- Email: [email protected]
Highly efficient adsorption behavior and mechanism of Urea-Fe3O4@LDH for triphenyl phosphate
Journal Pre-proof Highly efficient adsorption behavior and mechanism of Urea-Fe3O4@LDH for triphenyl phosphate Mengjie Hao, Pan Gao, Dian Yang, Xuanjin Chen, Feng Xiao, Shaoxia Yang PII:
S0269-7491(19)33921-1
DOI:
https://doi.org/10.1016/j.envpol.2020.114142
Reference:
ENPO 114142
To appear in:
Environmental Pollution
Received Date: 18 July 2019 Revised Date:
14 January 2020
Accepted Date: 5 February 2020
Please cite this article as: Hao, M., Gao, P., Yang, D., Chen, X., Xiao, F., Yang, S., Highly efficient adsorption behavior and mechanism of Urea-Fe3O4@LDH for triphenyl phosphate, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.114142. 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. © 2020 Published by Elsevier Ltd.
1 2
Highly efficient adsorption behavior and mechanism of
3
Urea-Fe3O4@LDH for Triphenyl phosphate
4 5
Mengjie Hao, Pan Gao, Dian Yang, Xuanjin Chen, Feng Xiao*, Shaoxia Yang*
6 7
National Engineering Laboratory for Biomass Power Generation Equipment, School
8
of Renewable Energy, North China Electric Power University, Beijing 102206, China
9 10 11 12 13
Corresponding author:
14
Dr. Feng XIAO
15
Dr. Shaoxia YANG
16 17
Tel: +8610 61772456
18
Fax: +8610 61772230
19 20 21
E-mail: [email protected]; [email protected]
22 23 24 1
25
Abstract
26
The emergence of organophosphorus flame retardants and the efficient removal from
27
aquatic environments have aroused increasing concerns. The Urea functionalized
28
Fe3O4@LDH (Urea-Fe3O4@LDH) was prepared and used to adsorb triphenyl
29
phosphate (tphp) for the first time. The tphp adsorption capacity was up to 589 mg g-1,
30
and the adsorption rate reached 49.9 mg g-1 min-1. Moreover, the influences of various
31
environmental factors (pH, ionic strength and organic matter) on the tphp adsorption
32
on the Urea-Fe3O4@LDH were investigated. The initial pH of the solution
33
significantly affected the tphp adsorption, whereas the ionic strength and HA slightly
34
affected the adsorption. The main adsorption mechanism was attributed to
35
electrostatic interaction and π-π interaction. We believe that urea is one of excellent
36
functional groups for the tphp adsorption removal and the materials with urea groups
37
as the adsorbents exhibit good prospects in the future.
38
Keywords:
39
Urea
functionalized
Fe3O4@LDH;
Triphenyl
phosphate
(tphp);
Organophosphorus flame retardants (OPFRs); Adsorption capacity
2
40
1. Introduction
41
Flame retardants (FRs) are extensively applied in plastic, rubber and fiber to
42
reduce materials’ flammability (Ji et al., 2019). Given their persistence and toxicity,
43
brominated FRs are gradually replaced by organophosphorus FRs (OPFRs). In 2012,
44
the production quantity of OPFRs in China was more than 179,000 t, and OPFRs were
45
extensively detected in soil, water and air (Gu et al., 2019). Among the OPFRs,
46
triphenyl phosphate (tphp) is a kind of high production quantity of OPFRs and
47
detected in water because of not chemically bonded to the end-use products. For
48
example, the tphp concentration in surface water in Australia was up to 150 ng L-1
49
(Teo et al., 2015). The tphp concentration can also reach 65 ng L-1 in the Songhua
50
River in China (Wang et al., 2011). In addition, the tphp was detected from
51
wastewater treatment plants in Henan province (Pang et al., 2016). The extensive
52
exposure of the tphp in the environment can lead to human health risks through diet
53
and inhalation. Studies have shown that tphp induced toxicological effects on
54
embryonic development, the neurological system and the immune system. The tphp
55
had great toxic effects on heart development on 0.10 mg L-1 exposure for model
56
organism zebrafish (Du et al., 2015). And the tphp concentration can be up to 3.12 μg
57
g-1 exposing to 100 μg L-1 though bioaccumulation and metabolism of adult zebrafish
58
(Wang et al., 2016). Therefore, developing an efficient processing method is an
59
important challenge for removing tphp in water.
60
Various technologies are investigated to remove OPFRs, including biochemical
61
or physicochemical methods, such as oxidation, coagulation, adsorption (Wei et al., 3
62
2018; Yan et al., 2014). Among these techniques, adsorption, as a green, economical
63
and efficient method, is extensively used for removing pollutants in different
64
concentration ranges. Adsorbents, with the rapid and efficient adsorption removal and
65
easy separation from water, are paid a lot attention for the application in the water
66
treatment. Conventional adsorbents (activated carbon, multi-walled carbon nanotube
67
and zeolite) have been used for removing organophosphorus pollutants by hydroxyl
68
groups (Wang et al., 2018a; Dehghani et al., 2017; Grieco and Ramarao, 2013).
69
Hydroxyl groups, one of the widest functional groups on the surface of the
70
adsorbents, were developed to remove organophosphorus pesticides. Hydroxyl groups
71
in silica exhibited a high adsorption capacity of 37.2-76.3 mg g-1 via H-bonding and
72
electrostatic interactions, while the adsorption of organophosphorus pesticides was
73
interfered by inorganic ions and other organic matters in water (Nodeh et al., 2017).
74
Therefore, the development of materials with favorable selectivity and rapid
75
adsorption has been focused on for removing tphp. Urea exhibits remarkable
76
selectivity toward organophosphorus pollutants (Cao et al., 2018). The groups as
77
active sites adsorbed dimethyl methylphosphonate vapors by identifying P=O, and
78
urea donors and P=O acceptors formed hydrogen bonding in gas reaction. In water,
79
adsorbents with urea groups can generate electrostatic attraction or hydrogen bonding
80
with tphp between NH-CO-NH and P=O. The result indicates that urea groups could
81
exhibit considerable potential in adsorbing organophosphorus pesticides or OPFRs.
82
However, the relative literatures, involving adsorbents with urea functional groups,
83
are not investigated for the tphp removal in water, and the adsorption mechanism of 4
84
the adsorbent is not clear.
85
In addition, separating and recovering the adsorbents are other important factors
86
for applying them in removing pollutants in water and wastewater treatment. Fe3O4
87
magnetic nanoparticles are biocompatible, non-toxic and easy to separate from
88
aqueous solutions. Moreover, the shell of coated Fe3O4 was confirmed as an effective
89
approach to preventing Fe3O4 aggregation and oxidation. Layered double hydroxides
90
(LDHs) have received considerable attention due to abundant hydroxyls,
91
extraordinary ion-exchange ability and simplicity of preparation. Moreover, a
92
Fe3O4@LDH core-shell material has shown favorable separation capability in the
93
reaction (Shan et al., 2014).
94
In the present work, the urea functionalized Fe3O4@LDH (Urea-Fe3O4@LDH)
95
with the core-shell was successful synthesized and used for the tphp removal from the
96
solution. The tphp adsorption kinetics and isotherm on the Urea-Fe3O4@LDH were
97
investigated. Moreover, the effect of experimental conditions (pH, ionic strength,
98
organic matter, tphp concentration and reaction temperature) on the tphp adsorption
99
was studied. And the adsorption efficiency in cycles of the Urea-Fe3O4@LDH and the
100
adsorption of other organophosphorus pollutants were discussed. Hence, the main aim
101
of the present work is to analyze the characteristics and adsorption performance of
102
Urea-Fe3O4@LDH, and also explain the mechanism for removing the tphp
103
2. Materials and methods
104
2.1. Chemicals
105
All chemicals were in an analytical grade, and were used without further 5
106
purification. The tphp (purity >98%), 4-tolyl isocyanate and tetrahydrofuran (THF)
107
were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Fe3O4 and
108
3-aminopropyltriethoxysilane (APTES) were obtained from Aladdin Industrial Co.,
109
Ltd.
110
USA). Mg(NO3)2•6H2O, Al(NO3)3•9H2O, NaOH and humic acid (HA) were produced
111
from Fuchen Chemical Reagent Factory (Tianjin, China).
112
2.2. Preparation of adsorbents
(Shanghai, China). Na2CO3 was supplied by Sigma-Aldrich (St. Louis, MO,
113
Preparation of Fe3O4@LDH. LDH was prepared via the coprecipitation method
114
(Zhang et al., 2019). Fe3O4 (1.5 mmol), NaOH (19.2 mmol) and Na2CO3 (6 mmol)
115
were ultrasonically dispersed in deionized water. The solution containing 9 mmol
116
Mg(NO3)2•6H2O and 3 mmol Al(NO3)3•9H2O, was added dropwise into the
117
abovementioned mixture under stirring and then kept for 20 min (Mg2+/Al3+=3). The
118
supernatant (20 mL) was removed using a pipette after separating by a magnet and
119
then added 20 mL ethylene glycol with stirring for 10 min. The mixture was
120
transferred to Teflon-lined autoclave and maintained at 110 °C for 12 h. The obtained
121
solid was washed with deionized water to neutrality and then dried at 60 °C.
122
Preparation of Urea-Fe3O4@LDH. The prepared Fe3O4@LDH was dispersed in
123
ethanol. APTES (0.1 g) were added to the abovementioned ethanol solution, they were
124
used as the functional reagent for introducing amino groups and connecting with
125
amide to form urea groups, and the mixtures were refluxed at 80 °C for 10 h to obtain
126
NH2-Fe3O4@LDH after separating, washing and drying. Then, the obtained materials
127
were dispersed in THF and 4-tolyl isocyanate (0.03 g) were added alone with stirring 6
128
for 24 h. The product was collected by a magnet, washed with ethanol and water, and
129
dried at 60 °C to obtain the Urea-Fe3O4@LDH. The preparation process of the
130
Urea-Fe3O4@SiO2 and Urea-Fe3O4@C were demonstrated in the supporting
131
information.
132
2.3. Analytical method
133
The surface morphologies of the adsorbents were measured by scanning electron
134
microscopy (SEM, JSM-7800, Japan) with an accelerating voltage of 10 kV and
135
X-MaxN50 Aztec X-ray energy-dispersion spectroscopy. The morphologies images of
136
the adsorbents were taken on the transmission electron microscope (TEM, JEM-2100
137
Japan). The Brunauer Emmett Teller (BET) surface areas and the pore size
138
distributions of the materials were determined by N2 adsorption/desorption isotherms
139
on an Autosorb iQ-MP system at 77 K. The crystalline structure of the materials was
140
conducted via X-ray diffraction (XRD, Rigaku S2, Japan) in the range of 20°-80° and
141
a scanning speed of 4 °/min. The characteristic functional groups of the materials
142
were recorded by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10, USA)
143
in the range of 400-4000 cm−1 at 32 scan times and 4 cm−1 resolution. X-ray
144
photoelectron spectroscopy (XPS) was performed using Thermo 250 XI analyzer
145
using C1s peak (Eb = 284.80 eV) for calibration binding energy. The zeta potentials
146
(Malvern, UK) of prepared samples were determined. Water contact angle
147
measurements were also detected.
148
2.4. Adsorption experiments
7
149
Adsorption experiments were performed in a batch reactor. The reaction was
150
conducted using a 3.0 mg L-1 adsorbent in a 500 mL tphp solution, and the mixed
151
solution was stirred at 200 rpm for 6 h. Adsorption isotherm experiments were
152
conducted at pH 6.0 under different temperatures (298, 313 and 328 K) and varying
153
tphp concentrations (0-1.9 mg L-1). The effect of the initial pH on the tphp removal
154
was investigated in the range of pH 3.0-8.0. Ionic strength as an interfering factor was
155
studied by adding NaNO3 solution (0.001, 0.01 and 0.1 M). The effect of organic
156
matter on the tphp removal was performed by adding HA (0-200 mg L-1). The used
157
material was regenerated with ethanol.
158
The tphp concentration in the solution was measured via HPLC at a wavelength
159
of 204 nm, and the mobile phase was methanol solution (v/v = 70:30) at a flow rate of
160
1.0 mL min-1. The removal efficiency and adsorption amount (qt) were calculated as
161
follows: Removal efficiency (%) = 𝑞𝑡 =
(𝐶0 − 𝐶𝑡 ) × 100% 𝐶0
(𝐶0 − 𝐶𝑡 ) × 𝑉 𝑚
162
Where C0 and Ct (mg L-1) were the tphp concentration in the solution at the
163
initial and t reaction time (min), respectively. V (L) was the total volume of the
164
solution, and qt (mg g-1) was the amount of the adsorbed tphp per unit mass of
165
adsorbent after t reaction time. m (g) was the mass adsorbent in the reaction.
166
3. Results and discussion
167
3.1. Characterization of the materials
168
The SEM morphologies of Fe3O4, LDH, Fe3O4@LDH and Urea-Fe3O4@LDH 8
169
were illustrated in Fig.1 and Fig.S1. The commercial Fe3O4 showed an octahedral
170
shape with the size of ca. 200 nm, thereby attaching the irregular small particles. The
171
functionalized material of the Urea-Fe3O4@LDH had a sheet structure, while the
172
layers had a length of ca. 90-200 nm and a thickness of ca.16 nm. The morphology of
173
the shell LDH was obtained similar to the literature (Abdolmohammad-Zadeh and
174
Zamani-Kalajahi, 2019). For the Fe3O4@LDH, the LDH coated Fe3O4 with the length
175
and thickness unchanged evidently, like as Urea-Fe3O4@LDH. The TEM images of
176
and Urea-Fe3O4@LDH indicated that the adsorbent was a core-shell structure, and
177
LDH encapsulated Fe3O4 particles.
178
EDX analysis indicated that there were Mg, Al, C, N and O on the surface of the
179
Urea-Fe3O4@LDH (Fig.S1), in accordance with the preparation process of the
180
materials (Fig.S2). The specific surface area of the materials decreased from 80 to 58
181
m2 g-1 after the functionalized process, thus implying that the pores were blocked for
182
the Urea-Fe3O4@LDH (Table S1). The N2 adsorption/desorption isotherms of the
183
Urea-Fe3O4@LDH exhibited in Fig.S3 could be identified as a Type IV isotherm H3
184
hysteresis loop, thereby indicating that a mesoporous structure with a pore size of
185
ca.19 nm was obtained. As shown in Fig.S4, the surface property of the
186
Urea-Fe3O4@LDH was further investigated by contact angle measurements using
187
water drops, and the water contact angle was determined to be 56.0.
188
The crystal structure of the materials was analyzed via XRD, as depicted in
189
Fig.2a. The diffraction peaks of pure Fe3O4 with a face-centered cubic crystal were
190
obtained at ca. 30.18°, 35.52°, 43.18°, 53.62°, 57.08° and 62.64° and indexed to (220), 9
191
(311), (400), (422), (511) and (440) facets of Fe3O4 (JCPDS no. 89-2355),
192
correspondingly. The particle size calculated by the Scherrer’s formula was ca. 33 nm,
193
which was close to the 30 nm domain size of Fe3O4 (Ge et al., 2007). Therefore,
194
Fe3O4 maintained superparamagnetic characteristics, in advantages of the separation
195
from the solution. The diffraction peaks of the LDH at ca.11.38°, 22.96°, 34.52°,
196
38.52°, 45.80°, 60.50° and 61.80° were attributed to typical layered double hydroxide
197
(JCPDS no. 35-0965). The XRD pattern of the Fe3O4@LDH contained the diffraction
198
peaks of Fe3O4 and LDH, and the intensity of the peaks became obviously weak. For
199
the Urea-Fe3O4@LDH, the average distance between two cations in the layer (a =
200
2d110) and three times the layer spacing (c = 3d003) of LDH was at 3.05 and 23.4 Å;
201
these values were the same as those of LDH and Fe3O4@LDH (Fig.S5) (Du et al.,
202
2016). The results indicated that the Urea-Fe3O4@LDH had a favorable crystal
203
structure and the LDH retained the layered structure after the functionalization
204
process.
205
In the FT-IR spectrum of Fe3O4, LDH, Fe3O4@LDH and Urea-Fe3O4@LDH (Fig.
206
2b), the broad and strong adsorption bands at ca. 3473 and 1633 cm−1 were assigned
207
to the stretching vibrations of hydroxyl groups and water on the surface of the
208
materials. For Fe3O4, Fe-O stretching vibration was observed at ca. 572 cm−1, while
209
the peak intensity weakened for Fe3O4 coated by LDH. The strong vibration of M-O,
210
O-M-O and M-O-M (M=Mg or Al) appeared at ca. 660 cm−1, and the strong peak at
211
ca. 1370 cm−1 was related to the interlayer carbonates of LDH (Wu et al., 2017). In
212
the Urea-Fe3O4@LDH spectrum, the peaks at ca. 2927 and 2850 cm−1 corresponded 10
213
to the asymmetric and symmetric stretching vibrations of C-H; furthermore, the new
214
bands at ca. 1544 and 1241 cm−1 were attributed to N-H in-plane bending vibration
215
and C-N stretching vibration (Seo et al., 2017). The peak at ca. 1519 cm−1 was the
216
vibration of the benzene ring. The above mentioned results revealed that the
217
Fe3O4@LDH and Urea-Fe3O4@LDH formed a core-shell structure and the
218
N-containing functional groups were attached to the Urea-Fe3O4@LDH.
219
The XPS analysis was used to further identify the N-containing functional groups
220
of the Urea-Fe3O4@LDH. In Fig.3a, the Mg, Al, C and O elements were observed on
221
the surface of Fe3O4@LDH, and the N peak was added after the functionalized
222
process of the Fe3O4@LDH. Moreover, the N1s peak of the Urea-Fe3O4@LDH was
223
fitted into four peaks at 398.8, 399.3, 400.0 and 401.1 eV (Fig. 3b). The peak at low
224
binding energy (398.8 eV) corresponded to the phenyl side C-N bond (Chen et al.,
225
2018), whereas the peaks at 399.3 and 400.0 eV were related to the amide group
226
CO-NH and the N-H of the C chain (Nouralishahi et al., 2019; Yang et al., 2018). The
227
peak at 401.1 eV indicated the -N+ presence, thereby suggesting that the N-containing
228
functional groups as charged species involved an electrostatic interaction (Sharma et
229
al., 2004). In accordance with the results of the FT-IR and XPS of the material, the
230
NH-CO-NH groups were grafted on the surface of the Urea-Fe3O4@LDH, and the
231
core-shell structure material was successfully achieved.
232
3.2. Adsorption performance
233
Fig.4a displayed the tphp removal on Fe3O4, LDH, Fe3O4@LDH and
234
Urea-Fe3O4@LDH. Ca. 40% tphp removal was obtained for Fe3O4, LDH and 11
235
Fe3O4@LDH, and long adsorption equilibrium was observed over 360 min. In
236
comparison with these materials, the Urea-Fe3O4@LDH showed the better tphp
237
removal. The adsorption removal efficiency achieved about 90% within 120 min.
238
Given the large molecule volume and the spatial effect of the tphp, the adsorption
239
preferentially occurred on the surface of the materials. The groups (NH-CO-NH) of
240
the Urea-Fe3O4@LDH were helpful to the adsorption performance than the hydroxyls
241
of Fe3O4 and LDH. The tphp adsorption capacity was closely related to different
242
functional groups. Therefore, the fast and high tphp removal was achieved on the
243
surface of the Urea-Fe3O4@LDH.
244
The tphp adsorption kinetics on the Urea-Fe3O4@LDH was illustrated in Fig.4b.
245
The pseudo-first-order and pseudo-second-order models were fitted in accordance
246
with the kinetic data of the tphp adsorption on the material (Table S2). Based on the
247
correlation coefficient R2, the pseudo-second-order model was suitable for simulating
248
the tphp adsorption on the material. The initial adsorption rate (V0) reached to 49.9
249
mg g-1 min-1 on the Urea-Fe3O4@LDH.
250
The adsorption of pollutants on the porous adsorbent mainly depends on pores.
251
In comparison with the molecular size of the tphp (ca. 1.1 nm), a higher pore size of
252
the mesoporous Urea-Fe3O4@LDH (ca. 19 nm) was obtained, thereby indicating that
253
the tphp could enter the pores and adsorb on the adsorbent. The intraparticle diffusion
254
model was used to fit the data of the tphp removal on the Urea-Fe3O4@LDH (Fig. 4c
255
and Table S3). The first stage did not pass the origin, thus implying that the
256
intraparticle diffusion was not the main step for the tphp adsorption on the material. 12
257
Moreover, the fast tphp adsorption was observed at the beginning of the adsorption
258
process, thereby denoting that external and boundary-layer diffusions might affect the
259
tphp adsorption on the Urea-Fe3O4@LDH (Wang et al., 2018c). The results further
260
confirmed that the tphp adsorption occurred mainly on the surface of the
261
Urea-Fe3O4@LDH.
262
The tphp adsorption isotherm on the Urea-Fe3O4@LDH was fitted with the
263
Langmuir and Freundlich equations, respectively (Fig.4d). The result indicated that
264
the tphp adsorption process on the Urea-Fe3O4@LDH was in accordance with the
265
Freundlich model. Moreover, the material exhibited favorable adsorption performance
266
at different operating concentrations. The tphp removal efficiency on the
267
Urea-Fe3O4@LDH rapidly increased with the increase of the tphp concentration. The
268
index in the Freundlich model was between 1.0 and 1.2, indicating that the adsorption
269
isotherm was closed to nonlinear (Table S4). The Langmuir model assumed
270
monolayer coverage on the surface of the adsorbents, there might be electrostatic
271
attraction in the adsorption process. Simultaneously, π-π interaction occurred between
272
aromatic pollutants and benzene on the surface of the material (Wang et al., 2018a).
273
The nonlinearity was due to the roles of heterogeneous adsorption sites and different
274
adsorption interactions like π-π interaction. In addition, the tphp adsorption capacity
275
of the Urea-Fe3O4@LDH increased with the increase of the reaction temperature,
276
confirming that the reaction was endothermic. The tphp adsorption capacities of the
277
Urea-Fe3O4@LDH were 501.18, 521.34 and 588.67 mg g-1 at 298-328 K. The
278
thermodynamic parameters were related to the adsorption isotherms at different 13
279
temperatures. 𝐾𝑑 = 𝑙𝑛𝐾𝑑 =
𝑞𝑒 𝑐𝑒
∆𝑆 ∆𝐻 − 𝑅 𝑅𝑇
∆𝐺 = −𝑅𝑇𝑙𝑛𝐾𝑑 280
Where ΔH (kJ mol-1), ΔS (J mol-1 K-1) and ΔG (kJ mol-1) were standard enthalpy,
281
entropy, and Gibbs free energy, respectively. T (K) was the temperature in Kelvin, R
282
was the gas constant and Kd was a distribution coefficient.
283
The thermodynamic parameters were listed in Table S5. The positive values of
284
ΔH indicated that the tphp adsorption process on the Urea-Fe3O4@LDH was
285
endothermic. ΔS (66.54 J mol−1K−1) reflected the increasing randomness at the solid
286
solution interface. The negative values of ΔG stated that the adsorption process was
287
spontaneous at experimental temperatures.
288
3.3. Effect of solution pH, ionic strength and organic matter
289
Effect of solution pH, ionic strength and organic matter on the tphp adsorption
290
were showed in Fig.5a and Fig.5b. The pH of the solution affects the surface charge of
291
the material and pollutant. The tphp adsorption capacity on the Urea-Fe3O4@LDH
292
was demonstrated under different pH conditions in Fig.5a. The tphp adsorption
293
removal gradually increased at pH 3.0-5.5 and then decreased with the pH increase to
294
8.0. As showed in Fig.5c, pH at zero charge point (pHZCP) of the Urea-Fe3O4@LDH
295
resulted to be equal to 10.5, the surface of the material was positively charged at pH
296
3-8, and the oxygen atom in phosphate had a high electron cloud density, thereby
297
resulting in the protonation of tphp (Vank et al., 2000). The protonation of the P=O 14
298
bond produced an electrostatic repulsion to the material surface with the positive
299
charge. With the pH increase of the solution, the protonation degree of tphp gradually
300
decreased, and the electrostatic attraction between O with a negative charge and the
301
material surface with a positive charge increased. Therefore, the tphp adsorption
302
removal increased rapidly as the pH increase. After the addition of the adsorbents, the
303
zeta potential change of the solution was shown in Fig.5d. The value of zeta potential
304
changed from negative to positive, demonstrating the electrostatic interaction between
305
the adsorbents and tphp. When the dosage of adsorbent was 3 mg L-1, the potential in
306
the solution was close to zero and the tphp was prone to adsorption. At above pH 5.5,
307
the deprotonation of tphp increased, and the positive charge of P in O=P was greater
308
than the negative charge of O (Lazarević-Pašti et al., 2018). The electrostatic
309
repulsion was generated between O=P and urea. Thus, the tphp adsorption on the
310
Urea-Fe3O4@LDH reduced partly at a high pH solution. The interactions between
311
Urea-Fe3O4@LDH and tphp under electrostatic repulsion were attributed to: π-π
312
interaction between the benzene ring on the surface of the material and tphp.
313
The effect of ionic strength conditions on the tphp removal on the
314
Urea-Fe3O4@LDH was investigated (Fig.5a). The change of the tphp adsorption
315
removal demonstrated a slight impact when the NaNO3 concentration increased from
316
0.001 to 0.1 M. For example, the tphp adsorption amount was 550 mg g-1 without
317
adding NaNO3 at pH 5.5. When the NaNO3 concentration increased to 0.1 M, the
318
adsorption capability was 464 mg L-1 at the same pH condition, and the adsorption
319
removal only reduced by ca. 12%. The NO3− could accumulate on the surface of the 15
320
Urea-Fe3O4@LDH with the positive charge, thereby resulting in the formation of the
321
charge screening effect by neutralizing electricity (Litke et al., 2019). Therefore, the
322
tphp adsorption removal reduced. The CO32− in the MgAl-LDH layers can be
323
converted by the ion exchange at acidic conditions to NO3−-LDH; this process was
324
called the “acid-salt method” (Iyi et al., 2004). The process reduced charge density on
325
the surface and weakened charge screening. Therefore, the increase of the ionic
326
strength could not significantly affect the tphp adsorption removal. For example, the
327
tphp adsorption capacity on the material was slightly enhanced at low pH. In addition,
328
NaNO3 had a “salting out” effect on the adsorption of hydrophobic organic pollutants
329
(Kalra et al., 2001), especially for highly hydrophobic and low solubility tphp. The
330
additional ions were contributed to reducing the tphp solubility. Therefore, the
331
increase in ionic strength slightly affected the tphp adsorption.
332
Fig.5b displayed the effect of HA, which was selected as a model of natural
333
organic matter, on the tphp adsorption removal on the Urea-Fe3O4@LDH at pH 7.5.
334
The tphp removal kept in a certain range on the Urea-Fe3O4@LDH when the HA
335
concentration was around 0-40 mg L-1. Moreover, the tphp adsorption removal
336
increased when the HA concentration ranged from 40 to 80 mg L-1. The tphp
337
adsorption capacity on the Urea-Fe3O4@LDH remained unchanged at 440 mg g-1
338
when HA exceeded 80 mg L-1. This was due to the reason that HA can adsorb
339
organophosphorus compounds (in Fig.S6). The adsorption efficiency of HA and the
340
binding ability increased with the increase in the hydrophobicity of pollutants. The
341
tphp was adsorbed by HA via hydrophobic and π-π interactions with the increase of 16
342
the HA concentration (Wang et al., 2017); thus, the amount of the tphp adsorption on
343
the material increased.
344
3.4. Comparison of adsorption performance
345
Different
shell
materials
(Urea-Fe3O4@LDH,
Urea-Fe3O4@SiO2
and
346
Urea-Fe3O4@C) with urea functional groups were prepared to investigate the tphp
347
adsorption
348
Urea-Fe3O4@LDH than in Urea-Fe3O4@SiO2 and Urea-Fe3O4@C under different pH
349
solutions and ionic strength. For example, the adsorption capacities for the
350
Urea-Fe3O4@LDH, Urea-Fe3O4@SiO2 and Urea-Fe3O4@C were 455, 285 and 206
351
mg g-1 at pH 6 with 0.001 M NaNO3, respectively. Then, the adsorption capacities
352
decreased to 428, 330 and 310 mg g-1 when the NaNO3 concentration increased to 0.1
353
M. The results indicated that LDH was the most effective shell for grafting urea
354
groups and removing the tphp. And the adsorption removal was slightly affected by
355
ionic strength, thereby indicating that urea functional groups had favorable efficient
356
adsorption for the tphp removal. The different shell materials that pH at pHZCP were
357
equal to 10.5, 10.1 and 9.4, carried on different charges and resulted in different
358
adsorption removal. The effects of pH values and surface charges indicated that
359
electrostatic attraction was the main way for the tphp removal, in which phosphate of
360
the tphp was negatively charged and urea was positively charged in the
361
Urea-Fe3O4@LDH adsorption process.
in
Fig.6a.
The
tphp
adsorption
removal
was
better
in
the
362
After the tphp adsorption reaction on the Urea-Fe3O4@LDH, Urea-Fe3O4@SiO2
363
and Urea-Fe3O4@C, the FT-IR spectrum demonstrated (in Fig.6b) that (1) some peaks 17
364
of the tphp appeared non-overlapping positions of the used adsorbents slightly; (2) the
365
intensity of the CO32− vibration of the Urea-Fe3O4@LDH remained unchanged, thus
366
representing that ion exchange reaction did not happen in LDH. The adsorption
367
mechanism was presented in Fig.7. In addition, the difference of removal efficiency
368
between the aryl-OPFR (tphp) and the alkyl-OPFR (triethyl phosphate) was about 30%
369
(in Fig.8a). The tphp had stronger hydrophobicity and aromaticity than triethyl
370
phosphate, and hydrophobic interaction was rare in adsorption process due to the
371
hydrophilicity of the adsorbents. Therefore, the benzene on the surface of the
372
adsorbents and the tphp exhibited π-π interaction.
373
In order to evaluate the Urea-Fe3O4@LDH adsorption performance, the
374
comparison of the adsorption capacities of different adsorbents was carried out (in
375
Table 1). It can be speculated that the Urea-Fe3O4@LDH was a better adsorbent
376
compared with some other materials.
377
3.5. Reusability and other organophosphorus pollutants adsorption
378
The Urea-Fe3O4@LDH was used to remove other organophosphorus pollutants,
379
and the experimental results were shown in Fig. 8a. Organophosphorus pesticides
380
such as malathion, dimethoate and dichlorvos, removal rates of 67.3%, 77.8% and
381
59.5% respectively. At the same time, the removal of triethyl phosphate, one of
382
OPFRs, reached 65.2%. The Urea-Fe3O4@LDH was a good adsorbent for removing
383
organophosphorus pollutants.
384
Fig.8b depicted the tphp adsorption removal of the reused Urea-Fe3O4@LDH
385
after regenerating ethanol. The Urea-Fe3O4@LDH exhibited favorable and stable tphp 18
386
removal in the consecutive five cycles. Moreover, in the sixth adsorption process, the
387
removal was slightly reduced (ca. 10%). In addition, the used material maintained
388
good magnetic separation from the solution (in Fig.S7). The results indicated that the
389
Urea-Fe3O4@LDH showed good reusability and magnetism for the tphp adsorption in
390
the regeneration cycle experiment.
391
The above results showed that the Urea-Fe3O4@LDH had a higher adsorption
392
performance and a better removal rate for the tphp, indicating that it can be a choice to
393
deal with the industrial wastewater with these contaminates, especially in some
394
emerging situations. Also, the optimization of the material will be investigated in the
395
future.
396
4. Conclusions
397
In summary, the core-shell Urea-Fe3O4@LDH was successfully synthesized and
398
urea groups were generated on the surface of the adsorbent. The tphp adsorption
399
capacity on the Urea-Fe3O4@LDH was up to 589 mg g-1. Moreover, ionic strength and
400
organic matter HA slightly affected the tphp adsorption on the adsorbent. The
401
adsorption process depended on the pH solution, indicating electrostatic interaction
402
was the main adsorption mechanism. Compared to the triethyl phosphate, the tphp
403
removal was higher due to the π-π interaction. The Urea-Fe3O4@LDH will have a
404
good prospect for removing different organophosphorus pollutants. In the future, we
405
will optimize the adsorbent and try to use it in the real wastewater.
406
Acknowledgments: The work was supported by National Major Science and
407
Technology Program for Water Pollution Control and Treatment (2017ZX07101-003) 19
408
and Fundamental Research Funds for Central Universities (2018ZD08, 2018MS033).
409
References
410
Abdolmohammad-Zadeh, H., Zamani-Kalajahi, M., 2019. A turn-on/off fluorescent
411
sensor based on nano-structured Mg-Al layered double hydroxide intercalated with
412
salicylic acid for monitoring of ferric ion in human serum samples. Anal. Chim. Acta
413
1061, 152-160.
414
Cao, Y., Fan, Y., Ma, Z., Cheng, Z., Xiang, Q., Duan, Z., Xu, J., 2018.
415
Urea-functionalized SBA-15 hybrids: Post-grafting synthesis, high-performance
416
organophosphorus sensing and their response mechanism. Sensor. Actuat. B-Chem.
417
273, 1162-1169.
418
Chen, Y., Zhang, Y., Luo, L., Shi, Y., Wang, S., Li, L., Long, Y., Jiang, F., 2018. A
419
novel templated synthesis of C/N-doped β-Bi2O3 nanosheets for synergistic rapid
420
removal of 17α-ethynylestradiol by adsorption and photocatalytic degradation. Ceram.
421
Int. 44, 2178-2185.
422
Dehghani, M.H., Niasar, Z.S., Mehrnia, M.R., Shayeghi, M., Al-Ghouti, M.A.,
423
Heibati, B., McKay, G., Yetilmezsoy, K., 2017. Optimizing the removal of
424
organophosphorus pesticide malathion from water using multi-walled carbon
425
nanotubes. Chem. Eng. J. 310, 22-32.
426
Du, W., Zheng, L., Li, X., Fu, J., Lu, X., Hou, Z., 2016. Plate-like Ni-Mg-Al layered
427
double hydroxide synthesized via a solvent-free approach and its application in
428
hydrogenolysis of D-sorbitol. Appl. Clay Sci. 123, 166-172.
429
Du, Z., Wang, G., Gao, S., Wang, Z., 2015. Aryl organophosphate flame retardants
430
induced cardiotoxicity during zebrafish embryogenesis: By disturbing expression of
431
the transcriptional regulators. Aquat. Toxicol. 161, 25-32.
432
Ge, J., Hu, Y., Biasini, M., Beyermann, W.P., Yin, Y., 2007. Superparamagnetic
433
magnetite colloidal nanocrystal clusters. Angew. Chem. Int. Edit. 46, 4342-4345. 20
434
Grieco, S.A., Ramarao, B.V., 2013. Removal of TCEP from aqueous solutions by
435
adsorption with zeolites. Colloid. Surface. A. 434, 329-338.
436
Gu, J., Su, F., Hong, P., Zhang, Q., Zhao, M., 2019. 1H NMR-based metabolomic
437
analysis of nine organophosphate flame retardants metabolic disturbance in Hep G2
438
cell line. Sci. Total Environ. 665, 162-170.
439
Iyi, N., Matsumoto, T., Kaneko, Y., Kitamura, K., 2004. Deintercalation of carbonate
440
ions from a hydrotalcite-like compound: Enhanced decarbonation using acid-salt
441
mixed solution. Chem. Mater. 16, 2926-2932.
442
Ji, Y., Wang, Y., Yao, Y., Ren, C., Lan, Z., Fang, X., Zhang, K., Sun, W., Alder, A.C.,
443
Sun, H., 2019. Occurrence of organophosphate flame retardants in farmland soils
444
from Northern China: Primary source analysis and risk assessment. Environ. Pollut.
445
247, 832-838.
446
Kalra, A., Tugcu, N., Cramer, S.M., Garde, S., 2001. Salting-in and salting-out of
447
hydrophobic solutes in aqueous salt solutions. J. Phys. Chem. B. 105, 6380-6386.
448
Lazarević-Pašti, T., Anicijevic, V., Baljozovic, M., Anicijevic, D.V., Gutic, S., Vasic,
449
V., Skorodumova, N.V., Pasti, I.A., 2018. The impact of the structure of
450
graphene-based materials on the removal of organophosphorus pesticides from water.
451
Environ. Sci-Nano 5, 1482-1494.
452
Litke, A., Frei, H., Hensen, E.J.M., Hofmann, J.P., 2019. Interfacial charge transfer in
453
Pt-loaded TiO2 P25 photocatalysts studied by in-situ diffuse reflectance FTIR
454
spectroscopy of adsorbed CO. J. Photoch. Photobio. A. 370, 84-88.
455
Nodeh, H.R., Ibrahim, W.A.W., Kamboh, M.A., Sanagi, M.M., 2017. New magnetic
456
graphene-based inorganic-organic sol-gel hybrid nanocomposite for simultaneous
457
analysis of polar and non-polar organophosphorus pesticides from water samples
458
using solid-phase extraction. Chemosphere 166, 21-30.
459
Nouralishahi, A., Mortazavi, Y., Khodadadi, A.A., Choolaei, M., Thompson, L.T., 21
460
Horri, B.A., 2019. Characteristics and performance of urea modified Pt-MWCNTs for
461
electro-oxidation of methanol. Appl. Surf. Sci. 467-468, 335-344.
462
Pang, L., Yuan, Y., He, H., Liang, K., Zhang, H., Zhao, J., 2016. Occurrence,
463
distribution, and potential affecting factors of organophosphate flame retardants in
464
sewage sludge of wastewater treatment plants in Henan Province, Central China.
465
Chemosphere 152, 245-251.
466
Seo, P.W., Khan, N.A., Jhung, S.H., 2017. Removal of nitroimidazole antibiotics from
467
water by adsorption over metal-organic frameworks modified with urea or melamine.
468
Chem. Eng. J. 315, 92-100.
469
Shan, R.R., Yan, L.G., Yang, K., Yu, S.J., Hao, Y.F., Yu, H.Q., Du, B., 2014. Magnetic
470
Fe3O4/MgAl-LDH composite for effective removal of three red dyes from aqueous
471
solution. Chem. Eng. J. 252, 38-46.
472
Sharma, J., Mahima, S., Kakade, B.A., Pasricha, R., Mandale, A.B., Vijayamohanan,
473
K.,
474
4-aminothiophenol-capped gold nanoparticles. J. Phys. Chem. B. 108, 13280-13286.
475
Teo, T.L.L., McDonald, J.A., Coleman, H.M., Khan, S.J., 2015. Analysis of
476
organophosphate flame retardants and plasticisers in water by isotope dilution gas
477
chromatography-electron ionisation tandem mass spectrometry. Talanta 143, 114-120.
478
Vank, J.C., Henry-Riyad, H., Csizmadia, I.G., 2000. Successive protonation of
479
phosphate (PO4/3-), thiophosphate (PSO3/3-), and selenophosphate (PSeO3/3-). J. Mol.
480
Struct. 504, 267-286.
481
Wang, F., Sun, H., Ren, X., Liu, Y., Zhu, H., Zhang, P., Ren, C., 2017. Effects of
482
humic acid and heavy metals on the sorption of polar and apolar organic pollutants
483
onto biochars. Environ. Pollut. 231, 229-236.
484
Wang, G., Du, Z., Chen, H., Su, Y., Gao, S., Mao, L., 2016. Tissue-specific
485
accumulation, depuration, and transformation of triphenyl phosphate (TPHP) in adult
2004.
Solvent-assisted
one-pot
22
synthesis
and
self-assembly
of
486
zebrafish (danio rerio). Environ. Sci. Technol. 50, 13555-13564.
487
Wang, W., Deng, S., Li, D., Ren, L., Shan, D., Wang, B., Huang, J., Wang, Y., Yu, G.,
488
2018a. Sorption behavior and mechanism of organophosphate flame retardants on
489
activated carbons. Chem. Eng. J. 332, 286-292.
490
Wang, W., Deng, S., Li, D., Ren, L., Wang, B., Huang, J., Wang, Y., Yu, G., 2018b.
491
Adsorptive removal of organophosphate flame retardants from water by non-ionic
492
resins. Chem. Eng. J. 354, 105-112.
493
Wang, W., Deng, S., Ren, L., Li, D., Wang, W., Vakili, M., Wang, B., Huang, J., Wang,
494
Y., Yu, G., 2018c. Stable covalent organic frameworks as efficient adsorbents for high
495
and selective removal of an aryl-organophosphorus flame retardant from water. ACS
496
Appl. Mater. Inter. 10, 30265-30272.
497
Wang, X.W., Liu, J.F., Yin, Y.G., 2011. Development of an ultra-high-performance
498
liquid chromatography-tandem mass spectrometry method for high throughput
499
determination of organophosphorus flame retardants in environmental water. J.
500
Chromatogr. A. 1218, 6705-6711.
501
Wei, K., Yin, H., Peng, H., Lu, G., Dang, Z., 2018. Bioremediation of triphenyl
502
phosphate by Brevibacillus brevis: Degradation characteristics and role of cytochrome
503
P450 monooxygenase. Sci. Total Environ. 627, 1389-1395.
504
Wu, X.G., Li, B., Wen, X.G., 2017. Synthesis and adsorption properties of
505
hierarchical Fe3O4@MgAl-LDH magnetic microspheres. J. Nanopart. Res. 19, 131.
506
Yan, W., Yan, L., Duan, J., Jing, C., 2014. Sorption of organophosphate esters by
507
carbon nanotubes. J. Hazard. Mater. 273, 53-60.
508
Yang, D., Wang, X., Wang, N., Zhao, G., Song, G., Chen, D., Liang, Y., Wen, T.,
509
Wang, H., Hayat, T., Alsaedi, A., Wang, X., Wang S., 2018. In-situ growth of
510
hierarchical layered double hydroxide on polydopamine-encapsulated hollow Fe3O4
511
microspheres for efficient removal and recovery of U(VI). J. Clean. Prod. 172, 23
512
2033-2044.
513
Zhang, W., Liang, Y., Wang, J., Zhang, Y., Gao, Z., Yang, Y., Yang, K., 2019.
514
Ultrasound-assisted adsorption of Congo red from aqueous solution using MgAlCO3
515
layered double hydroxide. Appl. Clay Sci. 174, 100-109.
24
Highlights
At the first time, the Urea functionalized Fe3O4@LDH was used for the tphp removal.
The Urea-Fe3O4@LDH exhibited the high TPhP adsorption removal and good cycles.
The tphp adsorption capacity of the Urea-Fe3O4@LDH was up to 589 mg g-1.
Ionic strength had little effect on the TPhP adsorption on the Urea-Fe3O4@LDH.
S0269-7491(19)33921-1
DOI:
https://doi.org/10.1016/j.envpol.2020.114142
Reference:
ENPO 114142
To appear in:
Environmental Pollution
Received Date: 18 July 2019 Revised Date:
14 January 2020
Accepted Date: 5 February 2020
Please cite this article as: Hao, M., Gao, P., Yang, D., Chen, X., Xiao, F., Yang, S., Highly efficient adsorption behavior and mechanism of Urea-Fe3O4@LDH for triphenyl phosphate, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.114142. 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. © 2020 Published by Elsevier Ltd.
1 2
Highly efficient adsorption behavior and mechanism of
3
Urea-Fe3O4@LDH for Triphenyl phosphate
4 5
Mengjie Hao, Pan Gao, Dian Yang, Xuanjin Chen, Feng Xiao*, Shaoxia Yang*
6 7
National Engineering Laboratory for Biomass Power Generation Equipment, School
8
of Renewable Energy, North China Electric Power University, Beijing 102206, China
9 10 11 12 13
Corresponding author:
14
Dr. Feng XIAO
15
Dr. Shaoxia YANG
16 17
Tel: +8610 61772456
18
Fax: +8610 61772230
19 20 21
E-mail: [email protected]; [email protected]
22 23 24 1
25
Abstract
26
The emergence of organophosphorus flame retardants and the efficient removal from
27
aquatic environments have aroused increasing concerns. The Urea functionalized
28
Fe3O4@LDH (Urea-Fe3O4@LDH) was prepared and used to adsorb triphenyl
29
phosphate (tphp) for the first time. The tphp adsorption capacity was up to 589 mg g-1,
30
and the adsorption rate reached 49.9 mg g-1 min-1. Moreover, the influences of various
31
environmental factors (pH, ionic strength and organic matter) on the tphp adsorption
32
on the Urea-Fe3O4@LDH were investigated. The initial pH of the solution
33
significantly affected the tphp adsorption, whereas the ionic strength and HA slightly
34
affected the adsorption. The main adsorption mechanism was attributed to
35
electrostatic interaction and π-π interaction. We believe that urea is one of excellent
36
functional groups for the tphp adsorption removal and the materials with urea groups
37
as the adsorbents exhibit good prospects in the future.
38
Keywords:
39
Urea
functionalized
Fe3O4@LDH;
Triphenyl
phosphate
(tphp);
Organophosphorus flame retardants (OPFRs); Adsorption capacity
2
40
1. Introduction
41
Flame retardants (FRs) are extensively applied in plastic, rubber and fiber to
42
reduce materials’ flammability (Ji et al., 2019). Given their persistence and toxicity,
43
brominated FRs are gradually replaced by organophosphorus FRs (OPFRs). In 2012,
44
the production quantity of OPFRs in China was more than 179,000 t, and OPFRs were
45
extensively detected in soil, water and air (Gu et al., 2019). Among the OPFRs,
46
triphenyl phosphate (tphp) is a kind of high production quantity of OPFRs and
47
detected in water because of not chemically bonded to the end-use products. For
48
example, the tphp concentration in surface water in Australia was up to 150 ng L-1
49
(Teo et al., 2015). The tphp concentration can also reach 65 ng L-1 in the Songhua
50
River in China (Wang et al., 2011). In addition, the tphp was detected from
51
wastewater treatment plants in Henan province (Pang et al., 2016). The extensive
52
exposure of the tphp in the environment can lead to human health risks through diet
53
and inhalation. Studies have shown that tphp induced toxicological effects on
54
embryonic development, the neurological system and the immune system. The tphp
55
had great toxic effects on heart development on 0.10 mg L-1 exposure for model
56
organism zebrafish (Du et al., 2015). And the tphp concentration can be up to 3.12 μg
57
g-1 exposing to 100 μg L-1 though bioaccumulation and metabolism of adult zebrafish
58
(Wang et al., 2016). Therefore, developing an efficient processing method is an
59
important challenge for removing tphp in water.
60
Various technologies are investigated to remove OPFRs, including biochemical
61
or physicochemical methods, such as oxidation, coagulation, adsorption (Wei et al., 3
62
2018; Yan et al., 2014). Among these techniques, adsorption, as a green, economical
63
and efficient method, is extensively used for removing pollutants in different
64
concentration ranges. Adsorbents, with the rapid and efficient adsorption removal and
65
easy separation from water, are paid a lot attention for the application in the water
66
treatment. Conventional adsorbents (activated carbon, multi-walled carbon nanotube
67
and zeolite) have been used for removing organophosphorus pollutants by hydroxyl
68
groups (Wang et al., 2018a; Dehghani et al., 2017; Grieco and Ramarao, 2013).
69
Hydroxyl groups, one of the widest functional groups on the surface of the
70
adsorbents, were developed to remove organophosphorus pesticides. Hydroxyl groups
71
in silica exhibited a high adsorption capacity of 37.2-76.3 mg g-1 via H-bonding and
72
electrostatic interactions, while the adsorption of organophosphorus pesticides was
73
interfered by inorganic ions and other organic matters in water (Nodeh et al., 2017).
74
Therefore, the development of materials with favorable selectivity and rapid
75
adsorption has been focused on for removing tphp. Urea exhibits remarkable
76
selectivity toward organophosphorus pollutants (Cao et al., 2018). The groups as
77
active sites adsorbed dimethyl methylphosphonate vapors by identifying P=O, and
78
urea donors and P=O acceptors formed hydrogen bonding in gas reaction. In water,
79
adsorbents with urea groups can generate electrostatic attraction or hydrogen bonding
80
with tphp between NH-CO-NH and P=O. The result indicates that urea groups could
81
exhibit considerable potential in adsorbing organophosphorus pesticides or OPFRs.
82
However, the relative literatures, involving adsorbents with urea functional groups,
83
are not investigated for the tphp removal in water, and the adsorption mechanism of 4
84
the adsorbent is not clear.
85
In addition, separating and recovering the adsorbents are other important factors
86
for applying them in removing pollutants in water and wastewater treatment. Fe3O4
87
magnetic nanoparticles are biocompatible, non-toxic and easy to separate from
88
aqueous solutions. Moreover, the shell of coated Fe3O4 was confirmed as an effective
89
approach to preventing Fe3O4 aggregation and oxidation. Layered double hydroxides
90
(LDHs) have received considerable attention due to abundant hydroxyls,
91
extraordinary ion-exchange ability and simplicity of preparation. Moreover, a
92
Fe3O4@LDH core-shell material has shown favorable separation capability in the
93
reaction (Shan et al., 2014).
94
In the present work, the urea functionalized Fe3O4@LDH (Urea-Fe3O4@LDH)
95
with the core-shell was successful synthesized and used for the tphp removal from the
96
solution. The tphp adsorption kinetics and isotherm on the Urea-Fe3O4@LDH were
97
investigated. Moreover, the effect of experimental conditions (pH, ionic strength,
98
organic matter, tphp concentration and reaction temperature) on the tphp adsorption
99
was studied. And the adsorption efficiency in cycles of the Urea-Fe3O4@LDH and the
100
adsorption of other organophosphorus pollutants were discussed. Hence, the main aim
101
of the present work is to analyze the characteristics and adsorption performance of
102
Urea-Fe3O4@LDH, and also explain the mechanism for removing the tphp
103
2. Materials and methods
104
2.1. Chemicals
105
All chemicals were in an analytical grade, and were used without further 5
106
purification. The tphp (purity >98%), 4-tolyl isocyanate and tetrahydrofuran (THF)
107
were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Fe3O4 and
108
3-aminopropyltriethoxysilane (APTES) were obtained from Aladdin Industrial Co.,
109
Ltd.
110
USA). Mg(NO3)2•6H2O, Al(NO3)3•9H2O, NaOH and humic acid (HA) were produced
111
from Fuchen Chemical Reagent Factory (Tianjin, China).
112
2.2. Preparation of adsorbents
(Shanghai, China). Na2CO3 was supplied by Sigma-Aldrich (St. Louis, MO,
113
Preparation of Fe3O4@LDH. LDH was prepared via the coprecipitation method
114
(Zhang et al., 2019). Fe3O4 (1.5 mmol), NaOH (19.2 mmol) and Na2CO3 (6 mmol)
115
were ultrasonically dispersed in deionized water. The solution containing 9 mmol
116
Mg(NO3)2•6H2O and 3 mmol Al(NO3)3•9H2O, was added dropwise into the
117
abovementioned mixture under stirring and then kept for 20 min (Mg2+/Al3+=3). The
118
supernatant (20 mL) was removed using a pipette after separating by a magnet and
119
then added 20 mL ethylene glycol with stirring for 10 min. The mixture was
120
transferred to Teflon-lined autoclave and maintained at 110 °C for 12 h. The obtained
121
solid was washed with deionized water to neutrality and then dried at 60 °C.
122
Preparation of Urea-Fe3O4@LDH. The prepared Fe3O4@LDH was dispersed in
123
ethanol. APTES (0.1 g) were added to the abovementioned ethanol solution, they were
124
used as the functional reagent for introducing amino groups and connecting with
125
amide to form urea groups, and the mixtures were refluxed at 80 °C for 10 h to obtain
126
NH2-Fe3O4@LDH after separating, washing and drying. Then, the obtained materials
127
were dispersed in THF and 4-tolyl isocyanate (0.03 g) were added alone with stirring 6
128
for 24 h. The product was collected by a magnet, washed with ethanol and water, and
129
dried at 60 °C to obtain the Urea-Fe3O4@LDH. The preparation process of the
130
Urea-Fe3O4@SiO2 and Urea-Fe3O4@C were demonstrated in the supporting
131
information.
132
2.3. Analytical method
133
The surface morphologies of the adsorbents were measured by scanning electron
134
microscopy (SEM, JSM-7800, Japan) with an accelerating voltage of 10 kV and
135
X-MaxN50 Aztec X-ray energy-dispersion spectroscopy. The morphologies images of
136
the adsorbents were taken on the transmission electron microscope (TEM, JEM-2100
137
Japan). The Brunauer Emmett Teller (BET) surface areas and the pore size
138
distributions of the materials were determined by N2 adsorption/desorption isotherms
139
on an Autosorb iQ-MP system at 77 K. The crystalline structure of the materials was
140
conducted via X-ray diffraction (XRD, Rigaku S2, Japan) in the range of 20°-80° and
141
a scanning speed of 4 °/min. The characteristic functional groups of the materials
142
were recorded by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10, USA)
143
in the range of 400-4000 cm−1 at 32 scan times and 4 cm−1 resolution. X-ray
144
photoelectron spectroscopy (XPS) was performed using Thermo 250 XI analyzer
145
using C1s peak (Eb = 284.80 eV) for calibration binding energy. The zeta potentials
146
(Malvern, UK) of prepared samples were determined. Water contact angle
147
measurements were also detected.
148
2.4. Adsorption experiments
7
149
Adsorption experiments were performed in a batch reactor. The reaction was
150
conducted using a 3.0 mg L-1 adsorbent in a 500 mL tphp solution, and the mixed
151
solution was stirred at 200 rpm for 6 h. Adsorption isotherm experiments were
152
conducted at pH 6.0 under different temperatures (298, 313 and 328 K) and varying
153
tphp concentrations (0-1.9 mg L-1). The effect of the initial pH on the tphp removal
154
was investigated in the range of pH 3.0-8.0. Ionic strength as an interfering factor was
155
studied by adding NaNO3 solution (0.001, 0.01 and 0.1 M). The effect of organic
156
matter on the tphp removal was performed by adding HA (0-200 mg L-1). The used
157
material was regenerated with ethanol.
158
The tphp concentration in the solution was measured via HPLC at a wavelength
159
of 204 nm, and the mobile phase was methanol solution (v/v = 70:30) at a flow rate of
160
1.0 mL min-1. The removal efficiency and adsorption amount (qt) were calculated as
161
follows: Removal efficiency (%) = 𝑞𝑡 =
(𝐶0 − 𝐶𝑡 ) × 100% 𝐶0
(𝐶0 − 𝐶𝑡 ) × 𝑉 𝑚
162
Where C0 and Ct (mg L-1) were the tphp concentration in the solution at the
163
initial and t reaction time (min), respectively. V (L) was the total volume of the
164
solution, and qt (mg g-1) was the amount of the adsorbed tphp per unit mass of
165
adsorbent after t reaction time. m (g) was the mass adsorbent in the reaction.
166
3. Results and discussion
167
3.1. Characterization of the materials
168
The SEM morphologies of Fe3O4, LDH, Fe3O4@LDH and Urea-Fe3O4@LDH 8
169
were illustrated in Fig.1 and Fig.S1. The commercial Fe3O4 showed an octahedral
170
shape with the size of ca. 200 nm, thereby attaching the irregular small particles. The
171
functionalized material of the Urea-Fe3O4@LDH had a sheet structure, while the
172
layers had a length of ca. 90-200 nm and a thickness of ca.16 nm. The morphology of
173
the shell LDH was obtained similar to the literature (Abdolmohammad-Zadeh and
174
Zamani-Kalajahi, 2019). For the Fe3O4@LDH, the LDH coated Fe3O4 with the length
175
and thickness unchanged evidently, like as Urea-Fe3O4@LDH. The TEM images of
176
and Urea-Fe3O4@LDH indicated that the adsorbent was a core-shell structure, and
177
LDH encapsulated Fe3O4 particles.
178
EDX analysis indicated that there were Mg, Al, C, N and O on the surface of the
179
Urea-Fe3O4@LDH (Fig.S1), in accordance with the preparation process of the
180
materials (Fig.S2). The specific surface area of the materials decreased from 80 to 58
181
m2 g-1 after the functionalized process, thus implying that the pores were blocked for
182
the Urea-Fe3O4@LDH (Table S1). The N2 adsorption/desorption isotherms of the
183
Urea-Fe3O4@LDH exhibited in Fig.S3 could be identified as a Type IV isotherm H3
184
hysteresis loop, thereby indicating that a mesoporous structure with a pore size of
185
ca.19 nm was obtained. As shown in Fig.S4, the surface property of the
186
Urea-Fe3O4@LDH was further investigated by contact angle measurements using
187
water drops, and the water contact angle was determined to be 56.0.
188
The crystal structure of the materials was analyzed via XRD, as depicted in
189
Fig.2a. The diffraction peaks of pure Fe3O4 with a face-centered cubic crystal were
190
obtained at ca. 30.18°, 35.52°, 43.18°, 53.62°, 57.08° and 62.64° and indexed to (220), 9
191
(311), (400), (422), (511) and (440) facets of Fe3O4 (JCPDS no. 89-2355),
192
correspondingly. The particle size calculated by the Scherrer’s formula was ca. 33 nm,
193
which was close to the 30 nm domain size of Fe3O4 (Ge et al., 2007). Therefore,
194
Fe3O4 maintained superparamagnetic characteristics, in advantages of the separation
195
from the solution. The diffraction peaks of the LDH at ca.11.38°, 22.96°, 34.52°,
196
38.52°, 45.80°, 60.50° and 61.80° were attributed to typical layered double hydroxide
197
(JCPDS no. 35-0965). The XRD pattern of the Fe3O4@LDH contained the diffraction
198
peaks of Fe3O4 and LDH, and the intensity of the peaks became obviously weak. For
199
the Urea-Fe3O4@LDH, the average distance between two cations in the layer (a =
200
2d110) and three times the layer spacing (c = 3d003) of LDH was at 3.05 and 23.4 Å;
201
these values were the same as those of LDH and Fe3O4@LDH (Fig.S5) (Du et al.,
202
2016). The results indicated that the Urea-Fe3O4@LDH had a favorable crystal
203
structure and the LDH retained the layered structure after the functionalization
204
process.
205
In the FT-IR spectrum of Fe3O4, LDH, Fe3O4@LDH and Urea-Fe3O4@LDH (Fig.
206
2b), the broad and strong adsorption bands at ca. 3473 and 1633 cm−1 were assigned
207
to the stretching vibrations of hydroxyl groups and water on the surface of the
208
materials. For Fe3O4, Fe-O stretching vibration was observed at ca. 572 cm−1, while
209
the peak intensity weakened for Fe3O4 coated by LDH. The strong vibration of M-O,
210
O-M-O and M-O-M (M=Mg or Al) appeared at ca. 660 cm−1, and the strong peak at
211
ca. 1370 cm−1 was related to the interlayer carbonates of LDH (Wu et al., 2017). In
212
the Urea-Fe3O4@LDH spectrum, the peaks at ca. 2927 and 2850 cm−1 corresponded 10
213
to the asymmetric and symmetric stretching vibrations of C-H; furthermore, the new
214
bands at ca. 1544 and 1241 cm−1 were attributed to N-H in-plane bending vibration
215
and C-N stretching vibration (Seo et al., 2017). The peak at ca. 1519 cm−1 was the
216
vibration of the benzene ring. The above mentioned results revealed that the
217
Fe3O4@LDH and Urea-Fe3O4@LDH formed a core-shell structure and the
218
N-containing functional groups were attached to the Urea-Fe3O4@LDH.
219
The XPS analysis was used to further identify the N-containing functional groups
220
of the Urea-Fe3O4@LDH. In Fig.3a, the Mg, Al, C and O elements were observed on
221
the surface of Fe3O4@LDH, and the N peak was added after the functionalized
222
process of the Fe3O4@LDH. Moreover, the N1s peak of the Urea-Fe3O4@LDH was
223
fitted into four peaks at 398.8, 399.3, 400.0 and 401.1 eV (Fig. 3b). The peak at low
224
binding energy (398.8 eV) corresponded to the phenyl side C-N bond (Chen et al.,
225
2018), whereas the peaks at 399.3 and 400.0 eV were related to the amide group
226
CO-NH and the N-H of the C chain (Nouralishahi et al., 2019; Yang et al., 2018). The
227
peak at 401.1 eV indicated the -N+ presence, thereby suggesting that the N-containing
228
functional groups as charged species involved an electrostatic interaction (Sharma et
229
al., 2004). In accordance with the results of the FT-IR and XPS of the material, the
230
NH-CO-NH groups were grafted on the surface of the Urea-Fe3O4@LDH, and the
231
core-shell structure material was successfully achieved.
232
3.2. Adsorption performance
233
Fig.4a displayed the tphp removal on Fe3O4, LDH, Fe3O4@LDH and
234
Urea-Fe3O4@LDH. Ca. 40% tphp removal was obtained for Fe3O4, LDH and 11
235
Fe3O4@LDH, and long adsorption equilibrium was observed over 360 min. In
236
comparison with these materials, the Urea-Fe3O4@LDH showed the better tphp
237
removal. The adsorption removal efficiency achieved about 90% within 120 min.
238
Given the large molecule volume and the spatial effect of the tphp, the adsorption
239
preferentially occurred on the surface of the materials. The groups (NH-CO-NH) of
240
the Urea-Fe3O4@LDH were helpful to the adsorption performance than the hydroxyls
241
of Fe3O4 and LDH. The tphp adsorption capacity was closely related to different
242
functional groups. Therefore, the fast and high tphp removal was achieved on the
243
surface of the Urea-Fe3O4@LDH.
244
The tphp adsorption kinetics on the Urea-Fe3O4@LDH was illustrated in Fig.4b.
245
The pseudo-first-order and pseudo-second-order models were fitted in accordance
246
with the kinetic data of the tphp adsorption on the material (Table S2). Based on the
247
correlation coefficient R2, the pseudo-second-order model was suitable for simulating
248
the tphp adsorption on the material. The initial adsorption rate (V0) reached to 49.9
249
mg g-1 min-1 on the Urea-Fe3O4@LDH.
250
The adsorption of pollutants on the porous adsorbent mainly depends on pores.
251
In comparison with the molecular size of the tphp (ca. 1.1 nm), a higher pore size of
252
the mesoporous Urea-Fe3O4@LDH (ca. 19 nm) was obtained, thereby indicating that
253
the tphp could enter the pores and adsorb on the adsorbent. The intraparticle diffusion
254
model was used to fit the data of the tphp removal on the Urea-Fe3O4@LDH (Fig. 4c
255
and Table S3). The first stage did not pass the origin, thus implying that the
256
intraparticle diffusion was not the main step for the tphp adsorption on the material. 12
257
Moreover, the fast tphp adsorption was observed at the beginning of the adsorption
258
process, thereby denoting that external and boundary-layer diffusions might affect the
259
tphp adsorption on the Urea-Fe3O4@LDH (Wang et al., 2018c). The results further
260
confirmed that the tphp adsorption occurred mainly on the surface of the
261
Urea-Fe3O4@LDH.
262
The tphp adsorption isotherm on the Urea-Fe3O4@LDH was fitted with the
263
Langmuir and Freundlich equations, respectively (Fig.4d). The result indicated that
264
the tphp adsorption process on the Urea-Fe3O4@LDH was in accordance with the
265
Freundlich model. Moreover, the material exhibited favorable adsorption performance
266
at different operating concentrations. The tphp removal efficiency on the
267
Urea-Fe3O4@LDH rapidly increased with the increase of the tphp concentration. The
268
index in the Freundlich model was between 1.0 and 1.2, indicating that the adsorption
269
isotherm was closed to nonlinear (Table S4). The Langmuir model assumed
270
monolayer coverage on the surface of the adsorbents, there might be electrostatic
271
attraction in the adsorption process. Simultaneously, π-π interaction occurred between
272
aromatic pollutants and benzene on the surface of the material (Wang et al., 2018a).
273
The nonlinearity was due to the roles of heterogeneous adsorption sites and different
274
adsorption interactions like π-π interaction. In addition, the tphp adsorption capacity
275
of the Urea-Fe3O4@LDH increased with the increase of the reaction temperature,
276
confirming that the reaction was endothermic. The tphp adsorption capacities of the
277
Urea-Fe3O4@LDH were 501.18, 521.34 and 588.67 mg g-1 at 298-328 K. The
278
thermodynamic parameters were related to the adsorption isotherms at different 13
279
temperatures. 𝐾𝑑 = 𝑙𝑛𝐾𝑑 =
𝑞𝑒 𝑐𝑒
∆𝑆 ∆𝐻 − 𝑅 𝑅𝑇
∆𝐺 = −𝑅𝑇𝑙𝑛𝐾𝑑 280
Where ΔH (kJ mol-1), ΔS (J mol-1 K-1) and ΔG (kJ mol-1) were standard enthalpy,
281
entropy, and Gibbs free energy, respectively. T (K) was the temperature in Kelvin, R
282
was the gas constant and Kd was a distribution coefficient.
283
The thermodynamic parameters were listed in Table S5. The positive values of
284
ΔH indicated that the tphp adsorption process on the Urea-Fe3O4@LDH was
285
endothermic. ΔS (66.54 J mol−1K−1) reflected the increasing randomness at the solid
286
solution interface. The negative values of ΔG stated that the adsorption process was
287
spontaneous at experimental temperatures.
288
3.3. Effect of solution pH, ionic strength and organic matter
289
Effect of solution pH, ionic strength and organic matter on the tphp adsorption
290
were showed in Fig.5a and Fig.5b. The pH of the solution affects the surface charge of
291
the material and pollutant. The tphp adsorption capacity on the Urea-Fe3O4@LDH
292
was demonstrated under different pH conditions in Fig.5a. The tphp adsorption
293
removal gradually increased at pH 3.0-5.5 and then decreased with the pH increase to
294
8.0. As showed in Fig.5c, pH at zero charge point (pHZCP) of the Urea-Fe3O4@LDH
295
resulted to be equal to 10.5, the surface of the material was positively charged at pH
296
3-8, and the oxygen atom in phosphate had a high electron cloud density, thereby
297
resulting in the protonation of tphp (Vank et al., 2000). The protonation of the P=O 14
298
bond produced an electrostatic repulsion to the material surface with the positive
299
charge. With the pH increase of the solution, the protonation degree of tphp gradually
300
decreased, and the electrostatic attraction between O with a negative charge and the
301
material surface with a positive charge increased. Therefore, the tphp adsorption
302
removal increased rapidly as the pH increase. After the addition of the adsorbents, the
303
zeta potential change of the solution was shown in Fig.5d. The value of zeta potential
304
changed from negative to positive, demonstrating the electrostatic interaction between
305
the adsorbents and tphp. When the dosage of adsorbent was 3 mg L-1, the potential in
306
the solution was close to zero and the tphp was prone to adsorption. At above pH 5.5,
307
the deprotonation of tphp increased, and the positive charge of P in O=P was greater
308
than the negative charge of O (Lazarević-Pašti et al., 2018). The electrostatic
309
repulsion was generated between O=P and urea. Thus, the tphp adsorption on the
310
Urea-Fe3O4@LDH reduced partly at a high pH solution. The interactions between
311
Urea-Fe3O4@LDH and tphp under electrostatic repulsion were attributed to: π-π
312
interaction between the benzene ring on the surface of the material and tphp.
313
The effect of ionic strength conditions on the tphp removal on the
314
Urea-Fe3O4@LDH was investigated (Fig.5a). The change of the tphp adsorption
315
removal demonstrated a slight impact when the NaNO3 concentration increased from
316
0.001 to 0.1 M. For example, the tphp adsorption amount was 550 mg g-1 without
317
adding NaNO3 at pH 5.5. When the NaNO3 concentration increased to 0.1 M, the
318
adsorption capability was 464 mg L-1 at the same pH condition, and the adsorption
319
removal only reduced by ca. 12%. The NO3− could accumulate on the surface of the 15
320
Urea-Fe3O4@LDH with the positive charge, thereby resulting in the formation of the
321
charge screening effect by neutralizing electricity (Litke et al., 2019). Therefore, the
322
tphp adsorption removal reduced. The CO32− in the MgAl-LDH layers can be
323
converted by the ion exchange at acidic conditions to NO3−-LDH; this process was
324
called the “acid-salt method” (Iyi et al., 2004). The process reduced charge density on
325
the surface and weakened charge screening. Therefore, the increase of the ionic
326
strength could not significantly affect the tphp adsorption removal. For example, the
327
tphp adsorption capacity on the material was slightly enhanced at low pH. In addition,
328
NaNO3 had a “salting out” effect on the adsorption of hydrophobic organic pollutants
329
(Kalra et al., 2001), especially for highly hydrophobic and low solubility tphp. The
330
additional ions were contributed to reducing the tphp solubility. Therefore, the
331
increase in ionic strength slightly affected the tphp adsorption.
332
Fig.5b displayed the effect of HA, which was selected as a model of natural
333
organic matter, on the tphp adsorption removal on the Urea-Fe3O4@LDH at pH 7.5.
334
The tphp removal kept in a certain range on the Urea-Fe3O4@LDH when the HA
335
concentration was around 0-40 mg L-1. Moreover, the tphp adsorption removal
336
increased when the HA concentration ranged from 40 to 80 mg L-1. The tphp
337
adsorption capacity on the Urea-Fe3O4@LDH remained unchanged at 440 mg g-1
338
when HA exceeded 80 mg L-1. This was due to the reason that HA can adsorb
339
organophosphorus compounds (in Fig.S6). The adsorption efficiency of HA and the
340
binding ability increased with the increase in the hydrophobicity of pollutants. The
341
tphp was adsorbed by HA via hydrophobic and π-π interactions with the increase of 16
342
the HA concentration (Wang et al., 2017); thus, the amount of the tphp adsorption on
343
the material increased.
344
3.4. Comparison of adsorption performance
345
Different
shell
materials
(Urea-Fe3O4@LDH,
Urea-Fe3O4@SiO2
and
346
Urea-Fe3O4@C) with urea functional groups were prepared to investigate the tphp
347
adsorption
348
Urea-Fe3O4@LDH than in Urea-Fe3O4@SiO2 and Urea-Fe3O4@C under different pH
349
solutions and ionic strength. For example, the adsorption capacities for the
350
Urea-Fe3O4@LDH, Urea-Fe3O4@SiO2 and Urea-Fe3O4@C were 455, 285 and 206
351
mg g-1 at pH 6 with 0.001 M NaNO3, respectively. Then, the adsorption capacities
352
decreased to 428, 330 and 310 mg g-1 when the NaNO3 concentration increased to 0.1
353
M. The results indicated that LDH was the most effective shell for grafting urea
354
groups and removing the tphp. And the adsorption removal was slightly affected by
355
ionic strength, thereby indicating that urea functional groups had favorable efficient
356
adsorption for the tphp removal. The different shell materials that pH at pHZCP were
357
equal to 10.5, 10.1 and 9.4, carried on different charges and resulted in different
358
adsorption removal. The effects of pH values and surface charges indicated that
359
electrostatic attraction was the main way for the tphp removal, in which phosphate of
360
the tphp was negatively charged and urea was positively charged in the
361
Urea-Fe3O4@LDH adsorption process.
in
Fig.6a.
The
tphp
adsorption
removal
was
better
in
the
362
After the tphp adsorption reaction on the Urea-Fe3O4@LDH, Urea-Fe3O4@SiO2
363
and Urea-Fe3O4@C, the FT-IR spectrum demonstrated (in Fig.6b) that (1) some peaks 17
364
of the tphp appeared non-overlapping positions of the used adsorbents slightly; (2) the
365
intensity of the CO32− vibration of the Urea-Fe3O4@LDH remained unchanged, thus
366
representing that ion exchange reaction did not happen in LDH. The adsorption
367
mechanism was presented in Fig.7. In addition, the difference of removal efficiency
368
between the aryl-OPFR (tphp) and the alkyl-OPFR (triethyl phosphate) was about 30%
369
(in Fig.8a). The tphp had stronger hydrophobicity and aromaticity than triethyl
370
phosphate, and hydrophobic interaction was rare in adsorption process due to the
371
hydrophilicity of the adsorbents. Therefore, the benzene on the surface of the
372
adsorbents and the tphp exhibited π-π interaction.
373
In order to evaluate the Urea-Fe3O4@LDH adsorption performance, the
374
comparison of the adsorption capacities of different adsorbents was carried out (in
375
Table 1). It can be speculated that the Urea-Fe3O4@LDH was a better adsorbent
376
compared with some other materials.
377
3.5. Reusability and other organophosphorus pollutants adsorption
378
The Urea-Fe3O4@LDH was used to remove other organophosphorus pollutants,
379
and the experimental results were shown in Fig. 8a. Organophosphorus pesticides
380
such as malathion, dimethoate and dichlorvos, removal rates of 67.3%, 77.8% and
381
59.5% respectively. At the same time, the removal of triethyl phosphate, one of
382
OPFRs, reached 65.2%. The Urea-Fe3O4@LDH was a good adsorbent for removing
383
organophosphorus pollutants.
384
Fig.8b depicted the tphp adsorption removal of the reused Urea-Fe3O4@LDH
385
after regenerating ethanol. The Urea-Fe3O4@LDH exhibited favorable and stable tphp 18
386
removal in the consecutive five cycles. Moreover, in the sixth adsorption process, the
387
removal was slightly reduced (ca. 10%). In addition, the used material maintained
388
good magnetic separation from the solution (in Fig.S7). The results indicated that the
389
Urea-Fe3O4@LDH showed good reusability and magnetism for the tphp adsorption in
390
the regeneration cycle experiment.
391
The above results showed that the Urea-Fe3O4@LDH had a higher adsorption
392
performance and a better removal rate for the tphp, indicating that it can be a choice to
393
deal with the industrial wastewater with these contaminates, especially in some
394
emerging situations. Also, the optimization of the material will be investigated in the
395
future.
396
4. Conclusions
397
In summary, the core-shell Urea-Fe3O4@LDH was successfully synthesized and
398
urea groups were generated on the surface of the adsorbent. The tphp adsorption
399
capacity on the Urea-Fe3O4@LDH was up to 589 mg g-1. Moreover, ionic strength and
400
organic matter HA slightly affected the tphp adsorption on the adsorbent. The
401
adsorption process depended on the pH solution, indicating electrostatic interaction
402
was the main adsorption mechanism. Compared to the triethyl phosphate, the tphp
403
removal was higher due to the π-π interaction. The Urea-Fe3O4@LDH will have a
404
good prospect for removing different organophosphorus pollutants. In the future, we
405
will optimize the adsorbent and try to use it in the real wastewater.
406
Acknowledgments: The work was supported by National Major Science and
407
Technology Program for Water Pollution Control and Treatment (2017ZX07101-003) 19
408
and Fundamental Research Funds for Central Universities (2018ZD08, 2018MS033).
409
References
410
Abdolmohammad-Zadeh, H., Zamani-Kalajahi, M., 2019. A turn-on/off fluorescent
411
sensor based on nano-structured Mg-Al layered double hydroxide intercalated with
412
salicylic acid for monitoring of ferric ion in human serum samples. Anal. Chim. Acta
413
1061, 152-160.
414
Cao, Y., Fan, Y., Ma, Z., Cheng, Z., Xiang, Q., Duan, Z., Xu, J., 2018.
415
Urea-functionalized SBA-15 hybrids: Post-grafting synthesis, high-performance
416
organophosphorus sensing and their response mechanism. Sensor. Actuat. B-Chem.
417
273, 1162-1169.
418
Chen, Y., Zhang, Y., Luo, L., Shi, Y., Wang, S., Li, L., Long, Y., Jiang, F., 2018. A
419
novel templated synthesis of C/N-doped β-Bi2O3 nanosheets for synergistic rapid
420
removal of 17α-ethynylestradiol by adsorption and photocatalytic degradation. Ceram.
421
Int. 44, 2178-2185.
422
Dehghani, M.H., Niasar, Z.S., Mehrnia, M.R., Shayeghi, M., Al-Ghouti, M.A.,
423
Heibati, B., McKay, G., Yetilmezsoy, K., 2017. Optimizing the removal of
424
organophosphorus pesticide malathion from water using multi-walled carbon
425
nanotubes. Chem. Eng. J. 310, 22-32.
426
Du, W., Zheng, L., Li, X., Fu, J., Lu, X., Hou, Z., 2016. Plate-like Ni-Mg-Al layered
427
double hydroxide synthesized via a solvent-free approach and its application in
428
hydrogenolysis of D-sorbitol. Appl. Clay Sci. 123, 166-172.
429
Du, Z., Wang, G., Gao, S., Wang, Z., 2015. Aryl organophosphate flame retardants
430
induced cardiotoxicity during zebrafish embryogenesis: By disturbing expression of
431
the transcriptional regulators. Aquat. Toxicol. 161, 25-32.
432
Ge, J., Hu, Y., Biasini, M., Beyermann, W.P., Yin, Y., 2007. Superparamagnetic
433
magnetite colloidal nanocrystal clusters. Angew. Chem. Int. Edit. 46, 4342-4345. 20
434
Grieco, S.A., Ramarao, B.V., 2013. Removal of TCEP from aqueous solutions by
435
adsorption with zeolites. Colloid. Surface. A. 434, 329-338.
436
Gu, J., Su, F., Hong, P., Zhang, Q., Zhao, M., 2019. 1H NMR-based metabolomic
437
analysis of nine organophosphate flame retardants metabolic disturbance in Hep G2
438
cell line. Sci. Total Environ. 665, 162-170.
439
Iyi, N., Matsumoto, T., Kaneko, Y., Kitamura, K., 2004. Deintercalation of carbonate
440
ions from a hydrotalcite-like compound: Enhanced decarbonation using acid-salt
441
mixed solution. Chem. Mater. 16, 2926-2932.
442
Ji, Y., Wang, Y., Yao, Y., Ren, C., Lan, Z., Fang, X., Zhang, K., Sun, W., Alder, A.C.,
443
Sun, H., 2019. Occurrence of organophosphate flame retardants in farmland soils
444
from Northern China: Primary source analysis and risk assessment. Environ. Pollut.
445
247, 832-838.
446
Kalra, A., Tugcu, N., Cramer, S.M., Garde, S., 2001. Salting-in and salting-out of
447
hydrophobic solutes in aqueous salt solutions. J. Phys. Chem. B. 105, 6380-6386.
448
Lazarević-Pašti, T., Anicijevic, V., Baljozovic, M., Anicijevic, D.V., Gutic, S., Vasic,
449
V., Skorodumova, N.V., Pasti, I.A., 2018. The impact of the structure of
450
graphene-based materials on the removal of organophosphorus pesticides from water.
451
Environ. Sci-Nano 5, 1482-1494.
452
Litke, A., Frei, H., Hensen, E.J.M., Hofmann, J.P., 2019. Interfacial charge transfer in
453
Pt-loaded TiO2 P25 photocatalysts studied by in-situ diffuse reflectance FTIR
454
spectroscopy of adsorbed CO. J. Photoch. Photobio. A. 370, 84-88.
455
Nodeh, H.R., Ibrahim, W.A.W., Kamboh, M.A., Sanagi, M.M., 2017. New magnetic
456
graphene-based inorganic-organic sol-gel hybrid nanocomposite for simultaneous
457
analysis of polar and non-polar organophosphorus pesticides from water samples
458
using solid-phase extraction. Chemosphere 166, 21-30.
459
Nouralishahi, A., Mortazavi, Y., Khodadadi, A.A., Choolaei, M., Thompson, L.T., 21
460
Horri, B.A., 2019. Characteristics and performance of urea modified Pt-MWCNTs for
461
electro-oxidation of methanol. Appl. Surf. Sci. 467-468, 335-344.
462
Pang, L., Yuan, Y., He, H., Liang, K., Zhang, H., Zhao, J., 2016. Occurrence,
463
distribution, and potential affecting factors of organophosphate flame retardants in
464
sewage sludge of wastewater treatment plants in Henan Province, Central China.
465
Chemosphere 152, 245-251.
466
Seo, P.W., Khan, N.A., Jhung, S.H., 2017. Removal of nitroimidazole antibiotics from
467
water by adsorption over metal-organic frameworks modified with urea or melamine.
468
Chem. Eng. J. 315, 92-100.
469
Shan, R.R., Yan, L.G., Yang, K., Yu, S.J., Hao, Y.F., Yu, H.Q., Du, B., 2014. Magnetic
470
Fe3O4/MgAl-LDH composite for effective removal of three red dyes from aqueous
471
solution. Chem. Eng. J. 252, 38-46.
472
Sharma, J., Mahima, S., Kakade, B.A., Pasricha, R., Mandale, A.B., Vijayamohanan,
473
K.,
474
4-aminothiophenol-capped gold nanoparticles. J. Phys. Chem. B. 108, 13280-13286.
475
Teo, T.L.L., McDonald, J.A., Coleman, H.M., Khan, S.J., 2015. Analysis of
476
organophosphate flame retardants and plasticisers in water by isotope dilution gas
477
chromatography-electron ionisation tandem mass spectrometry. Talanta 143, 114-120.
478
Vank, J.C., Henry-Riyad, H., Csizmadia, I.G., 2000. Successive protonation of
479
phosphate (PO4/3-), thiophosphate (PSO3/3-), and selenophosphate (PSeO3/3-). J. Mol.
480
Struct. 504, 267-286.
481
Wang, F., Sun, H., Ren, X., Liu, Y., Zhu, H., Zhang, P., Ren, C., 2017. Effects of
482
humic acid and heavy metals on the sorption of polar and apolar organic pollutants
483
onto biochars. Environ. Pollut. 231, 229-236.
484
Wang, G., Du, Z., Chen, H., Su, Y., Gao, S., Mao, L., 2016. Tissue-specific
485
accumulation, depuration, and transformation of triphenyl phosphate (TPHP) in adult
2004.
Solvent-assisted
one-pot
22
synthesis
and
self-assembly
of
486
zebrafish (danio rerio). Environ. Sci. Technol. 50, 13555-13564.
487
Wang, W., Deng, S., Li, D., Ren, L., Shan, D., Wang, B., Huang, J., Wang, Y., Yu, G.,
488
2018a. Sorption behavior and mechanism of organophosphate flame retardants on
489
activated carbons. Chem. Eng. J. 332, 286-292.
490
Wang, W., Deng, S., Li, D., Ren, L., Wang, B., Huang, J., Wang, Y., Yu, G., 2018b.
491
Adsorptive removal of organophosphate flame retardants from water by non-ionic
492
resins. Chem. Eng. J. 354, 105-112.
493
Wang, W., Deng, S., Ren, L., Li, D., Wang, W., Vakili, M., Wang, B., Huang, J., Wang,
494
Y., Yu, G., 2018c. Stable covalent organic frameworks as efficient adsorbents for high
495
and selective removal of an aryl-organophosphorus flame retardant from water. ACS
496
Appl. Mater. Inter. 10, 30265-30272.
497
Wang, X.W., Liu, J.F., Yin, Y.G., 2011. Development of an ultra-high-performance
498
liquid chromatography-tandem mass spectrometry method for high throughput
499
determination of organophosphorus flame retardants in environmental water. J.
500
Chromatogr. A. 1218, 6705-6711.
501
Wei, K., Yin, H., Peng, H., Lu, G., Dang, Z., 2018. Bioremediation of triphenyl
502
phosphate by Brevibacillus brevis: Degradation characteristics and role of cytochrome
503
P450 monooxygenase. Sci. Total Environ. 627, 1389-1395.
504
Wu, X.G., Li, B., Wen, X.G., 2017. Synthesis and adsorption properties of
505
hierarchical Fe3O4@MgAl-LDH magnetic microspheres. J. Nanopart. Res. 19, 131.
506
Yan, W., Yan, L., Duan, J., Jing, C., 2014. Sorption of organophosphate esters by
507
carbon nanotubes. J. Hazard. Mater. 273, 53-60.
508
Yang, D., Wang, X., Wang, N., Zhao, G., Song, G., Chen, D., Liang, Y., Wen, T.,
509
Wang, H., Hayat, T., Alsaedi, A., Wang, X., Wang S., 2018. In-situ growth of
510
hierarchical layered double hydroxide on polydopamine-encapsulated hollow Fe3O4
511
microspheres for efficient removal and recovery of U(VI). J. Clean. Prod. 172, 23
512
2033-2044.
513
Zhang, W., Liang, Y., Wang, J., Zhang, Y., Gao, Z., Yang, Y., Yang, K., 2019.
514
Ultrasound-assisted adsorption of Congo red from aqueous solution using MgAlCO3
515
layered double hydroxide. Appl. Clay Sci. 174, 100-109.
24
Highlights
At the first time, the Urea functionalized Fe3O4@LDH was used for the tphp removal.
The Urea-Fe3O4@LDH exhibited the high TPhP adsorption removal and good cycles.
The tphp adsorption capacity of the Urea-Fe3O4@LDH was up to 589 mg g-1.
Ionic strength had little effect on the TPhP adsorption on the Urea-Fe3O4@LDH.