bonding, hydrophobic and π−π interactions (Jiang
30
567
et al., 2017b; Akpotu and Moodley, 2018e). In a bid to obtain insight into the adsorption mechanism of
568
17β-estradiol on XSBA-rGO-mPEG, an examination of the effect of solution pH (Figure 4) was
569
carried out. Solution pH can change the adsorbent and adsorbate surface charges, consequently,
570
strongly influence the adsorption process via electrostatic interactions. From pH 2-10, there was
571
insignificant changes in adsorption which indicates that adsorption was not through electrostatic
572
interactions. XSBA-rGO-mPEG is a highly hydrophobic material because of the percentage carbon
573
(Table 1), and conjugated structure. On the other hand, 17β-estradiol is equally hydrophobic due to its
574
high octanol-water distribution coefficient (Kow). Consequently, hydrophobic interactions may be
575
regarded as a potential adsorption mechanism. A plot of log Kow against qm (not shown) was used to
576
account for the hydrophobicity of the XSBA-rGO-mPEG. The high sorption capacities of XSBA-rGO-
577
mPEG is further proof that hydrophobicity was mostly responsible for adsorption. Despite 0.5SBA-
578
rGO-mPEG having the least surface area, it was the most effective for adsorption. This may be
579
explained by a cursory examination of the chemical structures of 17β-estradiol, and mPEG used in
580
XSBA-rGO-mPEG (synthesis) revealed they both possess aromatic rings and that the GO was severely
581
reduced to rGO (Peng et al., 2016). Akpotu and Moodley (2018e) observed that hydrophobic carbon-
582
based adsorbents have a fast and high rate towards adsorbates with aromatic ring in their structures.
583
This explains why 0.5SBA-rO-mPEG had the highest sorption capacity and consequently a higher
584
reaction rate parameter (K1) (Table 2). The high sorption capacity of 0.5SBA-rGO-mPEG could be
585
due to multifaceted adsorption which is a consistent feature of graphene-based adsorbents. rGO-mPEG
586
anchored on the SBA surfaced led to increased sorption capacity of 17β-estradiol which can be
587
attributed to the alteration of GO surface. The alterations are in form of groove areas, high energy
588
surface and surface defects which results in adsorbed molecules having a strong attraction to occupy
589
these voids. The HRTEM images (Figure 2) of XSBA-rGO-mPEG) appeared wrinkled due to the
590
chemical reduction during the synthesis procedure. These wrinkles have non-uniform charge
591
distribution and are concentrated charge centre for intense chemical activity in charge, which leads to
31
592
higher adsorption capacity (Wang et al., 2014). The high sorption capacity of 0.5SBA-rGO-mPEG for
593
17β-estradiol may be due to the presence of π-conjugated structure or vacancies which favours
594
different types of interactions. Defective surface on the groove of XSBA-rGO-mPEG promotes the
595
adsorption of the 17β-estradiol via hydrophobic and π−π conjugation despite irrespective of the
596
adsorbate and adsorbent charge. Therefore, the increased adsorption by 0.5SBA-rGO-mPEG can be
597
due to π−π interaction between the adsorbent and 17β-estradiol. Therefore, π−π interaction can be said
598
to be a contributing factor to the adsorption mechanism. The XSBA-rGO-mPEG is a porous material
599
and pore filling by the 17β-estradiol molecules can be considered as a possible adsorption mechanism.
600
This is because there was a reduction in the surface area of (23-19 m2/g) and pore volumes (0.30-0.26
601
cm3/g) after adsorption. This implies that pore filling can be used to partly explain the adsorption
602
mechanism.
603
FTIR analysis was used to provide information on the interaction between 17β-estradiol and XSBA-
604
rGO-mPEG. An increase in peak intensity and frequency shift of the vibrational modes of functional
605
groups could be due to H-bonding (Tizaoui et al., 2017). A comparison of pristine 0.5SBA-rGO-
606
mPEG to its spent counterpart, showed that the spent adsorbent had increased intensity of vibration for
607
-C=O- (1634 cm-1) and -N-H- (3200, 1556 cm-1). This implied that hydrogen bond was involved in the
608
sorption mechanism with the strength of H-bond in this order N-H-O < O-H-O < O-H-N with a similar
609
observation by (Tizaoui et al. (2017)). Therefore, it is proposed that the mechanism of adsorption of
610
17β-estradiol onto XSBA-rGO-mPEG is firstly through π−π and hydrophobic interactions, increased
611
adsorption in the grooved section due to the morphological alteration of the adsorbents and
612
subsequently also via pore filling.
613
3.7. Adsorbent Regeneration. Adsorbent reusability after sorbate adsorption is important because it
614
provides insights into possible reuse of adsorbents. It impacts the costs of efficient remediation of 17β-
615
estradiol and reduces disposal of used adsorbents, consequently lessening secondary pollution. A
32
616
solution of acidic ethanol was chosen as the eluent for the dissolution of the adsorbent. The choice of
617
eluent is essential as it ensures high recyclability of the adsorbents (SBA-GO, XSBA-rGO-mPEG).
618
The adsorbate was stripped off the adsorbent and vacuumed at 60 °C and regeneration was carried out
619
4 times. The adsorbents SBA-GO and (0.5SBA-rGO-mPEG) after 4 adsorption-desorption cycles had
620
about 86% and 80% performance efficiency with a <5% decline in efficiency (Figure 6a). FTIR
621
spectra (Figure 6b) shows the same defining peaks as pristine 0.5SBA-rGO-mPEG after 4 cycles..
622
The efficiency of the sorbent adsorption-desorption process could be attributed to the various
623
functional groups and/or hydrophobic binding power of the adsorbate to adsorbent surface, thus leads
624
to high sorption/desorption in the recycling process. The slight reduction observed was due to the loss
625
of small portion of sorptive sites after each regeneration step. The results obtained showed that
626
0.5SBA-rGO-mPEG had excellent stability and reusability which significantly reduces the amount of
627
adsorbent spent in the remediation of 17β-estradiol from aqueous media. Thus, significantly reduces
628
the financial implications for treatment. In comparison to other sorbent materials for the sorption of
629
17β-estradiol, 0.5SBA-rGO-mPEG had a better sorption capacity, thus, reducing the economic cost of
630
remediating of 17β-estradiol from wastewater.
33
631
100
0.5SBA-rGO-mPEG SBA-GO
(b)
80 Transmittance/a.u
Desorption efficiency/%
(a)
60
40
20 Pristine 0.5SBA-rGO-mPEG Regenerated 0.5SBA-rGO-mPEG
0 1
632
2 3 Number of cycles
4000
4
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers (cm )
633
Figure 6 (a). Percentage desorption efficiency of the adsorbents SBA-GO and 0.5SBA-rGO-mPEG (Conditions: 20 mg
634
adsorbent dose, contact time 6 h, T = 25 °C, pH =4) (b) pristine and regenerated 0.5SBA-rGO-mPEG
635
3.8. Application study to real water sample. In a bid to determine the efficiency of 0.5SBA-rGO-
636
mPEG in actual application for water treatment, adsorption studies were carried out on water samples
637
collected from local river and spiked with 17β-estradiol and was compared with activated carbon and
638
GO which were applied as controls. River water is made up of a complicated matrix with several
639
organic and inorganic materials competing to bind onto the adsorbent active sites. Therefore, the
640
presence of other organic matter in natural aquatic systems supresses and complicates the adsorption
641
process. Nonetheless, results obtained indicated that the adsorption of 17β-estradiol on 0.5SBA-rGO-
642
mPEG was just slightly lower in this complex matrix as compared to simulated water samples. Despite
643
the low surface area of 0.5SBA-rGO-mPEG, a removal efficiency of 58% was recorded for 17β-
644
estradiol. SBA-15 had little to no significant adsorption effect on 17β-estradiol due to its high
645
hydrophilic nature. In contrast, the surface of XSBA-rGO-mPEG contains a variety of active
646
functional groups such as carboxyl, carbonyl, amine and the reduced chemical surface which resulted
647
in additional functional groups on the SBA-15 surface. Thus, accounting for the reduced pore volume
648
but highly efficient sorption of 0.5SBA-rGO-mPEG. Adsorption application from the real water 34
649
sample was carried at the natural pH of the river which was approximately 5. This is favourable in real
650
application scenario because no additional cost would be incurred from an induced pH change.
651
Ultimately, this material can be applied for the removal of 17β-estradiol from water samples with a
652
wide pH range, thus demonstrating the efficiency of this adsorbent in real water systems.
653
4. CONCLUSIONS
654
In this research, we successfully synthesised and modified highly hydrophobic SBA-15 with rGO and
655
mPEG. The rGO-mPEG was grown in-situ over the SBA-15 surface. Also, GO was modified with a
656
hyper branched polymeric material (mPEG) and was chemically reduced to rGO-mPEG with
657
hydrazine hydrate. Detailed characterisation of the synthesised materials revealed that the rGO-mPEG
658
was grafted onto the pores of the SBA-15 enabling it to form high surface area materials with vastly
659
improved textural properties.
660
The synthesised materials were applied as adsorbent for the removal of 17β-estradiol from solution
661
through batch adsorption method. pH 4 was preferred and equilibrium time was reached before 30
662
min. Kinetic data best suited the PSO model. It was noted that higher concentration of rGO-mPEG in
663
XSBA-rGO-mPEG resulted in increased sorption capacities. The equilibrium data was fitted both
664
Langmuir and Freundlich isotherm models. Adsorption was endothermic, feasible and spontaneous.
665
0.5SBA-rGO-mPEG was most efficient for the removal of 17β-estradiol. The adsorption mechanism
666
was through H-bonding, π−π and hydrophobic interactions and pore filling. Desorption experiments
667
revealed that after 4 regeneration cycles the adsorbents were still effective and not totally spent. The
668
adsorbents performed exceptionally when applied to real water samples. Consequently, these
669
adsorbents can be useful for the removal of organic pollutants from wastewater.
670
Notes
671
The authors declare no competing interest.
672 35
673
ACKNOWLEDGEMENTS
674
We acknowledge the funding of the Research Directorate, Vaal University of Technology,
675
Vanderbijlpark, South Africa.
676
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677
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41
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Covalently linked graphene oxide/reduced graphene oxidemethoxylether polyethylene glycol functionalized silica for scavenging of estrogen: Adsorption performance and mechanism
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DECLARATION OF INTEREST STATEMENT The authors of the manuscript entitled “Covalently linked reduced graphene oxidemethoxylether polyethylene glycol functionalised silica for estrogen scaavenging: Adsorption performance and mechanism” wish to submit our manuscript to Chemosphere for review and publication.
We confirm that this manuscript has not been submitted to any other journal for publication.
Yours sincerely, Dr S. Akpotu (PhD) (corresponding author) Email: [email protected]
1
Highlights •
Novel hydrophobic amido-carbonic groups from mPEG was covalently introduced to GO/SBA-15 surface
•
3-dimensional SBA-rGO-mPEG had high sorption capacity and fast removal for 17βestradiol
•
Maximum adsorption capacity of 17β-estradiol onto 0.5SBA-rGO-mPEG was 192.3 mg/g at pH 4 and 25 °C
•
Regenerated SBA-rGO-mPEG- performed excellently in sorption of 17β-estradiol from river water
•
Removal mechanism was due to synergistic 3 processes; H-bond, hydrophobic and π−π interactions
S0045-6535(19)32970-4
DOI:
https://doi.org/10.1016/j.chemosphere.2019.125729
Reference:
CHEM 125729
To appear in:
ECSN
Received Date: 21 October 2019 Revised Date:
17 December 2019
Accepted Date: 21 December 2019
Please cite this article as: Akpotu, S.O., Lawal, I.A., Moodley, B., Ofomaja, A.E., Covalently linked graphene oxide/reduced graphene oxide-methoxylether polyethylene glycol functionalised silica for scavenging of estrogen: Adsorption performance and mechanism, Chemosphere (2020), doi: https:// doi.org/10.1016/j.chemosphere.2019.125729. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Covalently linked graphene oxide/reduced graphene oxide-methoxylether polyethylene glycol
2
functionalized silica for scavenging of estrogen: Adsorption performance and mechanism
3
Samson O. Akpotu*,‡, Isiaka A. Lawal‡, Brenda Moodley† and Augustine E. Ofomaja‡
4
‡Wastewater Treatment Research Laboratory, Faculty of Applied and Computer Sciences, Department of
5
Chemistry, Vaal University of Technology, Vanderbijlpark 1911, South Africa.
6
†School of Chemistry and Physics, University of Kwazulu-Natal, Durban 4000, South Africa.
7
*Corresponding author: [email protected]
8
Abstract
9
Water pollution by pharmaceuticals is a global issue and its remediation is important. To overcome
10
this, we synthesised super hydrophobic nanoporous 3-dimensional ordered nanomaterials with multi-
11
functional binding chemistry for highly efficient adsorption of estrogen (17β-estradiol). Graphene
12
oxide (GO) was synthesised via Tours method and methoxylether polyethylene glycol (mPEG) was
13
covalently introduced onto GO surface via facile amidation mild process to give GO-mPEG. GO-
14
mPEG was anchored on nanoporous SBA-15 and homogenously reduced in-situ to SBA-rGO-mPEG.
15
XRD analysis confirmed successful synthesis of SBA-15 and cross-linked GO/rGO-mPEG on SBA-15
16
surface. Image analysis revealed the architecture of SBA-15 as porous 3-dimensional silica network
17
and presence of interwoven/crosslinked thin-films of GO-mPEG on SBA-15 surface. EDX
18
mapping/elemental analysis showed expected elements were present. FTIR and textural analysis
19
revealed the presence of different functional groups and high surface area as well as porosity,
20
respectively. Optimal molar ratio experiments showed that 0.5SBA-rGO-mPEG had the highest
21
sorption capacity. The relatively large surface area, 3-dimensional nanoprous silica structure and
22
excess of polyamide/amido-carbonic functional groups on nanocomposites were suited for adsorption
23
of 17β-estradiol. Equilibrium time was 30 min and effect of pH on adsorption was negligible.
24
Sorption kinetic process of SBA-rGO-mPEG suited the pseudo-second-order model and equilibrium 1
25
data fitted both Freundlich and Langmuir models. Qm values of 57.1, 78.5, 102.6 and 192.3 mg/g was
26
recorded for SBA-GO, 0.1SBA-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG,
27
respectively. H-bond, hydrophobic and π−π interactions were the sorption mechanism of SBA-rGO-
28
mPEG after detailed analysis of data. Adsorbents was regenerated/re-used after 4 cycles with high
29
remediation from environmental/real water samples.
30
Keywords: 17β-estradiol; adsorption; polyamide carbonic groups; reduced graphene oxide,
31
methoxylether polyethylene glycol; hydrophobicity; wastewater treatment
32 33
1. INTRODUCTION
34
Pharmaceuticals (endocrine disrupting chemicals (EDCs)) are present in the environment (Huerta-
35
Fontela et al., 2011). Estrogen, a class of EDC have caused endocrine disrupting function and
36
negatively impact human and aquatic health. EDCs and its metabolites pathway into the environment
37
are via domestic sewage treatment plants, seeps into lakes/rivers, ultimately polluting water bodies
38
(Okoro et al., 2017; Mnguni et al., 2018). 17 β-estradiol an EDCs have been found to be present in
39
drinking, surface, ground and wastewater (Huerta-Fontela et al., 2011). At concentration < 1 ng L-1,
40
studies by Tabata et al. (2001), Diamanti-Kandarakis et al. (2009), Han et al. (2015), have shown EDC
41
causes reproductive disorder, feminisation of aquatic species, malformation and cancer. 17 β-estradiol
42
(Figure Supplemental Information (S1)) is a steroid hormone produced from cholesterol via
43
androstenedione in body fat deposits, brain and ovaries (Mnguni et al., 2018). Medically, it is most
44
commonly used as an oral contraceptive pill. It possesses the most disruptive endocrine activity which
45
is 1000-10000 times the impact of nonylphenol (Jiang et al., 2017a). Hansen et al. (1998), found that
46
blood of males exposed to 5 ng L-1 of 17 β-estradiol for 2 weeks resulted in the inducement of a
47
specific female hormone. In South Africa, 0.24-0.35 ng L-1 of 17 β-estradiol was found in Vaal
48
catchment river (Mnguni et al., 2018). Conventional water treatment methods are ineffective for its 2
49
removal. Techniques such as photodegradation (Zhang et al., 2007), catalysis (Shappell et al., 2008),
50
photolysis (Rose et al., 2014), biodegradation (Li et al., 2018) and adsorption (Dong et al., 2018) have
51
been applied for the remediation of 17 β-estradiol. Adsorption is preferred because of ease of use, low
52
cost and high efficiency (Akpotu and Moodley, 2018c). 17 β-estradiol, has been adsorbed with
53
carbonaceous materials and their modified counterpart such as resin (Zhang and Zhou, 2005) biochar
54
(Dong et al., 2018) carbon nanotubes, activated carbon and graphene (Jiang et al., 2017b).
55
Nonetheless, these adsorbents are plagued with slow uptake rate, low sorption capacity and
56
interference from co-existing macromolecules. A cost efficient and rapid adsorbent for 17 β-estradiol
57
is required. To overcome these challenges, development of chemically versatile adsorbents from
58
natural /waste materials such as graphite, clay, zeolites with green production and sustainability is
59
deemed as a positive for the planet.
60
Graphene (G) and graphene-based nanomaterials are exceptional materials for adsorption due to large
61
theoretical surface area (SA) (~2630 m2/g) and a variety of easily modifiable oxygen and carbon
62
functional groups on their surface. However, graphene oxide (GO) and reduced GO (rGO) do not
63
actualise their sorption potential because G/rGO are aggregated in wastewater which reduces SA and
64
severely limit sorption ability (Akpotu and Moodley, 2018a). Aggregation during adsorption is due to
65
strong interplanar interaction of graphene sheet. GO is hydrophilic and its separation from adsorbate is
66
difficult which hinders adsorptive capacity. Consequently, GO and rGO modification is essential for
67
recovery and real-world application in wastewater remediation. Separation/aggregation challenges in
68
GO/rGO are remedied by anchoring on a support (polymer, mesoporous silica (MS)) which provides
69
an expanded structure. The structure in solution due to stearic hindrance restricts aggregation, ensuring
70
adsorbents achieve full sorption potential.
71
MS are chemically inert, possess large SA, high pore volume, extensive silanol network which permits
72
surface modification with organic species and applied as support for GO/rGO (Akpotu and Moodley,
73
2018e). Akpotu and Moodley (2018b) reported an increased sorption capacity of pharmaceuticals 3
74
when MS was organically modified. Gao et al. (2019), demonstrated that hydrophobic N-propyl
75
modified MCM-41 was effective in estrogen adsorption. Similarly, MS modification with GO/rGO has
76
been shown to increase sorption capacity toward pharmaceuticals in wastewater (Akpotu and
77
Moodley, 2018e). Polyethyleneimine (PEI), a hydrophilic polymer has been used to expand GO
78
structure and applied in adsorption of organic molecules. Reduced PEI interacts with organics through
79
hydrogen bonding and hydrophobic interaction (Geng et al., 2019). A polymer with similarities to PEI
80
is poly(ethylene glycol) (mPEG), which is a biological inert reagent with low toxicity, high
81
biocompatibility, excellent solubility in water and other solvents (Zalipsky, 1995; Xu et al., 2014).
82
mPEG modified nanomaterials has been widely used in drug delivery (Liu et al., 2016). Consequently,
83
mPEG is suited for water treatment because it does not leach or act as secondary pollutant. It is
84
therefore our opinion that mPEG functionalised nanomaterials would potentially be an efficient
85
adsorbent of organic pollutant from wastewater through an expansion of adsorbent structure and
86
functional groups manipulation. mPEG structure has an abundance of hyper branched network of C, H
87
and O with several routes to surface functionalisation/modification of the -O, -OH and C groups. The
88
presence of an extensive network of surface -O can be exploited through thermal/chemical reduction
89
resulting in a more hydrophobic molecule. Thus, mPEG is potentially an efficient adsorbent modifier
90
for the removal of water soluble/insoluble organic pollutants. This can be inferred from a study by Xu
91
et al. (2014) where mPEG was employed as a carrier for the delivery of hydrophobic anti-cancer drug
92
with high loading capacity. Liu et al. (2008), functionalised GO with mPEG which was effective in
93
uptake and delivery of aromatic anti-cancer drugs. GO and mPEG reaction is promoted by the
94
presence of hydroxyl and carboxylic acid groups interaction with GO planar structure resulting in an
95
expanded nucleated material highly ordered crystal structure (β or phase) (Lee et al., 2019). Effective
96
sorption of 17 β-estradiol requires a material with optimised surface functional groups, fast separation,
97
high SA, high sorption, easily separated and regeneratable.
4
98
In addressing these limitations, this article reports a facile approach for the development of multi-
99
functional groups adsorbents with a hybrid base of multi-carboxylic hyperbranched rGO-mPEG
100
modified mesoporous silica SBA-15 for efficient remediation of 17 β-estradiol. The ideation is based
101
on the covalent linkage of C and O from mPEG to GO via a simple polymerisation reaction. The
102
merits of the individual components of the hybrid adsorbents are (i) GO has a variety of oxygen
103
functional groups on its surface which are useful for adsorption, functionalisation and can be reduced
104
to rGO (ii) SBA-15 is an ordered material with large SA and pore volume which aids adsorbate
105
separation from adsorbent, also provide a platform for grafting and/or encapsulation of modifiers (iii)
106
mPEG is a large repeating hyperbranched polymer with an extensive network of carboxylic groups
107
that may significantly affect adsorption in its pristine condition or be a more effective medium towards
108
the removal of hydrophobic molecules when reduced. mPEG molecules are imprinted onto GO by a
109
ring opening polymerisation reaction ensuring an expanded structure and then grafted/encapsulated on
110
SBA-15. Subsequent in-situ chemical reduction of hybrid adsorbent (-OH groups) produces new
111
amine and carboxylic functionalisation ensuring improved hydrophobicity. This material has
112
advantages such as use of environmentally benign chemical and synthesis ease, thus making it cost
113
effective and easily scalable. To the best of the authors knowledge modification of silica with mPEG
114
and graphene oxide and subsequent in-situ reduction of the adsorbent materials and its application in
115
the adsorption of 17 β-estradiol from real water samples (river) and simulated water samples has not
116
been previously carried out. Synthesised materials were extensively characterised and applied for
117
batch sorption studies of 17 β-estradiol from simulated and real wastewater.
118
2. EXPERIMENTAL SECTION
119
2.1 Materials and Method. Pluronic P123 (average Mn ~5800, EO20PO70EO20), HCl (32%),
120
tetraethylorthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), methoxy polyethylene glycol
121
(mPEG),
122
(EDC.HCl), N-hydrosuccinimide (NHS), 17 β-estradiol, glutaraldehyde, and graphite powder were
hydrazine
hydrate,
N-(3-(dimethylamino)propyl)-N’-ethylcarbodiimide
5
hydrochloride
123
obtained from Aldrich and used without further purification. All reagents were used without further
124
purification.
125
2.2. Synthesis of SBA-15, NH2-SBA-15, GO, GO-mPEG, SBA-GO and XSBA-(r)GO-mPEG
126
SBA-15 was prepared by modifying the method of Chen et al. (2012). About 4 g of Pluronic P123
127
surfactant was dissolved in 30 mL of double-distilled deionised water and 120 mL of 2 mol L -1 HCl
128
under stirring. About 8.5 g of TEOS was added to the mixture at a temperature of 35-40 °C. The gel
129
was stirred continuously for 24 h and kept in a sealed bottle at 100 °C for 48 h. Afterwards, the
130
product was filtered and washed with double-distilled water. Thereafter, the material was dried in an
131
oven at 100 °C for 24 h and calcined at 550 °C for 6 h at a ramp rate of 2 °C min -1. This was labelled
132
as SBA-15.
133
Amine functionalised SBA-15 was synthesised with the method of Akpotu and Moodley (Akpotu and
134
Moodley, 2018e). About 1 g of SBA-15 was added to 20 mL of ethanol whilst stirring. Also, 1 mL of
135
APTES was added to the mixture and stirred for 24 h. Thereafter, the product was filtered and rinsed
136
with doubled-distilled water. The product was oven dried at 100 °C for 24 h and labelled as NH2-SBA-
137
15.
138
GO was synthesised using Tours’ method (Marcano et al., 2010; Akpotu and Moodley, 2018d),. A
139
specific amount of GO was dispersed in double-distilled deionised water placed in an ultrasonic bath
140
for 2 h. Thereafter, EDC.HCl and NHS were added as crosslinkers. Furthermore, certain amount
141
(ratio) of mPEG was added to the highly dispersed GO solution. The resulting hydrogel was stirred for
142
24 h at room temperature. The product was rinsed in a bid to get rid of any residual GO and mPEG
143
and subsequently freeze dried. mPEG molar ratio was varied and 0.10 mPEG, 0.25 mPEG and 0.5
144
mPEG and was used as a precursor for 0.10 GO-mPEG, 0.25 GO-mPEG 30 and 0.5 GO-mPEG.
145
mPEG ratio is denoted as X in the adsorbents.
6
146
XSBA-GO-mPEG was synthesised by stirring (1.0 g) NH2-SBA-15 with GO-mPEG at room
147
temperature. The pH was adjusted to 7 with the addition of NH4OH. Thereafter, 2 mL of
148
glutaraldehyde was added as a crosslinker. The product was filtered and washed with water and oven
149
dried for 6 h at 50 °C. XSBA-rGO-mPEG was synthesised with a similar procedure as XSBA-GO-
150
mPEG; however, the synthesis temperature was 80 °C. The mixture was stirred for 8 h and
151
subsequently filtered and washed with double-distilled water to a neutral pH (scheme 1). The product
152
was freeze dried and stored until further use.
7
153 154
Scheme 1. Schematic illustration of the synthesis of SBA-15-reduced graphene oxide-methyl ether polyethylene glycol
155
(SBA-rGO-mPEG). A = GO, B = mPEG, C = GO-mPEG, D = SBA-rGO-mPEG
156
2.3. Characterisation. Fourier-transform infrared (FTIR) spectra analysis of the samples were
157
recorded with a PerkinElmer Spectrum 100 spectrometer fitted with a universal ATR accessory, in the
158
range 4000−400 cm−1. Low and wide-angle X-ray diffraction (XRD) profiles of adsorbents were 8
159
recorded in the 2θ range of 1-5° and 10−90°, respectively with a Bruker D8 Advance instrument, using
160
a Cu Kα radiation source. The surface area, pore sizes and diameters of the adsorbents were
161
determined on a Micromeritics Tristar II 3020 instrument, and the surface area was calculated by BET
162
method. Prior to sample analysis, samples were degassed at 100 °C and under pressure for 12 h to
163
eliminate any physically adsorbed moisture. Morphological structural analyses of the samples were
164
carried out on a high-resolution transmission electron microscopy (HRTEM, JEOL 2100) and field
165
emission scanning electron microscopy (FESEM). The FESEM was used in energy dispersive X-ray
166
(EDX) analysis and elemental mapping. FESEM samples were gold coated on copper grids, while
167
samples for HRTEM analysis were kept in alcohol and sonicated for 30 min. Zeta potential of samples
168
were measured with a Malvern Zetasizer Nanoseries NanoZS instrument using a dip cell. Zeta
169
potential readings were obtained by adjusting the pH of the electrolyte (10 mM NaCl background
170
solution) with suitable quantities of 0.01 mol−3 HNO3 or NaOH. Elemental analyser of the samples
171
was determined with Thermo Scientific CHNS/O analyser. Thermogravimetric analysis of the
172
adsorbents were obtained by weighing a 5 mg sample mass in an aluminium pan (PerkinElmer) at a
173
temperature range of 50-900 °C and at a ramp rate of 5 °C min-1 in a nitrogen environment.
174
2.4. Analytical method. The 17 β-estradiol was quantified at a wavelength of 292 nm with a
175
Shimadzu Prominence UFLCxr (Shimadzu corporation, Japan) with two LC-20AD XR pump. An
176
Agilent C-18 Eclispe column (4.6 x 150 mm, 5 μm particle size) with a mobile flow rate of 1.0 mL
177
min⁻ 1 in isocratic conditions was used and an injection volume of 10 μL. The mobile phase was a
178
mixture of methanol/water (70:30; v/v). pH of the water used was adjusted to 2.3 with 1 mol L⁻ 1
179
phosphoric acid.
180
2.5. Adsorption studies. Adsorption experiments were performed in batch mode to investigate the
181
effects of pH, dose and contact time of 17 β-estradiol concentrations on SBA-15, SBA-GO, XSBA-15-
182
rGO-mPEG. Effect of dose was carried out by varying the amount of adsorbent from 10-60 mg in 20
183
mL, 15 mg L−1 of 17 β-estradiol solution. Effect of pH was carried out by adjusting the pH of 17 β9
184
estradiol (20 mL, 15 mg L−1) with 0.01 M NaOH/HCl from 2 – 10. This was carried out in a 50 mL
185
glass bottle with stopper containing the adsorbent materials. Blank sample analysis was carried out
186
under the same conditions as the samples without the adsorbents. For kinetic studies, experiments
187
were carried out by determining the amount of 17 β-estradiol adsorbed at optimum pH form different
188
solution concentration (50 – 200 mg L-1) while adding 150 mg of the adsorbents. Periodically, liquid
189
samples (2-1440 min) were withdrawn intermittently and concentration determined.
190
The concentration of 17 β-estradiol was determined and the amount adsorbed per unit of the adsorbent
191
mass, at a specific time, t (qt mg/L) was determined with the following equation 1:
192
(
)
(1)
193
V is solution volume (L), adsorbent mass is W (g) Co and Ce are initial and final 17 β-estradiol
194
concentration at a given time t (mg/L), V is solution volume (L). The percentage removal is calculated
195
with equation 2:
196
(2)
197 198
The sorption rate process was determined by fitting the experimental data obtained into pseudo first
199
order (PFO) (Lin and Wang, 2009), pseudo second order (PSO) (Ho, 2003), and intraparticle diffusion
200
models (Weber and Morris, 1963) with equation (SI, Kinetics).
201
Evaluation of equilibrium data from the sorption process of the adsorbents on 17 β-estradiol was
202
carried out by applying Langmuir (1918), Freundlich (1906), Dubinin-Radushkevich (DR) (Dubinin,
203
1947) and Temkin (Mane et al., 2007), isotherm models (SI, Isotherm).
204
Thermodynamics experiments was carried out by placing 20 mg of adsorbents in the 17 β-estradiol
205
solution which was agitated in a thermostatic shaker at temperatures of 298, 308 and 318 K for a 24 h
206
period. Thereafter, the solutions were centrifuged at 6000 rpm for 2 min and filtered with a 0.45 µm 10
207
cellulose acetate filter and the filtrate concentration was quantified with HPLC. Change in enthalpy
208
(ΔH°), Gibbs energy (ΔG°) and entropy (ΔS°) were calculated with data obtained from the
209
thermodynamics studies (SI, Thermodynamics).
210
2.6. Regeneration studies. To regenerate XSBA-rGO-mPEG, the adsorbents were preloaded with 17
211
β-estradiol, at neutral pH and ambient temperature. The preloaded adsorbents were washed with acidic
212
ethanol (adjusted with 0.1 M HCl) as eluent on a shaker for 180 min and dried under vacuum at 60 °C
213
to remove the bound 17 β-estradiol from the 0.5SBA-15-rGO-mPEG. The solution was filtered, and
214
filtrate concentration was determined with HPLC. Adsorption-desorption process was repeated 4
215
times.
216
2.7. Real water application study. In a bid to determine the efficiency of adsorption of the 0.5SBA-
217
15-rGO-mPEG on real water, river water was collected from Umgeni River (Durban, South Africa,
218
29° 37’16” S, 30° 40’ 46” E) in an amber bottle with an approximate pH of 6, filtered with a 0.45 µm
219
filter. A 10 mL aliquot of filtered real water sample was spiked with 0.150 mL (1000 mg/L) 17 β-
220
estradiol solution resulting in a 15 mg/L concentration. About 40 mg of the adsorbents were added to
221
the samples and shaken for 120 min at room temperature. The suspension obtained was filtered and the
222
concentration of the 17 β-estradiol left was determined with HPLC.
223
3. RESULTS AND DISCUSSION
224
3.1. Adsorbents characterisation.
225
FTIR analysis was carried out to validate functional groups attachment on SBA-15 surface as depicted
226
in Figure 1a. Absorption peak around 3200 cm-1 for all samples except SBA-15 and GO is attributed to
227
N-H stretching vibration (Tizaoui et al., 2017). However, the peak was relatively subdued by -OH
228
stretching vibration which arises from the interlayer water molecules present (Yap et al., 2018). GO
229
contains many oxygen functional groups on its basal plane which is attributed to oxidation. These
230
oxygen functionalities provides reactive sites for covalent modification by esterification or ring 11
231
opening reaction of the carboxyl and epoxy groups (Xu et al., 2014). For GO the vibration at 1729
232
cm-1 was assigned as -C=O stretch from the -COOH functional groups, and the peak at 1634 cm-1
233
emerged from the graphite framework vibration (-C=C-, -C-C-) (Akpotu and Moodley, 2018e). The
234
absorption vibrations at 1389 and 1068 cm-1 was assigned as stretching vibrations of -C-OH- and -C-O
235
groups, respectively. After the amidation reaction between GO and mPEG, the peak at 1732 cm-1
236
present in GO disappeared and a new peak appeared at 1643 cm-1 which signified the formation of
237
new amide functional groups (-NH-CO-). Prominent peaks at 2870 cm-1 and 1115 cm-1 showed the
238
presence of mPEG chains on the GO sheets surface. The peak at 2870 cm-1 is the symmetric stretch
239
mode of the -CH2 moiety on the carbon chain of the GO-mPEG and was found on the profile of
240
XSBA-15-rGO-mPEG. SBA-15, SBA-GO and XSBA-15-rGO-mPEG had peaks typical of siliceous
241
materials. This indicated that SBA-15 was successfully modified with GO and rGO-mPEG. The
242
absorption bands at 1642 and 1420 cm-1 on SBA-GO can be attributed to -C=O stretching vibrations.
243
The vibration present at 1026 cm-1 in XSBA-15-rGO-mPEG was attributed to Si-O-Si/Si-O-C
244
asymmetric vibration of which the latter vibration was covalently linked. Also, there was the
245
disappearance of the -C=O which was present in both GO and SBA-GO and a replacement with a new
246
amide peaks present at around and 1555 cm-1 and 1643 cm-1 due to the functionalisation reaction. The
247
peak at 1555 cm-1 is an amide group stretching vibration from the vinyl group which was successfully
248
introduced to the rGO-mPEG sheet due to the thermo-chemical reduction of SBA-GO-mPEG with a
249
reducing agent. The -N-C- bond enables the XSBA-rGO-mPEG to act as H-bond acceptor, also, the -
250
NH- bond allows the adsorbent to function as H-bond donor (Tizaoui et al., 2017). The presence of
251
rGO-mPEG in the interlayer of the silica sheet resulted in a symmetric vibration peak seen at 1370 cm -
252
1
253
surface of the SBA-15. The peaks at 430 and 790 cm-1 was due to Si-O-Si/silica-hydroxyl bending and
254
symmetric vibrations in the C-Si lattice, respectively. The obtained results demonstrated that the
. The absorption vibrations at 1132 and 2867 cm-1 was due to the presence of rGO-mPEG on the
12
successful grafting of rGO and mPEG in the XSBA-rGO-mPEG material through a ring opening
256
polymerisation.
(a)
(b)
(100)
SBA-15
(c)
Transmittance/a.u
Intensity/a.u
SBA-GO 0.1SBA-rGO-mPEG
0.25SBA-rGO-mPEG
0.5SBA-rGO-mPEG
OH stretch
CH2
stretch
rGO peak
Intensity/a.u
SBA SBA-GO 0.10SBA-rGO-mPEG 0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
GO
Intensity/a.u
255
-GO
10
20
30
40
2 Theta/
50
60
SBA-15
GO peak
SBA-GO 0.1SBA-rGO-mPEG 0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
(110) (200)
C=O NHCO
Si-O-Si
2000
1500
1
500
2
80 70 60 50
30 20
4
5
600
20
30
400
40
50
60
70
(e) 0.10SBA-rGO-mPEG
SBA-15 SBA-GO
200 0 0.0
0.2
0.4
0.6
0.8
1.0 o
GO 0.1SBA-rGO-mPEG 0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
10 0.0
10
2 Theta/
0.25SBA-rGO-mPEG 0.5SBA-rGO-mPEG
500
SBA-GO
GO
1000 1500 2000 2500 3000
Raman shift/cm-1
0
257
6
(d)
800
Relative Pressure/ P/P
40
3
2 Theta/
o
Wavenumbers/cm-1
1000
Intensity/a.u
2500
Intensity/a.u
3000
Quantity adsorbed/ P/P
3500
Quantity adsorbed/ cm3/g
4000
0.2
0.4
0.6
0.8
1.0
500
1000
1500
2000
2500
3000
Raman shift/cm-1
Relative Pressure/P/Po
258
Figure 1. (a) FTIR, (b) Low angle XRD, (c) Wide angle XRD, (d) N 2 adsorption desorption isotherm, (e) Raman for GO,
259
SBA-15, SBA-GO, 0.1SBA-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG.
260 261
X-ray diffraction profiles for SBA-15, GO, SBA-GO, and XSBA-15-rGO-mPEG are presented
262
(Figures 1b and c). Small angle XRD profile of SBA-15 obtained between 0-5(θ) was defined by three
263
characteristic diffraction peaks (2θ = 0.84° (100), 1.5° (110) and 1.8° (200)) and the corresponding d
264
spacings are 10.5, 6.0 and 5.3 nm, respectively, which revealed its hexagonal p6mm 2D well-ordered
265
hexagonal pore structure (Chen et al., 2012). The 3 peaks are indexed as 100, 110 and 200 have a
266
specific d spacing ratio of 1:1/√3:1/2. After modification with GO, there was a very slight change in
267
diffraction peak/intensity of SBA-GO which could be attributed to the presence of amorphous GO on 13
80
268
the internal and external surface of the SBA-15. In contrast, modification with rGO-mPEG, caused a
269
reduction in peak intensities but no significant shift in diffraction peaks between the 0.5-10°. The
270
reduction in the prevailing peak at 2θ = 0.8° (100) could be attributed to chemical reduction and the
271
introduction of new carbon functionalities from the bulky mPEG molecule. The main diffraction peaks
272
of pristine SBA-15, SBA-GO, and 0.1SBA-15-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-
273
mPEG are 0.84, 0.83, 0.85, 0.83 and 0.82, respectively. There was a shift to lower angles as compared
274
to pristine SBA as GO/rGO-mPEG was loaded and this could be attributed to contraction of the silica
275
framework. This was also observed by Lu et al. (2009). For GO, the intense diffraction (001) peak at
276
2θ = 9.76° with an interplanar basal spacing (8.71 Å) confirms the introduction of oxygen containing
277
functional groups and complete oxidation of graphite with peak which is present at 25.6° (Akpotu and
278
Moodley, 2018b). The decreased interlayer spacing (d001) of the reduced materials (XSBA-rGO-
279
mPEG) to 7.81 Å and the disappearance of the GO peak (9.76°) signified the partial reduction of GO
280
and the chemical functionalisation from the bulky mPEG molecules. This created a larger spacing
281
between the rGO-mPEG and the silica sheets. The wide angle XRD patterns of SBA-15, SBA-GO and
282
XSBA-rGO-mPEG had a broad amorphous siliceous peak between 2(Ɵ) 15-35°. SBA-GO had a peak
283
at 2(Ɵ) = 26.1° which is attributed to the partial transformation of the GO containing functional groups
284
on its surface as was also observed by Li et al. (2015). Whereas, XSBA-rGO-mPEG had a peak at
285
2(Ɵ) =25.1 which is attributed to the reduction of graphite as was also observed by Luo et al. (2013).
286
In addition, the disappearance of the 2θ = 9.76° peak is also evidence of the reduction of GO.
287
The elemental analyses (EA) of the adsorbents are presented in Table 1. For this calculation, it should
288
be noted that the percentage of silica was not considered, and this assumption was necessary to
289
calculate the ratio of O/C, N/C and H/C. SBA-GO had a higher H/C ratio but a lower O/C as compared
290
to XSBA-rGO-mPEG samples which implied that SBA-GO was more hydrophilic and less
291
hydrophobic. Interestingly, it was observed that as the mPEG ratio increased, the H/C decreased which
292
implied that the materials became more hydrophobic. Amide groups occurrence was verified by EA, 14
293
because prior to amidation, the N content in SBA-15 was negligible (< 0.05%) but after amidation it
294
increased to 1.92-2.03%. EA studies corroborated the FTIR (peaks around 1132 and 2867 cm-1) which
295
indicated the successful covalent grafting of the mPEG chains onto the surface of the reduced GO
296
sheets. For XSBA-rGO-mPEG materials, this proved that rGO-mPEG was successfully embedded in
297
the SBA-15 structure. An increase in mPEG ratio of the composites resulted to lower N/C ratio with
298
increased hydrophobicity. This corroborates the results obtained from the FTIR analysis.
299 300
Table 1. Elemental composition and textural characteristics of adsorbents H%
N%
O%
H/C
O/C
N/C
SA/m2/ g
Sample
C%
Pore volume/cm3/ g
GO SBA-15 SBA-GO 0.1SBA-rGOmPEG 0.25SBArGO-mPEG 0.5SBA-rGOmPEG
37.6 8.02 30.3
2.23 1.75 2.93 4.82
0.58 0.05 2.2 2.03
59.5 10.92 86.9 62.9
0.71 4.38 1.91
1.75 8.13 1.56
0.015 0.27 0.07
39 625 278 39
0.02 1.09 0.62 0.31
Average pore diamete r (nm) 38.2 7.9 6.54 4.6
31.9
5.47
1.95
60.9
2.06
1.43
0.061
29
0.3
4.3
32.5
5.63
1.92
60
2.07
1.38
0.059
23
0.3
4.1
301 302
N2 adsorption/desorption isotherms measurement were carried out and BET method was used to
303
calculate surface area and other textural characteristics of the adsorbents (Figure 1d) with detailed
304
analysis presented (Table 1). All synthesised materials had type IV isotherm as defined by IUPAC
305
(Kruk and Jaroniec, 2001). It was observed that increased rGO-mPEG ratio grafted onto silica resulted
306
in an inverse marked reduction of SA and mesoporous volume due to pore filling as compared to
307
pristine SBA-15. GO had a H3 hysteresis loop that do not level off at relative pressures close to
308
saturated vapour pressure which is characterised by loose aggregates of platelike particles forming
309
narrow slit like pores of irregular shape and broad size distribution. SBA-15, SBA-GO had H1
310
hysteresis loop which was between 0.5
311
mesopores. This loop is typical of a highly ordered cage-like cylindrical mesoporous material with
312
high pore size uniformity and interconnecting mesopores network (Mirzaie et al., 2017). XSBA-rGO-
313
mPEG had H3 hysteresis loop which is characterised by high porosity and interconnection. XSBA-
314
rGO-mPEG had lower point of inflection on the P/Po which confirms that rGO-mPEG has been
315
successfully grafted into the pores of SBA-15 as was also observed by Boukoussa et al. (2018). The
316
increased surface area of the XSBA-rGO-mPEG as compared to GO can be attributed to the spacing of
317
the GO sheet in the pores of SBA-15. The pore sizes of the modified materials were approximately 15
318
times larger than GO. The effective interlayer separation of rGO sheets resulted in SA enhancement
319
with more active sorption sites and larger pore volume which improves the adsorbents sorption
320
capabilities.
321
Raman spectra (Figure 1e) of GO, SBA-GO and XSBA-rGO-mPEG show 2 major bands at
322
approximately1334 and 1598 cm-1 which corresponds to the D and G bands, respectively. The G band
323
is the in-plane vibration of the sp2 C transitioning to sp3 hybridized C which could be attributed to the
324
destruction of the sp2 graphitic structure or the covalent attachment of functional groups. In contrast,
325
the D band represents the defects in the graphitic structure. There was a slight shift in the G band
326
values of GO as compared to SBA-GO and XSBA-rGO-mPEG which indicates functionalisation
327
(Kabiri et al., 2015). The ID/IG intensity ratio is a measure of the degree of functionalisation and an
328
evidence of carbon structural defects. XSBA-rGO-mPEG had a higher ID/IG of 1.08 which can be
329
attributed to the presence of the disordered partially reduced GO-mPEG as compared to SBA-GO and
330
GO of 1.02 and 0.95, respectively. The results obtained further reaffirmed the functionalisation of
331
SBA-15 with rGO-mPEG. The results of the Raman, TGA, XRD, EA and FTIR characterisation
332
provides irrefutable evidence of the synthesis of XSBA-rGO-mPEG through the formation of new
333
amine, amide and -CH2 group from the rGO-mPEG on the SBA.
334
FESEM and HRTEM was used to characterise the microstructure and morphological differences
335
between SBA-15, GO and XSBA-rGO-mPEG (Figure 2). The SEM image of SBA-15 appeared as 16
336
vertical stacks of lengthy interconnected tubular channel-like constrictions. GO (Figure 2a) appeared
337
as a transparent film with a rough wrinkled surface and very agglomerated. SBA-GO had a similar
338
appearance as SBA-15, however, uniform GO sheets can be seen on its surface. XSBA-rGO-mPEG
339
(Figures 2d-f) composed of SBA-15 which were grown and reduced in-situ in GO-mPEG. They
340
appeared to be highly crosslinked with a 3D porous architecture on the reduced GO sheets.
(a)
(c)
(b)
GO
(d)
rGO-mPEG
(e)
rGO-mPEG
(f)
(g)
(h)
(k)
(l)
rGO-mPEG
GO
(i)
(j)
rGO-mPEG
rGO-mPEG
rGO-mPEG
341 342
Figure 2. SEM micrographs of (a) GO (b) SBA-15 (c) SBA-GO (d) 0.1SBA-rGO-mPEG (e) 0.25SBA-rGO-mPEG (f)
343
0.5SBA-rGO-mPEG; HRTEM micrographs of (g) GO (h) SBA-15 (i) SBA-GO (j) 0.1SBA-rGO-mPEG (k) 0.25SBA-rGO-
344
mPEG (l) 0.5SBA-rGO-mPEG
345 346
HRTEM images (Figures 2g-l) showed the structure of SBA-15 to have an ordered 2-dimensional
347
(p6mm) hexagonal symmetry. The identical and uniform mesoporous channels which bares similarity
348
with a honeycomb is clearly seen. GO appeared as a smooth transparent surface with large pores. In
17
349
SBA-GO, the transparent GO film is visible as it appears well layered over the mesoporous channels
350
of the SBA-15. A similar observation was made for XSBA-rGO-mPEG, as the layered structure of the
351
reduced GO-mPEG can be seen over the mesopores.
352
Figure 3 shows the EDX profile and elemental mapping for 0.5SBA-rGO-mPEG. All the elements (C
353
44.7%, Si 11.7%, O 43.5%) were present in different percentages and uniformly distributed.
354
(a)
(b)
(c)
355 356
Figure 3. EDX image and mapping of 0.5SBA-rGO-mPEG
357
3.2. ADSORPTION STUDIES
358
Preliminary studies were carried out to determine the sorption potential of 17β-estradiol on SBA-15,
359
SBA-GO, 0.1SBA-rGO-mPEG, 0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG at pH 4. It was
360
observed that adsorption with SBA-15, was negligible, SBA-GO had ~13.4 %, 0.1SBA-rGO-mPEG
361
had ~30.2 %, 0.25SBA-rGO-mPEG had ~35.4 % and 0.5SBA-rGO-mPEG exhibited excellent
18
362
sorption capacity over 60%. Thus, SBA-15, SBA-GO and 0.5SBA-rGO-mPEG adsorption properties
363
were further investigated.
364 365 366
3.2.1. Influence of Solution pH.
367
The impact of solution pH towards 17β-estradiol removal by the sorbent materials was studied over a
368
pH range of 2-10 and an initial 17β-estradiol concentration of 15 mg/L (Figure 4). Our proposed
369
mechanism for the removal of the 17β-estradiol are (i) hydrophobic and π-π interactions (ii) hydrogen
370
bonding (iii) physical trapping by the amine-amide groups on the XSBA-rGO-mPEG adsorbents.
371
Removal efficiencies of 15.2, 39.5, 44, 65 % were observed for SBA-GO, 0.1SBA-rGO-mPEG,
372
0.25SBA-rGO-mPEG and 0.5SBA-rGO-mPEG, respectively. A direct relationship was observed
373
between adsorbent uptake and an increase in the rGO-mPEG concentration. This could be due to the
374
increasing non-polar nature of the adsorbents and also the lower oxygen content. Oxygen containing
375
functional groups could reduce the access to hydrophobic organic compounds, invariably reducing the
376
available number of adsorption sites due to water cluster formation on their surface (Jiang et al.,
377
2017b). This is because the water flux of the XSBA-rGO-mPEG is improved due to the reduction of
378
the rGO-mPEG sheets on the SBA-15. Thus, resulting in narrow planar channels and reduced
379
hydrophilic properties of the rGO-mPEG sheets with decreased water permeability. This phenomenon
380
partly explains the inverse relationship between the reducing oxygen content (Table 1) of the
381
adsorbents and increase in 17β-estradiol removal as the ratio of rGO-mPEG increased in XSBA-rGO-
382
mPEG. This observation is further supported by the negative zeta potential value of XSBA-rGO-
383
mPEG across the pH range of 2-10. Zeta-potential (Figure 4) values was used to provide further
384
insight into the effect of pH on the adsorption capacity of the adsorbents. The study showed that the
385
sorbent materials were effective for the removal of 17β-estradiol from solution with the highest 19
386
removal efficiency of approximately 60% at pH 2 for 0.5SBA-rGO-mPEG. This observation was
387
supported by the very negative zeta potential value of ~ -40 mV obtained for 0.5SBA-rGO-mPEG. At
388
a pH of 2, 17β-estradiol exists as neutral molecules in solution, therefore its high sorption efficiency
389
via hydrophobic interaction. Increased adsorption can be attributed to the electron rich surface of the
390
adsorbents which promotes the formation of H-bond between the adsorbent and the adsorbate.
391
Across the pH range, the differences in removal efficiency for all adsorbents was negligible with only
392
a slight dip occurring at neutral to basic pH. This is ascribed to the surface charge present on XSBA-
393
rGO-mPEG and the different molecular species of 17β-estradiol in solution at varying pH. This
394
behaviour could also be linked to the ionisation of 17β-estradiol in aqueous solution because 17β-
395
estradiol molecules are weak Lewis acid and strongly pH dependent during ionisation. Deprotonation
396
of 17β-estradiol molecules in solution starts at about pH 8 and at pH 9.2, it is apparent. Hydroxyl ions
397
are strong base and are proficient in proton extraction from the phenolic -OH moieties of 17β-estradiol
398
molecules from solution which results in deprotonation. Deprotonation increases the 17β-estradiol
399
solubility in aqueous solution and causes dissociation of H-bond between the protons on 17β-estradiol
400
and the adsorbents functional groups (Jiang et al., 2017a). For all adsorbents the zeta potential values
401
were negative range with very little impact on adsorption. Therefore, this implies that surface charge
402
does not play a significant role in adsorption. As a further confirmation that electrostatic interaction
403
did not significantly contribute to adsorption, the pKa of 17β-estradiol is 10.46 which signifies that its
404
impact on adsorption was minimal in the pH range of 2-7 because it was present as neutral molecules.
405
As the pH of the solution approached 9, most of the 17β-estradiol molecules have been deprotonated
406
which resulted in a slight dip in percentage removal and adsorbent hydrophobicity. Furthermore, an
407
increase in solution alkalinity resulted in several functional groups such as amide, hydroxyl and
408
carboxyl groups present on the XSBA-rGO-mPEG surface tends to be slightly deprotonated which
409
results in the adsorbents surface to be more negatively charged. An increase in anionic 17β-estradiol
410
molecules inhibits further sorption and uptake due to charge similarities/electronic repulsion between 20
411
the adsorbate and adsorbent. The adsorbent material was found to be significantly effective across a
412
wide pH range. Therefore, this further supports the potential application of this adsorbent as efficient
413
adsorbent of 17β-estradiol from real water sample via hydrophobic and π−π interactions.
-25 -30
0 -5
10
-10 -15
5
-20
0 30 -10 20 -20 10
-25
-35 0
2
4
6
8
2
10
-30
-30 4
6
8
0
10
2
4
pH
pH 50
70
10
(d) -10
20
-20
10
-30
0
Removal efficiency/%
30
Zeta Potential/ mV
0
4
414
6
8
(e)
10
-50
50
-40
40
-30
30
-20
20
-10
10
0
0
-40 2
8
-60
60
40
6
pH
10 2
10
Zeta Potential/ mV
-40
4
6
8
10
pH
pH
415
Figure 4. Effect of pH removal efficiency and zeta potential of 17β-estradiol carried out at different pH on (a) SBA-15 (b)
416
SBA-GO (c) 0.1SBA-rGO-mPEG (d) 0.25SBA-rGO-mPEG (e) 0.5SBA-rGO-mPEG. Conditions: 15 mg/L of 17β-
417
estradiol, T = 25 °C, n = 2.
418
3.3. Kinetics studies. Contact time effect of the adsorption of 17β-estradiol onto SBA-GO and
419
XSBA-rGO-mPEG results is presented (Figure 5). For all adsorbents, initially, there was rapid
420
removal of 17β-estradiol as the interaction period increased between the adsorbate and adsorbents. In
421
the next phase, the increment was gradual until it became negligible. At the initiation stage (2-30 min)
422
of the reaction, the availability of active sorption sites on the adsorbents resulted in the rapid removal
423
of 17β-estradiol. As the reaction progressed and contact time increased, sorption sites becomes
424
saturated and uptake became slower. Equilibrium was reached around the 30 min mark, thus, depicting
21
Zeta Potential/mV
-20
15
(c)
40
5
Zeta Potential/mV
Removal Efficiency/%
-15
10
10
(b)
-10
Removal Efficiency/%
Zeta Potential/mV
20
(a)
-5
Removal Efficiency/%
0
425
fast sorption rate by the adsorbents. XSBA-rGO-mPEG had higher sorption capacities as compared to
426
SBA-GO with 0.5SBA-rGO-mPEG having the highest efficiency which is attributed to its highly
427
hydrophobic nature (Table 2).
(b)
50
(a)
40
200 mg/L
20
150 mg/L
qt/ mg/g
qt/ mg/min
40 30
200 mg/L
30 150 mg/L 20 100 mg/L
100 mg/L 10
50 mg/L
50 mg/L
0 0
400
800
75 mg/L
10
75 mg/L
0 1200
0
1600
400
800
1200
1600
Time/min
Time/min 180
120
(d)
(c) 160
120
200 mg/L
80
150 mg/L
60
100 mg/L
100 mg/L
100 80
75 mg/L
60
40 75 mg/L
40
20
50 mg/L
20
50 mg/L
25 mg/L 0
0 0
428
200 mg/L
140
qt/ mg/g
qt/ mg/g
100
400
800
1200
0
1600
400
800
1200
1600
Time/min
Time/min
429
Figure 5. Effect of time on the adsorption of 17β-estradiol (a) SBA-GO (b) 0.1SBA-rGO-mPEG (c) 0.25SBA-rGO-mPEG
430
(d) 0.5SBA-rGO-mPEG. Conditions: T= 25 °C, time = 1440 min, concentration = 25-200 mg/L, dose = 150 mg, n=2.
431 432
In a bid to determine the adsorption dynamics, rate, mechanism and transport of 17β-estradiol onto the
433
various adsorbents, kinetics models such as PFO, PSO and IPD models were fitted against the
434
experimental data obtained from the time experiments.
435
The kinetics parameters for the models applied in the sorption studies of 17β-estradiol onto all the
436
adsorbents are presented in Table 2. The suitability of the model that describes the data was selected 22
437
based on the closeness of R2 value of the non-linear regression analysis to unity. The experimental
438
data obtained was for the sorption of 17β-estradiol onto SBA-GO, XSBA-rGO-mPEG best suited the
439
PSO model. Furthermore, qe values obtained from the PSO model was much closer to the
440
experimental values. PSO model is routinely applied to describe the adsorption of pollutants from
441
aqueous system in wastewater treatment process which occurs through chemisorption viz-a-viz the
442
number of sites accessible for the exchange process at solution-solid interface. It assumes that there is
443
bimolecular interaction between the adsorbent and the adsorbate molecule in solution which causes
444
adsorption. This process involves the exchange and sharing of electrons with the highly hydrophobic
445
17β-estradiol molecules and the amido-carbonic of the rGO-mPEG groups of the XSBA-rGO-mPEG
446
adsorbents. This implied that the adsorption rate was predominantly controlled through chemical
447
sorption and the number of active sites on the adsorbents determined the adsorption capacity (Akpotu
448
and Moodley, 2018a). An increase in the amount of rGO-mPEG in XSBA-rGO-mPEG resulted in the
449
creation of more active site on the adsorbents, consequently increasing the uptake of 17β-estradiol
450
from solution (Table 2). The rate constant value k2 of 0.5SBA-rGO-mPEG was 2.0761 g/mg.min and
451
higher than that of the other sorbent materials.
452
An increase in boundary thickness (Kl) of the rGO-mPEG adsorbents was observed as the
453
concentration of rGO-mPEG in SBA increases (Table 2). Increased thickness in l is likely to have
454
significantly impacted the adsorption of 17β-estradiol on the sorbent materials. Thus, implying the
455
adsorption of 17β-estradiol on the sorbent materials proceed through a multi-phase adsorption process.
456
This process may involve simultaneous chemical interaction such as hydrophobic, π−π and hydrogen
457
bonding between the adsorbate and the adsorbents.
458
The rate controlling step of the sorption process was determined using the IPD model. This model
459
assumes adsorption occurs through 4 processes; (a) diffusion of the 17β-estradiol molecules from the
460
bulk solution onto the surface of the adsorbents, (b) adsorbate diffusion through the boundary layer to
461
the surface of the adsorbents, (c) sorption of adsorbate onto the adsorbent active sites and, (d) initial 23
462
fast adsorption of adsorbate followed by slow intraparticle diffusion to the internal adsorbent surface.
463
The rate controlling step is normally the slowest step of the sorption process and this may stage can be
464
attributed to external mechanisms or intraparticle diffusion. An examination of the qt vs t1/2 plots
465
shows that it did not pass the origin. Hence, this affirms that IPD was not the only rate controlling step
466
(Sen et al., 2012). This further confirms that the sorption of 17β-estradiol onto SBA-GO and XSBA-
467
rGO-mPEG could be said to be a multistep adsorption process and of a heterogeneous nature.
468
Table 2. Kinetics Parameters for the Adsorption of 17β-estradiol on SBA-GO and XSBA-rGO-
469
mPEG adsorbents
Model
Parameters
SBA-GO
0.1SBA-rGO-
0.25SBA-rGO-
0.5SBA-rGO-
mPEG
mPEG
mPEG
Experimental
qexp/ mg/g
5.5800
8.5629
17.7000
25.4230
Pseudo-first-order
k1/ min-1
0.0067
0.0078
0.0079
0.0099
qeq/mg/g
0.6182
1.3897
2.2867
2.6325
R2
0.6020
0.8188
0.8990
0.9417
k2/ g/mg/min
0.6658
1.1626
1.7779
2.0761
qeq/mg/g
5.6200
8.6500
17.9000
25.7060
R2
0.9999
0.9999
0.9999
0.9999
Ki/ mg/g/min-0.5
0.0971
0.1523
0.5014
0.6610
C/ mg/g
3.0628
4.3616
7.6630
11.8250
R2
0.4044
0.6015
0.5846
0.5588
Pseudo-second-order
Intraparticle diffusion
470 471 24
472
3.4. Adsorption Isotherms. Equilibrium isotherms of 2 parameters (Langmuir, Freundlich, Temkin
473
and Dubinin-Radushkevich) were applied to study the adsorption interaction, mechanism and evaluate
474
the equilibrium between 17β-estradiol and the surface of the adsorbents. The suitability of the models
475
that best describes equilibrium data was obtained from the regression analysis that was closest to unity
476
(1). Based on Giles system for isotherm classification, the isotherms are classed as L type based on
477
shape (Figure S3) (Giles et al., 1960). This suggests that adsorbed solutes were vertically oriented and
478
there was strong interaction between the adsorbate molecules and the adsorption sites of the sorbent
479
materials (Okoli et al., 2014; Akpotu and Moodley, 2018b; Lawal and Moodley, 2018). The isotherm
480
parameters for the models that fits the equilibrium data for the adsorbents are presented (Table 3).
481
Langmuir isotherm was the most suited for the equilibrium data obtained for the adsorbents. This
482
model assumes that the adsorption of 17β-estradiol occurred on a monolayer surface with similar
483
energies without interaction between adsorbed entities. Langmuir model is usually suited to the
484
occurrence of adsorption on diverse sites with similar energies (Yap et al., 2018). Based on isotherm
485
and experimental data obtained, the interaction between the 17β-estradiol and the XSBA-rGO-mPEG
486
was principally through H-bonding, π−π and hydrophobic interactions because of the higher qm values
487
obtained. These can be explained by the hydrophobic nature of the XSBA-rGO-mPEG and the
488
corresponding chemical structure of 17β-estradiol which possesses phenolic moiety in position C3. The
489
17β-estradiol hydroxyl moieties have high H-bond donor activity towards XSBA-rGO-mPEG
490
molecules. The phenolic moiety in 17β-estradiol could also act as H-bond donor and acceptor to the
491
XSBA-rGO-mPEG (Duax et al., 1976; Fevig et al., 1988). The highest qm value was obtained for
492
0.5SBA-rGO-mPEG (192.3 mg/g). Non-dimensional separation factor (RL) (SI, isotherm) which is
493
obtained from the Langmuir isotherm is used to calculate the favourability factor (Hall et al., 1966).
494
RL values obtained was (0.012-0.092) which shows that the adsorption was favourable.
495 496 25
497
Table 3. Isotherm Parameters for the adsorption of 17β-estradiol onto SBA-GO and XSBA-
498
rGO-mPEG
Isotherm
Parameter
SBA-GO
0.1SBA-rGO-mPEG
0.25SBA-rGO-mPEG
0.5SBA-rGO-mPEG
Langmuir
qm (mg/g)
57.10
75.80
102.60
192.30
b (L/mg)
0.3538
0.3568
1.6333
3.0888
R2
0.985
0.982
0.982
0.980
Kf(mg/g/(mg/L)1/n)
3.02
4.28
6.18
9.35
N
1.26
1.36
1.37
1.64
R2
0.996
0.984
0.995
0.993
E (kJ/mol)
3.53
7.45
2.36
7.07
qD (mg/g)
0.96
2.09
2.65
3.01
B (mol/kJ2)
1.0X10-6
8.0X10-6
9.0X10-6
6.0X10-6
R2
0.9143
0.9956
0.9826
0.8628
B
36.7
71.9
149.7
193.6
b (j/mol)
12.8
16.5
34.4
67.5
A
34
37.7
52.8
137.7
R2
0.9558
0.849
0.9321
0.9521
Freundlich
DubininRadushkevich
Temkin
499 500
Langmuir maximum capacities (qm) obtained from this study was favourable when compared to that
501
obtained from similar adsorbents in other studies (Table 4). A comparison of the Langmuir parameters
502
to Freundlich shows consistency in value. This is further affirmed by the R 2 value obtained from the 26
503
Freundlich isotherm (Table 2). Freundlich model assumes that 17β-estradiol were absorbed onto a
504
non-uniform surface into multilayers of the adsorbents and adsorption involves several mechanisms.
505
For Freundlich model, metals are adsorbed on a non-uniform surface into multilayers, and the
506
adsorption capacity rises infinitely with increasing concentration. Highest KF value was obtained for
507
0.5SBA-rGO-mPEG which suggests that it had higher sorption capacity and affinity for 17β-estradiol
508
in solution. The KF values also corresponds with the qm value obtained from the Langmuir isotherm.
509
N>1 greater value shows that the adsorption of 17β-estradiol on the adsorbents was favourable.
510
D-R model was used to determine if the adsorption was a physical or chemical process (Dubinin,
511
1947). The E values (Table 3) for 17β-estradiol on the adsorbents was between the range of 2.36-7.45.
512
These values were less than E < 8.00 kJ mol−1 which indicated that the adsorption process was physical
513
(Akpotu and Moodley, 2016).
514 515 516 517 518 519 520 521 522
27
523
Table 4. Comparison of Langmuir (qm) values for the sorption of 17β-estradiol onto different
524
adsorbents
525
Adsorbent
SBET (cm3 g-1)
Isotherm model
β -CD-PLGraphene SiO2-GO
105.5
qm (mg.g-1)
pH
Reference
Langmuir 85.5
Equil. Kinetic Time model (min) 480 PSO
4.2-5.1
Jiang et al. (2017a)
132
Langmuir 101.48-141.89
480
PSO
9
Jiang et al. (2018)
Al2O3-GO
132
Langmuir 89.51-141.69
480
PSO
9
Jiang et al. (2018)
GO
92
Langmuir 149.4
480
PSO
7
Jiang et al. (2016)
GO
15.4
Langmuir 96.2
NA
NA
NA
Sun et al. (2017)
Magnetic GO1
247.4
Langmuir 52.9
720
PSO
3
Bai et al. (2017)
Magnetic GO2
251.6
Langmuir 106.38
720
PSO
3
Bai et al. (2017)
SBA-GO
278
Langmuir 57.1
30
PSO
4
This study
0.1SBA-rGOmPEG 0.25SBA-rGOmPEG 0.5SBA-rGOmpEG
39
Langmuir 75.8
30
PSO
4
This study
29
Langmuir 102.6
30
PSO
4
This study
23
Langmuir 192.3
30
PSO
4
This study
NA-not applicable, βCD-beta cyclodextrin
526 527
3.5. Effect of temperature and thermodynamics studies. Change in adsorbate temperature has been
528
shown to influence adsorption capacity of adsorbents (Khan et al., 2012). The resultant change in
529
sorption capacity of adsorbents can be attributed to changes in; rate of diffusion of the adsorbed
530
molecules; variation in viscosity of adsorbate; changes in adsorptive forces which plays a critical role
531
in adsorption and a variation in adsorbent textural properties (Srivastava and Rupainwar, 2009; Khan
532
et al., 2012). A temperature range of 298-318 K was considered for the adsorbents (Figure S4).
533
A slight increase in sorption capacity was noticed for SBA-GO and XSBA-rGO-mPEG when
534
adsorbate temperature was increased. Increase adsorbate temperature results in increased diffusion rate
535
across external boundary layer, decrease in solution viscosity, increased activation of sorbent active
536
sites, increased pore volume and adsorbent textural properties (Srivastava and Rupainwar, 2009; Khan
537
et al., 2012). The above-mentioned factors resulted in increased uptake of the 17β-estradiol molecules
28
538
onto the adsorbents active sites due to increased mobility of the adsorbate molecules. Thus,
539
demonstrating that adsorption was endothermic and shows the sorbents to be efficient for the
540
remediation point source pollution. Furthermore, the adsorbents showed effectiveness and
541
functionality across a range of low and high temperatures. Thermodynamics parameters such as
542
changes in mean free energy (ΔG°), entropy (ΔS°) and enthalpy (∆H°) were obtained for the
543
adsorption of 17β-estradiol onto SBA-GO, XSBA-rGO-mPEG adsorbents (Table 5). Negative ΔG°
544
values obtained established the spontaneity and thermodynamic favourability of the adsorption process
545
which increased with increase in adsorbate temperature. The adsorption of 17β-estradiol was favoured
546
at higher temperature. The positive ∆H° and ΔS° values obtained for the adsorbents, indicates the
547
adsorption process was endothermic and had increased randomness/disorderliness between the
548
solid/solution interface, respectively. This shows the adsorption process was entropy driven. 17β-
549
estradiol ions interaction with the adsorbents could be inferred from adsorption heat (∆H°). ∆H° value
550
between obtained for adsorbate-adsorbent interaction between 2-20 KJ is termed as physisorption, in
551
contrast values between 80-200 KJ is chemisorption. This implies that SBA-GO interaction was
552
physisorption, 0.1SBA-rGO-mPEG was partly phyisorption and chemisorption because the 30.7 KJ
553
value obtained was between physisorption and chemisorption. However, 0.25SBA-rGO-mPEG and
554
0.5SBA-rGO-mPEG lean towards to chemisorption as they had higher mPEG concentration. The
555
thermodynamic parameters show the adsorption process was spontaneous and was effective for the
556
removal of 17β-estradiol (organic pollutants) from wastewater.
557 558 559
29
560
Table 5. Thermodynamic parameters for the adsorption of 17β-estradiol on SBA-GO and XSBA-rGO-
561
mPEG
Adsorbents SBA-GO
0.1SBA-rGO-mPEG
0.25SBA-rGO-mPEG
0.5SBA-rGO-mPEG
T/K 298.15
ΔH°/kJ/mol 2.37
ΔS°/kJ/mol 0.05
ΔG°/kJ/mol -13.2
308.15
-13.7
318.15
-14.2
298.15
30.7
0.19
-25.9
308.15
-27.8
318.15
-29.7
298.15
161.5
0.59
-30.2
308.15
-31.8
318.15
-36.8
298.15
165.1
0.67
-36.4
308.15
-43.2
318.15
-49.9
562
563
3.6. Adsorption mechanism. An understanding of the sorption mechanism of 17β-estradiol onto
564
SBA-GO and XSBA-rGO-mPEG adsorbents is essential for future studies. Previous studies on
565
adsorption of estrogen onto silica, GO, rGO, organo-silica and silica-graphene composites are through
566
adsorption mechanisms such as; H-bond, coordinate bonding, hydrophobic and π−π interactions (Jiang
30
567
et al., 2017b; Akpotu and Moodley, 2018e). In a bid to obtain insight into the adsorption mechanism of
568
17β-estradiol on XSBA-rGO-mPEG, an examination of the effect of solution pH (Figure 4) was
569
carried out. Solution pH can change the adsorbent and adsorbate surface charges, consequently,
570
strongly influence the adsorption process via electrostatic interactions. From pH 2-10, there was
571
insignificant changes in adsorption which indicates that adsorption was not through electrostatic
572
interactions. XSBA-rGO-mPEG is a highly hydrophobic material because of the percentage carbon
573
(Table 1), and conjugated structure. On the other hand, 17β-estradiol is equally hydrophobic due to its
574
high octanol-water distribution coefficient (Kow). Consequently, hydrophobic interactions may be
575
regarded as a potential adsorption mechanism. A plot of log Kow against qm (not shown) was used to
576
account for the hydrophobicity of the XSBA-rGO-mPEG. The high sorption capacities of XSBA-rGO-
577
mPEG is further proof that hydrophobicity was mostly responsible for adsorption. Despite 0.5SBA-
578
rGO-mPEG having the least surface area, it was the most effective for adsorption. This may be
579
explained by a cursory examination of the chemical structures of 17β-estradiol, and mPEG used in
580
XSBA-rGO-mPEG (synthesis) revealed they both possess aromatic rings and that the GO was severely
581
reduced to rGO (Peng et al., 2016). Akpotu and Moodley (2018e) observed that hydrophobic carbon-
582
based adsorbents have a fast and high rate towards adsorbates with aromatic ring in their structures.
583
This explains why 0.5SBA-rO-mPEG had the highest sorption capacity and consequently a higher
584
reaction rate parameter (K1) (Table 2). The high sorption capacity of 0.5SBA-rGO-mPEG could be
585
due to multifaceted adsorption which is a consistent feature of graphene-based adsorbents. rGO-mPEG
586
anchored on the SBA surfaced led to increased sorption capacity of 17β-estradiol which can be
587
attributed to the alteration of GO surface. The alterations are in form of groove areas, high energy
588
surface and surface defects which results in adsorbed molecules having a strong attraction to occupy
589
these voids. The HRTEM images (Figure 2) of XSBA-rGO-mPEG) appeared wrinkled due to the
590
chemical reduction during the synthesis procedure. These wrinkles have non-uniform charge
591
distribution and are concentrated charge centre for intense chemical activity in charge, which leads to
31
592
higher adsorption capacity (Wang et al., 2014). The high sorption capacity of 0.5SBA-rGO-mPEG for
593
17β-estradiol may be due to the presence of π-conjugated structure or vacancies which favours
594
different types of interactions. Defective surface on the groove of XSBA-rGO-mPEG promotes the
595
adsorption of the 17β-estradiol via hydrophobic and π−π conjugation despite irrespective of the
596
adsorbate and adsorbent charge. Therefore, the increased adsorption by 0.5SBA-rGO-mPEG can be
597
due to π−π interaction between the adsorbent and 17β-estradiol. Therefore, π−π interaction can be said
598
to be a contributing factor to the adsorption mechanism. The XSBA-rGO-mPEG is a porous material
599
and pore filling by the 17β-estradiol molecules can be considered as a possible adsorption mechanism.
600
This is because there was a reduction in the surface area of (23-19 m2/g) and pore volumes (0.30-0.26
601
cm3/g) after adsorption. This implies that pore filling can be used to partly explain the adsorption
602
mechanism.
603
FTIR analysis was used to provide information on the interaction between 17β-estradiol and XSBA-
604
rGO-mPEG. An increase in peak intensity and frequency shift of the vibrational modes of functional
605
groups could be due to H-bonding (Tizaoui et al., 2017). A comparison of pristine 0.5SBA-rGO-
606
mPEG to its spent counterpart, showed that the spent adsorbent had increased intensity of vibration for
607
-C=O- (1634 cm-1) and -N-H- (3200, 1556 cm-1). This implied that hydrogen bond was involved in the
608
sorption mechanism with the strength of H-bond in this order N-H-O < O-H-O < O-H-N with a similar
609
observation by (Tizaoui et al. (2017)). Therefore, it is proposed that the mechanism of adsorption of
610
17β-estradiol onto XSBA-rGO-mPEG is firstly through π−π and hydrophobic interactions, increased
611
adsorption in the grooved section due to the morphological alteration of the adsorbents and
612
subsequently also via pore filling.
613
3.7. Adsorbent Regeneration. Adsorbent reusability after sorbate adsorption is important because it
614
provides insights into possible reuse of adsorbents. It impacts the costs of efficient remediation of 17β-
615
estradiol and reduces disposal of used adsorbents, consequently lessening secondary pollution. A
32
616
solution of acidic ethanol was chosen as the eluent for the dissolution of the adsorbent. The choice of
617
eluent is essential as it ensures high recyclability of the adsorbents (SBA-GO, XSBA-rGO-mPEG).
618
The adsorbate was stripped off the adsorbent and vacuumed at 60 °C and regeneration was carried out
619
4 times. The adsorbents SBA-GO and (0.5SBA-rGO-mPEG) after 4 adsorption-desorption cycles had
620
about 86% and 80% performance efficiency with a <5% decline in efficiency (Figure 6a). FTIR
621
spectra (Figure 6b) shows the same defining peaks as pristine 0.5SBA-rGO-mPEG after 4 cycles..
622
The efficiency of the sorbent adsorption-desorption process could be attributed to the various
623
functional groups and/or hydrophobic binding power of the adsorbate to adsorbent surface, thus leads
624
to high sorption/desorption in the recycling process. The slight reduction observed was due to the loss
625
of small portion of sorptive sites after each regeneration step. The results obtained showed that
626
0.5SBA-rGO-mPEG had excellent stability and reusability which significantly reduces the amount of
627
adsorbent spent in the remediation of 17β-estradiol from aqueous media. Thus, significantly reduces
628
the financial implications for treatment. In comparison to other sorbent materials for the sorption of
629
17β-estradiol, 0.5SBA-rGO-mPEG had a better sorption capacity, thus, reducing the economic cost of
630
remediating of 17β-estradiol from wastewater.
33
631
100
0.5SBA-rGO-mPEG SBA-GO
(b)
80 Transmittance/a.u
Desorption efficiency/%
(a)
60
40
20 Pristine 0.5SBA-rGO-mPEG Regenerated 0.5SBA-rGO-mPEG
0 1
632
2 3 Number of cycles
4000
4
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers (cm )
633
Figure 6 (a). Percentage desorption efficiency of the adsorbents SBA-GO and 0.5SBA-rGO-mPEG (Conditions: 20 mg
634
adsorbent dose, contact time 6 h, T = 25 °C, pH =4) (b) pristine and regenerated 0.5SBA-rGO-mPEG
635
3.8. Application study to real water sample. In a bid to determine the efficiency of 0.5SBA-rGO-
636
mPEG in actual application for water treatment, adsorption studies were carried out on water samples
637
collected from local river and spiked with 17β-estradiol and was compared with activated carbon and
638
GO which were applied as controls. River water is made up of a complicated matrix with several
639
organic and inorganic materials competing to bind onto the adsorbent active sites. Therefore, the
640
presence of other organic matter in natural aquatic systems supresses and complicates the adsorption
641
process. Nonetheless, results obtained indicated that the adsorption of 17β-estradiol on 0.5SBA-rGO-
642
mPEG was just slightly lower in this complex matrix as compared to simulated water samples. Despite
643
the low surface area of 0.5SBA-rGO-mPEG, a removal efficiency of 58% was recorded for 17β-
644
estradiol. SBA-15 had little to no significant adsorption effect on 17β-estradiol due to its high
645
hydrophilic nature. In contrast, the surface of XSBA-rGO-mPEG contains a variety of active
646
functional groups such as carboxyl, carbonyl, amine and the reduced chemical surface which resulted
647
in additional functional groups on the SBA-15 surface. Thus, accounting for the reduced pore volume
648
but highly efficient sorption of 0.5SBA-rGO-mPEG. Adsorption application from the real water 34
649
sample was carried at the natural pH of the river which was approximately 5. This is favourable in real
650
application scenario because no additional cost would be incurred from an induced pH change.
651
Ultimately, this material can be applied for the removal of 17β-estradiol from water samples with a
652
wide pH range, thus demonstrating the efficiency of this adsorbent in real water systems.
653
4. CONCLUSIONS
654
In this research, we successfully synthesised and modified highly hydrophobic SBA-15 with rGO and
655
mPEG. The rGO-mPEG was grown in-situ over the SBA-15 surface. Also, GO was modified with a
656
hyper branched polymeric material (mPEG) and was chemically reduced to rGO-mPEG with
657
hydrazine hydrate. Detailed characterisation of the synthesised materials revealed that the rGO-mPEG
658
was grafted onto the pores of the SBA-15 enabling it to form high surface area materials with vastly
659
improved textural properties.
660
The synthesised materials were applied as adsorbent for the removal of 17β-estradiol from solution
661
through batch adsorption method. pH 4 was preferred and equilibrium time was reached before 30
662
min. Kinetic data best suited the PSO model. It was noted that higher concentration of rGO-mPEG in
663
XSBA-rGO-mPEG resulted in increased sorption capacities. The equilibrium data was fitted both
664
Langmuir and Freundlich isotherm models. Adsorption was endothermic, feasible and spontaneous.
665
0.5SBA-rGO-mPEG was most efficient for the removal of 17β-estradiol. The adsorption mechanism
666
was through H-bonding, π−π and hydrophobic interactions and pore filling. Desorption experiments
667
revealed that after 4 regeneration cycles the adsorbents were still effective and not totally spent. The
668
adsorbents performed exceptionally when applied to real water samples. Consequently, these
669
adsorbents can be useful for the removal of organic pollutants from wastewater.
670
Notes
671
The authors declare no competing interest.
672 35
673
ACKNOWLEDGEMENTS
674
We acknowledge the funding of the Research Directorate, Vaal University of Technology,
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Vanderbijlpark, South Africa.
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41
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Covalently linked graphene oxide/reduced graphene oxidemethoxylether polyethylene glycol functionalized silica for scavenging of estrogen: Adsorption performance and mechanism
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1
Highlights •
Novel hydrophobic amido-carbonic groups from mPEG was covalently introduced to GO/SBA-15 surface
•
3-dimensional SBA-rGO-mPEG had high sorption capacity and fast removal for 17βestradiol
•
Maximum adsorption capacity of 17β-estradiol onto 0.5SBA-rGO-mPEG was 192.3 mg/g at pH 4 and 25 °C
•
Regenerated SBA-rGO-mPEG- performed excellently in sorption of 17β-estradiol from river water
•
Removal mechanism was due to synergistic 3 processes; H-bond, hydrophobic and π−π interactions