Photochemical treatment of As(III) with α-Fe₂O₃ synthesized from Jarosite Waste

Abstract

Endeavoring to mitigate and remedy Arsenic (As) in groundwater, we developed a one-step process for As(III) oxidation and subsequent adsorption by using α-Fe₂O₃ under UV irradiation. The α-Fe₂O₃ with high surface area was successfully prepared through a mild hydrothermal reaction using Jarosite waste as precursor. During the treatment of As(III) in aqueous solution, the as-obtained α-Fe₂O₃ can release ferric ions stimulated by UV irradiation. This led to the effective oxidation of As(III) to As(V) which is further adsorbed on α-Fe₂O₃, realizing the removal of As from water. This simple and efficient route may be potential for the remediation of polluted water containing As ions.

---

1. Introduction

Arsenic (As) is a significant contaminant in groundwater, which has been considered as a carcinogenic element to human beings and other living organisms.^1–3 Before the groundwater reaches safe drinking levels, some efforts need to be applied to remove arsenic and other toxic elements through adsorption, coagulation, ion exchange or membrane processes.^4–7 Although many researchers have exploited several efficient adsorbents, iron oxides are the most preferred one because it's cheap, innocuous and capable of being removed from the medium by a simple magnetic process.^8,9 However, the adsorption of As(III) is much lower than that of As(V) on iron oxide. As the toxicity of As(III) is several times higher than that of As(V), the oxidation of As(III) species is required in common removal technologies. Consequently, when arsenic species are present as As(III), chemical oxidants such as ozone, chlorine, hydrogen peroxide, manganese dioxide or potassium permanganate have been employed for a preoxidation step.^10–16 Since these procedures have their own limitations such as high cost and concomitant pollution, it's absolutely necessary to develop an efficient and mild technology to get rid of oxide As(III). Recently, attention has been focused on the oxidation of As(III) under photo irradiation, which is a much greener and more innocuous technology. The treatment methods such as TiO₂/UV, UV/Fe(III) and UV/iodide can oxidize As(III) to As(V).^17–19 However, excrescent processes are needed to separate the dissolved As(V) from the polluted water. Although much progress has been made in the treatment of As(III), it remains a major challenge to develop a fast and simple technique to remove As(III) from water in one step.

Currently, many efforts have been dedicated to curb the arsenite problem by developing composites through simultaneous oxidation and adsorption, especially for the TiO₂/Fe₂O₃ composites.²⁰ As reported in the literature, As(III) can be oxidized to As(V) through photocatalysis reaction by TiO₂, and then As(V) was removed by adsorption of α-Fe₂O₃. Although these composites are more efficient in the treatment of As(III), the synthesis process needs metal organic compounds and organic solvents, and the introduction of TiO₂ requires more energy and expense. Therefore, it is necessary to synthesize low-cost and high-efficiency sorbent with oxidation and adsorption bi-function for As(III) treatment.

Herein, we report a novel one-step process for As(III) oxidation and adsorption. The combined system of Fe₂O₃/UV is designed for the treatment of As(III). In aqueous solution, the α-Fe₂O₃ can release ferric ions stimulated by UV irradiation. This led to the effective oxidation of As(III) to As(V) which is further adsorbed on α-Fe₂O₃, thus achieving the aim to oxidize and remove As(III) from contaminated water in one step.

Furthermore, the α-Fe₂O₃ used for the treatment of As(III) is synthesized from Jarosite waste. The Jarosite is an iron rich hazard from the hydrometallurgy of zinc industry and can cause pollution towards both subsurface and underground water. Generally, the method for dealing with the Jarosite is sintering which generates acid and requires further treatment in the process.^21,22 In this article, the Jarosite is disposed by a simple hydrothermal reaction and then the Jarosite breaks down to produce α-Fe₂O₃ and dissolved sulfate. The sulfate is stripped from the liquid, which can be reutilized in other applications. The α-Fe₂O₃ solid, another main product of Jarosite, is used in the treatment of As(III). The mechanism of formation for α-Fe₂O₃ and the photo oxidation for As(III) are discussed in this article. Moreover, the obtained α-Fe₂O₃ may also be applied as potential treatment for other toxic elements.

2. Experimental section

2.1 Preparation of Jarosite and α-Fe₂O₃

All the reagents used in the experiments are of analytical grade without further purification. The Jarosite was prepared as follows: 0.01 mol FeCl₃·6H₂O and 0.02 mol Na₂SO₄ were added to 100 mL deionized water to form a homogeneous solution. The obtained red solution was stirred and refluxed at 80 °C for 2 h. The yellow Jarosite precipitate was collected by centrifugation and was redispersed into 60 ml water. The mixture was then transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated to 180 °C for 12 h. After hydrothermal reaction, the red α-Fe₂O₃ powder (marked as F1) was isolated by centrifugation and washed with deionized water and ethanol three times respectively. Finally, the product was dried in air at 80 °C for 3 h and then used for the treatment of As(III) and other characterizations.

The synthesis parameters, such as the raw material and reaction time were changed. The detailed experimental conditions for all the syntheses are listed in Table 1.

Table 1 Experimental conditions for the preparation of α-Fe₂O₃

Sample No.	Raw material	Reaction conditions	SBET (m^2 g^−1)
a Fe(OH)3 is synthesized by adding excessive Na2CO3 in 0.1 mol L^−1 FeCl3 solution.
F1	NaFe3(SO4)2(OH)6	180 °C, 12 h	22.5
F2	FeOOH	180 °C, 12 h	1.4
F3	(NH4)Fe3(SO4)2(OH)6	180 °C, 12 h	29.6
F4	KFe3(SO4)2(OH)6	180 °C, 12 h	8.06
F5	0.1 mol L^−1 FeCl3	180 °C, 12 h	1.2
F6	0.1 mol L^−1 FeCl3, 0.2 mol L^−1 CO (NH2)2	180 °C, 12 h	15.17
F7	0.1 mol L^−1 FeCl3, 0.2 mol L^−1 Na2SO4	180 °C, 24 h	12.65
F8	0.1 mol L^−1 FeCl3, 0.2 mol L^−1 (NH4) 2SO4	180 °C, 24 h	17.58
F9	0.1 mol L^−1 FeCl3, 0.2 mol L^−1 K2SO4	180 °C, 24 h	10.39
F10	Fe(OH)3^a	180 °C, 12 h	13.08

---

2.2 Characterizations

Powder X-ray diffraction (XRD) data were collected on Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation operated at 40 kV and 40 mA. A scan rate of 0.02°·s⁻¹ is applied to record the pattern in the 2θ range of 10–70°. The scanning electron microscopy (SEM) images were carried out with FESEM-6700 field-emission microscope. All the samples for the SEM characterization were ultrasonically dispersed in ethanol, and then one droplet of the suspension was dropped on silicon substrate allowing the solvent to evaporate for measurement. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermao Escalab 250 system at 1.2 × 10⁻⁹ m bar using Al Kα radiation (1486.6 eV). N₂ adsorption–desorption isotherms were measured on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA) at 77 K. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated from the adsorption branch using the Barett–Joyner–Halenda (BJH) theory. The concentration of Fe(III) in the supernatant liquid was determined by the phenanthroline spectrophotometric method (HJ/T 345–2007) with Varian Cary-50 spectrophotometer at 510 nm.

2.3 Water treatment experiments

Using the As(III) as a pollutant, the prepared α-Fe₂O₃ was used to investigate their application in water treatment. The stocked As(III) solution with a concentration of 1000 mg L⁻¹ was purchased from National Analysis Center for Iron and Steel (NACIS, Beijing, China). For the experiment, the standard stock solution was diluted to 2 mg L⁻¹ and adjusted to pH 4 with HCl or NaOH solution. Then, 0.05 g of α-Fe₂O₃ sample was added to 50 mL of the above solution under stirring. The suspension was stirred continuously under the photo illumination. At given irradiation time intervals, 1 mL of the suspension was collected, and then centrifuged to remove the solid. The total As concentration in the remaining solutions was detected by hydrid generation–atomic fluorescence spectrometry (HG–AFS) method (See the details in the ESI†).

3. Results and discussion

3.1 Characterizations of Jarosite and α-Fe₂O₃

Fig. 1a displays the XRD patterns of the jarosite precursor obtained by the hydrolysis reaction of FeCl₃ in the presence of Na₂SO₄ at 80 °C for 2 h. All the peaks can be indexed as hexagonal phase of natro-jarosite (NaFe₃(SO₄)₂(OH)₆), which are identical with the standard values of JCPDS No. 36-0425. Fig. 2a is the SEM image of the as-prepared precursors, showing that the as-prepared NaFe₃(SO₄)₂(OH)₆ particle is irregular aggregates. The SEM image of high magnification (Fig. 2b) indicates that the aggregates were mainly composed of irregular particles and finally congregated into micrometre big blocks with granulation surface.

Fig. 1 XRD pattern of the NaFe₃(SO₄)₂(OH)₆ precursor (a) and α-Fe₂O₃ (b) for F1.

Fig. 2 SEM images of the NaFe₃(SO₄)₂(OH)₆ precursor (a, b) and α-Fe₂O₃ (c, d) for F1.

Fig. 1b presents XRD patterns of the final product (marked as F1) obtained with hydrothermal treatment at 180 °C for 12 h. All the diffraction peaks of F1 can be readily assigned to the pure hexagonal phase of α-Fe₂O₃ with calculated lattice parameters a = 5.035 Å and c = 13.746 Å, which are in good accordance with standard values of JCPDS No. 33-0664. The average crystalline size is calculated to be 40.8 nm using the Debye-Scherrer formula. The SEM image of the F1 is shown in Fig. 2c. It shows that the size and morphology of the formed α-Fe₂O₃ changes dramatically after the NaFe₃(SO₄)₂(OH)₆ was decomposed by hydrothermal reaction. Further observation (from Fig. 2d) reveals that the average diameter of formed α-Fe₂O₃ is approximately 250–600 nm. Nitrogen adsorption/desorption isotherms are measured to determine the surface area and pore size of the as-obtained α-Fe₂O₃. The BET specific surface area and pore volume of the sample are 22.5 m² g⁻¹ and 0.059 cm³ g⁻¹ respectively.

3.2 Effects of Jarosite to the synthesis of α-Fe₂O₃

When iron ion hydrolyzes in the water, the state of the obtained product are closely related to the ions existing in the solution, such as H⁺, OH⁻, and other inorganic anions.^23–26 Therefore, the presence of Na₂SO₄, which affects the structure and surface area of the final product, is believed to be crucial for the formation of the NaFe₃(SO₄)₂(OH)₆ precursor. The following reactions may be involved during the hydrolysis reaction of FeCl₃ in the presence of Na₂SO₄:²⁷

Fe³⁺ + SO₄²⁻ + H₂O → Fe(OH)SO₄ + H⁺ (1)

2Fe(OH)SO₄ + 2H₂O → Fe₂(OH)₄SO₄ + 2H⁺ + SO₄²⁻ (2)

Fe(OH)SO₄ + Fe₂(OH)₄SO₄ + Na⁺ +H₂O → NaFe₃(SO₄)₂(OH)₆ (3)

The overall stoichiometry for precipitating NaFe₃(SO₄)₂(OH)₆ can be represented by the following reaction:

3Fe³⁺ + 2SO₄²⁻ +Na⁺ + 6H₂O → NaFe₃(SO₄)₂(OH)₆ + 6H⁺ (4)

In these reactions, excessive Na₂SO₄ (double amount more than the dosage of FeCl₃) and lower temperature (<100 °C) is the key to the synthesis of NaFe₃(SO₄)₂(OH)₆ in the hydrolysis process. Under hydrothermal treatment, the big block NaFe₃(SO₄)₂(OH)₆ decomposed and collapsed to smaller particles with the dissolve of sulphate and natrium. The formation process is similar to the osteoporosis caused by loss of phosphate and calcium in bones.

To illuminate the role of Na₂SO₄ in the reactions, the hydrolysis of FeCl₃ and the following hydrothermal treatment are conducted without Na₂SO₄. All the characterization such as XRD patterns and SEM images are presented in Fig. S2 (ESI†). Fig. S2 a displays the XRD patterns of the precursor and final product, respectively. In the curve I, all the peaks can be readily assigned to a tetragonal phase FeOOH (JCPDS No. 34-1266) that is most prevalent precursor as described in many literatures.^28–31 As can be seen in Fig. S2 b, the FeOOH exhibit spindle morphology with a width of 30–40 nm and a length of 400–500 nm. On the basis of the experimental results and relative reports, the hydrolysis process of FeCl₃ can be described as follows:^32,33

Fe³⁺ + 6H₂O → Fe(H₂O)₆³⁺ (5)

Fe(H₂O)₆³⁺ → FeOOH + 4H₂O + 3H⁺ (6)

After hydrothermal treatment, this obtained α-Fe₂O₃ (marked as F2) with a polyhedron structure is produced as shown in Fig. S2 c. During the hydrothermal reaction, the spindle-like FeOOH are transformed into α-Fe₂O₃ by dehydration process and induce low surface area according to the “oriented attachment”.³⁴ The formation of α-Fe₂O₃ can be summarized in the schematic illustration as shown in Fig. S3 (ESI†).

In this method, the formation of α-Fe₂O₃ is mainly determined with the assistance of NaFe₃(SO₄)₂(OH)₆. Therefore, it is expected that the result can recur by adjusting the species of Jarosite. Fig. 3 display the characterizations of the α-Fe₂O₃ obtained with different Jarosite. These curves in Fig. 3 a and e can be assigned to the hexagonal phase of ammonio-jarosite ((NH₄)Fe₃(SO₄)₂(OH)₆, JCPDS No. 26-1014, curve I) and jarosite (KFe₃(SO₄)₂(OH)₆, JCPDS No. 36-0427, curve III), respectively. In contrast with the precursor of NaFe₃(SO₄)₂(OH)₆, the aggregates of (NH₄)Fe₃(SO₄)₂(OH)₆ are also composed of irregular particles (Fig. 3b). With the releasing of ammonium and sulfate under high temperature and pressure, the (NH₄)Fe₃(SO₄)₂(OH)₆ is eroded and turned to α-Fe₂O₃ aggregates with smaller particles (Fig. 3c, marked as F3) in comparison with F1 in Fig. 2d. The BET surface area of F3 is 29.6 m² g⁻¹. The KFe₃(SO₄)₂(OH)₆ sample has the same decomposition process than the other two Jarosites. However, the surface area of the obtained α-Fe₂O₃ (Fig. 3f, marked as F4) is 8.06 m² g⁻¹, which is lower than that obtained from NaFe₃(SO₄)₂(OH)₆ or (NH₄)Fe₃(SO₄)₂(OH)₆. Much work is needed to know the exact mechanism.

Fig. 3 XRD patterns of the (NH₄)Fe₃(SO₄)₂(OH)₆ (a, curve I), KFe₃(SO₄)₂(OH)₆ (e, curve III) , F3 (a, curve II) and F4 (e, curve IV); SEM images of the (NH₄)Fe₃(SO₄)₂(OH)₆ (b), KFe₃(SO₄)₂(OH)₆ (f) and obtain α-Fe₂O₃ of F3 (c) and F4 (g).

3.3 Adsorption of As(III) under photo illumination

The obtained α-Fe₂O₃ was evaluated for the treatment of As(III) polluted water under photo illumination. Fig. 4a shows a comparison of the adsorbent activities under different conditions. In the absence of photo illumination, the prepared α-Fe₂O₃ (F1) shows weak adsorption activity to the As(III) (dash line in Fig. 4a). The adsorption rate is measured as 41.5%. In the presence of photo illumination, however, the α-Fe₂O₃ sample could remove most of the As(III) in the solution as indicated by the solid line in Fig. 4a. The removal rate increase to 91.1%, that is two times larger than the result without photo illumination. The α-Fe₂O₃ samples obtained from different Jarosites were also used for the treatment of As(III). As shown in Fig. 4c and d, sample F3 and F4 show better adsorption capability of As(III) under photo illumination than without photo illumination.

Fig. 4 Adsorption rate of As(III) with (solid line) and without (dashed line) photo illumination for the α-Fe₂O₃ (a for F1, c for F3 and e for F4) with As(III) = 2 mg L⁻¹, pH = 4. Adsorption isotherms of As(III) in Lagmuir model for the α-Fe₂O₃ (b for F1, d for F3 and f for F4) with (solid line) and without (dashed line) photo illumination.

The adsorption capacity at different As(III) concentrations can be illustrated by the adsorption isotherms, such as the Langmuir model. The Langmuir adsorption model, shown as eqn (7), is employed for the adsorption analysis to represent the correlation between the amount of poisonous element adsorbed at equilibrium (q_e, mg g⁻¹) and the equilibrium solute concentration (C_e, mg L⁻¹):³⁵

q_m (mg g⁻¹) is the maximum adsorption capacity corresponding to a complete monolayer coverage and b is the equilibrium constant (L mg⁻¹). Fig. 4b, d and f show the adsorption isotherms of As(III) on the α-Fe₂O₃ samples. All the experimental data fits the Langmuir adsorption isotherm well. The maximum adsorption capacity under photo illumination increased compared to that without photo illumination for all the α-Fe₂O₃ samples. The model parameters for the Langmuir models are listed in Table 2.

Table 2 Lagmuir parameters for As(III) adsorption on α-Fe₂O₃

Sample No.	Photo illumination	qmax (mg g^−1)	b (L mg^−1)	R^2
F1	No	1.706	1.090	0.986
	Yes	4.217	4.312	0.975
F3	No	2.439	0.879	0.981
	Yes	5.951	3.119	0.995
F4	No	0.713	0.830	0.989
	Yes	1.298	2.242	0.970

---

The variation of As(III) sorption with pH is investigated and the results obtained are shown in Fig. 5. The main trend observed in this figure is that the sorption of As(III) with photo illumination is always higher than that without photo illumination at either neutral or alkaline pH values. Acidic condition is favorable for the sorption of As(III) under photo illumination and little change in adsorption capacity is found when pH < 4. At alkaline pH values, moreover, the effect of illumination on the improvement of adsorption capacity is very low because iron oxides have a less effective adsorption for As(III) at alkaline pH values.³⁶

Fig. 5 Adsorption of As(III) with and without photo illumination at different pH values. Adsorption condition: 50 mL, As(III) = 2 mg L⁻¹, 0.05 g α-Fe₂O₃ (F1) for reaction time of 3 h.

Some comparative experiments were made to investigate the adsorption capability of the α-Fe₂O₃ obtained at different conditions (Table 1) and the results are shown in Fig. 6. It can be clearly seen that under photo illumination, all samples show much greater activity than without photo illumination. And the samples hydrothermally synthesized from FeCl₃ and sulphate directly (F7, F8 and F9) have a similar capability for the treatment of As(III).

Fig. 6 Adsorption of As(III) with and without photo illumination for different α-Fe₂O₃. Adsorption condition: As(III) = 2 mg L⁻¹, pH = 4 for reaction time of 3 h.

3.4 Photo oxidation mechanism of As(III) over α-Fe₂O₃

The XPS spectra of As3d on the α-Fe₂O₃ sorbent were obtained and the results are shown in Fig. 7. In curve a, a peak appears at 43.3 eV, which reveals the presence of As(III) on the surface of the α-Fe₂O₃ sorbent without photo illumination.^37,38 For the sample under photo illumination, the As3d peak locates at 44.4 eV as shown in curve b. In contrast to curve a, the As3d peak displays a remarkable disparity in location. As(V) is present on the surface of the α-Fe₂O₃ sorbent under photo illumination.^37,38 As the initial aqueous contained As(III) only, the observation of As(V) state suggest that α-Fe₂O₃ exhibits a combination of adsorptive and oxidative abilities of As(III) under photo illumination. This change may occur due to the presence of dissolved Fe(III) and illumination of UV light.^18,39

Fig. 7 As3d XPS spectra on the surface of F1: (a) with photo illumination, (b) without photo illumination.

Usually, Fe₂O₃ has a tendency to photocorrosion under photo illumination. Fig. 8 shows the concentration of ferric ion in different conditions. The data shown in Fig. 8a reveal that there is an insignificant release of ferric ion for F1 Sample in the absence of photo illumination. However, there is a significant release of ferric ion under the irradiation and absence of As(III) as shown in Fig. 8b. Because of the generation of ferric ion, the rate of oxidation of As(III) to As(V) is increased by several orders of magnitude with UV illumination.¹⁸ In contrast to the presence of As(III), shown in Fig. 8c, there is more ferric ion in the solution. Presumably, this is due to the photoinduced dissolution of α-Fe₂O₃ during the oxidation of As(III) to As(V).⁴⁰

Fig. 8 Concentration of soluble ferric ion at different conditions: (a) α-Fe₂O₃ suspension with pH 4 without photo illumination; (b) α-Fe₂O₃ suspension with pH 4 under photo illumination; (c) α-Fe₂O₃ in As(III) solution with pH 4 under photo illumination; (d) α-Fe₂O₃ suspension adding EDTA with pH 4 under photo illumination.

To validate the mechanism of these processes, therefore, scavengers were employed to determine the reactive species that played an important role in the reaction. As depicted in Fig. 9a, it was noted that when O₂ was bubbled in the reaction mixture, the adsorption rate of As(III) increased markedly. When dissolved O₂ is excluded by N₂ in the treatment process, Fig. 9b shows that the adsorption rate of As(III) decreased to 79.4%. The adsorption rate of As(III) was slightly weakened because of the redox reaction between As(III) and Fe(III) under the anoxic conditions.⁴¹ For further investigation, ethylenediaminetetraacetic acid disodium salt (EDTA) was introduced in the system. EDTA is widely used as chelating agent, as it has the capacity to sequester most metal positive ions. After being bound by EDTA, metal ions remain in solution but exhibit diminished reactivity. Data plotted in Fig. 8d show that the amount of ferric ion is lower than that in the absence of EDTA. After the addition of 1 g of EDTA to the reaction system with N₂ bubbled, the adsorption rate of As(III) decreased to 57% as shown in Fig. 9c. The addition of EDTA and bubbled N₂ seriously inhibit the adsorption efficiency under photo illumination. Therefore, the ferric ion released by α-Fe₂O₃ photocorrosion is the major responsible factor in the oxidation of As(III).

Fig. 9 Adsorption rate of As(III) with photo illumination for the α-Fe₂O₃ (F1) under different conditions: (a) O₂-bubbled (b) N₂-bubbled, (c) N₂-bubbled with addition of 1 g EDTA.

On the basis of the experimental results, a mechanism for As(III) treatment is proposed and illustrated in Fig. 10. In the first step, α-Fe₂O₃ particles release ferric ion under photo illumination. Then the As(III) can be oxidized to As(V) by oxygen in the presence of Fe(III) and UV illumination. As the adsorption of As(V) is much higher than that of As(III) on iron oxide, most of the toxic element is finally removed from water.

Fig. 10 Schematic illustration for As(III) oxidation and adsorption in one-step.

4. Conclusions

In summary, α-Fe₂O₃ with great adsorption capability was synthesized by hydrothermal treatment of Jarosite in the absence of any other organic compound. The as-obtained α-Fe₂O₃ exhibited better adsorption capability of As(III) under photo illumination than without photo illumination. Because the adsorption of As(III) is much lower than that of As(V) on iron oxide, it can be concluded that the photooxidation of As(III) in the presence of Fe(III) derived from the photocorrosion of α-Fe₂O₃ may be responsible for this phenomenon. α-Fe₂O₃ may have great potential in the purification of water polluted by toxic ions such as As(III) and other heavy metal elements.

Acknowledgements

The work was supported by National Natural Science Foundation of China (U1033603, 21033003 and 20977016), National Basic Research Program of China (973 Program: 2011CB612314 and 2010CB234604), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818). The authors also thank for the kind assistance from Dr Haitao Zhu, College of Materials Science and Engineering, Qingdao University of Science and Technology.

References

1. P. L. Smedley and D. G. Kinniburgh, Appl. Geochem., 2002, 17, 517.
2. Y. Xia and J. Liu, Toxicology, 2004, 198, 25.
3. K. C. Saha, Crit. Rev. Environ. Sci. Technol., 2003, 30, 127.
4. D. Mohan and C. U. Jr. Pittman, J. Hazard. Mater., 2007, 142, 1.
5. S. R. Wickramasinghe, B. Han, J. Zimbron, Z. Shen and M. N. Karim, Desalination, 2004, 169, 231.
6. B. Pakzadeh and J. R. Batista, J. Hazard. Mater., 2011, 188, 399.
7. P. Brandhuber and G. Amy, Desalination, 1998, 117, 1.
8. L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song and L. J. Wan, Adv. Mater., 2006, 18, 2426.
9. A. D. Ebner, J. A. Ritter, H. J. Ploehn, R. L. Kochen and J. D. Navratil, Separ. Sci. Technol., 1999, 34, 1277.
10. M. J. Kim and J. Nriagu, Sci. Total Environ., 2000, 247, 71.
11. M. C. Dodd, N. D. Vu, A. Ammann, V. C. Le, R. Kissner, H. V. Pham, T. H. Cao, M. Berg and U. V. Gunten, Environ. Sci. Technol., 2006, 40, 3285.
12. M. Pettine, L. Campanella and F. J. Millero, Geochim. Cosmochim. Acta, 1999, 63, 2727.
13. V. Q. Chiu and J. G. Hering, Environ. Sci. Technol., 2000, 34, 2029.
14. M. Borho and P. Wilderer, Water Sci. Technol., 1996, 34, 25.
15. D. W. Oscarson, P. M. Huang, W. K. Liaw and U. T. Hammer, Soil Sci. Soc. Am. J., 1983, 47, 644.
16. W. Driehaus, R. Seith and M. Jekel, Water Res., 1995, 29, 297.
17. Z. Xu and X. Meng, J. Hazard. Mater., 2009, 168, 747.
18. M. T. Emett and G. H. Khoe, Water Res., 2001, 35, 649.
19. J. Yeo and W. Choi, Environ. Sci. Technol., 2009, 43, 3784.
20. W. Zhou, H. Fu, K. Pan, C. Tian, Y. Qu, P. Lu and C.-C. Sun, J. Phys. Chem. C, 2008, 112, 19584.
21. J. L. T. Hage and R. D. Schuiling, Miner. Eng., 2000, 13, 287.
22. C. R. Cheeseman, A. Makinde and S. Bethanis, Resour., Conserv. Recycl., 2005, 43, 147.
23. L. Chen, X. Yang, J. Chen, J. Liu, H. Wu, H. Zhan, C. Liang and M. Wu, Inorg. Chem., 2010, 49, 8411.
24. S. Zeng, K. Tang, T. Li and Z. Liang, J. Phys. Chem. C, 2010, 114, 274.
25. X. L. Wu, Y. G. Guo, L. J. Wan and C. W. Hu, J. Phys. Chem. C, 2008, 112, 16824.
26. L. P. Zhu, H. M. Xiao and S. Y. Fu, Cryst. Growth Des., 2007, 7, 177.
27. G. A. Norton, R. G. Richardson, R. Markuszewski and A. D. Levine, Environ. Sci. Technol., 1991, 25, 449.
28. B. Tang, G. L. Wang, L. H. Zhuo, J. C. Ge and L. J. Cui, Inorg. Chem., 2006, 45, 5196.
29. X. L. Xie, H. Q. Yang, F. H. Zhang, L. Li, J. H. Ma, H. Jiao and J. Y. Zhang, J. Alloys Compd., 2009, 477, 90.
30. J. G. Yu, X. X. Yu, B. B. Huang, X. Y. Zhang and Y. Dai, Cryst. Growth Des., 2009, 9, 1474.
31. M. Zic, M. Ristic and S. Music, J. Alloys Compd., 2008, 464, 81.
32. X. Wang, X. Chen, L. Gao, H. Zheng, M. Ji, C. Tang, T. Shen and Z. Zhang, J. Mater. Chem., 2004, 14, 905.
33. C. Wei, X. Wang, Z. Nan and Z. Tan, J. Chem. Eng. Data, 2010, 55, 366.
34. B. P. Jia and L. Gao, Cryst. Growth Des., 2008, 8, 1372.
35. R. Wu, J. Qu and Y. Chen, Water Res., 2005, 39, 630.
36. J. Giménez, M. Martínez, J. de Pablo, M. Rovira and L. Duro, J. Hazard. Mater., 2007, 141, 575.
37. W. Yan, M. A. V. Ramos, B. E. Koel and W. X. Zhang, Chem. Commun., 2010, 46, 6995.
38. C. Su and A. W. Puls, Environ. Sci. Technol., 2001, 35, 1487.
39. B. D. Kocar and W. P. Inskeep, Environ. Sci. Technol., 2003, 37, 1581.
40. N. Bhandari, R. J. Reeder and D. R.Strongin, Environ. Sci. Technol., 2011, 45, 2783.
41. D. W. Oscarson, P. M. Huang, C. Defosse and A. Herbillon, Nature, 1981, 291, 50.

---

Footnote

† Electronic Supplementary Information (ESI) available: Experiment for the detective of total As. Experiment for the adsorption of As(III) under photo illumination. See DOI: 10.1039/c1ra00436k/

---