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Highly efficient adsorption of Cr(VI) from aqueous solutions by amino-functionalized titanate nanotubes
Accepted Manuscript Highly efficient adsorption of Cr(VI) from aqueous solutions by amino-func‐ tionalized titanate nanotubes Lei Wang, Wen Liu, Ting Wang, Jinren Ni PII: DOI: Reference:
S1385-8947(13)00406-3 http://dx.doi.org/10.1016/j.cej.2013.03.081 CEJ 10563
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
6 January 2013 16 March 2013 19 March 2013
Please cite this article as: L. Wang, W. Liu, T. Wang, J. Ni, Highly efficient adsorption of Cr(VI) from aqueous solutions by amino-functionalized titanate nanotubes, Chemical Engineering Journal (2013), doi: http://dx.doi.org/ 10.1016/j.cej.2013.03.081
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1
Highly efficient adsorption of Cr(VI) from aqueous
2
solutions by amino-functionalized titanate nanotubes
3
Lei Wanga, Wen Liua, Ting Wanga, Jinren Nib,*
4
a
5
Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China;
6
b
7
Sciences, Ministry of Education, Beijing 100871, China
Shenzhen Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and
Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment
*
Corresponding author. Tel.: +86-10-62751185; fax: +86-10-62756526. E-mail address: [email protected] (J.R. Ni). Page 1 of 33
8
Abstract
9
For highly efficient removal of Cr(VI) from aqueous solutions, amino-functionalized titanate
10
nanotubes (NH2-TNTs) with excellent adsorption performance have been synthesized by
11
covalently grafting [1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (AAPTS) onto protonated
12
titanate nanotubes (HTNTs) with great amounts of surface hydroxyl groups. TEM and XRD
13
results confirmed that the nanotubular morphology and crystal structure of HTNTs and NH2-TNTs
14
were preserved. FTIR spectra demonstrated that AAPTS was covalently bonded on the surface of
15
HTNTs. Batch adsorption experiments showed that pseudo-second-order kinetics model and
16
Langmuir isotherm model fitted the adsorption data very well for both materials, and the Cr(VI)
17
adsorption capacity on NH2-TNTs calculated by Langmuir model was up to 153.85 mg g-1 at
18
initial pH 5.4 and 30 oC, much larger than that on HTNTs (26.60 mg g-1). Moreover, uptake of
19
Cr(VI) ions onto NH2-TNTs could be completed within only 5 min for 95% adsorption of the
20
maximum. Influence of different species of Cr(VI) under varying pH was also considered. FTIR
21
and XPS analysis indicated that Cr(VI) ions were first exchanged with NO3- linked on the
22
positively charged amino groups and then partially reduced to Cr(III). Afterwards, Cr(III) were
23
then totally chelated with amino groups and no Cr(III) was detected in the solution after Cr(VI)
24
adsorption at pH range of 1-12.
25 26
Key words: Amino-functionalization; Titanate nanotubes; Hexavalent chromium; Adsorption;
27
Mechanism
28 29
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1. Introduction
31
Hexavalent chromium, Cr(VI), is highly toxic and carcinogenic. In this paper,
32
Cr(VI) is a general expression for H2CrO4(aq) and series of anion ions containing
33
Cr2O72-, HCrO4- and CrO42-, since it is difficult to give specific expression for
34
coexisted anion ions even under the same pH [1]. Most of the Cr(VI) pollutions are
35
from wastewater and waste materials of electroplating, tanning, metallurgic, chemical
36
and mining industries. Various methods have been used for Cr(VI) removal, such as
37
reduction, ion exchange, reverse osmosis, membrane separation, electrocoagulation
38
and adsorption. Among them, adsorption method is a particularly attractive option due
39
to its outstanding simplicity, low investment, high efficiency and potential recovery
40
[2,3]. Various kinds of materials have been employed for Cr(VI) adsorption, including
41
activated carbon [4], biomass materials [5], zeolite [6], chitosan [7] and ferriferrous
42
oxide [8]. However, the potential shortcomings of some absorbents prevented their
43
wide application, like impurities in the adsorbents, low adsorption capacity and slow
44
adsorption kinetics. Thus, searching for alternative materials with improved
45
adsorption capacity and high adsorption rate is urgently desired.
46
Recently, a lot of papers paid attention to using amino groups to modify various
47
categories of materials due to their special properties in improving Cr(VI) adsorption.
48
The amino groups can be easily protonated in acid conditions, forming a kind of
49
positively charged groups which is beneficial to the increase of the point of zero
50
charge (PZC) of modified materials [9]. By the effect of electrostatic attraction and
51
hydrogen bond between positively charged amino groups and negatively charged Page 3 of 33
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Cr(VI) ions, the adsorption capacity of Cr(VI) can be greatly enhanced [10]. The
53
materials used for modification should possess properties that are beneficial for
54
introducing functional groups, such as large surface area and pore volumes, high
55
density of surface available groups. These materials include activated carbon [11],
56
biomass materials [12], halloysite nanotubes [9] and mesoporous silica [13] etc.
57
Titanate nanotubes (TNTs) fabricated via hydrothermal method were first reported
58
by Kasuga et al. [14]. The lower fabrication cost, large specific surface area and pore
59
volumes, and nanotubular structure make titanate nanotubes a candidate as an
60
excellent adsorbent for organics and heavy metal removal in aqueous solutions [15].
61
Besides, the abundant surface hydroxyl groups with a density of 5.8 -OH/nm2 in
62
average [16] make it possible to graft amino groups on TNTs surface. The
63
amino-functionalized titanate nanotubes may be a good adsorbent for Cr(VI) removal
64
due to the combined action of electrostatic attraction and hydrogen bonding effect.
65
Moreover, the large specific surface area and pore volumes from TNTs provided
66
adsorption sites and diffusion channels for metals. Therefore, we herein synthesized
67
amino-functionalized titanate nanotubes to investigate the Cr(VI) adsorption capacity
68
in aqueous solutions.
69
[1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (AAPTS) is a kind of silane
70
coupling agent bearing two amino groups in one molecule. This silane coupling agent
71
is easily hydrolyzed to silanol group, and can be further dehydrated with surface
72
hydroxyl groups of TNTs [17,18]. Moreover, both the primary and secondary amino
73
groups of AAPTS could bind heavy metal anions and thus improve adsorption
Page 4 of 33
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capacity. In this paper, amino-functionalized TNTs is obtained using AAPTS as the
75
modification agent on protonated TNTs. The amino-functionalized TNTs and
76
protonated TNTs were characterized and further used for Cr(VI) adsorption. Important
77
factors such as pH, contact time, reaction temperature, initial Cr(VI) concentration,
78
ionic strength and coexisting anions were studied in detail. Furthermore, Cr(VI)
79
adsorption mechanism on AAPTS modified TNTs was discussed.
80
2. Materials and methods
81
2.1. Reagents
82
[1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (95%) was purchased from
83
Aladdin Chemistry Co. Ltd (Shanghai, China) and used without further purification.
84
Other reagents used in the experiments were of analytical grade, and purchased from
85
Tianjin Guang Fu Technology Development Co. Ltd (Tianjin, China).
86
2.2. Preparation of protonated titanate nanotubes
87
The protonated titanate nanotubes were synthesized by hydrothermal treatment of
88
TiO2 powder in concentrated NaOH solution as described by Chen et al. [19], and
89
then soaked with nitric acid to increase the amount of surface hydroxyl groups.
90
Typically, 1.2 g TiO2 nanoparticle powder (P25, Degussa, Germany) was added to 66
91
mL of 10 M NaOH solution. After vigorous stirring for 12 h, the mixture was
92
transferred into a 100 mL sealed teflon container and statically heated at 130 oC for 72
93
h. The precipitate was washed with 0.1 M HNO3 until the pH value of the rinsing Page 5 of 33
94
solution reached 7.0, then the products were soaked with 0.1 M HNO3 for 5 h. After
95
that, the products were centrifuged and washed with deionized water to neutral to
96
wipe out excessive nitric acid, and then dried in an oven at 105 oC for 12 h. The
97
protonated titanate nanotubes were labeled as HTNTs.
98
2.3. Amino-functionalization of titanate nanotubes
99
The amino-functionalization was performed as follows: 0.5 g HTNTs were
100
dispersed in 50 mL toluene solution, and then AAPTS with appropriate volume was
101
added. To detect the effect of AAPTS volume, 0.5 mL, 0.75 mL, 1 mL and 1.25 mL
102
AAPTS were separately added. The mixture was then kept under refluxing conditions
103
at 100 oC for 24 h. After separated by centrifugation at 5000 rpm for 3 min, the
104
as-synthesized powder was washed with toluene and ethanol to remove the excessive
105
AAPTS and hydrolyzed products. Finally, the products were washed twice with 0.1 M
106
nitric acid and dried at 60 oC. The amino-functionalized titanate nanotubes were
107
marked as NH2-TNTs.
108
2.4. Characterization and analyses
109
Transmission electron microscopy (TEM) analysis was conducted on a FEI Tecnai
110
F30 microscope equipped with energy dispersive X-ray spectroscopy (EDX)
111
spectrometer operating at 300 kV. X-ray diffraction (XRD) patterns were obtained on
112
a Rigaku Dmax/2400 X-ray diffractometer using Cu Kα radiation, with a scan rate (2θ)
113
of 4o/min (λ = 1.5418Å). Nitrogen adsorption-desorption isotherms were determined
Page 6 of 33
114
at -196 oC on an ASAP 2010 adsorption apparatus (Micromeritics, USA) to obtain
115
Brunauer-Emmett-Teller (BET) surface area and Barret-Joyner-Halender (BJH) pore
116
size distributions. The samples were degassed at 100 oC before adsorption. The
117
Fourier transform infrared spectroscopy (FTIR) of the samples were obtained by a
118
Tensor 27 FT-IR spectrometer (Bruker, Germany) at room temperature using the KBr
119
pellet method. For zeta potential measurements, 0.04 g samples were dispersed in 40
120
mL deionized water with ultrasonic radiation for 15 min before adjusting the pH
121
values, then the suspensions were shaken at 200 rpm for 24 h. The final pH was
122
measured before the zeta potential measurements using a Nano-ZS90 Zetasizer
123
(Malvern Instruments, UK). The surface elements and oxidation state were
124
investigated using X-ray photoelectron spectroscopy (XPS, AXIS-Ultra, Kratos
125
Analytical, Japan) with monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). All
126
binding energies were calibrated by C 1s hydrocarbon peaked at 284.80 eV for
127
compensation of surface charge effects. The calibrated high-resolution spectra were
128
analyzed by CasaXPS software. The surface elemental stoichiometries were
129
determined from the ratios of peak-area corrected by sensitivity factor. Distribution of
130
Cr(VI) species as a function of pH was calculated using Visual MINTEQ version 2.3.
131
2.5. Batch adsorption studies
132
A stock solution (1000 mg L-1) of Cr(VI) was prepared by dissolving a known
133
quantity of potassium dichromate (K2Cr2O7) in deionized water. All the adsorption
134
experiments were carried out on a rotary shaker at 200 rpm. The adsorption kinetics
Page 7 of 33
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experiments were first conducted to determine the equilibrium adsorption time.
136
Generally, 1 g L-1 NH2-TNTs or HTNTs were separately added to 200 mL of 50 mg
137
L-1, 100 mg L-1 and 150 mg L-1 Cr(VI) solutions with fixed pH, then aliquots of Cr(VI)
138
solution were withdrawn at different contact time intervals and filtered with 0.45 µm
139
hydrophilic membrane. The filtrate was diluted and analyzed to obtain total Cr and
140
Cr(VI) concentration. The concentration of total Cr was determined by inductively
141
coupled plasma-atomic emission spectroscopy (ICP-AES, Prodigy, Leeman, USA),
142
while Cr(VI) was determined with a diphenylcarbohydrazide spectrophotometric
143
method at 540 nm using UV-vis spectrophotometer (UV752, Shanghai Youke
144
Instrument Co. Ltd, China).
145
To investigate the effect of pH, 40 mL of 100 mg L-1 Cr(VI) solutions with various
146
initial pH at 1-12 were prepared. After adding 40 mg NH2-TNTs or HTNTs to the
147
solutions, the solutions were shaken at 30 oC for 1 h (determined by the adsorption
148
kinetics). To explore the adsorption isotherm at different temperatures, the initial
149
Cr(VI) concentration was varied from 25 to 400 mg L-1 at 30 oC, 40 oC and 50 oC,
150
respectively. Finally, different amounts of sodium chloride (NaCl), sodium nitrate
151
(NaNO3),
152
(Na3PO4·12H2O) were added to 100 mg L-1 Cr(VI) solutions to investigate the effect
153
of coexisting anions and ionic strength.
154
sodium sulfate
(Na2SO4)
and
sodium
phosphate
dodecahydrate
The adsorption capacity qe (mg g-1) was calculated by Eq. (1): (C i -C e ) V (1) W
155
qe =
156
where Ci and Ce (mg L-1) are the initial and equilibrium Cr(VI) concentration,
Page 8 of 33
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respectively. V (mL) is the volume of Cr(VI) solution, W (mg) is the mass of
158
adsorbent.
159
3. Results and discussion
160
3.1. Characterization of materials
161
3.1.1
Effect of AAPTS dosage
162
The elemental analysis of N, C and H for modified materials with different dosage
163
of AAPTS in the functionalization process is shown in Table 1. Samples with dosage
164
of 0.5 mL, 0.75 mL, 1 mL and 1.25 mL AAPTS were denoted as S0.5, S0.75, S1 and
165
S1.25 respectively. According to Table 1, the content of N, C and H enhanced with
166
the increase of AAPTS volume, followed by a decrease for S1.25. This can be
167
attributed to pH variation of nanotubes suspension [20]. When AAPTS volume was
168
less than 1 mL, Si-OH from AAPTS was limited and totally reacted with sufficient
169
Ti-OH; when AAPTS volume was above 1 mL, hydrolysis of amino groups from
170
AAPTS would increase nanotubes suspension pH, resulting in the decrease of Ti-OH
171
reactivity and thus N content of NH2-TNTs. Sample S1 shows the highest amounts of
172
N, C and H, i.e. 7.26 mmol N g-1, 9.63 mmol C g-1 and 37.2 mmol H g-1. The molar
173
ratio of N/C for S1 was 0.75, slightly lower than the theoretical value of 0.8, which
174
was calculated based on complete protonation of primary and secondary amino
175
groups. The results indicated that not all of the amino groups were protonated. As
176
sample S1 had the highest amount of N, that is, the most abundant amino groups on
177
NH2-TNTs, it was used in the following characterization and Cr(VI) adsorption Page 9 of 33
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experiments.
179
[Table 1]
180
3.1.2
TEM analysis
181
TEM image of HTNTs (Fig. 1(a)) shows cylindrical-shaped tubes with multilayer
182
walls and open-ended lumens along the nanotubes. The samples (Fig. 1(b)) contained
183
agglomerates of randomly tangled nanotubes with some irregularity in diameter
184
(typically 7.4 nm), wall thickness, and morphology. After modification (Fig. 1(c)), the
185
nanotubes still exhibited complete tubular structure, but the void due to the
186
aggregation of nanotubes significantly decreased. We can see from Fig. 1(d) that a
187
nanotube with a diameter of ca. 7.2 nm is mostly coated by organic-like materials
188
[21,22] with uneven thickness and less density than nanotubes along the external
189
surface, forming an uneven surface longitudinally and making the layered structure
190
invisible. The EDX spectra of nanotubes after modification (Fig. 1(c) inset) appeared
191
additional peaks corresponding to Si and N as compared to that of HTNTs (Fig. 1(a)
192
inset), indicating that the organic-like materials were silane, that is, the external
193
surface area of HTNTs was mostly coated with AAPTS.
194
[Fig. 1]
195
3.1.3 XRD patterns
196
XRD patterns of HTNTs and NH2-TNTs are shown in Fig. 2. Titanate nanotubes
197
were preferentially assigned for NaxH2-xTi3O7 [23], where x≈0 after protonation
Page 10 of 33
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process. The (200) reflection at around 2θ = 10o corresponded to interlayer spacing of
199
the samples. For HTNTs, the (200) reflection was not so obvious, which should be
200
attributed to some crystal defects caused by partial transformation of the titanate
201
structure to anatase TiO2 at pH 1 [24]. For NH2-TNTs, the position of (200) reflection
202
little shifted, indicating the interlayer distance of nanotubes didn’t change [25-27].
203
Therefore it was proved that amino-functionalization process didn’t occur in the
204
interlayer space of nanotubes. The (110) plane at ca. 24o and (211) plane at ca. 28o
205
could be assigned to hydrogen titanate nanotubes, and (020) plane at ca. 48o
206
corresponded to sodium titanate compounds [25,26]. The peak at ca. 28o was much
207
weaker than reported literatures [25-28], which was ascribed to the replacement of
208
Na+ with H+ during acid soaking [24]. The similar diffraction peaks of NH2-TNTs
209
with HTNTs proved that amino-functionalization process had little effect on HTNTs
210
crystal structure. Combined with TEM results, it can be concluded that the crystal
211
structure maintained after acid treatment and amino-functionalization.
212
[Fig. 2]
213
3.1.4 N2 adsorption-desorption studies
214
Fig. 3 shows the nitrogen adsorption-desorption isotherms and pore size
215
distributions of HTNTs and NH2-TNTs. Both samples exhibit type IV isotherms with
216
H3 type hysteresis loop according to IUPAC classification, indicating the presence of
217
mesopores (2-50 nm) [29]. The pore size of HTNTs (the inset) exhibited bimodal
218
distributions with peaks at 3-4 nm and 8-20 nm. The peak at 3-4 nm corresponded to
Page 11 of 33
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pores inside the nanotubes while the pores at 8-20 nm were ascribed to the voids in
220
the aggregation of nanotubes [28,30]. The silanization process resulted in a large
221
decrease of the volume of the smaller pores, while the larger pores were less affected,
222
indicating that the majority of nanotubes in the sample NH2-TNTs had closed ends,
223
presumably because of the blocking of nanotubes with polymerized silane during the
224
reaction of HTNTs with AAPTS [18]. This result was in consistent with the
225
observation of TEM images (Fig. 1). The surface properties of HTNTs and NH2-TNTs
226
were shown in Table 2. The BET surface area (243.3 m2 g-1) and pore volume (0.989
227
cm3 g-1 ) of NH2-TNTs were both smaller than those of HTNTs (343.5 m2 g-1 for BET
228
surface area and 1.015 cm3 g-1 for pore volume), proving that silane was coated on the
229
surface of nanotubes, thus reduced the surface area and pore volume.
230
[Fig. 3]
231
[Table 2]
232
3.1.5 FTIR studies
233
The FTIR spectra of HTNTs and NH2-TNTs are shown in Fig. 4. HTNTs exhibit
234
adsorption bands at 3300-3600, 1631, 1384, 913, 678 and 476 cm-1. The wide and
235
strong absorption bands in the region of 3300-3600 cm-1 was attributed to O-H
236
stretching vibration, suggesting the presence of huge amounts of surface hydroxyl
237
groups [31]. The band at 1631 cm-1 was ascribed to O-H stretching of water
238
molecules. The bands at 913, 678 and 476 cm-1 were all from titanate nanotubes,
239
corresponding to the stretching and bending vibration of Ti-O [26]. The band at 1384
Page 12 of 33
240
cm-1 corresponded to stretching vibration of nitrate ions. For NH2-TNTs, new
241
adsorption bands at 3376, 2973, 1211, 1133, 1039 and 826 cm-1 appeared as compared
242
to HTNTs. The relatively stronger adsorption band at 3376 cm-1 corresponded to N-H
243
stretching vibration, demonstrating the presence of amino groups. The existence of
244
aminosilane was further proved by C-H stretching at 2973 cm-1, C-N stretching at
245
1211 cm-1 and 1133 cm-1 and Si-C stretching at 826 cm-1 [32]. The band of Si-O-Si at
246
1039 cm-1 and Si-O-Ti at 945 cm-1 (shoulder) [18] was of great significance, as it
247
confirmed that AAPTS was covalently bonded with the surface of HTNTs, and
248
formed a polymerized network over the surface of nanotubes. The reaction of AAPTS
249
with surface OH groups of HTNTs can be assumed to conduct through the mechanism
250
similar to that found for the reaction of AAPTS with other oxide-based materials,
251
including (i) the hydrolysis of methoxy groups of AAPTS catalyzed by H2O/H3O+ and
252
the formation of silanol, i.e. (HO)3Si-C3H6NHC2H4NH2 and (ii) the condensation
253
between silanol OH groups and surface Ti-OH groups, with consequent formation of
254
Si-O-Ti bonds [33-35]. The greatly enhanced intensity at 1384 cm-1 indicated that
255
nitrate salt was formed.
256
[Fig. 4]
257
3.2 Adsorption behaviors of Cr(VI) on NH2-TNTs and HTNTs
258
3.2.1 Adsorption kinetics
259
The adsorption kinetics of NH2-TNTs and HTNTs with different initial Cr(VI)
260
concentrations are shown in Fig. 5. The kinetics displayed a very rapid initial uptake Page 13 of 33
261
and a subsequent stable stage. For NH2-TNTs, the equilibrium times for the three
262
concentrations were all less than 15 min. To be more precise, the adsorption capacity
263
reached up to 95% within only 5 min. The fast sorption ability was attributed to the
264
high porosity and surface area that facilitated the diffusion of Cr(VI) ions, combined
265
with the abundantly loaded amino groups, which can adsorb chromium with high
266
adsorption kinetics via electrostatic attraction. For HTNTs, the equilibrium times for
267
the three concentrations are all around 40 min, longer than those of NH2-TNTs. This
268
may be ascribed to the difference of adsorbing groups.
269
In order to make a better understanding of adsorption process, pseudo-first-order
270
kinetic and pseudo-second order kinetic models were used to describe the adsorption
271
kinetics. The pseudo-first-order model is expressed as Eq. (2) [36]:
272
ln (qe -qt )= ln qe -k1t (2)
273
where qe and qt (mg g-1) are the amounts of Cr(VI) ions adsorbed at equilibrium and
274
elapsed time t, respectively, t (min) is contact time, and k1 (min-1) is pseudo-first-order
275
rate constant. k1 and qe are calculated from the slope and intercept of the plot of log(qe
276
-qt) versus t.
277
The pseudo-second-order kinetic model is described by the Eq. (3) [37]:
278
t 1 1 = + t (3) 2 qt k2qe qe
279
where k2 (g mg-1 min-1) is pseudo-second-order rate constant. The values of k2 and qe
280
can be computed from the slope and intercept of the plot of t/qt versus t.
281
The results obtained from pseudo-first-order and pseudo-second-order models are
282
listed in Table 3. For NH2-TNTs, it can be seen that the correlation coefficients made Page 14 of 33
283
by pseudo-first-order model are smaller, and the calculated adsorption capacities
284
differ much from the measured results; while the correlation coefficients made by
285
pseudo-second-order model reach 0.99999, 0.99998 and 0.99991, and the calculated
286
adsorption capacities are almost identical to the measured adsorption capacities. This
287
suggested that the adsorption of Cr(VI) by NH2-TNTs followed pseudo-second-order
288
model very well, which corresponded to a chemisorption process. Moreover, the
289
pseudo-second-order rate constants at initial concentration of 99 and 157 mg L-1 are
290
smaller than that of 53 mg L-1, indicating that the uptake rate is faster at lower initial
291
concentration.
292
pseudo-second-order kinetic model at 98 mg L-1 is a little smaller than that of
293
pseudo-first-order kinetic model, the pseudo-second-order kinetic model is more
294
appropriate. This meant Cr(VI) adsorption with HTNTs tended to be a chemisorption
295
process.
296
[Fig. 5]
297
[Table 3]
298
3.2.2 Adsorption isotherm
HTNTs,
although
the
correlation
coefficient
of
Fig. 6 shows the adsorption isotherms by NH2-TNTs and HTNTs at 30, 40 and 50
299 300
For
o
C. The adsorption capacity of Cr(VI) enhanced gradually with the increase of the
301
equilibrium concentrations, and then kept stable both for NH2-TNTs and HTNTs.
302
Moreover, the adsorption capacity was higher at lower temperature for both materials.
303
To make a better understanding of the adsorption characteristics, Langmuir and
Page 15 of 33
304
Freundlich adsorption isotherm models were applied to fit the data. The Langmuir
305
model supposes that monolayer surface adsorption occurs on specific homogeneous
306
sites and there is no interaction between the adsorbed pollutants. The equation for this
307
model is expressed as Eq. (4) [38]:
308
Ce Ce 1 = + qe Q Qb
309
where Ce (mg L-1) is the equilibrium Cr(VI) concentration, qe (mg g -1) is the amount
310
of Cr(VI) ion adsorbed at equilibrium, Q (mg g-1) is the maximum adsorption capacity
311
of the adsorbent, and b (L mg-1) is the Langmuir constant corresponding to the free
312
energy of adsorption. By plotting Ce/qe versus Ce, the values of Q and b can be
313
computed from the slope and intercept of the linear plot.
314
(4)
The Freundlich isotherm is an empirical equation assuming heterogeneous surface
315
adsorption. The equation is usually expressed as the following linear form [39]:
316
ln qe = ln K F +
317
where KF (mg g-1) is the Freundlich constant corresponding to the adsorption capacity
318
of the adsorbent, and n is the heterogeneity factor related to the adsorption intensity of
319
the adsorbent. Both constants can be calculated from the slope and intercept of the
320
linear plot of log qe versus log Ce.
1 ln Ce (5) nF
321
The results calculated based on the above two models were listed in Table 4. For
322
NH2-TNTs, the correlation coefficients of Langmuir model were 0.99980, 0.99995,
323
and 0.99970 at 30 oC, 40 oC and 50 oC respectively, suggesting Langmuir model can
324
effectively describe the adsorption data, which confirmed the existence of
Page 16 of 33
325
homogeneous active sites within NH2-TNTs and monolayer adsorption of Cr(VI) ions
326
onto the adsorbent surface. For HTNTs, Langmuir model also fitted the adsorption
327
data better than Freundlich model. The maximum adsorption capacity by both
328
materials decreased with the increase of temperature, indicating the exothermic
329
characteristic of Cr(VI) sorption on both materials. The monolayer adsorption
330
capacity on NH2-TNTs computed by Langmuir model was 153.85 mg g-1 at 30 oC,
331
combined with the very fast uptake rate, making NH2-TNTs a promising Cr(VI)
332
adsorbent compared to other Cr(VI) adsorbents (Table 5). The monolayer adsorption
333
capacity on HTNTs computed by Langmuir model was only 26.60 mg g-1, much
334
smaller than that on NH2-TNTs, indicating that amino groups played a key role in the
335
Cr(VI) adsorption process on NH2-TNTs.
336
[Fig. 6]
337
[Table 4]
338
[Table 5]
339
3.2.3
Effect of pH
340
The effect of pH on Cr(VI) adsorption by NH2-TNTs was investigated with initial
341
pH ranged from pH 1 to 12 (Fig. 7). For comparison, the adsorption by HTNTs was
342
also conducted at the same pH range. The concentration of total Cr and Cr(VI) was
343
measured to be identical at all pH range, indicating that no Cr(VI) ion was reduced or
344
the reduced Cr(III) ions didn’t enter into the solution. Fig. 7(a) compares the
345
adsorption of Cr(VI) by HTNTs and NH2-TNTs at different equilibrium pH. It was
Page 17 of 33
346
revealed that HTNTs had only a little adsorption of Cr(VI) at equilibrium pH lower
347
than 4.28, while NH2-TNTs showed much larger adsorption capacity with equilibrium
348
pH ranged from pH 1 to 9.22. The adsorption capacity of Cr(VI) by NH2-TNTs was
349
largest at equilibrium pH 2.96 - 3.79 with a value of ca. 90 mg g-1 and a removal
350
efficiency of 90%, almost 6 times larger than that of HTNTs. For NH2-TNTs, when
351
the equilibrium pH was above pH 5, the adsorption capacity decreased gradually with
352
higher pH values.
353
The amino groups could adsorb protons easily during nitric acid washing of NH2+
/-NH3+. The
354
modification process, forming positively charged amino groups as
355
Cr(VI) ions mainly existed in the form of HCrO4- at pH 2-5 (Fig. 7(c)), which could
356
easily bond with positively charged amino groups via electrostatic attraction. At pH >
357
5, the concentration of divalent CrO42- increased gradually as the pH increased, which
358
consumed two positively charged amino group, making the adsorption capacity of
359
Cr(VI) decreased gradually. In addition, as the pH increased, more OH- ions
360
competed with Cr(VI) ions for positively charged amino groups. Similarly, at
361
equilibrium pH below 2.79, the adsorption capacity decreased with lower pH, due to
362
the increasing of H2CrO4 which had no attraction to positively charged amino groups.
363
For HTNTs, the adsorption capacity increased with the decease of pH. This should be
364
ascribed to that more -OH2+ groups which can attract negatively charged Cr(VI) ions
365
were formed at lower pH values.
366
Fig. 7(b) is the zeta potentials of HTNTs and NH2-TNTs at different solution pH.
367
The pHpzc of NH2-TNTs is 8.75, much larger than that of HTNTs at 4.00. When the
Page 18 of 33
368
solution pH is higher than the pHpzc, the material will be negatively charged,
369
otherwise it is positively charged. Fig. 7(a) showed that the adsorption capacities on
370
both materials are absolutely low when the solution pH was above their pHpzc. This
371
suggested that the primary driving force of both materials for adsorbing Cr(VI) ions
372
was electrostatic attraction. To be different, the active sites for NH2-TNTs were
373
NH2+
/-NH3+ while those for HTNTs were –OH2+ during Cr(VI) adsorption.
374
Fig. 7(d) is the change of pH during adsorption process. All the solution pH
375
decreased after Cr(VI) adsorption by the two materials. It was because the exchanged
376
protons in the interlayer space derived from acid soaking neutralized with OH- ions in
377
solutions. The variation of pH after adsorption by NH2-TNTs was larger than that by
378
HTNTs due to the bonding of OH- with positively charged amino groups.
379
[Fig. 7]
380
3.2.4 Effect of coexisting anions and ionic strength
381
According to the pH analysis, electrostatic attraction governs the adsorption
382
process, so the coexisting anions like nitrate, chloride, sulfate and phosphate in
383
wastewater samples would compete with Cr(VI) ions for active sites. Fig. 8(a) shows
384
the effect of coexisting anions on the Cr(VI) adsorption by NH2-TNTs and HTNTs.
385
The concentrations of coexisting anions were all 2.0 mM, which was comparable to
386
that of Cr(VI). For both materials, it can be seen that the adsorption capacity
387
decreased slightly after the addition of chloride, nitrate and phosphate, but greatly
388
decreased after sulfate addition. The different decreasing degrees caused by various
Page 19 of 33
389
anions might be related to the charge of the coexisting anions. The more charge of the
390
anions, the greater ability to compete with Cr(VI) ions [47]. At pH 5.4, the existing
391
form of phosphate mainly existed as monohydrogenphosphate (H2PO4-) while
392
phosphoric acid (H3PO4) is the main component at pH 1 (calculated by MINTEQ
393
version 2.3). Thus, the decreasing degree of adsorption capacity by SO42- was larger
394
than that by Cl-, NO3- and H2PO4-/H3PO4.
395
The effect of ionic strength in form of NaNO3 on the Cr(VI) adsorption capacity is
396
shown in Fig. 8(b). For both materials, the decreasing degree of Cr(VI) capacity
397
became larger as ionic strength increased, due to the enhanced competition between
398
NO3- and HCrO4- for positively charged amino groups or hydroxyl groups. It is
399
necessary to point out that when the concentration of NO3- was 0.01 M, which was
400
almost 5 times larger than that of Cr(VI), the adsorption capacity of Cr(VI) on
401
NH2-TNTs decreased only 12.5%, suggesting the good selectivity toward Cr(VI) by
402
NH2-TNTs.
403
[Fig. 8]
404
3.3 Adsorption mechanism
405
Fig. 9 shows the XPS spectra of N 1s before and after adsorption as well as Cr
406
2p3/2 after adsorption. Before Cr(VI) adsorption, N 1s spectra exhibited three peaks NH
/-NH2,
NH2+
/-NH3+ and
407
at 399.3, 401.5 and 406.7 eV, which were assigned to
408
NO3-, respectively [32,48]. After Cr(VI) adsorption at different initial pH, these three
409
peaks shifted to 399.85±0.05eV, 401.75±0.05eV and 407eV respectively. The content
Page 20 of 33
NH2+
NH
/-NH2 to
/-NH3+ became larger and increased from 3.42 to 21.32
410
ratio of
411
while the content of NO3- decreased when the pH increased from 2.04 to 10.40. It was
412
because that NO3- was adsorbed on the positively charged amino groups and formed a
413
salt-like material before adsorption. Once this material contacted with Cr(VI) solution,
414
NO3- would be exchanged by more attractive ions such as negatively charged Cr(VI)
415
ions and OH-, as depicted by Eq.(6) - Eq.(9) (-NH2 and -NH3+ as the representative):
416
-NH3+NO3- + HCrO4-
⇔
-NH3+HCrO4- + NO3-
(6)
417
2-NH3+NO3- + CrO42-
⇔
(-NH3+)2CrO42- + 2NO3-
(7)
418
2-NH3+NO3- + Cr2O72-
419
-NH3+NO3- + OH-
420
As the pH increased, the increasing OH- would exchange more NO3- and deprotonate
421
the positively charged amino groups, therefore the remaining NO3- and
422
decreased while the
⇒
⇔
(-NH3+)2Cr2O72- + 2NO3-
(8)
-NH2 + NO3- + H2O
(9)
NH2+
/-NH3+
NH
/-NH2 groups increased.
423
According to Fig. 9(b), after adsorption on NH2-TNTs, Cr 2p3/2 exhibited two
424
peaks at 577.0 and 579.4 eV corresponding to Cr(III) and Cr(VI) respectively [47] at
425
pH 2.04, 5.40 and 10.40, which meant that the adsorbed Cr(VI) ions were partially
426
reduced to Cr(III) at almost all pH ranges. It could be deduced that after Cr(VI) ions
427
were adsorbed by amino groups, the following reactions were occurred [49]:
428
Cr2O72- + 14H+ + 6e-
429
CrO42- + 8H+ + 3e-
430
HCrO4- + 7H+ + 3e-
431
The nanotubes was valence stable, thus the electron in the redox reaction was mainly
→
→ →
2Cr3+ + 7H2O
Cr3+ + 4H2O Cr3+ + 4H2O
E0 = +1.33V
(10)
E0 = +1.48 V
(11)
E0 = +1.35V
(12)
Page 21 of 33
432
from the amino groups of silane [50]. After adsorption by HTNTs, Cr 2p3/2 revealed
433
only one peak corresponding to Cr(VI). This meant the possibility of photocatalytic
434
reduction of Cr(VI) by TNTs was ruled out. As the pH increased from 2.04 to 10.40,
435
the proportion of Cr(III) to Cr(VI) increased from 1 to 3.25. This could be explained
436
that the negative charge of adsorbent increased as the pH increased, repelling
437
negatively charged Cr(VI) ions and adsorbing more positively charged Cr(III) ions.
438
The amino group is able to donate and share the lone electron pair with the empty
439
orbit of cations. Among the whole pH range from 1 to 12, no Cr(III) ion was detected
440
in solutions after adsorption, suggesting that Cr(III) ions were chelated on the amino
441
groups. Based on the Cr(III) species distribution at different solution pH [49], the
442
possible sorption mechanisms can be proposed as follows [12] (-NH2 as the
443
representative):
444
-NH2 + Cr3+
445
-NH2 + Cr(OH)2+
→
-NH2Cr(OH)2+
(14)
446
-NH2 + Cr(OH)2+
→
-NH2Cr(OH)2+
(15)
447
-NH2 + Cr(OH)3
→
-NH2Cr3+
→
-NH2Cr(OH)3
(13)
(16)
448
The FTIR spectra of NH2-TNTs before and after adsorption (Fig. 4) were in
449
agreement with the above analysis. After adsorption at initial pH 5.4, the peak at 1384
450
cm-1 ascribed to NO3- tremendously weakened, proving the exchange of NO3- during
451
Cr(VI) adsorption. The bending vibration of -CH2 at 1450 cm-1 which was hidden by
452
the intense band of 1384 cm-1 in the spectra before adsorption appeared in the spectra
453
after adsorption. The newly appeared peaks at 937 and 778 cm-1 were ascribed to
Page 22 of 33
454
Cr-O, proving Cr(VI) adsorption [51].
455
Based on the above analysis, an adsorption-reduction mechanism was proposed
456
(Fig. 10). At lower pH range, the Cr(VI) anions (mainly HCrO4-) exchanged with
457
NO3- on the adsorbent surface; At higher pH range, OH- competed with Cr(VI) anions
458
(mainly CrO42-) for the adsorption sites on the adsorbent surface, leading to increasing NO3-
and deprotonation of
NH2+
/-NH3+. Then, with the
459
ion exchange with
460
participation of H+ and electron from amino groups, Cr(VI) were partially reduced to
461
Cr(III) and chelated with amino groups, thus avoided entering into the solution.
462
[Fig. 9]
463
[Fig. 10]
464
4
Conclusion
465
After acid soaking and amino-functionalization processes, the nanotubular and
466
crystal structures of titanate nanotubes were preserved, while BET surface area and
467
pore volumes decreased obviously after amino-functionalization. FTIR spectra
468
demonstrated that AAPTS was covalently bonded on the surface of nanotubes and
469
abundant NO3- were linked with positively charged amino groups on the surface of
470
NH2-TNTs. The point of zero charge of NH2-TNTs was 8.75, much larger than that of
471
HTNTs at 4.00.
472
Pseudo-second-order model was found to fit kinetics data well and the adsorption
473
capacity on NH2-TNTs approached up to 95% of the maximum adsorption capacity
474
within only 5 min. For both materials, the adsorption isotherms at 30, 40 and 50 oC
Page 23 of 33
475
followed Langmuir model very well and adsorption capacity was higher at lower
476
temperature. The monolayer maximum adsorption capacity of Cr(VI) on NH2-TNTs
477
was 153.85 mg g-1 at 30 oC, combined with the very fast uptake rate, making
478
NH2-TNTs a promising Cr(VI) adsorbent compared to other Cr(VI) adsorbents. The
479
adsorption of Cr(VI) by NH2-TNTs was found to be strongly dependent on pH, with
480
maximum adsorption capacity obtained at equilibrium pH 2.96 - 3.79. The adsorption
481
capacity decreased after addition of ionic strength. The decline degree of Cr(VI)
482
adsorption capacity caused by coexisting anions followed the sequence: SO42- >
483
H2PO4-/H3PO4, Cl- and NO3-.
484
The XPS N1s spectra and FTIR spectra of Cr-laden NH2-TNTs revealed that
485
adsorption of Cr(VI) was occurred by exchanging with NO3-. XPS spectra of Cr2p3/2
486
on NH2-TNTs proved that the adsorbed Cr(VI) ions were partially reduced to Cr(III)
487
ions at pH range from 2.04 to 10.40. The absence of Cr(III) in the Cr(VI) solution
488
after adsorption indicated that Cr(III) were totally bonded with amino groups through
489
coordination effect. Interpretation of highly efficient adsorption process was
490
reasonably given with the proposed adsorption-reduction mechanism.
491
Acknowledgements
492
This work was supported by the Major Science and Technology Program for Water
493
Pollution Control and Treatment (2009ZX07212-001). The authors would like to
494
thank the anonymous reviewers for their constructive comments and suggestions.
495
References
Page 24 of 33
496
[1] W. F. Liu, J. Zhang, C. L. Zhang, L. Ren, Preparation and evaluation of activated carbon-based
497
iron-containing adsorbents for enhanced Cr(VI) removal: Mechanism study, Chem. Eng. J.
498
189-190 (2012) 295-302.
499
[2]
500
chromium-contaminated water and wastewater: A review, Water Air Soil Pollut. 200 (2009) 59-77.
501
[3]
502
poly(aniline-1,8-diaminonaphthalene) and its application as adsorbent for selective removal of Cr(VI)
503
ions, Chem. Eng. J. 173 (2011) 715-721.
504
[4] A.K. Giri, R. Patel, S. Mandal, Removal of Cr (VI) from aqueous solution by Eichhornia crassipes
505
root biomass-derived activated carbon, Chem. Eng. J. 185 (2012) 71-81.
506
[5] L.C. Hsu, S.L. Wang, Y.C. Lin, M.K. Wang, P.N. Chiang, J.C. Liu, W.H. Kuan, C.C. Chen, Y.M.
507
Tzou, Cr(VI) removal on fungal biomass of neurospora crassa: the importance of dissolved organic
508
carbons derived from the biomass to Cr(VI) reduction, Environ. Sci. Technol. 44 (2010) 6202-6208.
509
[6] A.M. Yusof, N.A.N.N. Malek, Removal of Cr(VI) and As(V) from aqueous solutions by
510
HDTMA-modified zeolite Y, J. Hazard. Mater. 162 (2009) 1019-1024.
511
[7] A.C. Zimmermann, A. Mecabo, T. Fagundes, C.A. Rodrigues, Adsorption of Cr(VI) using
512
Fe-crosslinked chitosan complex (Ch-Fe), J. Hazard. Mater. 179 (2010) 192-196.
513
[8] H. Shen, S. Pan, Y. Zhang, X. Huang, H. Gong, A new insight on the adsorption mechanism of
514
amino-functionalized nano-Fe3O4 magnetic polymers in Cu(II), Cr(VI) co-existing water system,
515
Chem. Eng. J. 183 (2012) 180-191.
516
[9] P. Luo, J.S. Zhang, B. Zhang, J.H. Wang, Y.F. Zhao, J.D. Liu, Preparation and characterization of
517
silane coupling agent modified halloysite for Cr(VI) removal, Ind. Eng. Chem. Res. 50 (2011)
M.
Q.
Owlad,
Li,
Y.
M.K.
Qian,
Aroua,
H.
W.A.W.
Cui,
Q.
Daud,
Zhang,
S.
R.
Baroutian,
Tang,
J.P.
Removal
Zhai,
of
hexavalent
Preparation
of
Page 25 of 33
518
10246-10252.
519
[10] B. Liu, Y. Huang, Polyethyleneimine modified eggshell membrane as a novel biosorbent for
520
adsorption and detoxification of Cr(VI) from water, J. Mater. Chem. 21 (2011) 17413-17418.
521
[11] D.W. Cho, C.M. Chon, Y. Kim, B.H. Jeon, F.W. Schwartz, E.S. Lee, H. Song, Adsorption of nitrate
522
and Cr(VI) by cationic polymer-modified granular activated carbon, Chem. Eng. J. 175 (2011)
523
298-305.
524
[12] S.B. Deng, Y.P. Ting, Polyethylenimine-modified fungal biomass as a high-capacity biosorbent for
525
Cr(VI) anions: Sorption capacity and uptake mechisms, Environ. Sci. Technol. 39 (2005) 8490-8496.
526
[13] J. Li, X. Miao, Y. Hao, J. Zhao, X. Sun, L. Wang, Synthesis, amino-functionalization of
527
mesoporous silica and its adsorption of Cr(VI), J. Colloid and Interf. Sci. 318 (2008) 309-314.
528
[14] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Formation of titanium oxide nanotube,
529
Langmuir, 14 (1998) 3160-3163.
530
[15] Q. Chen, L.M. Peng, Structure and applications of titanate and related nanostructures, Int. J.
531
Nanotechnol. 4 (2007) 44-65.
532
[16] D.V. Bavykin, F.C. Walsh, Titanate and Titania Nanotubes: Synthesis, Properties and Applications,
533
RSC, Cambridge, 2010.
534
[17] A. Rachini, M. Le Troedec, C. Peyratout, A. Smith, Chemical modification of hemp fibers by
535
silane coupling agents, J. Appl. Polym. Sci. 123 (2012) 601-610.
536
[18] D. Sovic, A. Gajovic, D. Ivekovic, Bioelectrocatalytic and biosensing properties of horseradish
537
peroxidase covalently immobilized on (3-aminopropyl)trimethoxysilane-modified titanate nanotubes,
538
Electrochim. Acta 56 (2011) 9953-9960.
539
[19] Q. Chen, W.Z. Zhou, G.H. Du, L.M. Peng, Trititanate nanotubes made via a single alkali treatment,
Page 26 of 33
540
Adv. Mater. 14 (2002) 1208-1212.
541
[20] M. Iijima, S. Takenouchi, I. W. Lenggoro, H. Kamiya, Effect of additive ratio of mixed silane
542
alkoxides on reactivity with TiO2 nanoparticle surface and their stability in organic solvents, Adv.
543
Powder Technol. 22 (2011) 663-668.
544
[21] R. J. Li, Q. He, Z. Hu, S. R. Zhang, L. J. Zhang, X. J. Chang, Highly selective solid-phase
545
extraction of trace Pd(II) by murexide functionalized halloysite nanotubes, Anal. Chim. Acta 713 (2012)
546
136-144.
547
[22] W. O. Yah, A. Takahara, Y. M. Lvov, Selective modification of halloysite lumen with
548
octadecylphosphonic acid: new inorganic tubular micelle, J. Am. Chem. Soc. 134 (2012) 1853-1859.
549
[23] H.H. Ou, C.H. Liao, Y.H. Liou, J.H. Hong, S.L. Lo, Photocatalytic oxidation of aqueous ammonia
550
over microwave-induced titanate nanotubes, Environ. Sci. Technol. 42 (2008) 4507-4512.
551
[24] C. C. Tasi, H. S. Teng, Structural features of nanotubes synthesized from NaOH treatment on TiO2
552
with different post-treatments, Chem. Mater. 18 (2006) 367-373.
553
[25] C.K. Lee, K.S. Lin, C.F. Wu, M.D. Lyu, C.C. Lo, Effects of synthesis temperature on the
554
microstructures and basic dyes adsorption of titanate nanotubes, J. Hazard. Mater. 150 (2008) 494-503.
555
[26] B.C. Viana, O.P. Ferreira, A.G. Souza Filho, A.A. Hidalgo, J. Mendes Filho, O.L. Alves, Alkali
556
metal intercalated titanate nanotubes: A vibrational spectroscopy study, Vib. Spectrosc. 55 (2011)
557
183-187.
558
[27] D.V. Bavykin, M. Carravetta, A.N. Kulak, F.C. Walsh, Application of magic-angle spinning NMR
559
to examine the nature of protons in titanate nanotubes, Chem. Mater. 22 (2010) 2458-2465.
560
[28] C.K. Lee, C.C. Wang, L.C. Juang, M.D. Lyu, S.H. Hung, S.S. Liu, Effects of sodium content on
561
the micro structures and basic dye cation exchange of titanate nanotubes, Colloid Surf., A 317 (2008)
Page 27 of 33
562
164-173.
563
[29] H. Kochkar, A. Turki, L. Bergaoui, G. Berhault, A. Ghorbel, Study of Pd(II) adsorption over
564
titanate nanotubes of different diameters, J. Colloid Interf. Sci. 331 (2009) 27-31.
565
[30] J.G. Yu, H.G. Yu, B. Cheng, C. Trapalis, Effects of calcination temperature on the microstructures
566
and photocatalytic activity of titanate nanotubes, J. Mol. Catal. A-Chem. 249 (2006) 135-142.
567
[31] L. Xiong, C. Chen, Q. Chen, J. Ni, Adsorption of Pb(II) and Cd(II) from aqueous solutions using
568
titanate nanotubes prepared via hydrothermal method, J. Hazard. Mater. 189 (2011) 741-748.
569
[32] H. Niu, Y. Cai, Preparation of octadecyl and amino mixed group modified titanate nanotubes and
570
its efficient adsorption to several ionic or ionizable organic analytes, Anal. Chem. 81 (2009)
571
9913-9920.
572
[33] Y. Ide, M. Ogawa, Surface modification of a layered alkali titanate with organosilanes, Chem.
573
Commun. (2003) 1262-1263.
574
[34] C. Gebald, J.A. Wurzbacher, P. Tingaut, T. Zimmermann, A. Steinfeld, Amine-based
575
nanofibrillated cellulose as adsorbent for CO2 capture from air, Environ. Sci. Technol. 45 (2011)
576
9101-9108.
577
[35] F. Beari, M. Brand, P. Jenkner, R. Lehnert, H.J. Metternich, J. Monkiewicz, H.W. Siesler,
578
Organofunctional alkoxysilanes in dilute aqueous solution: new accounts on the dynamic structural
579
mutability, J. Organomet. Chem. 625 (2001) 208-216.
580
[36] S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, K. Sven. Vetenskapsakad.
581
Handl. 24 (1898) 1-39.
582
[37] Y. Zeng, H. Woo, G. Lee, J. Park, Adsorption of Cr(VI) on hexadecylpyridinium bromide (HDPB)
583
modified natural zeolites, Microporous Mesoporous Mater. 130 (2010) 83-91.
Page 28 of 33
584
[38] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem.
585
Soc. 40 (1918) 1361-1403.
586
[39] H. Freundlich, Über die adsorption in lösungen, Z. Phys. Chem. 57 (1906) 385-470.
587
[40] A. Dong, J. Xie, W. Wang, L. Yu, Q. Liu, Y. Yin, A novel method for amino starch preparation and
588
its adsorption for Cu(II) and Cr(VI), J. Hazard. Mater. 181 (2010) 448-454.
589
[41] Y. Zhou, Q. Jin, T. Zhu, Y. Akama, Adsorption of chromium (VI) from aqueous solutions by
590
cellulose modified with beta-CD and quaternary ammonium groups, J. Hazard. Mater. 187 (2011)
591
303-310.
592
[42] Y.G. Abou El-Reash, M. Otto, I.M. Kenawy, A.M. Ouf, Adsorption of Cr(VI) and As(V) ions by
593
modified magnetic chitosan chelating resin, Int. J. Biol. Macromol. 49 (2011) 513-522.
594
[43] Y. Wen, Z. Tang, Y. Chen, Y. Gu, Adsorption of Cr(VI) from aqueous solutions using
595
chitosan-coated fly ash composite as biosorbent, Chem. Eng. J. 175 (2011) 110-116.
596
[44] X.S. Wang, L.F. Chen, F.Y. Li, K.L. Chen, W.Y. Wan, Y.J. Tang, Removal of Cr (VI) with
597
wheat-residue derived black carbon: Reaction mechanism and adsorption performance, J. Hazard.
598
Mater. 175 (2010) 816-822.
599
[45] S. Gupta, B.V. Babu, Removal of toxic metal Cr(VI) from aqueous solutions using sawdust as
600
adsorbent: Equilibrium, kinetics and regeneration studies, Chem. Eng. J. 150 (2009) 352-365.
601
[46] Y.G. Zhao, H.Y. Shen, S.D. Pan, M.Q. Hu, Synthesis, characterization and properties of
602
ethylenediamine-functionalized Fe(3)O(4) magnetic polymers for removal of Cr(VI) in wastewater, J.
603
Hazard. Mater. 182 (2010) 295-302.
604
[47] J. Fang, Z. Gu, D. Gang, C. Liu, E.S. Ilton, B. Deng, Cr(VI) removal from aqueous solution by
605
activated carbon coated with quaternized poly(4-vinylpyridine), Environ. Sci. Technol. 41 (2007)
Page 29 of 33
606
4748-4753.
607
[48] Y.T. Wei, Y.M. Zheng, J.P. Chen, Uptake of methylated arsenic by a polymeric adsorbent: Process
608
performance and adsorption chemistry, Water Res. 45 (2011) 2290-2296.
609
[49] X.F. Sun, Y. Ma, X.W. Liu, S.G. Wang, B.Y. Gao, X.M. Li, Sorption and detoxification of
610
chromium(VI) by aerobic granules functionalized with polyethylenimine, Water Res. 44 (2010)
611
2517-2524.
612
[50] J.P. Li, Q.Y. Lin, X.H. Zhang, Mechanism of electron transfer in the bioadsorption of hexavalent
613
chromium within Leersia hexandra Swartz granules by X-ray photoelectron spectroscopy, J. Hazard.
614
Mater. 182 (2010) 598-602.
615
[51] S.K. Das, M. Mukherjee, A.K. Guha, Interaction of chromium with resistant strain Aspergillus
616
versicolor: Investigation with atomic force microscopy and other physical studies, Langmuir, 24 (2008)
617
8643-8650.
618 619 620 621 622 623 624 625 626
627
Figure Captions Page 30 of 33
628
Fig. 1. TEM images of (a) HTNTs and (c) NH2-TNTs, HRTEM of (b) HTNTs and (d)
629
NH2-TNTs, EDX spectra of (inset a) HTNTs and (inset c) NH2-TNTs.
630
Fig. 2. XRD patterns of samples: (a) HTNTs and (b) NH2-TNTs.
631
Fig. 3. N2 adsorption-desorption isotherms and pore size distributions (the inset) of
632
the samples
633
Fig. 4. FTIR spectra of (a) HTNTs, (b) NH2-TNTs and (c) Cr(VI)-laden NH2-TNTs.
634
Fig. 5. Adsorption kinetics of Cr(VI) at different concentrations with NH2-TNTs
635
(m(NH2-TNTs) = 1 g L-1; T = 30 oC; pH =5.4) and HTNTs (m(HTNTs) = 1 g L-1; T =
636
30 oC; pH = 1).
637
Fig. 6. Adsorption isotherms of Cr(VI) with NH2-TNTs and HTNTs as a function of
638
temperature. (Cr(VI) = 25-400 mg L-1; m(NH2-TNTs) = 1 g L-1; pH =5.0; m(HTNTs)
639
= 1 g L-1; pH = 1; contact time = 1 h).
640
Fig. 7. (a) Adsorption of Cr(VI) onto HTNTs and NH2-TNTs as a function of
641
equilibrium pH, (b) Zeta potential of HTNTs and NH2-TNTs as a function of pH, (c)
642
Species distribution of Cr(VI) at different pH and (d) Effect of Cr(VI) adsorption
643
using HTNTs and NH2-TNTs on solution pH. (Cr(VI) = 100 mg L-1; m(HTNTs) =
644
m(NH2-TNTs) = 1 g L-1; T = 30 oC; contact time = 1 h).
645
Fig. 8. Effect of (a) coexisting anions and (b) ionic strength on the adsorption capacity
646
of Cr(VI) on NH2-TNTs and HTNTs. (Cr(VI) =100 mg L-1; T = 30 oC; m(NH2-TNTs)
647
= 1 g L-1; pH = 5.4; m(HTNTs) = 1 g L-1; pH = 1; contact time = 1 h; the
648
concentrations of coexisting anions were all 2.0 mM).
649
Fig. 9. (a) N 1s XPS spectra on the surface of NH2-TNTs before and after Cr(VI)
Page 31 of 33
650
adsorption at different initial solution pH and (b) Cr 2p3/2 XPS spectra on the surface
651
of NH2-TNTs and HTNTs after adsorption at different initial solution pH.
652
Fig. 10. Schematic illustration of preparation of NH2-TNTs and their Cr(VI)
653
adsorption-reduction mechanism.
654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671
Table Legends Page 32 of 33
672
Table 1. Elemental analysis of N, C and H for different samples.
673
Table 2. BET surface area and porosity of HTNTs and NH2-TNTs.
674
Table 3. Kinetic parameters for the adsorption of Cr(VI) onto NH2-TNTs and HTNTs
675
at different concentrations.
676
Table 4. List of model parameters of the adsorption isotherms onto NH2-TNTs and
677
HTNTs at different temperatures.
678
Table 5. Comparison of monolayer maximum capacities and equilibrium time of
679
some adsorbents to Cr(VI).
680 681 682 683
Page 33 of 33
Table(s)
Table 1 Elemental analysis of N, C and H for different samples. Percentage (wt.%) Samples N
C
H
S0.5
8.37
9.99
3.38
S0.75
8.87
10.24
3.41
S1
10.15
11.56
3.72
S1.25
9.15
10.80
3.51
Table 2 BET surface area and porosity of HTNTs and NH2-TNTs.
Samples
2
Single point total pore
Average pore diameter
volume (cm3 g-1)
(nm)
-1
BET surface area (m g )
HTNTs
343.5
1.015
11.82
NH2-TNTs
243.3
0.989
16.25
Table 3 Kinetic parameters for the adsorption of Cr(VI) onto NH2-TNTs and HTNTs at different concentrations. Kinetic models
Parameters
NH2-TNTs
HTNTs
Ci = 53 mg L-1
Ci = 99 mg L-1
Ci = 157mg L-1
Ci = 48 mg L-1
Ci = 98 mg L-1
Ci = 136mg L-1
Pseudo-first-order
qe,cala (mg g-1)
6.09
30.89
61.79
8.97
17.51
14.53
kinetic model
k1 (min-1)
0.522
0.461
0.583
0.0795
0.0733
0.0779
R2
0.5667
0.7740
0.9634
0.9568
0.9875
0.9504
Pseudo-second-order
qe,cala (mg g-1)
50.76
90.09
120.48
13.99
24.33
24.04
kinetic model
k2 (g mg-1 min-1)
1.94
0.088
0.172
0.0113
0.0036
0.0082
R2
0.99999
0.99998
0.99991
0.9866
0.9826
0.9907
qe,meab (mg g-1)
50.97
90.04
121.90
12.50
19.98
21.99
a
The calculated adsorption capacity at equilibrium.
b
The measured adsorption capacity at equilibrium.
Table 4 List of model parameters of the adsorption isotherms onto NH2-TNTs and HTNTs at different temperatures. Adsorption
Isotherm
NH2-TNTs
isotherm models
constants
30oC
40oC
50oC
30oC
40oC
50oC
Langmuir
Q (mg g-1)
153.85
140.85
129.87
26.60
22.73
21.10
b (L mg-1)
0.146
0.107
0.092
0.035
0.042
0.031
R2
0.99980
0.99995
0.99970
0.9931
0.9972
0.9894
KF (mg g-1)
33.12
27.16
23.95
2.41
2.08
1.02
nF
3.22
3.07
3.02
2.24
2.21
1.71
R2
0.8659
0.8676
0.8332
0.9750
0.9656
0.9641
Freundlich
HTNTs
Table 5 Comparison of monolayer maximum capacities and equilibrium time of some adsorbents to Cr(VI). Adsorbents
Monolayer
Equilibrium
maximum
time (min)
References
capacities (mg g-1) Amino starch
12.12
120
[40]
β-CD and quaternary ammonium groups
61.05
15
[41]
Modified magnetic chitosan chelating resin
58.48
120
[42]
Chitosan-coated fly ash
33.27
50
[43]
Hexadecylpyridinium bromide modified
14.31
240
[37]
36.34
30
[4]
Wheat-residue derived black carbon
21.34
240
[44]
Sawdust
41.5
1050
[45]
Ethylenediamine-functionalized Fe3O4
61.35
60
[46]
153.85
15
This study
modified cellulose
natural zeolites Eichhornia crassipes root biomass-derived activated carbon
magnetic polymers NH2-TNTs
Figure(s)
Figure Captions
Fig. 1. TEM images of (a) HTNTs and (c) NH2-TNTs, HRTEM of (b) HTNTs and (d) NH2-TNTs, EDX spectra of (inset a) HTNTs and (inset c) NH2-TNTs.
Fig. 2. XRD patterns of samples: (a) HTNTs and (b) NH2-TNTs.
Fig. 3. N2 adsorption-desorption isotherms and pore size distributions (the inset) of the samples
Fig. 4. FTIR spectra of (a) HTNTs, (b) NH2-TNTs and (c) Cr(VI)-laden NH2-TNTs.
Fig. 5. Adsorption kinetics of Cr(VI) at different concentrations with NH2-TNTs (m(NH2-TNTs) = 1 g L-1; T = 30 oC; pH =5.4) and HTNTs (m(HTNTs) = 1 g L-1; T = 30 oC; pH = 1).
Fig. 6. Adsorption isotherms of Cr(VI) with NH2-TNTs and HTNTs as a function of temperature. (Cr(VI) = 25-400 mg L-1; m(NH2-TNTs) = 1 g L-1; pH =5.0; m(HTNTs) = 1 g L-1; pH = 1; contact time = 1 h).
Fig. 7. (a) Adsorption of Cr(VI) onto HTNTs and NH2-TNTs as a function of equilibrium pH, (b) Zeta potential of HTNTs and NH2-TNTs as a function of pH, (c) Species distribution of Cr(VI) at different pH and (d) Effect of Cr(VI) adsorption using HTNTs and NH2-TNTs on solution pH. (Cr(VI) = 100 mg L-1; m(HTNTs) = m(NH2-TNTs) = 1 g L-1; T = 30 oC; contact time = 1 h).
Fig. 8. Effect of (a) coexisting anions and (b) ionic strength on the adsorption capacity of Cr(VI) on NH2-TNTs and HTNTs. (Cr(VI) =100 mg L-1; T = 30 oC; m(NH2-TNTs) = 1 g L-1; pH = 5.4; m(HTNTs) = 1 g L-1; pH = 1; contact time = 1 h; the concentrations of coexisting anions were all 2.0 mM).
Fig. 9. (a) N 1s XPS spectra on the surface of NH2-TNTs before and after Cr(VI) adsorption at different initial solution pH and (b) Cr 2p3/2 XPS spectra on the surface of NH2-TNTs and HTNTs after adsorption at different initial solution pH.
Fig. 10. Schematic illustration of preparation of NH2-TNTs and their Cr(VI) adsorption-reduction mechanism.
Highlights:
1. A new adsorbent for Cr(VI) with titanate nanotube structure was prepared. 2. Cr(VI) adsorption capacity on NH2-TNTs was much larger than that on HTNTs. 3. Ion-exchange with NO3- was responsible for efficient adsorption of Cr(VI). 4. The reduced Cr(III) was all bonded with amino groups.
S1385-8947(13)00406-3 http://dx.doi.org/10.1016/j.cej.2013.03.081 CEJ 10563
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
6 January 2013 16 March 2013 19 March 2013
Please cite this article as: L. Wang, W. Liu, T. Wang, J. Ni, Highly efficient adsorption of Cr(VI) from aqueous solutions by amino-functionalized titanate nanotubes, Chemical Engineering Journal (2013), doi: http://dx.doi.org/ 10.1016/j.cej.2013.03.081
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1
Highly efficient adsorption of Cr(VI) from aqueous
2
solutions by amino-functionalized titanate nanotubes
3
Lei Wanga, Wen Liua, Ting Wanga, Jinren Nib,*
4
a
5
Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China;
6
b
7
Sciences, Ministry of Education, Beijing 100871, China
Shenzhen Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and
Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment
*
Corresponding author. Tel.: +86-10-62751185; fax: +86-10-62756526. E-mail address: [email protected] (J.R. Ni). Page 1 of 33
8
Abstract
9
For highly efficient removal of Cr(VI) from aqueous solutions, amino-functionalized titanate
10
nanotubes (NH2-TNTs) with excellent adsorption performance have been synthesized by
11
covalently grafting [1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (AAPTS) onto protonated
12
titanate nanotubes (HTNTs) with great amounts of surface hydroxyl groups. TEM and XRD
13
results confirmed that the nanotubular morphology and crystal structure of HTNTs and NH2-TNTs
14
were preserved. FTIR spectra demonstrated that AAPTS was covalently bonded on the surface of
15
HTNTs. Batch adsorption experiments showed that pseudo-second-order kinetics model and
16
Langmuir isotherm model fitted the adsorption data very well for both materials, and the Cr(VI)
17
adsorption capacity on NH2-TNTs calculated by Langmuir model was up to 153.85 mg g-1 at
18
initial pH 5.4 and 30 oC, much larger than that on HTNTs (26.60 mg g-1). Moreover, uptake of
19
Cr(VI) ions onto NH2-TNTs could be completed within only 5 min for 95% adsorption of the
20
maximum. Influence of different species of Cr(VI) under varying pH was also considered. FTIR
21
and XPS analysis indicated that Cr(VI) ions were first exchanged with NO3- linked on the
22
positively charged amino groups and then partially reduced to Cr(III). Afterwards, Cr(III) were
23
then totally chelated with amino groups and no Cr(III) was detected in the solution after Cr(VI)
24
adsorption at pH range of 1-12.
25 26
Key words: Amino-functionalization; Titanate nanotubes; Hexavalent chromium; Adsorption;
27
Mechanism
28 29
Page 2 of 33
30
1. Introduction
31
Hexavalent chromium, Cr(VI), is highly toxic and carcinogenic. In this paper,
32
Cr(VI) is a general expression for H2CrO4(aq) and series of anion ions containing
33
Cr2O72-, HCrO4- and CrO42-, since it is difficult to give specific expression for
34
coexisted anion ions even under the same pH [1]. Most of the Cr(VI) pollutions are
35
from wastewater and waste materials of electroplating, tanning, metallurgic, chemical
36
and mining industries. Various methods have been used for Cr(VI) removal, such as
37
reduction, ion exchange, reverse osmosis, membrane separation, electrocoagulation
38
and adsorption. Among them, adsorption method is a particularly attractive option due
39
to its outstanding simplicity, low investment, high efficiency and potential recovery
40
[2,3]. Various kinds of materials have been employed for Cr(VI) adsorption, including
41
activated carbon [4], biomass materials [5], zeolite [6], chitosan [7] and ferriferrous
42
oxide [8]. However, the potential shortcomings of some absorbents prevented their
43
wide application, like impurities in the adsorbents, low adsorption capacity and slow
44
adsorption kinetics. Thus, searching for alternative materials with improved
45
adsorption capacity and high adsorption rate is urgently desired.
46
Recently, a lot of papers paid attention to using amino groups to modify various
47
categories of materials due to their special properties in improving Cr(VI) adsorption.
48
The amino groups can be easily protonated in acid conditions, forming a kind of
49
positively charged groups which is beneficial to the increase of the point of zero
50
charge (PZC) of modified materials [9]. By the effect of electrostatic attraction and
51
hydrogen bond between positively charged amino groups and negatively charged Page 3 of 33
52
Cr(VI) ions, the adsorption capacity of Cr(VI) can be greatly enhanced [10]. The
53
materials used for modification should possess properties that are beneficial for
54
introducing functional groups, such as large surface area and pore volumes, high
55
density of surface available groups. These materials include activated carbon [11],
56
biomass materials [12], halloysite nanotubes [9] and mesoporous silica [13] etc.
57
Titanate nanotubes (TNTs) fabricated via hydrothermal method were first reported
58
by Kasuga et al. [14]. The lower fabrication cost, large specific surface area and pore
59
volumes, and nanotubular structure make titanate nanotubes a candidate as an
60
excellent adsorbent for organics and heavy metal removal in aqueous solutions [15].
61
Besides, the abundant surface hydroxyl groups with a density of 5.8 -OH/nm2 in
62
average [16] make it possible to graft amino groups on TNTs surface. The
63
amino-functionalized titanate nanotubes may be a good adsorbent for Cr(VI) removal
64
due to the combined action of electrostatic attraction and hydrogen bonding effect.
65
Moreover, the large specific surface area and pore volumes from TNTs provided
66
adsorption sites and diffusion channels for metals. Therefore, we herein synthesized
67
amino-functionalized titanate nanotubes to investigate the Cr(VI) adsorption capacity
68
in aqueous solutions.
69
[1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (AAPTS) is a kind of silane
70
coupling agent bearing two amino groups in one molecule. This silane coupling agent
71
is easily hydrolyzed to silanol group, and can be further dehydrated with surface
72
hydroxyl groups of TNTs [17,18]. Moreover, both the primary and secondary amino
73
groups of AAPTS could bind heavy metal anions and thus improve adsorption
Page 4 of 33
74
capacity. In this paper, amino-functionalized TNTs is obtained using AAPTS as the
75
modification agent on protonated TNTs. The amino-functionalized TNTs and
76
protonated TNTs were characterized and further used for Cr(VI) adsorption. Important
77
factors such as pH, contact time, reaction temperature, initial Cr(VI) concentration,
78
ionic strength and coexisting anions were studied in detail. Furthermore, Cr(VI)
79
adsorption mechanism on AAPTS modified TNTs was discussed.
80
2. Materials and methods
81
2.1. Reagents
82
[1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (95%) was purchased from
83
Aladdin Chemistry Co. Ltd (Shanghai, China) and used without further purification.
84
Other reagents used in the experiments were of analytical grade, and purchased from
85
Tianjin Guang Fu Technology Development Co. Ltd (Tianjin, China).
86
2.2. Preparation of protonated titanate nanotubes
87
The protonated titanate nanotubes were synthesized by hydrothermal treatment of
88
TiO2 powder in concentrated NaOH solution as described by Chen et al. [19], and
89
then soaked with nitric acid to increase the amount of surface hydroxyl groups.
90
Typically, 1.2 g TiO2 nanoparticle powder (P25, Degussa, Germany) was added to 66
91
mL of 10 M NaOH solution. After vigorous stirring for 12 h, the mixture was
92
transferred into a 100 mL sealed teflon container and statically heated at 130 oC for 72
93
h. The precipitate was washed with 0.1 M HNO3 until the pH value of the rinsing Page 5 of 33
94
solution reached 7.0, then the products were soaked with 0.1 M HNO3 for 5 h. After
95
that, the products were centrifuged and washed with deionized water to neutral to
96
wipe out excessive nitric acid, and then dried in an oven at 105 oC for 12 h. The
97
protonated titanate nanotubes were labeled as HTNTs.
98
2.3. Amino-functionalization of titanate nanotubes
99
The amino-functionalization was performed as follows: 0.5 g HTNTs were
100
dispersed in 50 mL toluene solution, and then AAPTS with appropriate volume was
101
added. To detect the effect of AAPTS volume, 0.5 mL, 0.75 mL, 1 mL and 1.25 mL
102
AAPTS were separately added. The mixture was then kept under refluxing conditions
103
at 100 oC for 24 h. After separated by centrifugation at 5000 rpm for 3 min, the
104
as-synthesized powder was washed with toluene and ethanol to remove the excessive
105
AAPTS and hydrolyzed products. Finally, the products were washed twice with 0.1 M
106
nitric acid and dried at 60 oC. The amino-functionalized titanate nanotubes were
107
marked as NH2-TNTs.
108
2.4. Characterization and analyses
109
Transmission electron microscopy (TEM) analysis was conducted on a FEI Tecnai
110
F30 microscope equipped with energy dispersive X-ray spectroscopy (EDX)
111
spectrometer operating at 300 kV. X-ray diffraction (XRD) patterns were obtained on
112
a Rigaku Dmax/2400 X-ray diffractometer using Cu Kα radiation, with a scan rate (2θ)
113
of 4o/min (λ = 1.5418Å). Nitrogen adsorption-desorption isotherms were determined
Page 6 of 33
114
at -196 oC on an ASAP 2010 adsorption apparatus (Micromeritics, USA) to obtain
115
Brunauer-Emmett-Teller (BET) surface area and Barret-Joyner-Halender (BJH) pore
116
size distributions. The samples were degassed at 100 oC before adsorption. The
117
Fourier transform infrared spectroscopy (FTIR) of the samples were obtained by a
118
Tensor 27 FT-IR spectrometer (Bruker, Germany) at room temperature using the KBr
119
pellet method. For zeta potential measurements, 0.04 g samples were dispersed in 40
120
mL deionized water with ultrasonic radiation for 15 min before adjusting the pH
121
values, then the suspensions were shaken at 200 rpm for 24 h. The final pH was
122
measured before the zeta potential measurements using a Nano-ZS90 Zetasizer
123
(Malvern Instruments, UK). The surface elements and oxidation state were
124
investigated using X-ray photoelectron spectroscopy (XPS, AXIS-Ultra, Kratos
125
Analytical, Japan) with monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). All
126
binding energies were calibrated by C 1s hydrocarbon peaked at 284.80 eV for
127
compensation of surface charge effects. The calibrated high-resolution spectra were
128
analyzed by CasaXPS software. The surface elemental stoichiometries were
129
determined from the ratios of peak-area corrected by sensitivity factor. Distribution of
130
Cr(VI) species as a function of pH was calculated using Visual MINTEQ version 2.3.
131
2.5. Batch adsorption studies
132
A stock solution (1000 mg L-1) of Cr(VI) was prepared by dissolving a known
133
quantity of potassium dichromate (K2Cr2O7) in deionized water. All the adsorption
134
experiments were carried out on a rotary shaker at 200 rpm. The adsorption kinetics
Page 7 of 33
135
experiments were first conducted to determine the equilibrium adsorption time.
136
Generally, 1 g L-1 NH2-TNTs or HTNTs were separately added to 200 mL of 50 mg
137
L-1, 100 mg L-1 and 150 mg L-1 Cr(VI) solutions with fixed pH, then aliquots of Cr(VI)
138
solution were withdrawn at different contact time intervals and filtered with 0.45 µm
139
hydrophilic membrane. The filtrate was diluted and analyzed to obtain total Cr and
140
Cr(VI) concentration. The concentration of total Cr was determined by inductively
141
coupled plasma-atomic emission spectroscopy (ICP-AES, Prodigy, Leeman, USA),
142
while Cr(VI) was determined with a diphenylcarbohydrazide spectrophotometric
143
method at 540 nm using UV-vis spectrophotometer (UV752, Shanghai Youke
144
Instrument Co. Ltd, China).
145
To investigate the effect of pH, 40 mL of 100 mg L-1 Cr(VI) solutions with various
146
initial pH at 1-12 were prepared. After adding 40 mg NH2-TNTs or HTNTs to the
147
solutions, the solutions were shaken at 30 oC for 1 h (determined by the adsorption
148
kinetics). To explore the adsorption isotherm at different temperatures, the initial
149
Cr(VI) concentration was varied from 25 to 400 mg L-1 at 30 oC, 40 oC and 50 oC,
150
respectively. Finally, different amounts of sodium chloride (NaCl), sodium nitrate
151
(NaNO3),
152
(Na3PO4·12H2O) were added to 100 mg L-1 Cr(VI) solutions to investigate the effect
153
of coexisting anions and ionic strength.
154
sodium sulfate
(Na2SO4)
and
sodium
phosphate
dodecahydrate
The adsorption capacity qe (mg g-1) was calculated by Eq. (1): (C i -C e ) V (1) W
155
qe =
156
where Ci and Ce (mg L-1) are the initial and equilibrium Cr(VI) concentration,
Page 8 of 33
157
respectively. V (mL) is the volume of Cr(VI) solution, W (mg) is the mass of
158
adsorbent.
159
3. Results and discussion
160
3.1. Characterization of materials
161
3.1.1
Effect of AAPTS dosage
162
The elemental analysis of N, C and H for modified materials with different dosage
163
of AAPTS in the functionalization process is shown in Table 1. Samples with dosage
164
of 0.5 mL, 0.75 mL, 1 mL and 1.25 mL AAPTS were denoted as S0.5, S0.75, S1 and
165
S1.25 respectively. According to Table 1, the content of N, C and H enhanced with
166
the increase of AAPTS volume, followed by a decrease for S1.25. This can be
167
attributed to pH variation of nanotubes suspension [20]. When AAPTS volume was
168
less than 1 mL, Si-OH from AAPTS was limited and totally reacted with sufficient
169
Ti-OH; when AAPTS volume was above 1 mL, hydrolysis of amino groups from
170
AAPTS would increase nanotubes suspension pH, resulting in the decrease of Ti-OH
171
reactivity and thus N content of NH2-TNTs. Sample S1 shows the highest amounts of
172
N, C and H, i.e. 7.26 mmol N g-1, 9.63 mmol C g-1 and 37.2 mmol H g-1. The molar
173
ratio of N/C for S1 was 0.75, slightly lower than the theoretical value of 0.8, which
174
was calculated based on complete protonation of primary and secondary amino
175
groups. The results indicated that not all of the amino groups were protonated. As
176
sample S1 had the highest amount of N, that is, the most abundant amino groups on
177
NH2-TNTs, it was used in the following characterization and Cr(VI) adsorption Page 9 of 33
178
experiments.
179
[Table 1]
180
3.1.2
TEM analysis
181
TEM image of HTNTs (Fig. 1(a)) shows cylindrical-shaped tubes with multilayer
182
walls and open-ended lumens along the nanotubes. The samples (Fig. 1(b)) contained
183
agglomerates of randomly tangled nanotubes with some irregularity in diameter
184
(typically 7.4 nm), wall thickness, and morphology. After modification (Fig. 1(c)), the
185
nanotubes still exhibited complete tubular structure, but the void due to the
186
aggregation of nanotubes significantly decreased. We can see from Fig. 1(d) that a
187
nanotube with a diameter of ca. 7.2 nm is mostly coated by organic-like materials
188
[21,22] with uneven thickness and less density than nanotubes along the external
189
surface, forming an uneven surface longitudinally and making the layered structure
190
invisible. The EDX spectra of nanotubes after modification (Fig. 1(c) inset) appeared
191
additional peaks corresponding to Si and N as compared to that of HTNTs (Fig. 1(a)
192
inset), indicating that the organic-like materials were silane, that is, the external
193
surface area of HTNTs was mostly coated with AAPTS.
194
[Fig. 1]
195
3.1.3 XRD patterns
196
XRD patterns of HTNTs and NH2-TNTs are shown in Fig. 2. Titanate nanotubes
197
were preferentially assigned for NaxH2-xTi3O7 [23], where x≈0 after protonation
Page 10 of 33
198
process. The (200) reflection at around 2θ = 10o corresponded to interlayer spacing of
199
the samples. For HTNTs, the (200) reflection was not so obvious, which should be
200
attributed to some crystal defects caused by partial transformation of the titanate
201
structure to anatase TiO2 at pH 1 [24]. For NH2-TNTs, the position of (200) reflection
202
little shifted, indicating the interlayer distance of nanotubes didn’t change [25-27].
203
Therefore it was proved that amino-functionalization process didn’t occur in the
204
interlayer space of nanotubes. The (110) plane at ca. 24o and (211) plane at ca. 28o
205
could be assigned to hydrogen titanate nanotubes, and (020) plane at ca. 48o
206
corresponded to sodium titanate compounds [25,26]. The peak at ca. 28o was much
207
weaker than reported literatures [25-28], which was ascribed to the replacement of
208
Na+ with H+ during acid soaking [24]. The similar diffraction peaks of NH2-TNTs
209
with HTNTs proved that amino-functionalization process had little effect on HTNTs
210
crystal structure. Combined with TEM results, it can be concluded that the crystal
211
structure maintained after acid treatment and amino-functionalization.
212
[Fig. 2]
213
3.1.4 N2 adsorption-desorption studies
214
Fig. 3 shows the nitrogen adsorption-desorption isotherms and pore size
215
distributions of HTNTs and NH2-TNTs. Both samples exhibit type IV isotherms with
216
H3 type hysteresis loop according to IUPAC classification, indicating the presence of
217
mesopores (2-50 nm) [29]. The pore size of HTNTs (the inset) exhibited bimodal
218
distributions with peaks at 3-4 nm and 8-20 nm. The peak at 3-4 nm corresponded to
Page 11 of 33
219
pores inside the nanotubes while the pores at 8-20 nm were ascribed to the voids in
220
the aggregation of nanotubes [28,30]. The silanization process resulted in a large
221
decrease of the volume of the smaller pores, while the larger pores were less affected,
222
indicating that the majority of nanotubes in the sample NH2-TNTs had closed ends,
223
presumably because of the blocking of nanotubes with polymerized silane during the
224
reaction of HTNTs with AAPTS [18]. This result was in consistent with the
225
observation of TEM images (Fig. 1). The surface properties of HTNTs and NH2-TNTs
226
were shown in Table 2. The BET surface area (243.3 m2 g-1) and pore volume (0.989
227
cm3 g-1 ) of NH2-TNTs were both smaller than those of HTNTs (343.5 m2 g-1 for BET
228
surface area and 1.015 cm3 g-1 for pore volume), proving that silane was coated on the
229
surface of nanotubes, thus reduced the surface area and pore volume.
230
[Fig. 3]
231
[Table 2]
232
3.1.5 FTIR studies
233
The FTIR spectra of HTNTs and NH2-TNTs are shown in Fig. 4. HTNTs exhibit
234
adsorption bands at 3300-3600, 1631, 1384, 913, 678 and 476 cm-1. The wide and
235
strong absorption bands in the region of 3300-3600 cm-1 was attributed to O-H
236
stretching vibration, suggesting the presence of huge amounts of surface hydroxyl
237
groups [31]. The band at 1631 cm-1 was ascribed to O-H stretching of water
238
molecules. The bands at 913, 678 and 476 cm-1 were all from titanate nanotubes,
239
corresponding to the stretching and bending vibration of Ti-O [26]. The band at 1384
Page 12 of 33
240
cm-1 corresponded to stretching vibration of nitrate ions. For NH2-TNTs, new
241
adsorption bands at 3376, 2973, 1211, 1133, 1039 and 826 cm-1 appeared as compared
242
to HTNTs. The relatively stronger adsorption band at 3376 cm-1 corresponded to N-H
243
stretching vibration, demonstrating the presence of amino groups. The existence of
244
aminosilane was further proved by C-H stretching at 2973 cm-1, C-N stretching at
245
1211 cm-1 and 1133 cm-1 and Si-C stretching at 826 cm-1 [32]. The band of Si-O-Si at
246
1039 cm-1 and Si-O-Ti at 945 cm-1 (shoulder) [18] was of great significance, as it
247
confirmed that AAPTS was covalently bonded with the surface of HTNTs, and
248
formed a polymerized network over the surface of nanotubes. The reaction of AAPTS
249
with surface OH groups of HTNTs can be assumed to conduct through the mechanism
250
similar to that found for the reaction of AAPTS with other oxide-based materials,
251
including (i) the hydrolysis of methoxy groups of AAPTS catalyzed by H2O/H3O+ and
252
the formation of silanol, i.e. (HO)3Si-C3H6NHC2H4NH2 and (ii) the condensation
253
between silanol OH groups and surface Ti-OH groups, with consequent formation of
254
Si-O-Ti bonds [33-35]. The greatly enhanced intensity at 1384 cm-1 indicated that
255
nitrate salt was formed.
256
[Fig. 4]
257
3.2 Adsorption behaviors of Cr(VI) on NH2-TNTs and HTNTs
258
3.2.1 Adsorption kinetics
259
The adsorption kinetics of NH2-TNTs and HTNTs with different initial Cr(VI)
260
concentrations are shown in Fig. 5. The kinetics displayed a very rapid initial uptake Page 13 of 33
261
and a subsequent stable stage. For NH2-TNTs, the equilibrium times for the three
262
concentrations were all less than 15 min. To be more precise, the adsorption capacity
263
reached up to 95% within only 5 min. The fast sorption ability was attributed to the
264
high porosity and surface area that facilitated the diffusion of Cr(VI) ions, combined
265
with the abundantly loaded amino groups, which can adsorb chromium with high
266
adsorption kinetics via electrostatic attraction. For HTNTs, the equilibrium times for
267
the three concentrations are all around 40 min, longer than those of NH2-TNTs. This
268
may be ascribed to the difference of adsorbing groups.
269
In order to make a better understanding of adsorption process, pseudo-first-order
270
kinetic and pseudo-second order kinetic models were used to describe the adsorption
271
kinetics. The pseudo-first-order model is expressed as Eq. (2) [36]:
272
ln (qe -qt )= ln qe -k1t (2)
273
where qe and qt (mg g-1) are the amounts of Cr(VI) ions adsorbed at equilibrium and
274
elapsed time t, respectively, t (min) is contact time, and k1 (min-1) is pseudo-first-order
275
rate constant. k1 and qe are calculated from the slope and intercept of the plot of log(qe
276
-qt) versus t.
277
The pseudo-second-order kinetic model is described by the Eq. (3) [37]:
278
t 1 1 = + t (3) 2 qt k2qe qe
279
where k2 (g mg-1 min-1) is pseudo-second-order rate constant. The values of k2 and qe
280
can be computed from the slope and intercept of the plot of t/qt versus t.
281
The results obtained from pseudo-first-order and pseudo-second-order models are
282
listed in Table 3. For NH2-TNTs, it can be seen that the correlation coefficients made Page 14 of 33
283
by pseudo-first-order model are smaller, and the calculated adsorption capacities
284
differ much from the measured results; while the correlation coefficients made by
285
pseudo-second-order model reach 0.99999, 0.99998 and 0.99991, and the calculated
286
adsorption capacities are almost identical to the measured adsorption capacities. This
287
suggested that the adsorption of Cr(VI) by NH2-TNTs followed pseudo-second-order
288
model very well, which corresponded to a chemisorption process. Moreover, the
289
pseudo-second-order rate constants at initial concentration of 99 and 157 mg L-1 are
290
smaller than that of 53 mg L-1, indicating that the uptake rate is faster at lower initial
291
concentration.
292
pseudo-second-order kinetic model at 98 mg L-1 is a little smaller than that of
293
pseudo-first-order kinetic model, the pseudo-second-order kinetic model is more
294
appropriate. This meant Cr(VI) adsorption with HTNTs tended to be a chemisorption
295
process.
296
[Fig. 5]
297
[Table 3]
298
3.2.2 Adsorption isotherm
HTNTs,
although
the
correlation
coefficient
of
Fig. 6 shows the adsorption isotherms by NH2-TNTs and HTNTs at 30, 40 and 50
299 300
For
o
C. The adsorption capacity of Cr(VI) enhanced gradually with the increase of the
301
equilibrium concentrations, and then kept stable both for NH2-TNTs and HTNTs.
302
Moreover, the adsorption capacity was higher at lower temperature for both materials.
303
To make a better understanding of the adsorption characteristics, Langmuir and
Page 15 of 33
304
Freundlich adsorption isotherm models were applied to fit the data. The Langmuir
305
model supposes that monolayer surface adsorption occurs on specific homogeneous
306
sites and there is no interaction between the adsorbed pollutants. The equation for this
307
model is expressed as Eq. (4) [38]:
308
Ce Ce 1 = + qe Q Qb
309
where Ce (mg L-1) is the equilibrium Cr(VI) concentration, qe (mg g -1) is the amount
310
of Cr(VI) ion adsorbed at equilibrium, Q (mg g-1) is the maximum adsorption capacity
311
of the adsorbent, and b (L mg-1) is the Langmuir constant corresponding to the free
312
energy of adsorption. By plotting Ce/qe versus Ce, the values of Q and b can be
313
computed from the slope and intercept of the linear plot.
314
(4)
The Freundlich isotherm is an empirical equation assuming heterogeneous surface
315
adsorption. The equation is usually expressed as the following linear form [39]:
316
ln qe = ln K F +
317
where KF (mg g-1) is the Freundlich constant corresponding to the adsorption capacity
318
of the adsorbent, and n is the heterogeneity factor related to the adsorption intensity of
319
the adsorbent. Both constants can be calculated from the slope and intercept of the
320
linear plot of log qe versus log Ce.
1 ln Ce (5) nF
321
The results calculated based on the above two models were listed in Table 4. For
322
NH2-TNTs, the correlation coefficients of Langmuir model were 0.99980, 0.99995,
323
and 0.99970 at 30 oC, 40 oC and 50 oC respectively, suggesting Langmuir model can
324
effectively describe the adsorption data, which confirmed the existence of
Page 16 of 33
325
homogeneous active sites within NH2-TNTs and monolayer adsorption of Cr(VI) ions
326
onto the adsorbent surface. For HTNTs, Langmuir model also fitted the adsorption
327
data better than Freundlich model. The maximum adsorption capacity by both
328
materials decreased with the increase of temperature, indicating the exothermic
329
characteristic of Cr(VI) sorption on both materials. The monolayer adsorption
330
capacity on NH2-TNTs computed by Langmuir model was 153.85 mg g-1 at 30 oC,
331
combined with the very fast uptake rate, making NH2-TNTs a promising Cr(VI)
332
adsorbent compared to other Cr(VI) adsorbents (Table 5). The monolayer adsorption
333
capacity on HTNTs computed by Langmuir model was only 26.60 mg g-1, much
334
smaller than that on NH2-TNTs, indicating that amino groups played a key role in the
335
Cr(VI) adsorption process on NH2-TNTs.
336
[Fig. 6]
337
[Table 4]
338
[Table 5]
339
3.2.3
Effect of pH
340
The effect of pH on Cr(VI) adsorption by NH2-TNTs was investigated with initial
341
pH ranged from pH 1 to 12 (Fig. 7). For comparison, the adsorption by HTNTs was
342
also conducted at the same pH range. The concentration of total Cr and Cr(VI) was
343
measured to be identical at all pH range, indicating that no Cr(VI) ion was reduced or
344
the reduced Cr(III) ions didn’t enter into the solution. Fig. 7(a) compares the
345
adsorption of Cr(VI) by HTNTs and NH2-TNTs at different equilibrium pH. It was
Page 17 of 33
346
revealed that HTNTs had only a little adsorption of Cr(VI) at equilibrium pH lower
347
than 4.28, while NH2-TNTs showed much larger adsorption capacity with equilibrium
348
pH ranged from pH 1 to 9.22. The adsorption capacity of Cr(VI) by NH2-TNTs was
349
largest at equilibrium pH 2.96 - 3.79 with a value of ca. 90 mg g-1 and a removal
350
efficiency of 90%, almost 6 times larger than that of HTNTs. For NH2-TNTs, when
351
the equilibrium pH was above pH 5, the adsorption capacity decreased gradually with
352
higher pH values.
353
The amino groups could adsorb protons easily during nitric acid washing of NH2+
/-NH3+. The
354
modification process, forming positively charged amino groups as
355
Cr(VI) ions mainly existed in the form of HCrO4- at pH 2-5 (Fig. 7(c)), which could
356
easily bond with positively charged amino groups via electrostatic attraction. At pH >
357
5, the concentration of divalent CrO42- increased gradually as the pH increased, which
358
consumed two positively charged amino group, making the adsorption capacity of
359
Cr(VI) decreased gradually. In addition, as the pH increased, more OH- ions
360
competed with Cr(VI) ions for positively charged amino groups. Similarly, at
361
equilibrium pH below 2.79, the adsorption capacity decreased with lower pH, due to
362
the increasing of H2CrO4 which had no attraction to positively charged amino groups.
363
For HTNTs, the adsorption capacity increased with the decease of pH. This should be
364
ascribed to that more -OH2+ groups which can attract negatively charged Cr(VI) ions
365
were formed at lower pH values.
366
Fig. 7(b) is the zeta potentials of HTNTs and NH2-TNTs at different solution pH.
367
The pHpzc of NH2-TNTs is 8.75, much larger than that of HTNTs at 4.00. When the
Page 18 of 33
368
solution pH is higher than the pHpzc, the material will be negatively charged,
369
otherwise it is positively charged. Fig. 7(a) showed that the adsorption capacities on
370
both materials are absolutely low when the solution pH was above their pHpzc. This
371
suggested that the primary driving force of both materials for adsorbing Cr(VI) ions
372
was electrostatic attraction. To be different, the active sites for NH2-TNTs were
373
NH2+
/-NH3+ while those for HTNTs were –OH2+ during Cr(VI) adsorption.
374
Fig. 7(d) is the change of pH during adsorption process. All the solution pH
375
decreased after Cr(VI) adsorption by the two materials. It was because the exchanged
376
protons in the interlayer space derived from acid soaking neutralized with OH- ions in
377
solutions. The variation of pH after adsorption by NH2-TNTs was larger than that by
378
HTNTs due to the bonding of OH- with positively charged amino groups.
379
[Fig. 7]
380
3.2.4 Effect of coexisting anions and ionic strength
381
According to the pH analysis, electrostatic attraction governs the adsorption
382
process, so the coexisting anions like nitrate, chloride, sulfate and phosphate in
383
wastewater samples would compete with Cr(VI) ions for active sites. Fig. 8(a) shows
384
the effect of coexisting anions on the Cr(VI) adsorption by NH2-TNTs and HTNTs.
385
The concentrations of coexisting anions were all 2.0 mM, which was comparable to
386
that of Cr(VI). For both materials, it can be seen that the adsorption capacity
387
decreased slightly after the addition of chloride, nitrate and phosphate, but greatly
388
decreased after sulfate addition. The different decreasing degrees caused by various
Page 19 of 33
389
anions might be related to the charge of the coexisting anions. The more charge of the
390
anions, the greater ability to compete with Cr(VI) ions [47]. At pH 5.4, the existing
391
form of phosphate mainly existed as monohydrogenphosphate (H2PO4-) while
392
phosphoric acid (H3PO4) is the main component at pH 1 (calculated by MINTEQ
393
version 2.3). Thus, the decreasing degree of adsorption capacity by SO42- was larger
394
than that by Cl-, NO3- and H2PO4-/H3PO4.
395
The effect of ionic strength in form of NaNO3 on the Cr(VI) adsorption capacity is
396
shown in Fig. 8(b). For both materials, the decreasing degree of Cr(VI) capacity
397
became larger as ionic strength increased, due to the enhanced competition between
398
NO3- and HCrO4- for positively charged amino groups or hydroxyl groups. It is
399
necessary to point out that when the concentration of NO3- was 0.01 M, which was
400
almost 5 times larger than that of Cr(VI), the adsorption capacity of Cr(VI) on
401
NH2-TNTs decreased only 12.5%, suggesting the good selectivity toward Cr(VI) by
402
NH2-TNTs.
403
[Fig. 8]
404
3.3 Adsorption mechanism
405
Fig. 9 shows the XPS spectra of N 1s before and after adsorption as well as Cr
406
2p3/2 after adsorption. Before Cr(VI) adsorption, N 1s spectra exhibited three peaks NH
/-NH2,
NH2+
/-NH3+ and
407
at 399.3, 401.5 and 406.7 eV, which were assigned to
408
NO3-, respectively [32,48]. After Cr(VI) adsorption at different initial pH, these three
409
peaks shifted to 399.85±0.05eV, 401.75±0.05eV and 407eV respectively. The content
Page 20 of 33
NH2+
NH
/-NH2 to
/-NH3+ became larger and increased from 3.42 to 21.32
410
ratio of
411
while the content of NO3- decreased when the pH increased from 2.04 to 10.40. It was
412
because that NO3- was adsorbed on the positively charged amino groups and formed a
413
salt-like material before adsorption. Once this material contacted with Cr(VI) solution,
414
NO3- would be exchanged by more attractive ions such as negatively charged Cr(VI)
415
ions and OH-, as depicted by Eq.(6) - Eq.(9) (-NH2 and -NH3+ as the representative):
416
-NH3+NO3- + HCrO4-
⇔
-NH3+HCrO4- + NO3-
(6)
417
2-NH3+NO3- + CrO42-
⇔
(-NH3+)2CrO42- + 2NO3-
(7)
418
2-NH3+NO3- + Cr2O72-
419
-NH3+NO3- + OH-
420
As the pH increased, the increasing OH- would exchange more NO3- and deprotonate
421
the positively charged amino groups, therefore the remaining NO3- and
422
decreased while the
⇒
⇔
(-NH3+)2Cr2O72- + 2NO3-
(8)
-NH2 + NO3- + H2O
(9)
NH2+
/-NH3+
NH
/-NH2 groups increased.
423
According to Fig. 9(b), after adsorption on NH2-TNTs, Cr 2p3/2 exhibited two
424
peaks at 577.0 and 579.4 eV corresponding to Cr(III) and Cr(VI) respectively [47] at
425
pH 2.04, 5.40 and 10.40, which meant that the adsorbed Cr(VI) ions were partially
426
reduced to Cr(III) at almost all pH ranges. It could be deduced that after Cr(VI) ions
427
were adsorbed by amino groups, the following reactions were occurred [49]:
428
Cr2O72- + 14H+ + 6e-
429
CrO42- + 8H+ + 3e-
430
HCrO4- + 7H+ + 3e-
431
The nanotubes was valence stable, thus the electron in the redox reaction was mainly
→
→ →
2Cr3+ + 7H2O
Cr3+ + 4H2O Cr3+ + 4H2O
E0 = +1.33V
(10)
E0 = +1.48 V
(11)
E0 = +1.35V
(12)
Page 21 of 33
432
from the amino groups of silane [50]. After adsorption by HTNTs, Cr 2p3/2 revealed
433
only one peak corresponding to Cr(VI). This meant the possibility of photocatalytic
434
reduction of Cr(VI) by TNTs was ruled out. As the pH increased from 2.04 to 10.40,
435
the proportion of Cr(III) to Cr(VI) increased from 1 to 3.25. This could be explained
436
that the negative charge of adsorbent increased as the pH increased, repelling
437
negatively charged Cr(VI) ions and adsorbing more positively charged Cr(III) ions.
438
The amino group is able to donate and share the lone electron pair with the empty
439
orbit of cations. Among the whole pH range from 1 to 12, no Cr(III) ion was detected
440
in solutions after adsorption, suggesting that Cr(III) ions were chelated on the amino
441
groups. Based on the Cr(III) species distribution at different solution pH [49], the
442
possible sorption mechanisms can be proposed as follows [12] (-NH2 as the
443
representative):
444
-NH2 + Cr3+
445
-NH2 + Cr(OH)2+
→
-NH2Cr(OH)2+
(14)
446
-NH2 + Cr(OH)2+
→
-NH2Cr(OH)2+
(15)
447
-NH2 + Cr(OH)3
→
-NH2Cr3+
→
-NH2Cr(OH)3
(13)
(16)
448
The FTIR spectra of NH2-TNTs before and after adsorption (Fig. 4) were in
449
agreement with the above analysis. After adsorption at initial pH 5.4, the peak at 1384
450
cm-1 ascribed to NO3- tremendously weakened, proving the exchange of NO3- during
451
Cr(VI) adsorption. The bending vibration of -CH2 at 1450 cm-1 which was hidden by
452
the intense band of 1384 cm-1 in the spectra before adsorption appeared in the spectra
453
after adsorption. The newly appeared peaks at 937 and 778 cm-1 were ascribed to
Page 22 of 33
454
Cr-O, proving Cr(VI) adsorption [51].
455
Based on the above analysis, an adsorption-reduction mechanism was proposed
456
(Fig. 10). At lower pH range, the Cr(VI) anions (mainly HCrO4-) exchanged with
457
NO3- on the adsorbent surface; At higher pH range, OH- competed with Cr(VI) anions
458
(mainly CrO42-) for the adsorption sites on the adsorbent surface, leading to increasing NO3-
and deprotonation of
NH2+
/-NH3+. Then, with the
459
ion exchange with
460
participation of H+ and electron from amino groups, Cr(VI) were partially reduced to
461
Cr(III) and chelated with amino groups, thus avoided entering into the solution.
462
[Fig. 9]
463
[Fig. 10]
464
4
Conclusion
465
After acid soaking and amino-functionalization processes, the nanotubular and
466
crystal structures of titanate nanotubes were preserved, while BET surface area and
467
pore volumes decreased obviously after amino-functionalization. FTIR spectra
468
demonstrated that AAPTS was covalently bonded on the surface of nanotubes and
469
abundant NO3- were linked with positively charged amino groups on the surface of
470
NH2-TNTs. The point of zero charge of NH2-TNTs was 8.75, much larger than that of
471
HTNTs at 4.00.
472
Pseudo-second-order model was found to fit kinetics data well and the adsorption
473
capacity on NH2-TNTs approached up to 95% of the maximum adsorption capacity
474
within only 5 min. For both materials, the adsorption isotherms at 30, 40 and 50 oC
Page 23 of 33
475
followed Langmuir model very well and adsorption capacity was higher at lower
476
temperature. The monolayer maximum adsorption capacity of Cr(VI) on NH2-TNTs
477
was 153.85 mg g-1 at 30 oC, combined with the very fast uptake rate, making
478
NH2-TNTs a promising Cr(VI) adsorbent compared to other Cr(VI) adsorbents. The
479
adsorption of Cr(VI) by NH2-TNTs was found to be strongly dependent on pH, with
480
maximum adsorption capacity obtained at equilibrium pH 2.96 - 3.79. The adsorption
481
capacity decreased after addition of ionic strength. The decline degree of Cr(VI)
482
adsorption capacity caused by coexisting anions followed the sequence: SO42- >
483
H2PO4-/H3PO4, Cl- and NO3-.
484
The XPS N1s spectra and FTIR spectra of Cr-laden NH2-TNTs revealed that
485
adsorption of Cr(VI) was occurred by exchanging with NO3-. XPS spectra of Cr2p3/2
486
on NH2-TNTs proved that the adsorbed Cr(VI) ions were partially reduced to Cr(III)
487
ions at pH range from 2.04 to 10.40. The absence of Cr(III) in the Cr(VI) solution
488
after adsorption indicated that Cr(III) were totally bonded with amino groups through
489
coordination effect. Interpretation of highly efficient adsorption process was
490
reasonably given with the proposed adsorption-reduction mechanism.
491
Acknowledgements
492
This work was supported by the Major Science and Technology Program for Water
493
Pollution Control and Treatment (2009ZX07212-001). The authors would like to
494
thank the anonymous reviewers for their constructive comments and suggestions.
495
References
Page 24 of 33
496
[1] W. F. Liu, J. Zhang, C. L. Zhang, L. Ren, Preparation and evaluation of activated carbon-based
497
iron-containing adsorbents for enhanced Cr(VI) removal: Mechanism study, Chem. Eng. J.
498
189-190 (2012) 295-302.
499
[2]
500
chromium-contaminated water and wastewater: A review, Water Air Soil Pollut. 200 (2009) 59-77.
501
[3]
502
poly(aniline-1,8-diaminonaphthalene) and its application as adsorbent for selective removal of Cr(VI)
503
ions, Chem. Eng. J. 173 (2011) 715-721.
504
[4] A.K. Giri, R. Patel, S. Mandal, Removal of Cr (VI) from aqueous solution by Eichhornia crassipes
505
root biomass-derived activated carbon, Chem. Eng. J. 185 (2012) 71-81.
506
[5] L.C. Hsu, S.L. Wang, Y.C. Lin, M.K. Wang, P.N. Chiang, J.C. Liu, W.H. Kuan, C.C. Chen, Y.M.
507
Tzou, Cr(VI) removal on fungal biomass of neurospora crassa: the importance of dissolved organic
508
carbons derived from the biomass to Cr(VI) reduction, Environ. Sci. Technol. 44 (2010) 6202-6208.
509
[6] A.M. Yusof, N.A.N.N. Malek, Removal of Cr(VI) and As(V) from aqueous solutions by
510
HDTMA-modified zeolite Y, J. Hazard. Mater. 162 (2009) 1019-1024.
511
[7] A.C. Zimmermann, A. Mecabo, T. Fagundes, C.A. Rodrigues, Adsorption of Cr(VI) using
512
Fe-crosslinked chitosan complex (Ch-Fe), J. Hazard. Mater. 179 (2010) 192-196.
513
[8] H. Shen, S. Pan, Y. Zhang, X. Huang, H. Gong, A new insight on the adsorption mechanism of
514
amino-functionalized nano-Fe3O4 magnetic polymers in Cu(II), Cr(VI) co-existing water system,
515
Chem. Eng. J. 183 (2012) 180-191.
516
[9] P. Luo, J.S. Zhang, B. Zhang, J.H. Wang, Y.F. Zhao, J.D. Liu, Preparation and characterization of
517
silane coupling agent modified halloysite for Cr(VI) removal, Ind. Eng. Chem. Res. 50 (2011)
M.
Q.
Owlad,
Li,
Y.
M.K.
Qian,
Aroua,
H.
W.A.W.
Cui,
Q.
Daud,
Zhang,
S.
R.
Baroutian,
Tang,
J.P.
Removal
Zhai,
of
hexavalent
Preparation
of
Page 25 of 33
518
10246-10252.
519
[10] B. Liu, Y. Huang, Polyethyleneimine modified eggshell membrane as a novel biosorbent for
520
adsorption and detoxification of Cr(VI) from water, J. Mater. Chem. 21 (2011) 17413-17418.
521
[11] D.W. Cho, C.M. Chon, Y. Kim, B.H. Jeon, F.W. Schwartz, E.S. Lee, H. Song, Adsorption of nitrate
522
and Cr(VI) by cationic polymer-modified granular activated carbon, Chem. Eng. J. 175 (2011)
523
298-305.
524
[12] S.B. Deng, Y.P. Ting, Polyethylenimine-modified fungal biomass as a high-capacity biosorbent for
525
Cr(VI) anions: Sorption capacity and uptake mechisms, Environ. Sci. Technol. 39 (2005) 8490-8496.
526
[13] J. Li, X. Miao, Y. Hao, J. Zhao, X. Sun, L. Wang, Synthesis, amino-functionalization of
527
mesoporous silica and its adsorption of Cr(VI), J. Colloid and Interf. Sci. 318 (2008) 309-314.
528
[14] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Formation of titanium oxide nanotube,
529
Langmuir, 14 (1998) 3160-3163.
530
[15] Q. Chen, L.M. Peng, Structure and applications of titanate and related nanostructures, Int. J.
531
Nanotechnol. 4 (2007) 44-65.
532
[16] D.V. Bavykin, F.C. Walsh, Titanate and Titania Nanotubes: Synthesis, Properties and Applications,
533
RSC, Cambridge, 2010.
534
[17] A. Rachini, M. Le Troedec, C. Peyratout, A. Smith, Chemical modification of hemp fibers by
535
silane coupling agents, J. Appl. Polym. Sci. 123 (2012) 601-610.
536
[18] D. Sovic, A. Gajovic, D. Ivekovic, Bioelectrocatalytic and biosensing properties of horseradish
537
peroxidase covalently immobilized on (3-aminopropyl)trimethoxysilane-modified titanate nanotubes,
538
Electrochim. Acta 56 (2011) 9953-9960.
539
[19] Q. Chen, W.Z. Zhou, G.H. Du, L.M. Peng, Trititanate nanotubes made via a single alkali treatment,
Page 26 of 33
540
Adv. Mater. 14 (2002) 1208-1212.
541
[20] M. Iijima, S. Takenouchi, I. W. Lenggoro, H. Kamiya, Effect of additive ratio of mixed silane
542
alkoxides on reactivity with TiO2 nanoparticle surface and their stability in organic solvents, Adv.
543
Powder Technol. 22 (2011) 663-668.
544
[21] R. J. Li, Q. He, Z. Hu, S. R. Zhang, L. J. Zhang, X. J. Chang, Highly selective solid-phase
545
extraction of trace Pd(II) by murexide functionalized halloysite nanotubes, Anal. Chim. Acta 713 (2012)
546
136-144.
547
[22] W. O. Yah, A. Takahara, Y. M. Lvov, Selective modification of halloysite lumen with
548
octadecylphosphonic acid: new inorganic tubular micelle, J. Am. Chem. Soc. 134 (2012) 1853-1859.
549
[23] H.H. Ou, C.H. Liao, Y.H. Liou, J.H. Hong, S.L. Lo, Photocatalytic oxidation of aqueous ammonia
550
over microwave-induced titanate nanotubes, Environ. Sci. Technol. 42 (2008) 4507-4512.
551
[24] C. C. Tasi, H. S. Teng, Structural features of nanotubes synthesized from NaOH treatment on TiO2
552
with different post-treatments, Chem. Mater. 18 (2006) 367-373.
553
[25] C.K. Lee, K.S. Lin, C.F. Wu, M.D. Lyu, C.C. Lo, Effects of synthesis temperature on the
554
microstructures and basic dyes adsorption of titanate nanotubes, J. Hazard. Mater. 150 (2008) 494-503.
555
[26] B.C. Viana, O.P. Ferreira, A.G. Souza Filho, A.A. Hidalgo, J. Mendes Filho, O.L. Alves, Alkali
556
metal intercalated titanate nanotubes: A vibrational spectroscopy study, Vib. Spectrosc. 55 (2011)
557
183-187.
558
[27] D.V. Bavykin, M. Carravetta, A.N. Kulak, F.C. Walsh, Application of magic-angle spinning NMR
559
to examine the nature of protons in titanate nanotubes, Chem. Mater. 22 (2010) 2458-2465.
560
[28] C.K. Lee, C.C. Wang, L.C. Juang, M.D. Lyu, S.H. Hung, S.S. Liu, Effects of sodium content on
561
the micro structures and basic dye cation exchange of titanate nanotubes, Colloid Surf., A 317 (2008)
Page 27 of 33
562
164-173.
563
[29] H. Kochkar, A. Turki, L. Bergaoui, G. Berhault, A. Ghorbel, Study of Pd(II) adsorption over
564
titanate nanotubes of different diameters, J. Colloid Interf. Sci. 331 (2009) 27-31.
565
[30] J.G. Yu, H.G. Yu, B. Cheng, C. Trapalis, Effects of calcination temperature on the microstructures
566
and photocatalytic activity of titanate nanotubes, J. Mol. Catal. A-Chem. 249 (2006) 135-142.
567
[31] L. Xiong, C. Chen, Q. Chen, J. Ni, Adsorption of Pb(II) and Cd(II) from aqueous solutions using
568
titanate nanotubes prepared via hydrothermal method, J. Hazard. Mater. 189 (2011) 741-748.
569
[32] H. Niu, Y. Cai, Preparation of octadecyl and amino mixed group modified titanate nanotubes and
570
its efficient adsorption to several ionic or ionizable organic analytes, Anal. Chem. 81 (2009)
571
9913-9920.
572
[33] Y. Ide, M. Ogawa, Surface modification of a layered alkali titanate with organosilanes, Chem.
573
Commun. (2003) 1262-1263.
574
[34] C. Gebald, J.A. Wurzbacher, P. Tingaut, T. Zimmermann, A. Steinfeld, Amine-based
575
nanofibrillated cellulose as adsorbent for CO2 capture from air, Environ. Sci. Technol. 45 (2011)
576
9101-9108.
577
[35] F. Beari, M. Brand, P. Jenkner, R. Lehnert, H.J. Metternich, J. Monkiewicz, H.W. Siesler,
578
Organofunctional alkoxysilanes in dilute aqueous solution: new accounts on the dynamic structural
579
mutability, J. Organomet. Chem. 625 (2001) 208-216.
580
[36] S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, K. Sven. Vetenskapsakad.
581
Handl. 24 (1898) 1-39.
582
[37] Y. Zeng, H. Woo, G. Lee, J. Park, Adsorption of Cr(VI) on hexadecylpyridinium bromide (HDPB)
583
modified natural zeolites, Microporous Mesoporous Mater. 130 (2010) 83-91.
Page 28 of 33
584
[38] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem.
585
Soc. 40 (1918) 1361-1403.
586
[39] H. Freundlich, Über die adsorption in lösungen, Z. Phys. Chem. 57 (1906) 385-470.
587
[40] A. Dong, J. Xie, W. Wang, L. Yu, Q. Liu, Y. Yin, A novel method for amino starch preparation and
588
its adsorption for Cu(II) and Cr(VI), J. Hazard. Mater. 181 (2010) 448-454.
589
[41] Y. Zhou, Q. Jin, T. Zhu, Y. Akama, Adsorption of chromium (VI) from aqueous solutions by
590
cellulose modified with beta-CD and quaternary ammonium groups, J. Hazard. Mater. 187 (2011)
591
303-310.
592
[42] Y.G. Abou El-Reash, M. Otto, I.M. Kenawy, A.M. Ouf, Adsorption of Cr(VI) and As(V) ions by
593
modified magnetic chitosan chelating resin, Int. J. Biol. Macromol. 49 (2011) 513-522.
594
[43] Y. Wen, Z. Tang, Y. Chen, Y. Gu, Adsorption of Cr(VI) from aqueous solutions using
595
chitosan-coated fly ash composite as biosorbent, Chem. Eng. J. 175 (2011) 110-116.
596
[44] X.S. Wang, L.F. Chen, F.Y. Li, K.L. Chen, W.Y. Wan, Y.J. Tang, Removal of Cr (VI) with
597
wheat-residue derived black carbon: Reaction mechanism and adsorption performance, J. Hazard.
598
Mater. 175 (2010) 816-822.
599
[45] S. Gupta, B.V. Babu, Removal of toxic metal Cr(VI) from aqueous solutions using sawdust as
600
adsorbent: Equilibrium, kinetics and regeneration studies, Chem. Eng. J. 150 (2009) 352-365.
601
[46] Y.G. Zhao, H.Y. Shen, S.D. Pan, M.Q. Hu, Synthesis, characterization and properties of
602
ethylenediamine-functionalized Fe(3)O(4) magnetic polymers for removal of Cr(VI) in wastewater, J.
603
Hazard. Mater. 182 (2010) 295-302.
604
[47] J. Fang, Z. Gu, D. Gang, C. Liu, E.S. Ilton, B. Deng, Cr(VI) removal from aqueous solution by
605
activated carbon coated with quaternized poly(4-vinylpyridine), Environ. Sci. Technol. 41 (2007)
Page 29 of 33
606
4748-4753.
607
[48] Y.T. Wei, Y.M. Zheng, J.P. Chen, Uptake of methylated arsenic by a polymeric adsorbent: Process
608
performance and adsorption chemistry, Water Res. 45 (2011) 2290-2296.
609
[49] X.F. Sun, Y. Ma, X.W. Liu, S.G. Wang, B.Y. Gao, X.M. Li, Sorption and detoxification of
610
chromium(VI) by aerobic granules functionalized with polyethylenimine, Water Res. 44 (2010)
611
2517-2524.
612
[50] J.P. Li, Q.Y. Lin, X.H. Zhang, Mechanism of electron transfer in the bioadsorption of hexavalent
613
chromium within Leersia hexandra Swartz granules by X-ray photoelectron spectroscopy, J. Hazard.
614
Mater. 182 (2010) 598-602.
615
[51] S.K. Das, M. Mukherjee, A.K. Guha, Interaction of chromium with resistant strain Aspergillus
616
versicolor: Investigation with atomic force microscopy and other physical studies, Langmuir, 24 (2008)
617
8643-8650.
618 619 620 621 622 623 624 625 626
627
Figure Captions Page 30 of 33
628
Fig. 1. TEM images of (a) HTNTs and (c) NH2-TNTs, HRTEM of (b) HTNTs and (d)
629
NH2-TNTs, EDX spectra of (inset a) HTNTs and (inset c) NH2-TNTs.
630
Fig. 2. XRD patterns of samples: (a) HTNTs and (b) NH2-TNTs.
631
Fig. 3. N2 adsorption-desorption isotherms and pore size distributions (the inset) of
632
the samples
633
Fig. 4. FTIR spectra of (a) HTNTs, (b) NH2-TNTs and (c) Cr(VI)-laden NH2-TNTs.
634
Fig. 5. Adsorption kinetics of Cr(VI) at different concentrations with NH2-TNTs
635
(m(NH2-TNTs) = 1 g L-1; T = 30 oC; pH =5.4) and HTNTs (m(HTNTs) = 1 g L-1; T =
636
30 oC; pH = 1).
637
Fig. 6. Adsorption isotherms of Cr(VI) with NH2-TNTs and HTNTs as a function of
638
temperature. (Cr(VI) = 25-400 mg L-1; m(NH2-TNTs) = 1 g L-1; pH =5.0; m(HTNTs)
639
= 1 g L-1; pH = 1; contact time = 1 h).
640
Fig. 7. (a) Adsorption of Cr(VI) onto HTNTs and NH2-TNTs as a function of
641
equilibrium pH, (b) Zeta potential of HTNTs and NH2-TNTs as a function of pH, (c)
642
Species distribution of Cr(VI) at different pH and (d) Effect of Cr(VI) adsorption
643
using HTNTs and NH2-TNTs on solution pH. (Cr(VI) = 100 mg L-1; m(HTNTs) =
644
m(NH2-TNTs) = 1 g L-1; T = 30 oC; contact time = 1 h).
645
Fig. 8. Effect of (a) coexisting anions and (b) ionic strength on the adsorption capacity
646
of Cr(VI) on NH2-TNTs and HTNTs. (Cr(VI) =100 mg L-1; T = 30 oC; m(NH2-TNTs)
647
= 1 g L-1; pH = 5.4; m(HTNTs) = 1 g L-1; pH = 1; contact time = 1 h; the
648
concentrations of coexisting anions were all 2.0 mM).
649
Fig. 9. (a) N 1s XPS spectra on the surface of NH2-TNTs before and after Cr(VI)
Page 31 of 33
650
adsorption at different initial solution pH and (b) Cr 2p3/2 XPS spectra on the surface
651
of NH2-TNTs and HTNTs after adsorption at different initial solution pH.
652
Fig. 10. Schematic illustration of preparation of NH2-TNTs and their Cr(VI)
653
adsorption-reduction mechanism.
654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671
Table Legends Page 32 of 33
672
Table 1. Elemental analysis of N, C and H for different samples.
673
Table 2. BET surface area and porosity of HTNTs and NH2-TNTs.
674
Table 3. Kinetic parameters for the adsorption of Cr(VI) onto NH2-TNTs and HTNTs
675
at different concentrations.
676
Table 4. List of model parameters of the adsorption isotherms onto NH2-TNTs and
677
HTNTs at different temperatures.
678
Table 5. Comparison of monolayer maximum capacities and equilibrium time of
679
some adsorbents to Cr(VI).
680 681 682 683
Page 33 of 33
Table(s)
Table 1 Elemental analysis of N, C and H for different samples. Percentage (wt.%) Samples N
C
H
S0.5
8.37
9.99
3.38
S0.75
8.87
10.24
3.41
S1
10.15
11.56
3.72
S1.25
9.15
10.80
3.51
Table 2 BET surface area and porosity of HTNTs and NH2-TNTs.
Samples
2
Single point total pore
Average pore diameter
volume (cm3 g-1)
(nm)
-1
BET surface area (m g )
HTNTs
343.5
1.015
11.82
NH2-TNTs
243.3
0.989
16.25
Table 3 Kinetic parameters for the adsorption of Cr(VI) onto NH2-TNTs and HTNTs at different concentrations. Kinetic models
Parameters
NH2-TNTs
HTNTs
Ci = 53 mg L-1
Ci = 99 mg L-1
Ci = 157mg L-1
Ci = 48 mg L-1
Ci = 98 mg L-1
Ci = 136mg L-1
Pseudo-first-order
qe,cala (mg g-1)
6.09
30.89
61.79
8.97
17.51
14.53
kinetic model
k1 (min-1)
0.522
0.461
0.583
0.0795
0.0733
0.0779
R2
0.5667
0.7740
0.9634
0.9568
0.9875
0.9504
Pseudo-second-order
qe,cala (mg g-1)
50.76
90.09
120.48
13.99
24.33
24.04
kinetic model
k2 (g mg-1 min-1)
1.94
0.088
0.172
0.0113
0.0036
0.0082
R2
0.99999
0.99998
0.99991
0.9866
0.9826
0.9907
qe,meab (mg g-1)
50.97
90.04
121.90
12.50
19.98
21.99
a
The calculated adsorption capacity at equilibrium.
b
The measured adsorption capacity at equilibrium.
Table 4 List of model parameters of the adsorption isotherms onto NH2-TNTs and HTNTs at different temperatures. Adsorption
Isotherm
NH2-TNTs
isotherm models
constants
30oC
40oC
50oC
30oC
40oC
50oC
Langmuir
Q (mg g-1)
153.85
140.85
129.87
26.60
22.73
21.10
b (L mg-1)
0.146
0.107
0.092
0.035
0.042
0.031
R2
0.99980
0.99995
0.99970
0.9931
0.9972
0.9894
KF (mg g-1)
33.12
27.16
23.95
2.41
2.08
1.02
nF
3.22
3.07
3.02
2.24
2.21
1.71
R2
0.8659
0.8676
0.8332
0.9750
0.9656
0.9641
Freundlich
HTNTs
Table 5 Comparison of monolayer maximum capacities and equilibrium time of some adsorbents to Cr(VI). Adsorbents
Monolayer
Equilibrium
maximum
time (min)
References
capacities (mg g-1) Amino starch
12.12
120
[40]
β-CD and quaternary ammonium groups
61.05
15
[41]
Modified magnetic chitosan chelating resin
58.48
120
[42]
Chitosan-coated fly ash
33.27
50
[43]
Hexadecylpyridinium bromide modified
14.31
240
[37]
36.34
30
[4]
Wheat-residue derived black carbon
21.34
240
[44]
Sawdust
41.5
1050
[45]
Ethylenediamine-functionalized Fe3O4
61.35
60
[46]
153.85
15
This study
modified cellulose
natural zeolites Eichhornia crassipes root biomass-derived activated carbon
magnetic polymers NH2-TNTs
Figure(s)
Figure Captions
Fig. 1. TEM images of (a) HTNTs and (c) NH2-TNTs, HRTEM of (b) HTNTs and (d) NH2-TNTs, EDX spectra of (inset a) HTNTs and (inset c) NH2-TNTs.
Fig. 2. XRD patterns of samples: (a) HTNTs and (b) NH2-TNTs.
Fig. 3. N2 adsorption-desorption isotherms and pore size distributions (the inset) of the samples
Fig. 4. FTIR spectra of (a) HTNTs, (b) NH2-TNTs and (c) Cr(VI)-laden NH2-TNTs.
Fig. 5. Adsorption kinetics of Cr(VI) at different concentrations with NH2-TNTs (m(NH2-TNTs) = 1 g L-1; T = 30 oC; pH =5.4) and HTNTs (m(HTNTs) = 1 g L-1; T = 30 oC; pH = 1).
Fig. 6. Adsorption isotherms of Cr(VI) with NH2-TNTs and HTNTs as a function of temperature. (Cr(VI) = 25-400 mg L-1; m(NH2-TNTs) = 1 g L-1; pH =5.0; m(HTNTs) = 1 g L-1; pH = 1; contact time = 1 h).
Fig. 7. (a) Adsorption of Cr(VI) onto HTNTs and NH2-TNTs as a function of equilibrium pH, (b) Zeta potential of HTNTs and NH2-TNTs as a function of pH, (c) Species distribution of Cr(VI) at different pH and (d) Effect of Cr(VI) adsorption using HTNTs and NH2-TNTs on solution pH. (Cr(VI) = 100 mg L-1; m(HTNTs) = m(NH2-TNTs) = 1 g L-1; T = 30 oC; contact time = 1 h).
Fig. 8. Effect of (a) coexisting anions and (b) ionic strength on the adsorption capacity of Cr(VI) on NH2-TNTs and HTNTs. (Cr(VI) =100 mg L-1; T = 30 oC; m(NH2-TNTs) = 1 g L-1; pH = 5.4; m(HTNTs) = 1 g L-1; pH = 1; contact time = 1 h; the concentrations of coexisting anions were all 2.0 mM).
Fig. 9. (a) N 1s XPS spectra on the surface of NH2-TNTs before and after Cr(VI) adsorption at different initial solution pH and (b) Cr 2p3/2 XPS spectra on the surface of NH2-TNTs and HTNTs after adsorption at different initial solution pH.
Fig. 10. Schematic illustration of preparation of NH2-TNTs and their Cr(VI) adsorption-reduction mechanism.
Highlights:
1. A new adsorbent for Cr(VI) with titanate nanotube structure was prepared. 2. Cr(VI) adsorption capacity on NH2-TNTs was much larger than that on HTNTs. 3. Ion-exchange with NO3- was responsible for efficient adsorption of Cr(VI). 4. The reduced Cr(III) was all bonded with amino groups.