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Direct removal of aqueous As(III) and As(V) by amorphous titanium dioxide nanotube arrays Shaolin Wu Lixia Yang
a b
a b
, Wentao Hu
, Xinman Tu
a b
a b
, Xubiao Luo
a b
& Guisheng Zeng
, Fang Deng
a b
c
, Kai Yu , Shenglian Luo
a b
,
a b
a
Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, PR China b
College of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang, PR China c
Institute of Wastes and Soil Environment, Shanghai Academy of Environmental Sciences, Shanghai, China Published online: 11 Feb 2013.
To cite this article: Shaolin Wu , Wentao Hu , Xubiao Luo , Fang Deng , Kai Yu , Shenglian Luo , Lixia Yang , Xinman Tu & Guisheng Zeng (2013): Direct removal of aqueous As(III) and As(V) by amorphous titanium dioxide nanotube arrays, Environmental Technology, DOI:10.1080/09593330.2013.765923 To link to this article: http://dx.doi.org/10.1080/09593330.2013.765923
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.765923
Direct removal of aqueous As(III) and As(V) by amorphous titanium dioxide nanotube arrays Shaolin Wua,b , Wentao Hua,b , Xubiao Luoa,b∗ , Fang Denga,b , Kai Yuc , Shenglian Luoa,b∗ , Lixia Yanga,b , Xinman Tua,b and Guisheng Zenga,b a Key
Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, PR China; b College of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang, PR China; c Institute of Wastes and Soil Environment, Shanghai Academy of Environmental Sciences, Shanghai, China
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(Received 15 September 2012; final version received 7 January 2013 ) Amorphous titanium dioxide nanotube arrays (TiO2 NTs) were prepared by a simple anodization process without subsequent calcination at high temperature, and the effectiveness of amorphous TiO2 NTs as adsorbents in removing arsenite (As(III)) and arsenate (As(V)) was investigated. The TiO2 NTs were not only effective for arsenic removal without a pre-oxidation of As(III) to As(V) and/or adjusting the pH value of water before the adsorption process, but also can be separated and recovered easily from the solution. The adsorption kinetics and adsorption capacity of the amorphous TiO2 NTs for As(III) and As(V) were studied separately by batch experiments. The apparent values for Langmuir monolayer sorption capacities were 28.9 mg/g for As(III) and 24.7 mg/g for As(V) at pH 7. Kinetics studies indicated that the adsorption process on TiO2 NTs followed a pseudo-second-order kinetics model. Arsenic adsorption of TiO2 NTs remains stable over a broad pH range. Moreover, the TiO2 NTs have excellent stability and regeneration, and they can be used repeatedly at least five times. Keywords: adsorption; titanium dioxide nanotubes array; arsenic; regeneration
1. Introduction Arsenic, a ubiquitous metalloid that is widely found in soils and groundwater, has endangered human health and attracted great concern because of its toxic and carcinogenic properties [1]. In natural waters, inorganic arsenic occurs primarily as arsenite (As(III)) and arsenate (As(V)). At neutral pH levels, the dominant As(III) exists mainly as non-ionic H3 AsO3 , while the primary As(V) species are 2− the monovalent (H2 AsO− 4 ) and divalent (HAsO4 ). Chronic arsenic poisoning can cause various types of cancer and black foot disease through either contaminated drinking water or agriculture products irrigated by contaminated water [2]. In order to reduce the health risk, the US Environmental Protection Agency (USEPA) revised the guideline for drinking water in which the maximum contaminant level for arsenic was reduced from 50 μg/L to 10 μg/L in 2001 and required this standard to be achieved from 2006 [3]. Therefore, an effective technology for arsenic removal is urgently required to provide safe drinking water for affected people. Until the present time, several techniques for removing arsenic from contaminated water have been developed, such as oxidation/precipitation [4,5], coagulation/coprecipitation [6,7], adsorption [8], ion exchange [9] and membrane filtration [10–13]. Among the above techniques, adsorption is considered to be one of the most promising methods ∗ Corresponding
for arsenic removal from aqueous solution because it is effective, efficient and economic for water treatment [14]. A large number of adsorbents have been widely used in arsenic removal, such as activated carbons, activated alumina, oxides/hydroxides, tombarthite, zeolite clay minerals and biosorbents [15]. However, As(III) is generally reported to have low affinity to the surface of various adsorbents, and it is also difficult to remove using these conventional adsorbents such as activated carbon or activated alumina when compared with As(V). Moreover, As(III) is significantly more toxic and mobile than As(V). Therefore, if the conventional adsorbents are used for arsenic removal, it is necessary to oxidize As(III) to As(V) and/or adjust the pH value of water before the adsorption process for effective arsenic removal from water. After the adsorption process, the pH of treated water requires to be readjusted back to neutral. So it is worthwhile to develop an effective adsorbent for As(III) removal without a pre-oxidation/pH adjustment, which could greatly simplify the process of treatment and cut down the cost of treatment. At present, titanium dioxide (TiO2 ), commonly used for environmental applications, has attracted considerable attention due to its good adsorption activity, physical and chemical stability, non-toxicity, resistance to corrosion, commercial availability and inexpensiveness [16,17]. Studies have demonstrated that TiO2 exhibits high arsenic
authors. Emails: [email protected], [email protected]
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removal capability. For example, Pirilä et al. [18] reported that hydrous TiO2 is effective for arsenic removal from aqueous solution. Jing et al. [19] researched a nanocrystalline TiO2 -based adsorbent for the simultaneous removal of As(V), As(III), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in contaminated groundwater. In addition, Yao et al. [20] had synthesized a composite photocatalyst for arsenic removal by loading titanium dioxide onto activated carbon fibre (TiO2 /ACF). Bang et al. [21] reported that a novel granular TiO2 can effectively remove arsenic from groundwater. The above research and several other studies [22,23] have proved the potential of TiO2 for arsenic removal. However, practical applications of TiO2 powers are limited by the tedious process of separation after the treatment process. There is now a creative way to solve this problem, which is to prepare highly ordered TiO2 nanotube arrays (TiO2 NTs) on titanium (Ti) substrates. It has been reported that anodic oxidation is a simple, cheap and straight-forward method to prepare highly-ordered TiO2 NTs on Ti substrates. Because the TiO2 NTs grow from the Ti substrate, the TiO2 NTs can be easily separated and recovered in practical application. To the authors’ knowledge, however, no work has been reported on the research of TiO2 NTs as an adsorbent for arsenic removal from water. In the present study, amorphous TiO2 NTs were prepared by a simple anodization process without subsequent calcination at high temperature. The adsorption kinetics and adsorption capacity of the amorphous TiO2 NTs for As(III) and As(V) were studied separately by batch experiments. Moreover, the effect of pH on the adsorption capacity of amorphous TiO2 NTs for As(V) and As(III) was investigated. The TiO2 NTs not only effectively removed As(V) and As(III), especially As(III) without pre-oxidation/pH adjustment, but can also be removed easily from the aqueous solution after treatment.
2. Experimental methods 2.1. Materials All chemicals were analytical grade except hydrochloric acid (Guaranteed reagent, Shantou Xilong Chemical Co. Ltd, Shantou, China), and all stock solutions were prepared with ultrapure water which was produced using a Milli-Q water system (EMD Millipore Corporation, Bedford, MA, USA). Stock solutions of As(III) and As(V) (1000 mg/L) were prepared using NaAsO2 (Beijing Chemicals Corporation, PR China) and Na2 HAsO4 ·12H2 O (Beijing Chemicals Corporation, PR China), respectively. These stock solutions were further diluted for experimental use.
2.2.
Preparation of TiO2 NTs Titanium foil (250 μm thick, 99.8%; Sigma Aldrich, St. Louis, MO, USA) was cut into 1.0 × 4.0 cm strips. Strips
were ultrasonically cleaned in ethanol and then in acetone for 5 min. These strips were anodized at 30 V for 2 h in a two-electrode electrochemical cell with titanium foil anode and platinum foil cathode in an ethylene glycol solution containing 0.3 wt% NH4 F and 1% volume H2 O [24]. 2.3.
Characterization of the TiO2 NTs The morphology of TiO2 NTs was observed on a SSX-550 SEM instrument (Shimadzu, kyoto, Japan). The specific surface area and pore of the TiO2 NTs were determined by NOVA 2000e surface area and pore size analyzer (Quantachrome, Boynton Beach, FL, USA). The zeta potential of TiO2 NTs was measured by SurPASS electrokinetic analyzer for solid samples (Anton Paar, Austria). The composition of adsorbents was analyzed by X-ray powder diffractometer (XRD, Rigaku D/Max-B, Cu Kα, Tokyo, Japan). 2.4.
Adsorption kinetics
All experiments were conducted for As(III) or As(V) separately unless otherwise indicated. Batch sorption experiments were conducted to investigate the adsorption kinetics. After the pH of 1.0 mg/L arsenic solution (either As(III) or As(V)) was adjusted to 7.0 with 0.5 mol/L hydrochloric acid or sodium hydroxide, 10 mL arsenic solution and 9 mg of TiO2 (two pieces of TiO2 NTs) were added into each 100 mL conical flask. The conical flasks were oscillated in a thermostatic oscillator (Changzhou Guohua Instrument Company, Changzhou, China) at 25◦ C for different times (0.25 h, 0.5 h, 0.75 h, 1 h, 1.5 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, 10 h, 12 h, 18 h and 24 h), then the TiO2 NTs were immediately taken from the solution. The solution was acidified using HCl, and the total soluble arsenic concentrations were determined by an atomic fluorescence spectrometer (AFS8220) coupled with a hydride generator. To detect As(III) selectively, the working solution was prepared with a mixture of 2% KBH4 and 0.5% KOH as reducing solution and 5% hydrochloric acid as carrier solution. Under these conditions, only As(III) was converted to AsH3 and detected on an AFS instrument (Beijing Jitian Instrument Company, Beijing, China). 2.5.
Effect of pH on arsenic adsorption
Arsenic solution (10 mL 1.0 mg/L) (either As(III) or As(V)) with specific initial pH ranging from 3 to 11 and 9 mg of TiO2 (two pieces of TiO2 NTs) were placed in conical flasks. The samples were oscillated in a thermostatic oscillator for 12 h and analyzed in similar fashion to the adsorption kinetics studies described above. 2.6.
Adsorption isotherms
In the investigation of adsorption isotherms, the initial arsenic concentrations ranged from 0.5 mg/L to 30 mg/L,
Environmental Technology and pH values were adjusted to 7.0 by adding HCl and NaOH solutions. Arsenic solution (10 mL) (either As(III) or As(V)) with specific initial concentrations and 9 mg of TiO2 (two pieces of TiO2 NTs) were placed in conical flasks. The samples were oscillated in a thermostatic oscillator (Changzhou Guohua Instrument Company, Changzhou, China) at 25◦ C for 12 h and analyzed in similar fashion to the adsorption kinetics studies described above. The equilibrium adsorption capacity (qe (mg/g)) was calculated using the following equation: (c0 − ce ) × V mads
(1)
where c0 and ce are the initial and equilibrium concentration (mg/L) of arsenic in solution, respectively, V is the total volume of the solution and mads is the mass of the TiO2 adsorbent. 2.7. Desorption experiment The desorption study is very important since the regeneration of adsorbent decides the economic success of the adsorption process. In this study, 2 mol/L NaOH was used as eluent to regenerate the TiO2 NTs. Nine mg of TiO2 (two pieces of TiO2 NTs) was added into 10 mL of 1.0 mg/L As(III) or As(V) solution at pH 7.0, and the flask was oscillated in a thermostatic oscillator at 25◦ C for 12 h. After adsorption, the as-loaded TiO2 NTs were taken from the solution and rinsed with water. In the desorption test, the saturated TiO2 NTs were immersed in 2 mol/L NaOH solution and oscillated for 1 h in a thermostatic oscillator. The above procedure was repeated many times until arsenic could not be detected in the eluent. After the desorption process, the TiO2 NTs were taken from the solution and the arsenic concentration in the eluent was analyzed. The regenerated TiO2 NTs were washed with hydrochloric acid and de-ionized water until neutral pH was reached, and then the TiO2 NTs and dried at 50◦ C for reuse in the next cycle. The adsorption–desorption cycle were repeated five times, and the adsorption capacity for arsenic in each adsorption experiment was calculated.
and (102) planes of Ti, respectively. Because TiO2 NTs are amorphous, diffraction peaks are not present in the XRD patterns. The surface area and pore size are crucial from the view of the adsorption, because higher surface area and proper pore size often increase adsorption capacity. In our case, the specific surface area of TiO2 NTs is 49.5 m2 /g and the pore diameter is about 4 nm. The zeta-potential of amorphous TiO2 NTs is shown in the supplementary material (Figure S3). The isoelectric point (IEP) of these TiO2 NTs is determined at about pH 5.39, which is larger than the IEP value of TiO2 •xH2 O nanoparticles (pH 3.8) in the literature [3] due to the well-ordered structure of TiO2 NTs. In a near-neutral environment TiO2 NTs are negatively charged with a zeta potential of about −20.53 mV, indicating the existence of the surface hydroxyl groups.
3.2. Adsorption kinetics Adsorption kinetics are one of the most important characters that represent adsorption efficiency. Figure 1 shows the dynamic curves for the adsorption of As(III) and As(V) on the amorphous TiO2 NTs at pH 7.0. It can be seen from Figure 1 that initial adsorption of arsenic is rapid, then adsorption becomes slow until adsorption equilibrium is reached, and arsenic removal efficiency is approximately 99%. Moreover, the adsorption of As(V) and As(III) on TiO2 NTs requires 180 min and 240 min to reach equilibrium, respectively, which indicates that the removal of As(V) was faster than As(III). The results were in agreement with those reported by Bang et al. [21]. Longer contact time would be required for the effective removal of As(III) in comparison with As(V) due to the slower rate of As(III) adsorption. Based on the results of adsorption kinetics, 12 h was used as the contact time in the other adsorption experiments.
100
90 25
The surface morphologies of the TiO2 NTs are shown in the supplementary material (Figure S1). The TiO2 nanotube arrays are composed of high-density, well-ordered, vertically-oriented and uniform TiO2 nanotubes. The length of TiO2 NTs is 4 μm, the pore sizes range from 90–100 nm and wall thickness is about 20 nm. XRD patterns of TiO2 NTs are shown in the supplementary material (Figure S2). The spectrum of Ti foil is shown in the figure as a reference. The dominant peaks at 2θ about 38.3◦ , 40.1◦ and 52.8◦ represent the indices of (002), (101)
80
t /qt (h g/mg)
3. Results and discussion 3.1. The characterization of TiO2 NTs
Removal (%)
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qe =
3
70
20 15 10
As(III) As(IV)
5
60
0
As(III) As(IV)
0
4
8
12 t (h)
16
20
24
50 0
4
8
12 t (h)
16
20
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Figure 1. Adsorption kinetics of As(III) and As(V) on TiO2 NTs (9 mg TiO2 ). Initial arsenic concentration is 1 mg/L. Inner illustration is the pseudo-second-order rate kinetic model fitted to the adsorption kinetics studies.
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Table 1. Kinetic parameters of the pseudo-first-order and pseudo-second-order rate equation for As(III) and As(V) adsorption on TiO2 NTs.
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qe k1 (mg/g) (L/min) As(III) 0.5644 As(V) 0.0189
R2
Pseudo-second-order qe k2 (mg/g) ((g/mg)/h)
0.0082 0.9360 1.1138 0.0042 0.3842 1.067
2.46 50.082
R2 0.9999 1.0000
Removal (%)
90
Pseudo-first-order
80
70 As(III) As(IV)
60
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To determine the mechanism of the adsorption process of TiO2 NTs, the pseudo-first-order and pseudo-secondorder equations were used to fit the experiment data. The pseudo-first-order kinetic equation [25] is defined by Equation (2): ln(qe − qt ) = ln qe − k1 t
(2)
where qe and qt are the amount of arsenic adsorbed at equilibrium and at any time t, respectively (mg/g) and k1 is the constant rate of pseudo-first-order adsorption (L/min). The pseudo-second-order rate expression [26] is defined by Equation (3) and its integrated form is given in Equation (4): dqt = Kad (qe − qt )2 dt t 1 t = + 2 qt k2 q e qe
(3) (4)
where qe and qt are the amount (mg/g) of arsenic adsorbed at equilibrium and at time t, respectively, and k2 is the rate constant of adsorption (g/(mg min)). The kinetics parameters that are obtained by fitting the experimental data are summarized in Table 1. The value of k1 that was used to calculate the slope of the curve is 0.0082 L/min for As(III) and 0.0042 L/min for As(V) (curve not shown), and the correlation coefficients (R2 ) of As(III) and As(V) are 0.9360 and 0.3842, respectively. Moreover, the experimental value did not agree with the calculated value. These results indicated that the adsorption dynamics of arsenic on TiO2 NTs did not accord with the first-order kinetic model. As demonstrated in Figure 1 and Table 1, the experimental results fit well with the pseudo-second-order rate kinetic model through the data analysis. The applicability of the pseudo-second-order rate model was estimated by the square of the correlation coefficient (R2 ), and the R2 from the curve (insert, Figure 1) was close to 1, indicating that experimental data fit well with the model. In our study, the initial concentration of As(III) and As(V) solution was 1.0 mg/L and the pH value of the solution was 7.0, 0.9 g/L of TiO2 was used as adsorbent in each conical flask, the k2 was 2.46 mg−1 g h−1 and 53.082 mg−1 g h−1 for As(III) and As(V), respectively. Although the experimental conditions are different from those of Xu et al. [3] and Pena et al. [27],
2
3
4
5
6
7 pH
8
9
10
11
Figure 2. Removal of arsenic as a function of initial pH on TiO2 NTs (9 mg TiO2 ). Initial arsenic concentration is 1 mg/L.
we can conclude that TiO2 NTs are effective for arsenic removal. 3.3.
Effect of pH on As adsorption
The effect of pH on As(III) and As(V) removal is shown in Figure 2. The results indicated that arsenic removal of As(III) and As(V) were similar in a pH range between 3 and 7, and arsenic removal efficiency is very high, close to 100%. When pH value ranges from 7 to 10, the removal efficiency of As(V) and As(III) decreased slowly to 95% and 90%, respectively. When pH is above 10, a steep decrease occurred in the removal of As(III) and As(V). It can be concluded from the above results that TiO2 NTs can effectively remove As(III) and As(V) in pH range 3–10. Similar arsenic adsorption behaviour was observed when nanocrystalline titanium dioxide [19] and granular titanium dioxide [21] were used as arsenic adsorbent. The possible reason for high adsorption capacity of TiO2 NTs for As(V) and As(III) in acidic solution and decrease of adsorption capacity at pH above 7.0 is as follows. (1) The hydroxyl groups on the adsorbent can be protonated in acidic solution and become positive-charged, the negative-charged H2 AsO− 4 and HAsO2− 4 can be adsorbed on the positive sites on TiO2 NTs via electrostatic attraction. (2) The Ti-OH groups on the TiO2 NTs surface can bind As(V) and As(III) through the formation of monodentate and bidentate complexes between As−OH and Ti−OH [28–30]. 3.4. Adsorption isotherms Adsorption isotherms are important factors that depict how the solutes interact with adsorbents. The adsorption equilibrium experiments of As(III) and As(V) onto TiO2 NTs were carried out under near-neutral pH conditions, and the adsorption isotherms are shown in Figure 3. The adsorption data were fitted with both the Langmuir isotherm and the
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Table 2. The parameters of Langmuir and Freundlich isotherms for adsorption of As(III) and As(V) on TiO2 NTs. Langmuir isotherm qe KL (mg/g) (L/mg)
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As(III) As(V)
28.9 24.7
R2
0.1117 0.9626 4.4562 0.9857
Freundlich isotherm KF ((mg/g)1−1/n ) 4.0494 16.6177
n
R2
1.7274 0.9857 3.6576 0.8975
Figure 4. Stability and potential regeneration of TiO2 NTs. Initial arsenic concentration is 1.0 mg/L.
Figure 3. Equilibrium adsorption isotherms (equilibrium adsorption capacity qe as a function of liquid-phase equilibrium concentration (Ce ) for (a) As(III); (b) As(V).
Freundlich isotherm [31,32] as given in Equations (5) and (6), respectively: qe =
qmax KL Ce 1 + K L Ce
(5)
1
qe = KF × Cen
(6)
where qe is the amount (mg/g) of As(III) and As(V) adsorbed at equilibrium, Ce is the equilibrium As(III) and As(V) concentration (mg/L) in water samples, KL and qmax (maximum adsorption capability) are the Langmuir constants of adsorption, and KF and n are the Freundlich constants of adsorption. The parameters obtained by fitting the experimental data are summarized in Table 2. The applicability of two isotherm models was evaluated through the square of the correlation coefficient R (R2 ). It is clear that the adsorption data could be fitted well with the Freundlich isotherm for As(III) because the R2 fitted with this model is close to 1, compared with that fitted by the Langmuir isotherm. It indicated that the adsorption of As(III) was a multi-site adsorption process due to the heterogeneity of the surface.
On the contrary, the adsorption data of As(V) fitted well with the Langmuir isotherm, compared with that fitted by the Freundlich isotherm. The results indicated that it tended to monolayer adsorption for As(V). Moreover, the maximum adsorption capacities (qmax ) of As(III) and As(V) were calculated to be 28.9 and 24.7 mg/g for the Langmuir model, respectively. This is close to the experimental values. 3.5.
Desorption and repeated use
The regeneration of the adsorbent is likely to be a key factor in estimating the wastewater process economics. The effect of desorption time on TiO2 NTs desorption efficiency was observed using 10 mL 2 M NaOH solutions as eluent. Adsorption-desorption results from using TiO2 NTs repeatedly are shown in Figure 4. Results illustrated that the TiO2 NTs can be used repeatedly at least five times without significant decrease of their adsorption capacities. The results indicated that the TiO2 NTs had excellent regeneration ability. 4.
Conclusions
The amorphous TiO2 NTs were prepared by a simple anodization process. The TiO2 NTs adsorbent exhibits a high adsorption capacity for both As(III) and As(V) over a broad pH range, and it is especially effective for As(III) removal without a pre-oxidation of As(III) to As(V) and/or
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the pH adjustment of water before the adsorption process. Moreover, TiO2 NTs can be separated and recovered easily from the aqueous solution, and can be used at least five times without an obvious decrease of adsorption capacities. Acknowledgements
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This work was financially supported by Natural Science Foundation of China (50978132, 51178213, 51238002, 51272099, 51008149), Program for New Century Excellent Talents in University (NCET-11-1004), Cultivating Program for Young Scientists of Jiangxi Province of China (20112BCB23016), Natural Science Foundation of Jiangxi Province (20122BAB213014, 20114BAB203018), National High Technology Research and Development Program of China (2009AA062905, 2011AA0606 04) and Department of Education Fund of Jiangxi Province (Grant GJJ11508).
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[13] Iqbal J, Kim HJ, Yang JS, Baek K, Yang JW. Removal of arsenic from groundwater by micellar-enhanced ultrafiltration (MEUF). Chemosphere. 2007;66:970–976. [14] Qu JH. Research progress of novel adsorption processes in water purification: a review. J Environ Sci. 2008;20:1–13. [15] Cullen WR, Reimer KJ. Arsenic speciation in the environment. Chem Rev. 1989;89:713–764. [16] Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem. 2004;32:33–177. [17] Hung WC, Fu SH, Tseng JJ, Chu H, Ko TH. Study on photocatalytic degradation of gaseous dichloromethane using pure and iron ion-doped TiO2 prepared by the sol–gel method. Chemosphere. 2007;66:2142–2151. [18] Pirilä M, Martikainen M, Ainassaari K, Kuokkanen T, Keiski RL. Removal of aqueous As(III) and As(V) by hydrous titanium dioxide. J Coll Interf Sci. 2011;353:257–262. [19] Jing CY, Meng XG, Calvache E, Jiang G. Remediation of organic and inorganic arsenic contaminated groundwater using a nanocrystalline TiO2 -based adsorbent. Environ Pollut. 2009;157:2514–2519. [20] Yao SH, Jia YF, Shi ZL, Zhao SL. Photocatalytic oxidation of arsenite by a composite of titanium dioxide and activated carbon fiber. Photochem Photobiol. 2010;86:1215–1221. [21] Bang S, Patel M, Lippincott L, Meng XG. Removal of arsenic from groundwater by granular titanium dioxide adsorbent. Chemosphere. 2005;60:389–397. [22] Nabi D, Aslam I, Qasi IA. Evaluation of the adsorption potential of titanium dioxide nanoparticles for arsenic removal. J Environ Sci. 2009;21:402–408. [23] Jing C, Meng X, Liu S, Baidas S, Patraju R, Christodoulatos C, Korfiatis GP. Surface complexation of organic arsenic on nanocrystalline titanium oxide. J Coll Interf Sci. 2005;290:14–21. [24] Yang LX, Luo SL, Li Y, Xiao Y, Kang Q, Cai QY. High efficient photocatalytic degradation of p-nitrophenol on a unique Cu2 O/TiO2 p-n heterojunction network catalyst. Environ Sci Technol. 2010;44:7641–7646. [25] Zhan YC, Luo XB, Nie SS, Huang YN, Tu TM, Luo SL. Selective separation of Cu(II) from aqueous solution with a novel Cu(II) surface magnetic ion-imprinted polymer. Ind Eng Chem Res. 2011;50:6355–6361. [26] Sag Y, Aktay Y. Kinetic studies on sorption of Cr(VI) and Cu(II) ions by chitin, chitosan and Rhizopus arrhizus. Biochem Eng J. 2002;12:143–153. [27] Pena ME, Korfiatis GP, Patel M, Lippincott L, Meng XG. Adsorption of As(V) and As(III) by nanocrystalline titanium dioxide. Water Res. 2005;39:2327–2337. [28] Li ZJ, Deng SB, Yu G, Huang J, Lim VC. As(V) and As(III) removal from water by a Ce-Ti oxide adsorbent: behavior and mechanism. Chem Eng J. 2010;161:106–113. [29] Zhang Y, Yang M, Dou XM, He H, Wang DS. As(V) adsorption on an Fe–Ce bimetal oxide adsorbent: role of surface properties. Environ Sci Technol. 2005;39:7246–7253. [30] Goh KH, Lim TT, Dong ZL. Enhanced arsenic removal by hydrothermally treated nanocrystalline Mg/Al layered double hydroxide with nitrate intercalation. Environ Sci Technol. 2009;43:2537–2543. [31] Wu H, Zhao YY, Nie MC, Jiang ZY. Molecularly imprinted organic inorganic hybrid membranes for selective separation of phenylalanine isomers and its analogue. Sep Purif Technol. 2009;68:97–104. [32] Shaw DJ, Costello B. Introduction to colloid and surface chemistry. Tribol Int. 1993;26:222.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Direct removal of aqueous As(III) and As(V) by amorphous titanium dioxide nanotube arrays Shaolin Wu Lixia Yang
a b
a b
, Wentao Hu
, Xinman Tu
a b
a b
, Xubiao Luo
a b
& Guisheng Zeng
, Fang Deng
a b
c
, Kai Yu , Shenglian Luo
a b
,
a b
a
Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, PR China b
College of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang, PR China c
Institute of Wastes and Soil Environment, Shanghai Academy of Environmental Sciences, Shanghai, China Published online: 11 Feb 2013.
To cite this article: Shaolin Wu , Wentao Hu , Xubiao Luo , Fang Deng , Kai Yu , Shenglian Luo , Lixia Yang , Xinman Tu & Guisheng Zeng (2013): Direct removal of aqueous As(III) and As(V) by amorphous titanium dioxide nanotube arrays, Environmental Technology, DOI:10.1080/09593330.2013.765923 To link to this article: http://dx.doi.org/10.1080/09593330.2013.765923
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.765923
Direct removal of aqueous As(III) and As(V) by amorphous titanium dioxide nanotube arrays Shaolin Wua,b , Wentao Hua,b , Xubiao Luoa,b∗ , Fang Denga,b , Kai Yuc , Shenglian Luoa,b∗ , Lixia Yanga,b , Xinman Tua,b and Guisheng Zenga,b a Key
Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, PR China; b College of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang, PR China; c Institute of Wastes and Soil Environment, Shanghai Academy of Environmental Sciences, Shanghai, China
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(Received 15 September 2012; final version received 7 January 2013 ) Amorphous titanium dioxide nanotube arrays (TiO2 NTs) were prepared by a simple anodization process without subsequent calcination at high temperature, and the effectiveness of amorphous TiO2 NTs as adsorbents in removing arsenite (As(III)) and arsenate (As(V)) was investigated. The TiO2 NTs were not only effective for arsenic removal without a pre-oxidation of As(III) to As(V) and/or adjusting the pH value of water before the adsorption process, but also can be separated and recovered easily from the solution. The adsorption kinetics and adsorption capacity of the amorphous TiO2 NTs for As(III) and As(V) were studied separately by batch experiments. The apparent values for Langmuir monolayer sorption capacities were 28.9 mg/g for As(III) and 24.7 mg/g for As(V) at pH 7. Kinetics studies indicated that the adsorption process on TiO2 NTs followed a pseudo-second-order kinetics model. Arsenic adsorption of TiO2 NTs remains stable over a broad pH range. Moreover, the TiO2 NTs have excellent stability and regeneration, and they can be used repeatedly at least five times. Keywords: adsorption; titanium dioxide nanotubes array; arsenic; regeneration
1. Introduction Arsenic, a ubiquitous metalloid that is widely found in soils and groundwater, has endangered human health and attracted great concern because of its toxic and carcinogenic properties [1]. In natural waters, inorganic arsenic occurs primarily as arsenite (As(III)) and arsenate (As(V)). At neutral pH levels, the dominant As(III) exists mainly as non-ionic H3 AsO3 , while the primary As(V) species are 2− the monovalent (H2 AsO− 4 ) and divalent (HAsO4 ). Chronic arsenic poisoning can cause various types of cancer and black foot disease through either contaminated drinking water or agriculture products irrigated by contaminated water [2]. In order to reduce the health risk, the US Environmental Protection Agency (USEPA) revised the guideline for drinking water in which the maximum contaminant level for arsenic was reduced from 50 μg/L to 10 μg/L in 2001 and required this standard to be achieved from 2006 [3]. Therefore, an effective technology for arsenic removal is urgently required to provide safe drinking water for affected people. Until the present time, several techniques for removing arsenic from contaminated water have been developed, such as oxidation/precipitation [4,5], coagulation/coprecipitation [6,7], adsorption [8], ion exchange [9] and membrane filtration [10–13]. Among the above techniques, adsorption is considered to be one of the most promising methods ∗ Corresponding
for arsenic removal from aqueous solution because it is effective, efficient and economic for water treatment [14]. A large number of adsorbents have been widely used in arsenic removal, such as activated carbons, activated alumina, oxides/hydroxides, tombarthite, zeolite clay minerals and biosorbents [15]. However, As(III) is generally reported to have low affinity to the surface of various adsorbents, and it is also difficult to remove using these conventional adsorbents such as activated carbon or activated alumina when compared with As(V). Moreover, As(III) is significantly more toxic and mobile than As(V). Therefore, if the conventional adsorbents are used for arsenic removal, it is necessary to oxidize As(III) to As(V) and/or adjust the pH value of water before the adsorption process for effective arsenic removal from water. After the adsorption process, the pH of treated water requires to be readjusted back to neutral. So it is worthwhile to develop an effective adsorbent for As(III) removal without a pre-oxidation/pH adjustment, which could greatly simplify the process of treatment and cut down the cost of treatment. At present, titanium dioxide (TiO2 ), commonly used for environmental applications, has attracted considerable attention due to its good adsorption activity, physical and chemical stability, non-toxicity, resistance to corrosion, commercial availability and inexpensiveness [16,17]. Studies have demonstrated that TiO2 exhibits high arsenic
authors. Emails: [email protected], [email protected]
© 2013 Taylor & Francis
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removal capability. For example, Pirilä et al. [18] reported that hydrous TiO2 is effective for arsenic removal from aqueous solution. Jing et al. [19] researched a nanocrystalline TiO2 -based adsorbent for the simultaneous removal of As(V), As(III), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in contaminated groundwater. In addition, Yao et al. [20] had synthesized a composite photocatalyst for arsenic removal by loading titanium dioxide onto activated carbon fibre (TiO2 /ACF). Bang et al. [21] reported that a novel granular TiO2 can effectively remove arsenic from groundwater. The above research and several other studies [22,23] have proved the potential of TiO2 for arsenic removal. However, practical applications of TiO2 powers are limited by the tedious process of separation after the treatment process. There is now a creative way to solve this problem, which is to prepare highly ordered TiO2 nanotube arrays (TiO2 NTs) on titanium (Ti) substrates. It has been reported that anodic oxidation is a simple, cheap and straight-forward method to prepare highly-ordered TiO2 NTs on Ti substrates. Because the TiO2 NTs grow from the Ti substrate, the TiO2 NTs can be easily separated and recovered in practical application. To the authors’ knowledge, however, no work has been reported on the research of TiO2 NTs as an adsorbent for arsenic removal from water. In the present study, amorphous TiO2 NTs were prepared by a simple anodization process without subsequent calcination at high temperature. The adsorption kinetics and adsorption capacity of the amorphous TiO2 NTs for As(III) and As(V) were studied separately by batch experiments. Moreover, the effect of pH on the adsorption capacity of amorphous TiO2 NTs for As(V) and As(III) was investigated. The TiO2 NTs not only effectively removed As(V) and As(III), especially As(III) without pre-oxidation/pH adjustment, but can also be removed easily from the aqueous solution after treatment.
2. Experimental methods 2.1. Materials All chemicals were analytical grade except hydrochloric acid (Guaranteed reagent, Shantou Xilong Chemical Co. Ltd, Shantou, China), and all stock solutions were prepared with ultrapure water which was produced using a Milli-Q water system (EMD Millipore Corporation, Bedford, MA, USA). Stock solutions of As(III) and As(V) (1000 mg/L) were prepared using NaAsO2 (Beijing Chemicals Corporation, PR China) and Na2 HAsO4 ·12H2 O (Beijing Chemicals Corporation, PR China), respectively. These stock solutions were further diluted for experimental use.
2.2.
Preparation of TiO2 NTs Titanium foil (250 μm thick, 99.8%; Sigma Aldrich, St. Louis, MO, USA) was cut into 1.0 × 4.0 cm strips. Strips
were ultrasonically cleaned in ethanol and then in acetone for 5 min. These strips were anodized at 30 V for 2 h in a two-electrode electrochemical cell with titanium foil anode and platinum foil cathode in an ethylene glycol solution containing 0.3 wt% NH4 F and 1% volume H2 O [24]. 2.3.
Characterization of the TiO2 NTs The morphology of TiO2 NTs was observed on a SSX-550 SEM instrument (Shimadzu, kyoto, Japan). The specific surface area and pore of the TiO2 NTs were determined by NOVA 2000e surface area and pore size analyzer (Quantachrome, Boynton Beach, FL, USA). The zeta potential of TiO2 NTs was measured by SurPASS electrokinetic analyzer for solid samples (Anton Paar, Austria). The composition of adsorbents was analyzed by X-ray powder diffractometer (XRD, Rigaku D/Max-B, Cu Kα, Tokyo, Japan). 2.4.
Adsorption kinetics
All experiments were conducted for As(III) or As(V) separately unless otherwise indicated. Batch sorption experiments were conducted to investigate the adsorption kinetics. After the pH of 1.0 mg/L arsenic solution (either As(III) or As(V)) was adjusted to 7.0 with 0.5 mol/L hydrochloric acid or sodium hydroxide, 10 mL arsenic solution and 9 mg of TiO2 (two pieces of TiO2 NTs) were added into each 100 mL conical flask. The conical flasks were oscillated in a thermostatic oscillator (Changzhou Guohua Instrument Company, Changzhou, China) at 25◦ C for different times (0.25 h, 0.5 h, 0.75 h, 1 h, 1.5 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, 10 h, 12 h, 18 h and 24 h), then the TiO2 NTs were immediately taken from the solution. The solution was acidified using HCl, and the total soluble arsenic concentrations were determined by an atomic fluorescence spectrometer (AFS8220) coupled with a hydride generator. To detect As(III) selectively, the working solution was prepared with a mixture of 2% KBH4 and 0.5% KOH as reducing solution and 5% hydrochloric acid as carrier solution. Under these conditions, only As(III) was converted to AsH3 and detected on an AFS instrument (Beijing Jitian Instrument Company, Beijing, China). 2.5.
Effect of pH on arsenic adsorption
Arsenic solution (10 mL 1.0 mg/L) (either As(III) or As(V)) with specific initial pH ranging from 3 to 11 and 9 mg of TiO2 (two pieces of TiO2 NTs) were placed in conical flasks. The samples were oscillated in a thermostatic oscillator for 12 h and analyzed in similar fashion to the adsorption kinetics studies described above. 2.6.
Adsorption isotherms
In the investigation of adsorption isotherms, the initial arsenic concentrations ranged from 0.5 mg/L to 30 mg/L,
Environmental Technology and pH values were adjusted to 7.0 by adding HCl and NaOH solutions. Arsenic solution (10 mL) (either As(III) or As(V)) with specific initial concentrations and 9 mg of TiO2 (two pieces of TiO2 NTs) were placed in conical flasks. The samples were oscillated in a thermostatic oscillator (Changzhou Guohua Instrument Company, Changzhou, China) at 25◦ C for 12 h and analyzed in similar fashion to the adsorption kinetics studies described above. The equilibrium adsorption capacity (qe (mg/g)) was calculated using the following equation: (c0 − ce ) × V mads
(1)
where c0 and ce are the initial and equilibrium concentration (mg/L) of arsenic in solution, respectively, V is the total volume of the solution and mads is the mass of the TiO2 adsorbent. 2.7. Desorption experiment The desorption study is very important since the regeneration of adsorbent decides the economic success of the adsorption process. In this study, 2 mol/L NaOH was used as eluent to regenerate the TiO2 NTs. Nine mg of TiO2 (two pieces of TiO2 NTs) was added into 10 mL of 1.0 mg/L As(III) or As(V) solution at pH 7.0, and the flask was oscillated in a thermostatic oscillator at 25◦ C for 12 h. After adsorption, the as-loaded TiO2 NTs were taken from the solution and rinsed with water. In the desorption test, the saturated TiO2 NTs were immersed in 2 mol/L NaOH solution and oscillated for 1 h in a thermostatic oscillator. The above procedure was repeated many times until arsenic could not be detected in the eluent. After the desorption process, the TiO2 NTs were taken from the solution and the arsenic concentration in the eluent was analyzed. The regenerated TiO2 NTs were washed with hydrochloric acid and de-ionized water until neutral pH was reached, and then the TiO2 NTs and dried at 50◦ C for reuse in the next cycle. The adsorption–desorption cycle were repeated five times, and the adsorption capacity for arsenic in each adsorption experiment was calculated.
and (102) planes of Ti, respectively. Because TiO2 NTs are amorphous, diffraction peaks are not present in the XRD patterns. The surface area and pore size are crucial from the view of the adsorption, because higher surface area and proper pore size often increase adsorption capacity. In our case, the specific surface area of TiO2 NTs is 49.5 m2 /g and the pore diameter is about 4 nm. The zeta-potential of amorphous TiO2 NTs is shown in the supplementary material (Figure S3). The isoelectric point (IEP) of these TiO2 NTs is determined at about pH 5.39, which is larger than the IEP value of TiO2 •xH2 O nanoparticles (pH 3.8) in the literature [3] due to the well-ordered structure of TiO2 NTs. In a near-neutral environment TiO2 NTs are negatively charged with a zeta potential of about −20.53 mV, indicating the existence of the surface hydroxyl groups.
3.2. Adsorption kinetics Adsorption kinetics are one of the most important characters that represent adsorption efficiency. Figure 1 shows the dynamic curves for the adsorption of As(III) and As(V) on the amorphous TiO2 NTs at pH 7.0. It can be seen from Figure 1 that initial adsorption of arsenic is rapid, then adsorption becomes slow until adsorption equilibrium is reached, and arsenic removal efficiency is approximately 99%. Moreover, the adsorption of As(V) and As(III) on TiO2 NTs requires 180 min and 240 min to reach equilibrium, respectively, which indicates that the removal of As(V) was faster than As(III). The results were in agreement with those reported by Bang et al. [21]. Longer contact time would be required for the effective removal of As(III) in comparison with As(V) due to the slower rate of As(III) adsorption. Based on the results of adsorption kinetics, 12 h was used as the contact time in the other adsorption experiments.
100
90 25
The surface morphologies of the TiO2 NTs are shown in the supplementary material (Figure S1). The TiO2 nanotube arrays are composed of high-density, well-ordered, vertically-oriented and uniform TiO2 nanotubes. The length of TiO2 NTs is 4 μm, the pore sizes range from 90–100 nm and wall thickness is about 20 nm. XRD patterns of TiO2 NTs are shown in the supplementary material (Figure S2). The spectrum of Ti foil is shown in the figure as a reference. The dominant peaks at 2θ about 38.3◦ , 40.1◦ and 52.8◦ represent the indices of (002), (101)
80
t /qt (h g/mg)
3. Results and discussion 3.1. The characterization of TiO2 NTs
Removal (%)
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qe =
3
70
20 15 10
As(III) As(IV)
5
60
0
As(III) As(IV)
0
4
8
12 t (h)
16
20
24
50 0
4
8
12 t (h)
16
20
24
Figure 1. Adsorption kinetics of As(III) and As(V) on TiO2 NTs (9 mg TiO2 ). Initial arsenic concentration is 1 mg/L. Inner illustration is the pseudo-second-order rate kinetic model fitted to the adsorption kinetics studies.
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Table 1. Kinetic parameters of the pseudo-first-order and pseudo-second-order rate equation for As(III) and As(V) adsorption on TiO2 NTs.
100
qe k1 (mg/g) (L/min) As(III) 0.5644 As(V) 0.0189
R2
Pseudo-second-order qe k2 (mg/g) ((g/mg)/h)
0.0082 0.9360 1.1138 0.0042 0.3842 1.067
2.46 50.082
R2 0.9999 1.0000
Removal (%)
90
Pseudo-first-order
80
70 As(III) As(IV)
60
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To determine the mechanism of the adsorption process of TiO2 NTs, the pseudo-first-order and pseudo-secondorder equations were used to fit the experiment data. The pseudo-first-order kinetic equation [25] is defined by Equation (2): ln(qe − qt ) = ln qe − k1 t
(2)
where qe and qt are the amount of arsenic adsorbed at equilibrium and at any time t, respectively (mg/g) and k1 is the constant rate of pseudo-first-order adsorption (L/min). The pseudo-second-order rate expression [26] is defined by Equation (3) and its integrated form is given in Equation (4): dqt = Kad (qe − qt )2 dt t 1 t = + 2 qt k2 q e qe
(3) (4)
where qe and qt are the amount (mg/g) of arsenic adsorbed at equilibrium and at time t, respectively, and k2 is the rate constant of adsorption (g/(mg min)). The kinetics parameters that are obtained by fitting the experimental data are summarized in Table 1. The value of k1 that was used to calculate the slope of the curve is 0.0082 L/min for As(III) and 0.0042 L/min for As(V) (curve not shown), and the correlation coefficients (R2 ) of As(III) and As(V) are 0.9360 and 0.3842, respectively. Moreover, the experimental value did not agree with the calculated value. These results indicated that the adsorption dynamics of arsenic on TiO2 NTs did not accord with the first-order kinetic model. As demonstrated in Figure 1 and Table 1, the experimental results fit well with the pseudo-second-order rate kinetic model through the data analysis. The applicability of the pseudo-second-order rate model was estimated by the square of the correlation coefficient (R2 ), and the R2 from the curve (insert, Figure 1) was close to 1, indicating that experimental data fit well with the model. In our study, the initial concentration of As(III) and As(V) solution was 1.0 mg/L and the pH value of the solution was 7.0, 0.9 g/L of TiO2 was used as adsorbent in each conical flask, the k2 was 2.46 mg−1 g h−1 and 53.082 mg−1 g h−1 for As(III) and As(V), respectively. Although the experimental conditions are different from those of Xu et al. [3] and Pena et al. [27],
2
3
4
5
6
7 pH
8
9
10
11
Figure 2. Removal of arsenic as a function of initial pH on TiO2 NTs (9 mg TiO2 ). Initial arsenic concentration is 1 mg/L.
we can conclude that TiO2 NTs are effective for arsenic removal. 3.3.
Effect of pH on As adsorption
The effect of pH on As(III) and As(V) removal is shown in Figure 2. The results indicated that arsenic removal of As(III) and As(V) were similar in a pH range between 3 and 7, and arsenic removal efficiency is very high, close to 100%. When pH value ranges from 7 to 10, the removal efficiency of As(V) and As(III) decreased slowly to 95% and 90%, respectively. When pH is above 10, a steep decrease occurred in the removal of As(III) and As(V). It can be concluded from the above results that TiO2 NTs can effectively remove As(III) and As(V) in pH range 3–10. Similar arsenic adsorption behaviour was observed when nanocrystalline titanium dioxide [19] and granular titanium dioxide [21] were used as arsenic adsorbent. The possible reason for high adsorption capacity of TiO2 NTs for As(V) and As(III) in acidic solution and decrease of adsorption capacity at pH above 7.0 is as follows. (1) The hydroxyl groups on the adsorbent can be protonated in acidic solution and become positive-charged, the negative-charged H2 AsO− 4 and HAsO2− 4 can be adsorbed on the positive sites on TiO2 NTs via electrostatic attraction. (2) The Ti-OH groups on the TiO2 NTs surface can bind As(V) and As(III) through the formation of monodentate and bidentate complexes between As−OH and Ti−OH [28–30]. 3.4. Adsorption isotherms Adsorption isotherms are important factors that depict how the solutes interact with adsorbents. The adsorption equilibrium experiments of As(III) and As(V) onto TiO2 NTs were carried out under near-neutral pH conditions, and the adsorption isotherms are shown in Figure 3. The adsorption data were fitted with both the Langmuir isotherm and the
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Table 2. The parameters of Langmuir and Freundlich isotherms for adsorption of As(III) and As(V) on TiO2 NTs. Langmuir isotherm qe KL (mg/g) (L/mg)
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As(III) As(V)
28.9 24.7
R2
0.1117 0.9626 4.4562 0.9857
Freundlich isotherm KF ((mg/g)1−1/n ) 4.0494 16.6177
n
R2
1.7274 0.9857 3.6576 0.8975
Figure 4. Stability and potential regeneration of TiO2 NTs. Initial arsenic concentration is 1.0 mg/L.
Figure 3. Equilibrium adsorption isotherms (equilibrium adsorption capacity qe as a function of liquid-phase equilibrium concentration (Ce ) for (a) As(III); (b) As(V).
Freundlich isotherm [31,32] as given in Equations (5) and (6), respectively: qe =
qmax KL Ce 1 + K L Ce
(5)
1
qe = KF × Cen
(6)
where qe is the amount (mg/g) of As(III) and As(V) adsorbed at equilibrium, Ce is the equilibrium As(III) and As(V) concentration (mg/L) in water samples, KL and qmax (maximum adsorption capability) are the Langmuir constants of adsorption, and KF and n are the Freundlich constants of adsorption. The parameters obtained by fitting the experimental data are summarized in Table 2. The applicability of two isotherm models was evaluated through the square of the correlation coefficient R (R2 ). It is clear that the adsorption data could be fitted well with the Freundlich isotherm for As(III) because the R2 fitted with this model is close to 1, compared with that fitted by the Langmuir isotherm. It indicated that the adsorption of As(III) was a multi-site adsorption process due to the heterogeneity of the surface.
On the contrary, the adsorption data of As(V) fitted well with the Langmuir isotherm, compared with that fitted by the Freundlich isotherm. The results indicated that it tended to monolayer adsorption for As(V). Moreover, the maximum adsorption capacities (qmax ) of As(III) and As(V) were calculated to be 28.9 and 24.7 mg/g for the Langmuir model, respectively. This is close to the experimental values. 3.5.
Desorption and repeated use
The regeneration of the adsorbent is likely to be a key factor in estimating the wastewater process economics. The effect of desorption time on TiO2 NTs desorption efficiency was observed using 10 mL 2 M NaOH solutions as eluent. Adsorption-desorption results from using TiO2 NTs repeatedly are shown in Figure 4. Results illustrated that the TiO2 NTs can be used repeatedly at least five times without significant decrease of their adsorption capacities. The results indicated that the TiO2 NTs had excellent regeneration ability. 4.
Conclusions
The amorphous TiO2 NTs were prepared by a simple anodization process. The TiO2 NTs adsorbent exhibits a high adsorption capacity for both As(III) and As(V) over a broad pH range, and it is especially effective for As(III) removal without a pre-oxidation of As(III) to As(V) and/or
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the pH adjustment of water before the adsorption process. Moreover, TiO2 NTs can be separated and recovered easily from the aqueous solution, and can be used at least five times without an obvious decrease of adsorption capacities. Acknowledgements
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This work was financially supported by Natural Science Foundation of China (50978132, 51178213, 51238002, 51272099, 51008149), Program for New Century Excellent Talents in University (NCET-11-1004), Cultivating Program for Young Scientists of Jiangxi Province of China (20112BCB23016), Natural Science Foundation of Jiangxi Province (20122BAB213014, 20114BAB203018), National High Technology Research and Development Program of China (2009AA062905, 2011AA0606 04) and Department of Education Fund of Jiangxi Province (Grant GJJ11508).
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