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Electrosorption of fluoride on TiO2-loaded activated carbon in water
Accepted Manuscript Title: Electrosorption of Fluoride on TiO2 -Loaded Activated Carbon in Water Author: Peng Wu Ling Xia Min Dai Liliang Lin Shaoxian Song PII: DOI: Reference:
S0927-7757(16)30328-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.05.020 COLSUA 20644
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
27-1-2016 29-4-2016 3-5-2016
Please cite this article as: Peng Wu, Ling Xia, Min Dai, Liliang Lin, Shaoxian Song, Electrosorption of Fluoride on TiO2-Loaded Activated Carbon in Water, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.05.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrosorption of Fluoride on TiO2-Loaded Activated Carbon in Water Peng Wu, Ling Xia, Min Dai, Liliang Lin, Shaoxian Song* School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China
* Corresponding author. E-mail: [email protected]
Graphical abstract
Highlights
Presented a promising method for removing fluoride from water. Electrosorption with TiO2-loaded AC achieved a higher defluoridation. Presented a possible mechanism for the elctrosorption of fluoride on Ti-AC
Abstract In this work the electrosorption of fluoride ions on TiO2-loaded activated carbon (Ti-AC) in water has been studied, in order to develop a novel process to remove fluoride from water. The loading of TiO2 on activated carbon (AC) was realized with sol–gel method. The experimental results have shown that the capacity of the electrosorption on the Ti-AC was two folds of that on the original AC. The sorption fitted to the Langmuir isotherm with the maximum capacity of 157.8 μmol/g, and the sorption kinetics followed pseudo-second-order model. The pH study showed the adsorption capacity of Ti-AC by electrosorption had a good performance at pH range of 6-9. By means of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, it was found that the fluoride existed on the Ti-AC surfaces through ligand exchange to replace the -OH group. The mechanism of the sorption might be attributed to that fluoride first entered the electric double layer of the AC through electrosorption and led to the great increase of fluoride equilibrium concentration near the electrode surfaces, followed by the chemisorption of fluoride on the surfaces, which would greatly improve the sorption capacity.
Keywords: Electrosorption; Fluoride; Defluoridation; TiO2-loaded activated carbon; Chemisorption
1. Introduction Fluoride contamination in drinking water has become a worldwide concern in recent years due to its severe threat to public health. There are more than 20 developed and developing nations where people suffered from endemic fluorosis with a fluoride concentration higher than 1.5 mg/L, the guideline limitation for fluoride in drinking water set by World Health Organization [1, 2] . Various technologies and methods have been developed to remove fluoride from water, such as, adsorption, ion exchange, electrodialysis, precipitation and reverse osmosis [3, 4]. Electrosorption is a good desalination process, in which ions were attracted and concentrated in the electrical double layers of electrodes when an electrical potential is applied [5, 6]. It is effective for removing many kinds of ions from brackish waters or other organic pollutants [7-9]. Commonly, activated carbon (AC) is more widely applied to the electrode than other carbon-based materials such as, carbon nanotubes, carbon fiber, carbon aerogel and grapheme due to AC was of high specific surface area, cheap and easy to get [10]. In the case of fluoride removal, AC is not efficient enough, so that a modification is needed. It has been proved that titanium dioxide is a potential selective adsorbent for fluoride due to the high stability, nontoxicity, hydrophilicity and strong affinity with fluoride [11]. Accordingly, the AC with TiO2 loading might be better electrode for the electrosorption of fluoride. In the present work, the electrosorption of fluoride using TiO2-loaded activated carbon (Ti-AC) as the electrode was studied, in order to develop a novel process of removing fluoride from water. This study was performed on the measurements of the
kinetics and isotherms of the electrosorption. The mechanism by which high defluoridation efficiency was achieved by means of Ti-AC electrode was revealed through the measurements of FTIR and XPS.
2. Materials and methods 2.1 Materials Activated carbon was bought from Sinopharm Chemical Regent Co., Ltd. (China). Its average pore volume, specific surface area and pore radius were 0.869 cm3/g, 1487.3 m2/g and 2.34 nm, respectively. All the chemicals used in the present study were of analytical reagent grade. Ultrapure water was used throughout the experiments. 2.2 TiO2-loaded AC preparation AC powder was firstly treated in boiling water for 2 h, and then washed using ultrapure water until the supernatant conductivity was＜20 μS/cm. The precipitate was dried at 80 °C for 24 h in preparation for TiO2 loading. For loading preparation, solution I (6 ml tetrabutyl titanate in 20 ml ethyl alcohol) and solution II (a mixture of 2 ml glacial acetic acid, 0.3 ml hydrochloric acid, 4.5 ml ultrapure water and 10 ml ethyl alcohol) were first prepared, and then 2 g pretreated activated carbon powder was immersed in solution I. Solution II was added dropwise to solution I with AC with magnetic stirring at 30 °C until a white sol-gel formed, then triturated into powder after dried in the oven at 70 °C for 8 h, finally calcinated at 450 °C under a N2 atmosphere for 2 h. The obtained Ti-AC powder was stored in a desiccator. 2.3 Electrode preparation For preparation of electrodes, the slurry was prepared by mixing the activated
carbon or as-obtained Ti-AC powder, conductive carbon black and the binder polyvinylidene fluoride (PVDF) in the weight ratio of 85:5:10 in dimethylacetamide (DMAC) with magnetic stirring for 4 h. After then, the slurry was symmetrically casted onto a graphite sheet (8 × 4 cm) with a blade to form an electrode. The electrodes were dried in a vacuum drying oven at 65 °C for 4 h to volatilize DMAC residual on the electrodes. 2.4 Characterization of the electrode materials The crystallization feature of Ti-AC was determined by X Ray Diffraction (XRD) analysis (D8, Brucker, Germany). A Fourier transform infrared spectroscopy (FTIR) (2000，Thermo Nicolet, America) was used to identify the chemical structure and the functional groups presented on the samples. X-ray photoelectron spectroscopy (XPS) (2000，Thermo Electron VG Multilab, America) for determination of functional groups and the chemicalstate of titanium incorporated into AC surface. 2.5 Electrosorption test Adsorption experiments of sodium fluoride (NaF) on the electrodes were performed using an electric adsorption system, presenting in Fig. 1. The system was consisted of an electric adsorption cell, an external power supply, and a peristaltic pump. Graphite sheets with electrode materials were installed in the polymethyl methacrylate cell which containing 160 ml of NaF solution, and every electrode was spaced at 2 mm intervals to create a flow channel. Physicochemical adsorbed amounts of fluoride were measured without applying electric potential.
Power supply
Electrosorption cell
Peristaltic pump
Fig. 1 Schematic diagram of the electrosorption unit cell A series of NaF solutions with concentrations of 526, 789, 1053, 1579, 2105, 2632, 3158, 3684 μmol/L were passed through the system in the flow rate of 25 ml/min at the direct potential of 1.5 V. The concentration of the 𝐹 − at a given time was recorded using an ultraviolet spectrophotometer (Orion Aquamate 8000, Thermo Fisher Scientific, America), by which defluoridation efficiency can be calculated according to following equation:
qe
(c0 ce ) V 1000 M m
(1)
Where q e is the adsorption capacity of the electrodes, 𝑐0 and 𝑐𝑒 are the initial concentration and the fluoride ions concentration at the duration of time t, respectively. V is the initial volume of solution, m is the electrodes weight used in the electrosorption test and M is the relative molecular mass of fluoride. The adsorption isotherms experiment was conducted with aforementioned concentrations. The adsorption kinetics experiment run at an initial concentration of 526 μmol/L, and the results were recorded at different time intervals until adsorption equilibrium.
3. Results and discussion 3.1 Characterization of electrode materials Fig. 2 shows XRD spectra of AC and Ti-AC powders. AC powder revealed two broad characteristic peaks positioned at 2θ=24.1° and 43.3° corresponding to diffraction from (002) and (100) crystal planes of graphite, respectively. This broad diffraction peaks reflected an amorphous framework and highly disordered of AC [12]. By contrast, Ti-AC powder presented many new narrow and sharp diffraction peaks. The angle 2θ=25.8°,37.8°,48.05°,55.1°,and 62.14°, corresponding to the typical diffraction peak of the (101), (004), (200), (105), and (211) for pure anatase TiO2, indicating the formation of anatase incorporated into AC, the specific surface area analysis shown the TiO2 was porous which consistent with other studies by sol-gel method[13]. T
Ti-AC AC T Anatase TiO2
T
TT
T
Intensity
T
10
20
30
40
50
60
2 Theta
Fig. 2 XRD patterns of Ti-AC and AC powders
3.2 Defluoridation performance
70
Since Ti-AC electrode had polar functional groups and porous structure on its surface, fluoride can be adsorbed on the Ti-AC surface without applying any electric field by –either physically or chemically– adsorption. As shown in the Fig. 3, a small amount of fluoride ions were adsorbed on the Ti-AC electrodes in the open circuit, with a capacity of 18.6 μmol/g. Expectedly, the adsorption performance of Ti-AC electrode was obviously improved by applying a potential, over 4 times higher at 1.5 V than that at open circuit. The significant increase of fluoride adsorption capacity with electric potential can be explained that force of electric drive the fluoride ions toward oppositely charged electrodes and increase the thickness of electric double-layer, which is beneficial for the 𝐹 − adsorption onto Ti-AC electrodes.
adsorption capacity (mol/g)
80
60
40
At potential of 1.5V No electric field 20
0 0
20
40
60
80
Time (min)
Fig. 3 Ti-AC electrodes adsorption performance at a direct voltage of 1.5 V and without electricity, with initial fluoride concentration of 10 ppm
In order to compare adsorption efficiency of the AC and Ti-AC electrode materials, both adsorption kinetics and isotherms of fluoride onto the two electrodes were
investigated in this study. The fluoride adsorption kinetics and isotherms of AC and Ti-AC under the identity conditions were shown in Fig. 4 and Fig. 5, respectively. The adsorption experiments were operated in NaF solution under a flow rate of 25 ml/min and a direct voltage of 1.5 V. The effect of contact time on the amount of fluoride adsorbed by the two electrode materials was studied at initial concentration of 526 μmol/L. Various kinetic models such as pseudo-first-order Eq. (2), pseudo-second-order models Eq. (3) and intra-particle pore diffusion model Eq. (4) were mathematically simulate the kinetics experimental data, the equations are given as: ln(𝑞𝑒− 𝑞𝑡 ) = 𝑙𝑛𝑞𝑒 − 𝑘1 𝑡 𝑡 𝑞𝑡
=𝑘
1 2 2 𝑞𝑒
(2)
𝑡
+𝑞
(3)
𝑒
𝑞𝑡 = 𝐶 + 𝑘𝑛 𝑡 0.5
(4)
Where 𝑞𝑡 is the amount of fluoride adsorbed at time t, 𝑞𝑒 is the amount of fluoride adsorbed at equilibrium, kn is the intra-particle diffusion rate constant, k1 and k2 is the equilibrium rate constants
of pseudo-first-order and pseudo-second-order adsorption
model, respectively. 1.8
6.0
(a)
(b)
4.5
Ti-AC AC
1.2
1.5
t/qt
ln (qe-qt)
3.0
1.5
0.0
0.9
-1.5 0.6
-3.0 -4.5
Ti-AC AC
0.3
0
20
40
Time (min）
60
80
0
15
30
45
Time（ min)
60
75
75
(c)
qt (mol/g)
60
45
30
Ti-AC AC
15
0 3.0
4.5
6.0
t
7.5
9.0
0.5
Fig. 4 Kinetics of fluoride removal by AC and Ti-AC electrodes. The pseudo-first-order (a), pseudo-second-order (b) and intra-particle pore diffusion model (c) sorption kinetics curves were fitted with experimental data The kinetics results of electrosoption by AC and Ti-AC electrodes were illustrated in Fig. 4 and the associated kinetic parameters and regression coefficients (R2) were summarized in table 1. They showed that the experimental results were in better agreement with the pseudo second-order kinetics equation than pseudo first-order equation or intra-particle diffusion model for fluoride sorption on both AC and Ti-AC electrode materials according to the R2. In addition, the experimental adsorption capacity (qe,exp) values of the two materials (45.3, 76.8 μmol/L) both match well with the theoretical adsorption capacity (qe,cal) values calculated from the pseudo second-order model (52.9, 109 μmol/L) rather than that calculated from the pseudo first-order model (84.5, 118μmol/L). The adsorption kinetics results indicate that the electrosorption process of fluoride fits well by pseudo second-order equation and likely accompanied chemisorption.
Table 1 Pseudo-first-order kinetics, pseudo-second-order kinetic and intra-particle pore diffusion model parameters for AC and Ti-AC electrodes. Pseudo-second-order
Intra-particle pore diffusion
kinetics
model
Pseudo-first-order kinetics qe,exp Electrode (μmol/g)
𝑞𝑒
K1
R2
-1
(μmol/g)
(min )
𝑞𝑒 (μmol/g)
K2
R2
-1
(min )
Kn C
R2
(μmol/g·min)
Ti-AC
76.8
118.35
0.0656
0.928
110.51
0.23
−4
0.983
9.4
-0.3637
0.947
AC
45.3
84.53
0.1115
0.925
57.955
0.73
−4
0.989
5.0784
6.9924
0.741
The adsorption isotherm of fluoride on AC and Ti-AC electrodes were illustrated in Fig. 5. Two commonly used models, the Langmuir Eq. (5) and Freundlich Eq. (6) model, were used to stimulate the isotherms. 𝑞𝑒 =
𝑞𝑚 𝑏 𝑒 1+𝑏 𝑒
𝑞𝑒 = 𝐾𝐹 𝑐𝑒 -n
(5) (6)
Where 𝑐𝑒 is the equilibrium concentration, 𝑞𝑒 is the amount of adsorbed 𝐹 − , and 𝑞𝑚 is the maximum adsorption capacity. As showed in Fig. 5 and Table 2, both Langmuir and Freundlich model were fitting well for fluoride sorption on the electrode according to the R2. Notably, the Ti-AC electrode showed a much higher adsorption capacity than the AC electrode. The maximum adsorption capacity on TiO2-loated AC electrode was 157.8 μmol/g, which was twice as high as that on the pristine AC electrode (81.9 μmol/g). The results suggested that the TiO2 loading can largely enhance the AC electrode adsorption capacity.
Electrosorptive capacity (mol/g)
150
120
90
60
Ti-AC AC Langmuir isotherm Freundlich isotherm
30
0 0
500
1000
1500
2000
2500
3000
Equilibrium concentration (mol/L）
Fig. 5 Adsorption isotherms of fluoride on AC and Ti-AC electrodes, the data were fitted with the Langmuir model and Freundlich model
Table 2 Determined parameters and regression coefficients R2, b, and KF of Langmuir and Freundlich isotherms of AC and Ti-AC electrodes with an electric potential of 1.5 V. Value Isotherm
Langmuir
Freundlich
Parameter Ti-AC
AC
qm (μmol/g)
157.8
81.91
b
0.02449
0.00425
R2
0.9842
0.9556
KF
58.26
12.59
n
7.509
4.345
R2
0.9391
0.9864
In order to estimate the efficiency and performance in the process of electrosorption
of fluoride ions on Ti-AC electrode, a comparison with other adsorbents based on carbon, titanium and aluminum was conducted and listed in Table 3. The Ti-AC showed much higher adsorption capacity than that most of the adsorbents based on Ti or Al. Although fluoride adsorption capacity of some adsorbents like bone char[14], carbon nanotubes [15] is higher than that of Ti-AC, the reaction time were many times longer than that in Ti-AC. Thus, Ti-AC as the electrode material for removing fluoride had a larger adsorption capacity at shorter equilibrium time. Moreover, regeneration of adsorbent is usually treated with acid and alkali after adsorption, but electrosorption has the advantage of simply opening circuit or reversing the electrode for generation to increase life of media.
Table 3 Comparison of the adsorption capacity of fluoride in this study with other carbon,
titanium and aluminum-based materials adsorption capacity
Contact
pH adsorbent
references （mg/g）
time
bone char
7
5.2
3h
[14]
activated alumina
7
1.45
25 h
[16]
7
1.88
250 min
[17]
3-4
1.07
_
[18]
carbon nanotubes
7
4.5
180 min
[15]
titanium hydroxide
3
2.8
30 min
[19]
4-7
2.85
12 h
[20]
6.68
157.8 μmol/g=3 mg/g
60 min
this study
zirconium impregnated nut shell carbon aluminum-impregnated carbon
Manganese-oxide-coated alumina Ti-AC
3.3 Effect of pH The initial pH in solution is an important factor influencing the fluoride adsorption performances on Ti-AC, which controls the interface at the electrode materials and water. Hence, the effect of different initial pH values covering a range from 3.2 to 11.32 on the Ti-AC electrode for an initial fluoride concentration of 10 mg/L under a direct voltage of 1.5 V were investigated. As shown in Fig. 6, the adsorption capacity increased with the increasing pH, peaked at pH 9.05 and then sharply decreased when pH further increased. Lower fluoride uptake in the acidic solution can be ascribed to the existence type of
fluoride. When the solution pH is at 2～6, the mainly type of fluoride is 𝐹 − and 𝐻𝐹 2− , but only 𝐹 − in the solution when the pH >6. The Ti-AC electrode had a prior sorption on 𝐹 − , the amount of 𝐹 − increasing with the increasing pH led to a better fluoride uptake. On the other hand, the adsorption capacity decreased when the pH value was above 9.05, which might be attributed to the competition between fluoride and hydroxyl ions on the Ti-AC electrode active adsorption sites. As shown in Fig. 6, Ti-AC as the electrode by electrosorption had a good performance at the pH range of 6-11, and the maximum adsorption capacity was realized at pH 9. As reported, the pH value of groundwater is about 7.5[21]. The adsorption isotherms experiment was run at pH value of 6.68, with the maximum capacity of 3mg/g, larger than most of materials and good enough for ground water treatment.
Adsorption capacity (mol/g)
180
150
120
90
60
30
0 2
4
6
8
10
12
Initial pH
Fig. 6 Electrosorption of fluoride by Ti-AC under different initial pH values with the initial concentration of 10 ppm 3.4 Adsorption mechanism To further learn the adsorption mechanism of fluoride on Ti-AC electrodes, the
materials were further investigated using FTIR and XPS analytical technique. Fig. 7 shows the FTIR spectra of the AC, Ti-AC electrodes before sorption and Ti-AC electrodes after sorption. The peaks around 3433 and 1584 cm−1, were due to the stretching vibration of hydroxyl or carboxyl groups and bending vibration of water molecules [22]. The relative intensities of the peaks at 3433 cm−1 were increased significantly after loading TiO2 as a result of inducing more hydroxyl and carboxyl groups [23, 24]. However, after adsorption, the peak of –OH onto Ti-AC was decreased and shifted, indicating that the hydroxyl group was involved in the fluoride adsorption [25]. The peak at 482 cm−1 in the spectra of Ti-AC was the bending vibration of the bonds of Ti-O, this suggested that TiO2 have a strong chemical binding on the surface of AC, and further indicating the successful loading of TiO2 on activated carbon [26]. It is worth noting that the peak intensity had a significantly decreased after fluoride sorption, suggesting there was a chemical reaction happened between fluoride and Ti-AC material。 Thus, the peaks at 3433 and 482 cm−1 can be considered as the active sites of the Ti-AC electrode for fluoride adsorption. In addition, the FTIR spectra of Ti-AC show obvious absorption bands at 1397, 1184 and 874 cm−1, which represented Ti oxide，C-O group and –F bond bending vibration, respectively[27, 28]. The variation of the three bands after fluoride adsorption indicated the interaction of fluoride with Ti-AC electrodes.
AC Ti-AC Ti-AC after sorption
1397
Transmittance
874
482
1577
3433
3600
3000
2400
1800
1187
1200
600
Wavenumbers (cm-1) Fig. 7 FTIR spectra of pure AC, Ti-AC and Ti-AC after fluoride sorption The Ti-AC materials before and after adsorbed fluoride ions were analyzed by XPS and presented in Fig. 8. A new F1s peak was found at 687.4 eV, which was generally considered as the evidence for the presence of –F bonds formed. The main peak located at 459.11 eV was attributed to the Ti–O group of TiO2. After adsorption, the peak strength of the Ti 2p had a significant decrease, inferring that a Ti–F bond was formed. It was indicated that the fluoride was effective adsorbed on the Ti-AC electrodes [29]. The relative amounts (%) of the atoms, Ti, oxygen (O) and F, were obviously changed after sorption as showed in Table 4. In particularly, the amount of O atom was reduced from 28.48 to14.08 %, so a detailed analysis of the changes of O atom was performed and shown in Fig. 9. The O1s region was composed of two contributions, namely, 530.2 eV assigned to –OH groups and 532.39 eV attributed to O2−on the surface [30-32]. The
proportion of –OH and O2− remarkably changed before and after sorption, as listed in Table 5. After fluoride adsorption, the relative area ratio of O2- increased from 41.2% to 66.2%, whereas the percentage of –OH decreased from 58.8% to 33.8%. The results further confirmed that the high adsorption efficiency of fluoride on Ti-AC can be attributed to hydroxyl groups.
F1s
Intensity
O1s
Ti2p C1s
After sorption Before sorption 150
300
450
600
750
Binding energy (eV)
Fig. 8 XPS spectra of Ti-AC before and after fluoride sorption
Table 4 Atomic ratios of the Ti-AC materials before and after fluoride adsorption. Atomic ratios O
Ti
F
Others
Ti-AC
28.48
8.94
0
62.3
Ti-AC-F
14.08
2.4
12.55
69.94
(%)
(a)
70000
60000
-OH O 2-
28000
Counts
50000
Counts
(b)
32000
-OH O2-
40000
24000
20000
30000 16000
20000 525
530
535
540
12000 525
530
535
540
Binding energy (eV)
Binding energy (eV)
Fig. 9 XPS spectra of the O1s of the Ti-AC electrodes and the fitted distribution of O2and –OH (a) before adsorption; (b) after adsorption
Table 5 The percentage of O2- and –OH distribution from the O1s peak of the Ti-AC electrodes before and after adsorption. Sample
Peak
Binding energy（eV）
Percent (%)
O2-
531.86
41.2
–OH
530.29
58.8
O2-
532.35
66.2
–OH
530.16
33.8
Ti-AC
Ti-AC-F
Based on the above results, the possible mechanism for higher fluoride adsorption efficiency on the Ti-AC electrodes than that on AC electrodes can be hypothesized as the following reactions: ≡ MOH + 𝐻3 𝑂+ + 𝐹 − ⇔≡ 𝑀𝑂𝐻2 + − 𝐹 − + 𝐻2 𝑂
(7)
≡ MOH + 𝐹 − ⇔ MF + O𝐻 −
(8)
Where ≡ M represents the Ti-AC electrodes surface. When the aqueous solution under
acidic conditions, the electrodes adsorbed fluoride with electrostatic force of attraction (Eq. (7)); at alkaline solution, ion-exchange mechanism dominated the fluoride sorption (Eq. (8)) [25]. The interaction between TiO2 and carbon can enhance the content of hydroxyl. Fluoride ions and -OH can isomorphously replace each other through ligand exchange due to they have similar dimensions [33]. Fig. 10 showed the changes in pH values during the process of fluoride adsorption, and the pH value increased from 6.68 to 9.02 after 80 min further confirmed that hydroxide released from the electrode into the solution during the adsorption process, which is alkalescence. The adsorption kinetics experiment result showed the process reached adsorption equilibrium at 60 min, with a pH value of 7.8, which inferred not effect on water quality after treatment.
12
10
pH
8
6
4
2 0
20
40
60
80
Time (min)
Fig. 10 The pH value in the process of fluoride adsorption on Ti-AC
On the other hand, the increased of adsorption capacity of fluoride was attributed to external electric field. When an electric field was applied on the Ti-AC electrodes, many electrons and vacancies were generated in TiO2. Via external circuit the electrons on the
anode move to cathode, so, on the surface of anode were more positive charges and the anode can adsorb more fluoride ions [33]. Given that under the electrosoption condition as shown in Fig. 11, the concentration of fluoride ions on the Ti-AC surface was much higher, and it is the enhancement of fluoride equilibrium concentration on TiO2 surface that led the better performances of chemisorption. Therefore, electrosorption collected fluoride ions in the double layer to promote chemisorption on Ti-AC electrodes.
Fig. 11 Adsorbed fluoride mechanism diagram of used Ti-AC electrode by electrosorption
In order to prove the assumption that ion-exchange happened in the electrosorption process, the adsorption equilibrium data were fitted with Dubinin–Radushkevich (D–R) adsorption equation to determine the mean free energy of the adsorption and the process mechanism onto a heterogeneous surface [34]. The linearized form is expressed as: 𝑙𝑛𝑞𝑒 = 𝑙𝑛𝑞𝑚 − 𝐾𝐷𝑅
𝑅2
𝑇2
1
𝑙𝑛2 ( + ) 𝑒
(10)
Where qe is the fluoride adsorption capacity under different initial concentrations (μmol/g), qm is the equilibrium adsorption capacity (μmol/g), KDR is the activity coefficient related to adsorption free energy (mol2/kJ2), and R and T are the gas constant and temperature (K), respectively. The type of adsorption reaction can be determined by the mean free energy (E, kJ/mol), which is calculated by the following equation: E=
1 √2𝐾𝐷𝑅
(11)
When the E value is less than 8 kJ/mol, it means that adsorption is physical sorption. When the E value which lies between 8-16 kJ/mol, the chemisorption occurs [35]. From the adsorption experimental data of AC fitted by D-R equation, the E obtained was 10.91 kJ/mol, which supporting the assumption that ion-exchange reaction did occur during the electrosorption.
4. Conclusions (1) The TiO2-loaded activated carbon (Ti-AC) achieved two folds of defluoridation higher than the original AC electrode, showing that the electrosorption with Ti-AC as the electrodes might be an effective process for removing fluoride from water. The adsorption kinetics results indicate that the electrosorption process of fluoride fits well by pseudo second-order equation and likely accompanied chemisorption. The sorption followed the Langmuir isotherm with the maximum sorption capacity of 157.8 μmol/g. Furthermore, the adsorption capacity with a potential of 1.5 V was four times higher than that without any electric potential applied. (2) pH study show the adsorption capacity of Ti-AC by this method had a better
performance at pH value 7-9 ascribed to the existence type of fluoride and the competition between fluoride and hydroxyl ions on the Ti-AC electrode active adsorption sites. (3) The mechanism of the sorption might be attributed to that fluoride first entered the electrical double layer of the Ti-AC through electrosorption and greatly increased fluoride equilibrium concentration near the electrode surfaces, followed by the chemisorption of fluoride on the surfaces, leading to the great improvement of the sorption capacity.
Acknowledgements The financial support for this work from the National Natural Science Foundation of China under the project No. 51474167 is gratefully acknowledged.
References:
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S0927-7757(16)30328-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.05.020 COLSUA 20644
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
27-1-2016 29-4-2016 3-5-2016
Please cite this article as: Peng Wu, Ling Xia, Min Dai, Liliang Lin, Shaoxian Song, Electrosorption of Fluoride on TiO2-Loaded Activated Carbon in Water, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.05.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrosorption of Fluoride on TiO2-Loaded Activated Carbon in Water Peng Wu, Ling Xia, Min Dai, Liliang Lin, Shaoxian Song* School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China
* Corresponding author. E-mail: [email protected]
Graphical abstract
Highlights
Presented a promising method for removing fluoride from water. Electrosorption with TiO2-loaded AC achieved a higher defluoridation. Presented a possible mechanism for the elctrosorption of fluoride on Ti-AC
Abstract In this work the electrosorption of fluoride ions on TiO2-loaded activated carbon (Ti-AC) in water has been studied, in order to develop a novel process to remove fluoride from water. The loading of TiO2 on activated carbon (AC) was realized with sol–gel method. The experimental results have shown that the capacity of the electrosorption on the Ti-AC was two folds of that on the original AC. The sorption fitted to the Langmuir isotherm with the maximum capacity of 157.8 μmol/g, and the sorption kinetics followed pseudo-second-order model. The pH study showed the adsorption capacity of Ti-AC by electrosorption had a good performance at pH range of 6-9. By means of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, it was found that the fluoride existed on the Ti-AC surfaces through ligand exchange to replace the -OH group. The mechanism of the sorption might be attributed to that fluoride first entered the electric double layer of the AC through electrosorption and led to the great increase of fluoride equilibrium concentration near the electrode surfaces, followed by the chemisorption of fluoride on the surfaces, which would greatly improve the sorption capacity.
Keywords: Electrosorption; Fluoride; Defluoridation; TiO2-loaded activated carbon; Chemisorption
1. Introduction Fluoride contamination in drinking water has become a worldwide concern in recent years due to its severe threat to public health. There are more than 20 developed and developing nations where people suffered from endemic fluorosis with a fluoride concentration higher than 1.5 mg/L, the guideline limitation for fluoride in drinking water set by World Health Organization [1, 2] . Various technologies and methods have been developed to remove fluoride from water, such as, adsorption, ion exchange, electrodialysis, precipitation and reverse osmosis [3, 4]. Electrosorption is a good desalination process, in which ions were attracted and concentrated in the electrical double layers of electrodes when an electrical potential is applied [5, 6]. It is effective for removing many kinds of ions from brackish waters or other organic pollutants [7-9]. Commonly, activated carbon (AC) is more widely applied to the electrode than other carbon-based materials such as, carbon nanotubes, carbon fiber, carbon aerogel and grapheme due to AC was of high specific surface area, cheap and easy to get [10]. In the case of fluoride removal, AC is not efficient enough, so that a modification is needed. It has been proved that titanium dioxide is a potential selective adsorbent for fluoride due to the high stability, nontoxicity, hydrophilicity and strong affinity with fluoride [11]. Accordingly, the AC with TiO2 loading might be better electrode for the electrosorption of fluoride. In the present work, the electrosorption of fluoride using TiO2-loaded activated carbon (Ti-AC) as the electrode was studied, in order to develop a novel process of removing fluoride from water. This study was performed on the measurements of the
kinetics and isotherms of the electrosorption. The mechanism by which high defluoridation efficiency was achieved by means of Ti-AC electrode was revealed through the measurements of FTIR and XPS.
2. Materials and methods 2.1 Materials Activated carbon was bought from Sinopharm Chemical Regent Co., Ltd. (China). Its average pore volume, specific surface area and pore radius were 0.869 cm3/g, 1487.3 m2/g and 2.34 nm, respectively. All the chemicals used in the present study were of analytical reagent grade. Ultrapure water was used throughout the experiments. 2.2 TiO2-loaded AC preparation AC powder was firstly treated in boiling water for 2 h, and then washed using ultrapure water until the supernatant conductivity was＜20 μS/cm. The precipitate was dried at 80 °C for 24 h in preparation for TiO2 loading. For loading preparation, solution I (6 ml tetrabutyl titanate in 20 ml ethyl alcohol) and solution II (a mixture of 2 ml glacial acetic acid, 0.3 ml hydrochloric acid, 4.5 ml ultrapure water and 10 ml ethyl alcohol) were first prepared, and then 2 g pretreated activated carbon powder was immersed in solution I. Solution II was added dropwise to solution I with AC with magnetic stirring at 30 °C until a white sol-gel formed, then triturated into powder after dried in the oven at 70 °C for 8 h, finally calcinated at 450 °C under a N2 atmosphere for 2 h. The obtained Ti-AC powder was stored in a desiccator. 2.3 Electrode preparation For preparation of electrodes, the slurry was prepared by mixing the activated
carbon or as-obtained Ti-AC powder, conductive carbon black and the binder polyvinylidene fluoride (PVDF) in the weight ratio of 85:5:10 in dimethylacetamide (DMAC) with magnetic stirring for 4 h. After then, the slurry was symmetrically casted onto a graphite sheet (8 × 4 cm) with a blade to form an electrode. The electrodes were dried in a vacuum drying oven at 65 °C for 4 h to volatilize DMAC residual on the electrodes. 2.4 Characterization of the electrode materials The crystallization feature of Ti-AC was determined by X Ray Diffraction (XRD) analysis (D8, Brucker, Germany). A Fourier transform infrared spectroscopy (FTIR) (2000，Thermo Nicolet, America) was used to identify the chemical structure and the functional groups presented on the samples. X-ray photoelectron spectroscopy (XPS) (2000，Thermo Electron VG Multilab, America) for determination of functional groups and the chemicalstate of titanium incorporated into AC surface. 2.5 Electrosorption test Adsorption experiments of sodium fluoride (NaF) on the electrodes were performed using an electric adsorption system, presenting in Fig. 1. The system was consisted of an electric adsorption cell, an external power supply, and a peristaltic pump. Graphite sheets with electrode materials were installed in the polymethyl methacrylate cell which containing 160 ml of NaF solution, and every electrode was spaced at 2 mm intervals to create a flow channel. Physicochemical adsorbed amounts of fluoride were measured without applying electric potential.
Power supply
Electrosorption cell
Peristaltic pump
Fig. 1 Schematic diagram of the electrosorption unit cell A series of NaF solutions with concentrations of 526, 789, 1053, 1579, 2105, 2632, 3158, 3684 μmol/L were passed through the system in the flow rate of 25 ml/min at the direct potential of 1.5 V. The concentration of the 𝐹 − at a given time was recorded using an ultraviolet spectrophotometer (Orion Aquamate 8000, Thermo Fisher Scientific, America), by which defluoridation efficiency can be calculated according to following equation:
qe
(c0 ce ) V 1000 M m
(1)
Where q e is the adsorption capacity of the electrodes, 𝑐0 and 𝑐𝑒 are the initial concentration and the fluoride ions concentration at the duration of time t, respectively. V is the initial volume of solution, m is the electrodes weight used in the electrosorption test and M is the relative molecular mass of fluoride. The adsorption isotherms experiment was conducted with aforementioned concentrations. The adsorption kinetics experiment run at an initial concentration of 526 μmol/L, and the results were recorded at different time intervals until adsorption equilibrium.
3. Results and discussion 3.1 Characterization of electrode materials Fig. 2 shows XRD spectra of AC and Ti-AC powders. AC powder revealed two broad characteristic peaks positioned at 2θ=24.1° and 43.3° corresponding to diffraction from (002) and (100) crystal planes of graphite, respectively. This broad diffraction peaks reflected an amorphous framework and highly disordered of AC [12]. By contrast, Ti-AC powder presented many new narrow and sharp diffraction peaks. The angle 2θ=25.8°,37.8°,48.05°,55.1°,and 62.14°, corresponding to the typical diffraction peak of the (101), (004), (200), (105), and (211) for pure anatase TiO2, indicating the formation of anatase incorporated into AC, the specific surface area analysis shown the TiO2 was porous which consistent with other studies by sol-gel method[13]. T
Ti-AC AC T Anatase TiO2
T
TT
T
Intensity
T
10
20
30
40
50
60
2 Theta
Fig. 2 XRD patterns of Ti-AC and AC powders
3.2 Defluoridation performance
70
Since Ti-AC electrode had polar functional groups and porous structure on its surface, fluoride can be adsorbed on the Ti-AC surface without applying any electric field by –either physically or chemically– adsorption. As shown in the Fig. 3, a small amount of fluoride ions were adsorbed on the Ti-AC electrodes in the open circuit, with a capacity of 18.6 μmol/g. Expectedly, the adsorption performance of Ti-AC electrode was obviously improved by applying a potential, over 4 times higher at 1.5 V than that at open circuit. The significant increase of fluoride adsorption capacity with electric potential can be explained that force of electric drive the fluoride ions toward oppositely charged electrodes and increase the thickness of electric double-layer, which is beneficial for the 𝐹 − adsorption onto Ti-AC electrodes.
adsorption capacity (mol/g)
80
60
40
At potential of 1.5V No electric field 20
0 0
20
40
60
80
Time (min)
Fig. 3 Ti-AC electrodes adsorption performance at a direct voltage of 1.5 V and without electricity, with initial fluoride concentration of 10 ppm
In order to compare adsorption efficiency of the AC and Ti-AC electrode materials, both adsorption kinetics and isotherms of fluoride onto the two electrodes were
investigated in this study. The fluoride adsorption kinetics and isotherms of AC and Ti-AC under the identity conditions were shown in Fig. 4 and Fig. 5, respectively. The adsorption experiments were operated in NaF solution under a flow rate of 25 ml/min and a direct voltage of 1.5 V. The effect of contact time on the amount of fluoride adsorbed by the two electrode materials was studied at initial concentration of 526 μmol/L. Various kinetic models such as pseudo-first-order Eq. (2), pseudo-second-order models Eq. (3) and intra-particle pore diffusion model Eq. (4) were mathematically simulate the kinetics experimental data, the equations are given as: ln(𝑞𝑒− 𝑞𝑡 ) = 𝑙𝑛𝑞𝑒 − 𝑘1 𝑡 𝑡 𝑞𝑡
=𝑘
1 2 2 𝑞𝑒
(2)
𝑡
+𝑞
(3)
𝑒
𝑞𝑡 = 𝐶 + 𝑘𝑛 𝑡 0.5
(4)
Where 𝑞𝑡 is the amount of fluoride adsorbed at time t, 𝑞𝑒 is the amount of fluoride adsorbed at equilibrium, kn is the intra-particle diffusion rate constant, k1 and k2 is the equilibrium rate constants
of pseudo-first-order and pseudo-second-order adsorption
model, respectively. 1.8
6.0
(a)
(b)
4.5
Ti-AC AC
1.2
1.5
t/qt
ln (qe-qt)
3.0
1.5
0.0
0.9
-1.5 0.6
-3.0 -4.5
Ti-AC AC
0.3
0
20
40
Time (min）
60
80
0
15
30
45
Time（ min)
60
75
75
(c)
qt (mol/g)
60
45
30
Ti-AC AC
15
0 3.0
4.5
6.0
t
7.5
9.0
0.5
Fig. 4 Kinetics of fluoride removal by AC and Ti-AC electrodes. The pseudo-first-order (a), pseudo-second-order (b) and intra-particle pore diffusion model (c) sorption kinetics curves were fitted with experimental data The kinetics results of electrosoption by AC and Ti-AC electrodes were illustrated in Fig. 4 and the associated kinetic parameters and regression coefficients (R2) were summarized in table 1. They showed that the experimental results were in better agreement with the pseudo second-order kinetics equation than pseudo first-order equation or intra-particle diffusion model for fluoride sorption on both AC and Ti-AC electrode materials according to the R2. In addition, the experimental adsorption capacity (qe,exp) values of the two materials (45.3, 76.8 μmol/L) both match well with the theoretical adsorption capacity (qe,cal) values calculated from the pseudo second-order model (52.9, 109 μmol/L) rather than that calculated from the pseudo first-order model (84.5, 118μmol/L). The adsorption kinetics results indicate that the electrosorption process of fluoride fits well by pseudo second-order equation and likely accompanied chemisorption.
Table 1 Pseudo-first-order kinetics, pseudo-second-order kinetic and intra-particle pore diffusion model parameters for AC and Ti-AC electrodes. Pseudo-second-order
Intra-particle pore diffusion
kinetics
model
Pseudo-first-order kinetics qe,exp Electrode (μmol/g)
𝑞𝑒
K1
R2
-1
(μmol/g)
(min )
𝑞𝑒 (μmol/g)
K2
R2
-1
(min )
Kn C
R2
(μmol/g·min)
Ti-AC
76.8
118.35
0.0656
0.928
110.51
0.23
−4
0.983
9.4
-0.3637
0.947
AC
45.3
84.53
0.1115
0.925
57.955
0.73
−4
0.989
5.0784
6.9924
0.741
The adsorption isotherm of fluoride on AC and Ti-AC electrodes were illustrated in Fig. 5. Two commonly used models, the Langmuir Eq. (5) and Freundlich Eq. (6) model, were used to stimulate the isotherms. 𝑞𝑒 =
𝑞𝑚 𝑏 𝑒 1+𝑏 𝑒
𝑞𝑒 = 𝐾𝐹 𝑐𝑒 -n
(5) (6)
Where 𝑐𝑒 is the equilibrium concentration, 𝑞𝑒 is the amount of adsorbed 𝐹 − , and 𝑞𝑚 is the maximum adsorption capacity. As showed in Fig. 5 and Table 2, both Langmuir and Freundlich model were fitting well for fluoride sorption on the electrode according to the R2. Notably, the Ti-AC electrode showed a much higher adsorption capacity than the AC electrode. The maximum adsorption capacity on TiO2-loated AC electrode was 157.8 μmol/g, which was twice as high as that on the pristine AC electrode (81.9 μmol/g). The results suggested that the TiO2 loading can largely enhance the AC electrode adsorption capacity.
Electrosorptive capacity (mol/g)
150
120
90
60
Ti-AC AC Langmuir isotherm Freundlich isotherm
30
0 0
500
1000
1500
2000
2500
3000
Equilibrium concentration (mol/L）
Fig. 5 Adsorption isotherms of fluoride on AC and Ti-AC electrodes, the data were fitted with the Langmuir model and Freundlich model
Table 2 Determined parameters and regression coefficients R2, b, and KF of Langmuir and Freundlich isotherms of AC and Ti-AC electrodes with an electric potential of 1.5 V. Value Isotherm
Langmuir
Freundlich
Parameter Ti-AC
AC
qm (μmol/g)
157.8
81.91
b
0.02449
0.00425
R2
0.9842
0.9556
KF
58.26
12.59
n
7.509
4.345
R2
0.9391
0.9864
In order to estimate the efficiency and performance in the process of electrosorption
of fluoride ions on Ti-AC electrode, a comparison with other adsorbents based on carbon, titanium and aluminum was conducted and listed in Table 3. The Ti-AC showed much higher adsorption capacity than that most of the adsorbents based on Ti or Al. Although fluoride adsorption capacity of some adsorbents like bone char[14], carbon nanotubes [15] is higher than that of Ti-AC, the reaction time were many times longer than that in Ti-AC. Thus, Ti-AC as the electrode material for removing fluoride had a larger adsorption capacity at shorter equilibrium time. Moreover, regeneration of adsorbent is usually treated with acid and alkali after adsorption, but electrosorption has the advantage of simply opening circuit or reversing the electrode for generation to increase life of media.
Table 3 Comparison of the adsorption capacity of fluoride in this study with other carbon,
titanium and aluminum-based materials adsorption capacity
Contact
pH adsorbent
references （mg/g）
time
bone char
7
5.2
3h
[14]
activated alumina
7
1.45
25 h
[16]
7
1.88
250 min
[17]
3-4
1.07
_
[18]
carbon nanotubes
7
4.5
180 min
[15]
titanium hydroxide
3
2.8
30 min
[19]
4-7
2.85
12 h
[20]
6.68
157.8 μmol/g=3 mg/g
60 min
this study
zirconium impregnated nut shell carbon aluminum-impregnated carbon
Manganese-oxide-coated alumina Ti-AC
3.3 Effect of pH The initial pH in solution is an important factor influencing the fluoride adsorption performances on Ti-AC, which controls the interface at the electrode materials and water. Hence, the effect of different initial pH values covering a range from 3.2 to 11.32 on the Ti-AC electrode for an initial fluoride concentration of 10 mg/L under a direct voltage of 1.5 V were investigated. As shown in Fig. 6, the adsorption capacity increased with the increasing pH, peaked at pH 9.05 and then sharply decreased when pH further increased. Lower fluoride uptake in the acidic solution can be ascribed to the existence type of
fluoride. When the solution pH is at 2～6, the mainly type of fluoride is 𝐹 − and 𝐻𝐹 2− , but only 𝐹 − in the solution when the pH >6. The Ti-AC electrode had a prior sorption on 𝐹 − , the amount of 𝐹 − increasing with the increasing pH led to a better fluoride uptake. On the other hand, the adsorption capacity decreased when the pH value was above 9.05, which might be attributed to the competition between fluoride and hydroxyl ions on the Ti-AC electrode active adsorption sites. As shown in Fig. 6, Ti-AC as the electrode by electrosorption had a good performance at the pH range of 6-11, and the maximum adsorption capacity was realized at pH 9. As reported, the pH value of groundwater is about 7.5[21]. The adsorption isotherms experiment was run at pH value of 6.68, with the maximum capacity of 3mg/g, larger than most of materials and good enough for ground water treatment.
Adsorption capacity (mol/g)
180
150
120
90
60
30
0 2
4
6
8
10
12
Initial pH
Fig. 6 Electrosorption of fluoride by Ti-AC under different initial pH values with the initial concentration of 10 ppm 3.4 Adsorption mechanism To further learn the adsorption mechanism of fluoride on Ti-AC electrodes, the
materials were further investigated using FTIR and XPS analytical technique. Fig. 7 shows the FTIR spectra of the AC, Ti-AC electrodes before sorption and Ti-AC electrodes after sorption. The peaks around 3433 and 1584 cm−1, were due to the stretching vibration of hydroxyl or carboxyl groups and bending vibration of water molecules [22]. The relative intensities of the peaks at 3433 cm−1 were increased significantly after loading TiO2 as a result of inducing more hydroxyl and carboxyl groups [23, 24]. However, after adsorption, the peak of –OH onto Ti-AC was decreased and shifted, indicating that the hydroxyl group was involved in the fluoride adsorption [25]. The peak at 482 cm−1 in the spectra of Ti-AC was the bending vibration of the bonds of Ti-O, this suggested that TiO2 have a strong chemical binding on the surface of AC, and further indicating the successful loading of TiO2 on activated carbon [26]. It is worth noting that the peak intensity had a significantly decreased after fluoride sorption, suggesting there was a chemical reaction happened between fluoride and Ti-AC material。 Thus, the peaks at 3433 and 482 cm−1 can be considered as the active sites of the Ti-AC electrode for fluoride adsorption. In addition, the FTIR spectra of Ti-AC show obvious absorption bands at 1397, 1184 and 874 cm−1, which represented Ti oxide，C-O group and –F bond bending vibration, respectively[27, 28]. The variation of the three bands after fluoride adsorption indicated the interaction of fluoride with Ti-AC electrodes.
AC Ti-AC Ti-AC after sorption
1397
Transmittance
874
482
1577
3433
3600
3000
2400
1800
1187
1200
600
Wavenumbers (cm-1) Fig. 7 FTIR spectra of pure AC, Ti-AC and Ti-AC after fluoride sorption The Ti-AC materials before and after adsorbed fluoride ions were analyzed by XPS and presented in Fig. 8. A new F1s peak was found at 687.4 eV, which was generally considered as the evidence for the presence of –F bonds formed. The main peak located at 459.11 eV was attributed to the Ti–O group of TiO2. After adsorption, the peak strength of the Ti 2p had a significant decrease, inferring that a Ti–F bond was formed. It was indicated that the fluoride was effective adsorbed on the Ti-AC electrodes [29]. The relative amounts (%) of the atoms, Ti, oxygen (O) and F, were obviously changed after sorption as showed in Table 4. In particularly, the amount of O atom was reduced from 28.48 to14.08 %, so a detailed analysis of the changes of O atom was performed and shown in Fig. 9. The O1s region was composed of two contributions, namely, 530.2 eV assigned to –OH groups and 532.39 eV attributed to O2−on the surface [30-32]. The
proportion of –OH and O2− remarkably changed before and after sorption, as listed in Table 5. After fluoride adsorption, the relative area ratio of O2- increased from 41.2% to 66.2%, whereas the percentage of –OH decreased from 58.8% to 33.8%. The results further confirmed that the high adsorption efficiency of fluoride on Ti-AC can be attributed to hydroxyl groups.
F1s
Intensity
O1s
Ti2p C1s
After sorption Before sorption 150
300
450
600
750
Binding energy (eV)
Fig. 8 XPS spectra of Ti-AC before and after fluoride sorption
Table 4 Atomic ratios of the Ti-AC materials before and after fluoride adsorption. Atomic ratios O
Ti
F
Others
Ti-AC
28.48
8.94
0
62.3
Ti-AC-F
14.08
2.4
12.55
69.94
(%)
(a)
70000
60000
-OH O 2-
28000
Counts
50000
Counts
(b)
32000
-OH O2-
40000
24000
20000
30000 16000
20000 525
530
535
540
12000 525
530
535
540
Binding energy (eV)
Binding energy (eV)
Fig. 9 XPS spectra of the O1s of the Ti-AC electrodes and the fitted distribution of O2and –OH (a) before adsorption; (b) after adsorption
Table 5 The percentage of O2- and –OH distribution from the O1s peak of the Ti-AC electrodes before and after adsorption. Sample
Peak
Binding energy（eV）
Percent (%)
O2-
531.86
41.2
–OH
530.29
58.8
O2-
532.35
66.2
–OH
530.16
33.8
Ti-AC
Ti-AC-F
Based on the above results, the possible mechanism for higher fluoride adsorption efficiency on the Ti-AC electrodes than that on AC electrodes can be hypothesized as the following reactions: ≡ MOH + 𝐻3 𝑂+ + 𝐹 − ⇔≡ 𝑀𝑂𝐻2 + − 𝐹 − + 𝐻2 𝑂
(7)
≡ MOH + 𝐹 − ⇔ MF + O𝐻 −
(8)
Where ≡ M represents the Ti-AC electrodes surface. When the aqueous solution under
acidic conditions, the electrodes adsorbed fluoride with electrostatic force of attraction (Eq. (7)); at alkaline solution, ion-exchange mechanism dominated the fluoride sorption (Eq. (8)) [25]. The interaction between TiO2 and carbon can enhance the content of hydroxyl. Fluoride ions and -OH can isomorphously replace each other through ligand exchange due to they have similar dimensions [33]. Fig. 10 showed the changes in pH values during the process of fluoride adsorption, and the pH value increased from 6.68 to 9.02 after 80 min further confirmed that hydroxide released from the electrode into the solution during the adsorption process, which is alkalescence. The adsorption kinetics experiment result showed the process reached adsorption equilibrium at 60 min, with a pH value of 7.8, which inferred not effect on water quality after treatment.
12
10
pH
8
6
4
2 0
20
40
60
80
Time (min)
Fig. 10 The pH value in the process of fluoride adsorption on Ti-AC
On the other hand, the increased of adsorption capacity of fluoride was attributed to external electric field. When an electric field was applied on the Ti-AC electrodes, many electrons and vacancies were generated in TiO2. Via external circuit the electrons on the
anode move to cathode, so, on the surface of anode were more positive charges and the anode can adsorb more fluoride ions [33]. Given that under the electrosoption condition as shown in Fig. 11, the concentration of fluoride ions on the Ti-AC surface was much higher, and it is the enhancement of fluoride equilibrium concentration on TiO2 surface that led the better performances of chemisorption. Therefore, electrosorption collected fluoride ions in the double layer to promote chemisorption on Ti-AC electrodes.
Fig. 11 Adsorbed fluoride mechanism diagram of used Ti-AC electrode by electrosorption
In order to prove the assumption that ion-exchange happened in the electrosorption process, the adsorption equilibrium data were fitted with Dubinin–Radushkevich (D–R) adsorption equation to determine the mean free energy of the adsorption and the process mechanism onto a heterogeneous surface [34]. The linearized form is expressed as: 𝑙𝑛𝑞𝑒 = 𝑙𝑛𝑞𝑚 − 𝐾𝐷𝑅
𝑅2
𝑇2
1
𝑙𝑛2 ( + ) 𝑒
(10)
Where qe is the fluoride adsorption capacity under different initial concentrations (μmol/g), qm is the equilibrium adsorption capacity (μmol/g), KDR is the activity coefficient related to adsorption free energy (mol2/kJ2), and R and T are the gas constant and temperature (K), respectively. The type of adsorption reaction can be determined by the mean free energy (E, kJ/mol), which is calculated by the following equation: E=
1 √2𝐾𝐷𝑅
(11)
When the E value is less than 8 kJ/mol, it means that adsorption is physical sorption. When the E value which lies between 8-16 kJ/mol, the chemisorption occurs [35]. From the adsorption experimental data of AC fitted by D-R equation, the E obtained was 10.91 kJ/mol, which supporting the assumption that ion-exchange reaction did occur during the electrosorption.
4. Conclusions (1) The TiO2-loaded activated carbon (Ti-AC) achieved two folds of defluoridation higher than the original AC electrode, showing that the electrosorption with Ti-AC as the electrodes might be an effective process for removing fluoride from water. The adsorption kinetics results indicate that the electrosorption process of fluoride fits well by pseudo second-order equation and likely accompanied chemisorption. The sorption followed the Langmuir isotherm with the maximum sorption capacity of 157.8 μmol/g. Furthermore, the adsorption capacity with a potential of 1.5 V was four times higher than that without any electric potential applied. (2) pH study show the adsorption capacity of Ti-AC by this method had a better
performance at pH value 7-9 ascribed to the existence type of fluoride and the competition between fluoride and hydroxyl ions on the Ti-AC electrode active adsorption sites. (3) The mechanism of the sorption might be attributed to that fluoride first entered the electrical double layer of the Ti-AC through electrosorption and greatly increased fluoride equilibrium concentration near the electrode surfaces, followed by the chemisorption of fluoride on the surfaces, leading to the great improvement of the sorption capacity.
Acknowledgements The financial support for this work from the National Natural Science Foundation of China under the project No. 51474167 is gratefully acknowledged.
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