Water Air Soil Pollut (2016) 227:257 DOI 10.1007/s11270-016-2955-3

Adsorption Behavior and Removal Mechanism of Arsenic from Water by Fe(III)-Modified 13X Molecular Sieves Yulong Wang & Shaofeng Wang & Xin Wang & Zhanhua Zhang & Yongfeng Jia

Received: 22 October 2015 / Accepted: 28 June 2016 # Springer International Publishing Switzerland 2016

Abstract Presented are the synthesis and characterization of Fe(III)-modified 13X molecular sieves and their application as a novel adsorbent for removing arsenic from aqueous solutions. Batch experimental results showed that Fe(III) adsorption by 13X molecular sieves matched well with the Langmuir adsorption isotherm. The adsorption kinetics of arsenic on the Fe(III)-modified molecular sieves fit well with a pseudo-secondorder model. The Langmuir adsorption isotherms of arsenic adsorption indicated the highest adsorption capacities of 1167.79 for As(V) at pH 4 and 731.56 mg/kg for As(III) at pH 9. The Fe(III)-modified 13X molecular sieves removed much more As(V) than As(III) at equivalent arsenic concentrations, regardless of the pH conditions. After As(V) removal, the Fe(III)-modified 13X molecular sieves were characterized by PXRD, SEMEDX, and ATR-FTIR to analyze the morphology and arsenic speciation. The results of PXRD and SEM-EDX spectroscopy indicated that the material was physically

Electronic supplementary material The online version of this article (doi:10.1007/s11270-016-2955-3) contains supplementary material, which is available to authorized users. Y. Wang : S. Wang (*) : X. Wang : Z. Zhang : Y. Jia Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, No. 72, Wenhua Road, Shenyang 110016, China e-mail: [email protected] Y. Wang (*) : Z. Zhang University of the Chinese Academy of Sciences, Beijing 100049, China e-mail: [email protected]

stable after As(V) adsorption. ATR-FTIR spectroscopy showed that the formation of inner-sphere surface complexations between Fe hydroxide on the surface of the molecular sieves and As(V) could be a plausible mechanism for the uptake of arsenic by the Fe(III)-modified 13X molecular sieves. Therefore, the relatively low cost and remarkable arsenic-adsorption performance make the title material a promising absorbent for the treatment of arsenic in wastewater. Keywords 13X molecular sieves . Fe-modified . Arsenic . Adsorption . Removal

1 Introduction Arsenic (As) is a highly toxic and carcinogenic metalloid that is ubiquitous in the environment and organisms. Natural processes and anthropogenic activities, such as weathering reactions, geochemical reactions, and mining and agricultural activities, have important impacts on the release and accumulation of As in the environment, increasing the extent of pollution by wastewater containing high levels of arsenic (Mohan and Pittman 2007; Smedley and Kinniburgh 2002). The mobilization of arsenic may lead to numerous arsenic environmental and health problems. One of the major issues is that several diseases, such as cancers of the skin, lung, bladder, and kidney; angiocardiopathy; neurological disorders; muscular weakness; and pigmentation changes, can arise as a result of the longterm uptake of arsenic-bearing groundwater. It is

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reported that approximately 20 million people in China are under the threat of chronic exposure to arseniccontaminated groundwater (Rodriguez-Lado et al. 2013). Therefore, there is great demand for developing cost-effective technologies to remove arsenic from drinking water (Mohan and Pittman 2007). In most aquatic environments, arsenic is primarily present in inorganic forms, and the dominant species are arsenate [As(V)] and arsenite [As(III)] (Smedley and Kinniburgh 2002). The speciation of As in the environment is mainly governed by the redox potential (Eh) and pH conditions (Mohan and Pittman 2007; Smedley and Kinniburgh 2002). In general, As(V) is the major species under well-oxygenated conditions and exists as the oxyanion forms (H2AsO4−, HAsO42−, and AsO43− with pKa1 = 3.60, pKa2 = 7.25, and pKa3 = 12.25, respectively) at various pH values. However, under reducing conditions (such as in groundwater), As(III) is the predominant form, present as the neutral species H3AsO3 at pH less than approximately 9.2. Hitherto, various effective arsenic removal methods including coprecipitation, coagulation, ion exchange, adsorption, reverse osmosis, and electrodialysis have been developed and compared (Dixit and Hering 2003; Jia and Demopoulos 2008; Jia et al. 2007; Mertens et al. 2012; Mohan and Pittman 2007; Tuna et al. 2013). Among them, adsorption is an extremely promising technology because of its relatively low cost, effectiveness, and convenient operation. Fe- or Al-hydroxides as adsorbents have been systematically studied and widely used to remove arsenic because of their high affinity for As species and low operating costs (Dixit and Hering 2003; Mohan and Pittman 2007; Zhang et al. 2007). However, all of these adsorbents suffer from one or more drawbacks or limitations, such as separation due to the fine powdered forms of iron hydroxide or a high tendency to agglomerate (Mohan and Pittman 2007; Tuna et al. 2013). To overcome these problems, an inexpensive and effective method involves modification of the support material with iron hydroxide to remove As (Tuna et al. 2013). Activated carbon and graphene have been used as good candidate support media for preparing hybrid adsorbents because of their excellent chemical and physical properties and wide potential applications (Guo et al. 2015; Tuna et al. 2013; Vadahanambi et al. 2013; Zhu et al. 2009; Zhu et al. 2015). Aluminosilicate zeolites, such as clinoptilolite and chabazite, are gaining attention because of their great

Water Air Soil Pollut (2016) 227:257

ion exchange capacity and easy modification (Mohan and Pittman 2007; Payne and Abdel-Fattah 2005). Both natural and synthetic zeolites have been studied in terms of their arsenic adsorption (Elizalde-Gonzalez et al. 2001a; Elizalde-Gonzalez et al. 2001b; ElizaldeGonzalez et al. 2001c; Melo et al. 2012; Payne and Abdel-Fattah 2005; Ruggieri et al. 2008; Shevade and Ford 2004). The high aluminum content of synthetic zeolites may be a significant factor governing the As removal efficiency (Melo et al. 2012; Shevade and Ford 2004). Ruggieri et al. investigated the application of natural common zeolitic rocks to the treatment of arsenic-contaminated water (Ruggieri et al. 2008). Recently, some studies of natural zeolites modified with Fe to remove As have also been reported. The presence of Fe in zeolites could improve the uptake of arsenic (Baskan and Pala 2013; Jeon et al. 2009; Li et al. 2011; Payne and Abdel-Fattah 2005; Qiu and Zheng 2007; Simsek et al. 2013a; Simsek et al. 2013b; Stanić et al. 2008). However, the results of these investigations were inconsistent, which could be attributed to the variation in the Fe content and the solution pH and the different performances of the natural zeolites used (Lv et al. 2014; Simsek et al. 2013b; Stanić et al. 2008; Tuna et al. 2013). Macroscopic and spectroscopic techniques have been used extensively to investigate the adsorption mechanisms of arsenate and arsenite on minerals (such as goethite and 6-L ferrihydrite) (Gao et al. 2013; Neupane et al. 2014; O’Reilly et al. 2001; Swedlund et al. 2014). Both arsenate and arsenite can form predominantly inner-sphere surface complexes through ligand exchange reactions. The proposed mechanisms involve arsenic removal by iron-coated zeolite through ligand exchange reactions with the hydroxide groups, which results in an increase in solution pH after arsenic adsorption (Jeon et al. 2009; Li et al. 2011). However, these mechanisms are assumptions and lack strong evidence. Therefore, it is necessary to examine the reliability and validity of these mechanisms. However, to the best of our knowledge, there are very few results on the adsorption characteristics and removal mechanisms of arsenic by iron-coated synthetic zeolites, especially those with an ordered and uniform pore structure, which warrant further research. In 2005, Payne and Abdel-Fattah compared the arsenic adsorption on Faujasite (13X) and Linde (5A) molecular sieves with that on Fe-treated natural zeolites and activated carbon, which represents the only literature concerning synthetic iron-coated zeolites to date (Payne and Abdel-Fattah

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2005). However, the adsorption kinetics and adsorption edges were not investigated due to the poor arsenic adsorptive performance, which may have been attributed to their Fe treatment processes. Herein, the main objectives of the present work were (i) to modify 13X molecular sieves, whose effective pore diameter is approximately 10 Å, with Fe(III); (ii) to investigate the adsorption properties of arsenate and arsenite from water; and (iii) to characterize the samples before or after arsenic adsorption using PXRD, SEM-EDX, and ATRFTIR. The arsenate adsorption mechanism of the formation of inner-sphere complexations on the surface of zeolite modified with Fe(III) was observed by ATRFTIR spectroscopy for the first time.

2 Materials and Methods 2.1 Materials The 13X molecular sieves were obtained from the Hengye Group (Shanghai, China) with a particle size range of 1.6–2.5 mm and a chemical formula of Na2O·Al2O3·(2.8 ± 0.2)SiO2·(6-7)H2O. The average pore size was 10 Å. All the chemicals purchased from commercial sources were of analytical reagent grade and used without further purification. All volumetric flasks and vessels were cleaned by soaking in 10 % HNO3 for at least 24 h and rinsing several times with distilled water. The iron used for the adsorption and modification of the molecular sieves was Fe(NO3)3·9H2O. An artificial As stock solution for batch studies was prepared by dissolving Na2HAsO4·7H2O and NaAsO2 in deionized (DI) water, respectively, and was used to prepare separate working solutions by diluting the stock arsenate solution with deionized (DI) water. 2.2 Batch Experiments of Fe(III) Uptake by Molecular Sieves To each 50-mL centrifuge tube, 1.0 g of molecular sieves and 40.0 mL of a Fe(III) solution at concentrations from 0.1 to 15 mmol/L were combined. After equilibration on a platform shaker at room temperature for 24 h, an aliquot of each supernatant was extracted by a syringe and filtered through a 0.22-μm membrane (Millipore) for the analysis of equilibrium Fe(III) concentrations. The amount of Fe adsorbed/exchanged was

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calculated from the difference between the initial and equilibrium concentrations. 2.3 Preparation of Fe-Modified Molecular Sieves For the preparation of Fe-modified molecular sieves, 50 g of molecular sieves and 500 mL of a 20 mmol/L Fe(III) solution were combined. The pH of the mixture was adjusted to 10 using 0.5 M NaOH and shaken on a platform shaker at room temperature (25 °C). During this procedure, the NaOH or HCl solution was added to maintain the pH of the system at the target value for 24 h. Then, the mixtures were allowed to settle, and the supernatant was removed, followed by washing of the molecular sieves with DI water several times until the Fe(III) concentration of the filtrate was below 0.5 mg/L. The modified molecular sieves were allowed to dry in a vacuum oven at 60 °C and were stored in an N2-purged desiccator. 2.4 Adsorption Experiments The sorption of As(III) and As(V) onto the Fe-modified molecular sieves was initiated by the addition of the solid to the arsenic solutions. To study the arsenic adsorption edges, 2.0 g of Fe-modified molecular sieves was mixed with 100 mL of a 25 mg/L As solution at different pH values from 3 to 12. To study the sorption isotherms, 2.0 g of Fe-modified molecular sieves was mixed with 100 mL of the As solutions with initial concentrations of 0.1 to 50 mg/L at pH 4.0 ± 0.2 for As(V) and at pH 9.0 ± 0.2 for As(III), corresponding to the maximum sorption of the arsenic species at different pH values. The kinetic studies of arsenic removal were conducted at pH 4.0 ± 0.2 for As(V) and at pH 9.0 ± 0.2 for As(III). The initial concentration of arsenic was 25 mg/L in 300 mL of solution, which was mixed with 6.0 g of the Fe-modified molecular sieves. The pH of the system was readjusted using 1 M HCl or 1 M NaOH solutions to keep constant throughout the adsorption tests. The mixture was shaken on a platform shaker for 24 h at room temperature (25 °C), except in the kinetic studies in which the mixtures were shaken from 0.25 to 30 h. At the end of the experiments, the mixtures were allowed to settle and the supernatant was passed through a 0.22-μm membrane (Millipore) with a syringe filter before being analyzed for their equilibrium As solution concentrations; solutions were withdrawn at selected time intervals for the kinetic adsorption experiments.

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The uptake of As by the Fe-modified molecular sieves was determined based on the difference between the initial and final arsenic concentrations in solution. All batch experiments were performed in duplicate, and the mean value was used to analyze the results of the study.

the Fe(III) adsorption. The Langmuir equation is expressed as follows:

2.5 Analytical Methods

where qe (mmol/kg) is the amount of Fe(III) adsorption, qmax (mmol/kg) is the maximum adsorption amount of Fe(III) ions per unit weight of adsorbent for complete monolayer coverage, Ce (mmol/L) is the equilibrium concentration in the solution phase, and k (L/mmol) is the equilibrium adsorption constant. The isotherm for iron (III) adsorption on 13X molecular sieves, obtained through batch experiments, was fitted using the Langmuir sorption isotherm model with a coefficient of determination of R2 = 0.98 (Fig. 1). Based on the Langmuir fit to the data, the calculated Fe(III) sorption capacity was 357.14 mmol/kg. However, with a trivalent charge, the total charge of Fe(III) (Lv et al. 2014) adsorbed was much less than the total cation exchange capacity (TCEC) of 4366.8 mequiv/kg calculated from the chemical formula of the 13X molecular sieves. However, the total charge of Fe(III) adsorbed was much greater than the external cation exchange capacity (ECEC) of the natural zeolite, suggesting that the internal sorption sites were probably partially responsible for the Fe(III) uptake (Li et al. 2007). The amount of Fe(III) adsorbed was much different from the amounts on natural zeolite (Li et al. 2011; Stanić et al. 2008). The larger difference in Fe contents could be attributed to the initial Fe(III) concentrations and the effective pore diameter (Simsek et al. 2013b).

3 Results and Discussion 3.1 Batch Fe(III) Adsorption

400

Fe(III) adsorbed (mmol/kg)

The pH of the solutions was measured using a pH electrode (Eutech, America), which was calibrated using commercial pH 4.01, 6.86, and 9.18 buffers. Quantification of the total iron in the Fe(III)-modified 13X molecular sieves was analyzed by the acid leaching procedure using a 5 M HNO3 solution. The concentration of Fe in the supernatant was determined on a flame atomic absorption spectrophotometer (AA240, Varian) at 248.3-nm wavelength with a detection limit of 0.05 mg/L, and duplicate analyses agreed within 7 %. The concentration of As was measured on a hydridegeneration atomic fluorescence spectrophotometer (AFS-2202E, Haiguang Corp., Beijing) with a detection limit of 10 μg/L, and duplicate analyses agreed within 5 %. Proper dilution was performed for higher FeTOT and As concentrations. The morphological structures and elemental compositions of the 13X molecular sieves, Fe(III)-modified molecular sieves, and the sieves after arsenic adsorption were examined by a scanning electron microscope combined with an energy-dispersive X-ray spectrometer (SEM-EDX, S-3400N, Hitachi, Japan). Powder X-ray diffraction (P-XRD) patterns were recorded on a Rigaku Miniflex II diffractometer using CuKα radiation to determine the crystalline structures of the samples. Attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) of the samples was conducted on a Bio-Rad FTS 60 Fourier transform infrared spectrometer with an MCT liquid nitrogencooled detector. The measurement resolution was set at 4 cm−1, and the spectra were obtained in the range of 400–4000 cm−1 with 200 co-added scans.

Ce 1 1 ¼ Ce þ qmax bqmax qe

300

200

100

0 0

Adsorption isotherms are usually determined to study the equilibrium relationships between an adsorbent and an adsorbate. The Langmuir model was used to describe

1

2

3

4

5

6

7

Equilibrium Fe concentration (mmom/L) Fig. 1 Adsorption of Fe(III) on 13X molecular sieves. The solid line is the Langmuir fit to the experiment data

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3.2 Characterization of the Samples The surface structure and stability of the Fe-modified molecular sieves in the arsenic solutions was considered before the subsequent systematic arsenic adsorption study. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectrometric microanalysis (EDX) of the surfaces are shown in Fig. 2 and Table 1. The major constituents of the molecular sieves are silicate, sodium, and alumina, based on the EDX analysis described in Fig. 2a. The superficial iron contents of the Fe-modified molecular sieves and Femodified molecular sieves after arsenate adsorption were 35.14 and 34.75 wt%, respectively (Fig. 2b, c). The sodium content of the Fe-modified molecular sieves decreased from 12.56 to 0.91 wt% with the modification by iron of the molecular sieves. The iron content was 35.14 wt% and agglomerated as a spherical shape on the surface of the molecular sieves, which is consistent with the macroscopic data of Fe(III) adsorption (Fig. 1). This indicated that the cation exchange and formation of iron hydroxide on the surface of the molecular sieves may occur simultaneously, combined with the results of the Fe(III) Langmuir adsorption isotherm (Fig. 1). The SEM micrographs show evidence of adsorbed arsenate distributed in a bulky manner on the surface. Meanwhile, the appearance of the new arsenate peaks in the EDX

Fig. 2 SEM images and EDX surface analysis of the initial molecular sieves (a), Fe-modified molecular sieves (b), and Femodified molecular sieves after As(V) adsorption (c). Adsorption

Page 5 of 10 257 Table 1 The element constituents based on the quantitative EDX results Element (wt%) O

Na

Si

Al

Fe

As

1

33.72

12.56

18.66

35.05

–

–

2

33.33

0.91

6.82

23.80

35.14

–

3

30.57

1.33

6.90

22.38

34.75

4.05

The numbers 1, 2, and 3 represent the initial molecular sieves, Femodified molecular sieves, and Fe-modified molecular sieves after arsenate adsorption, respectively

spectrum after arsenate adsorption reveals that the As(V) adsorbed on the surface was approximately 4.05 wt% (Fig. 2c and Table 1). As clearly depicted in Fig. 3, the powder X-ray diffraction patterns of the Fe-modified molecular sieves and Fe-modified molecular sieves after arsenate adsorption were similar to that of the initial molecular sieves, which indicates that the crystal structure of the molecular sieves did not change after Fe exchange and arsenate adsorption. Figure S1 in the Electronic Supporting Information (ESI) presents the ATR-FTIR spectra of the Fe-modified molecular sieves and Fe-modified molecular sieves after arsenate adsorption. According to Fig. S1, the new absorbance peak that appeared at 877.7 cm−1 was

conditions: pH = 4; initial arsenate concentration, C0 = 50 mg/L; and solid/solution, 20 g L−1

23.23 26.56

Relative intensity / a.u.

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6.18 15.55 10.07

a 30.89

20.05

1000

800

33.63 Fe-modified Molecular Sieves

Molecular Sieves

As(V) Adsorption by Fe-modified Molecular Sieves

As Adsorbed (mg/kg)

257

600

400

As(V) As(III)

200

10

20

30

40

50

60

0 0

2θ / °

assigned to the symmetric vibration of surfacecomplexed As–O− bonds, similar to the previous spectroscopic studies of arsenate adsorption on feroxyhyte, ferrihydrite, and hematite and consistent with the formation of inner-sphere As(V) surface complexations (Gao et al. 2013; Muller et al. 2010). Overall, the invariance of crystallinity and crystal morphology among the molecular sieves, Fe(III)-exchanged molecular sieves, and Fe(III)-exchanged molecular sieves after arsenate adsorption suggests that the molecular sieves exchanged with Fe(III) were physically stable and that ferric hydroxides could precipitate on the surface of the molecular sieves. Inner-sphere complexations between Fe hydroxides on the surface of the molecular sieves and arsenate formed on the surface of the Fe-modified molecular sieves. 3.3 Kinetics of Arsenic Adsorption To evaluate the sorption process, the adsorption kinetics of arsenic on the Fe-modified molecular sieves were studied by adding Fe-modified molecular sieves to a solution of arsenic. As shown in Fig. 4, the uptake of As(V) in solution was extremely fast in the initial 15 min, followed by a much slower adsorption process. Similar trends, but with lower adsorption, occurred for the uptake of As(III). Slow adsorption kinetics were also reported previously and may be attributed to the decrease in adsorption sites of Fe-modified molecular sieves and/or the decrease in the concentration gradient

10

15

20

25

30

35

30

35

Time (h)

b

0.08

t/qt of As(V) t/qt of As(III) 0.06

t/qt (h•kg•mg-1)

Fig. 3 Comparison of the powder X-ray diffraction patterns of the initial molecular sieves, Fe-modified molecular sieves, and Femodified molecular sieves after As(V) adsorption. Adsorption conditions: pH = 4; initial arsenate concentration, C0 = 50 mg/L; and solid/solution, 20 g L−1

5

Linear fit of t/qt of As(V) Linear fit of t/qt of As(III)

0.04

0.02

0.00 0

5

10

15

20

25

Time (h)

Fig. 4 Adsorption of arsenate and arsenite onto Fe-modified molecular sieves with time (a) and their fit with a pseudosecond-order rate model (b). Experimental conditions: pH 4.0 for As(V) and pH 9.0 for As(III)

of As (Neupane et al. 2014; Swedlund et al. 2014). A pseudo-second-order rate equation model was applied to describe the As adsorption kinetic data in order to investigate the mechanism of As adsorption on the Femodified molecular sieves. The adsorption constant (KC) can be calculated using the following equation: . . .  t qt ¼ 1 K C  qe 2 þ t q e where qt (mg/kg) is the amount of adsorption at any time t, and qe (mg/kg) is the amount of adsorption at equilibrium (Lin et al. 2015). The parameters obtained from the kinetic model are listed in Table 2. Remarkably, the data for arsenate and arsenite adsorption fit well to the pseudo-second-order

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Table 2 Kinetic parameters for arsenate and arsenite adsorption by Fe-modified molecular sieves using a pseudo-second-order equation model Parameters

qe (mg/kg)

Kc [mg/(kg h)]

R2

As(V)

833.33

2.400 × 10−3

0.9987

As(III)

454.55

1.862 × 10−3

0.9966

equation model, with high coefficients of determination (R2) of 0.9987 and 0.9975 for As(V) and As(III) adsorption, respectively. These results are similar to As adsorption on Fe-modified natural zeolite (Jeon et al. 2009; Li et al. 2011). Furthermore, the uptake of arsenate and arsenite at equilibrium qe calculated from the pseudosecond-order equation model is 833.33 and 454.55 mg/kg, respectively, agreeing well with the results from the batch isotherm study (Fig. 6). 3.4 Effect of Absorbent Dosage

3.5 Adsorption Isotherms Arsenic adsorption isotherms were obtained at pH 4.0 for arsenate and at pH 8.0 for arsenite (Fig. 6). It is clear that As(V) had higher adsorption on the Fe-modified molecular sieves than As(III) under all arsenic concentrations used. This result is consistent with the arsenic adsorption edges (Fig. 7). The Langmuir isotherm model was employed to study the arsenic adsorption data. The adsorption constants obtained from the Langmuir model are listed in Table 3. As shown in Table 3, the Langmuir adsorption isotherms matched well with the experimental data, with R2 = 0.95 and 0.99 for As(V) and As(III) adsorption, respectively. The calculated As(V) and As(III) adsorption capacities were 1167.79 and 731.56 mg/kg, respectively (Fig. 6). In similar studies, the As(V) adsorption capacity was 40, 680, and 100 mg/kg on Fe(II)-modified clinoptilolite (Payne and Abdel-Fattah 2005), Fe(III)-modified clinoptilolite (Jeon et al. 2009) and Fe(III)-exchanged nature zeolite (Li et al. 2011), respectively. These large discrepancies

100

1200

80

1000

As Adsorbed (mg/kg)

Arsenic removal (%)

The effect of the adsorbent dosage on the removal of As(V) and As(III) is shown in Fig. 5. From Fig. 5, the removal efficiency of both arsenate and arsenite increased from 8.7 and 7.5 % to 99.9 % as the adsorbent dosage of the Fe-modified molecular sieves increased from 5 to 100 g/L. This increase could be attributed to the increase in exchangeable sites and surface area at higher concentrations of the absorbent. However, further increases in the absorbent dosage (>60 g/L) did not

cause significant improvements in arsenic removal. Compared with our experimental results, iron-treated 13X molecular sieves were ineffective in both arsenate and arsenic removal at the initial arsenic concentrations of C0 = 50 μg/L with 0.1000 g iron-treated 13X molecular sieves in a previous study (Payne and Abdel-Fattah 2005). This dramatic discrepancy could be attributed to the different methods of iron treatment.

60

40

As(V) As(III) 20

800 600 400

As(V) As(III)

200

0 0

20

40

60

80

100

Adsorbent dose (g/L)

Fig. 5 Effect of the adsorbent dosage on the arsenic removal by Fe-modified molecular sieves from arsenate and arsenite in solution. Experimental conditions: initial arsenic concentrations, 25 mg/L; pH 4.0 for As(V) and pH 9.0 for As(III); temperature, 25 °C; and equilibration time, 24 h

0 0

10

20

30

40

Equilibrium As concentration (mg/L)

Fig. 6 As(V) and As(III) sorption isotherms for the Fe-modified molecular sieves. The solid line is the Langmuir fit to the experiment data. Experimental conditions: pH 4.0 for the As(V) isotherm and pH 9.0 for the As(III) isotherm

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As Adsorbed (mg/kg)

700 600 500 400 300 200

As(V) As(III)

100 0 2

4

6

8

10

12

pH

Fig. 7 Comparison of the As(V) and As(III) adsorption edges on Fe-modified molecular sieves

can be partially explained by the various Fe contents and the diverse surface properties of zeolite. Note that the arsenic adsorption isotherms of the iron-treated 13X molecular sieves were not fitted by Langmuir or Freundlich models for the arsenic adsorptive performance in previous work (Payne and Abdel-Fattah 2005).

3.6 Arsenic Adsorption Edges The pH of the solution, which is one of the most important factors in the treatment of groundwater for arsenic, can affect the surface charge of adsorbents and the speciation of arsenic due to deprotonation and protonation reactions. Therefore, it is necessary to investigate the effect of pH on the arsenic adsorption process. The influence of pH on arsenate and arsenite adsorption on Fe-modified molecular sieves is compared in Fig. 7. Batch experiments were carried out at ambient temperature and the initial As concentrations of 25 mg/L with a 24-h reaction time. As a result, the uptake of arsenate by the Fe-modified molecular sieves was higher than arsenite under the same conditions with respect to pH, which

Table 3 Langmuir isotherm parameters for arsenate and arsenite adsorption on Fe-modified molecular sieves Parameters

qm (mg/kg)

b (L/mg)

R2

As(V)

1167.79

0.23

0.9544

As(III)

731.56

0.035

0.9968

is similar to a previous observation of As adsorption by clinoptilolite-rich tuffs (Elizalde-Gonzalez et al. 2001c). In the pH range of the experiments, the uptake of arsenate decreased with increasing pH. The maximum As(V) adsorption capacity was observed at lower pH, similar to that found by other researchers (Dixit and Hering 2003; Jia et al. 2007; Zhang et al. 2007). On the other hand, the effect of pH on arsenite adsorption was different from that of arsenate; arsenite removal increased with increasing pH to a maximum adsorption at pH 9.0 and then decreased with further increases in pH in our experiments. The effect of pH on arsenate and arsenite adsorption is consistent with previously published studies (Dixit and Hering 2003; Li et al. 2011). This may be attributed to the surface properties and different adsorption behaviors of arsenate and arsenite (Baskan and Pala 2013; Payne and Abdel-Fattah 2005). 3.7 Possible Mechanisms of As Adsorption Previous studies have shown that the widely accepted mechanisms of arsenate and arsenite removal by iron hydroxides specifically involve adsorption consisting of ligand exchange in the mode of both monodentate and bidentate binuclear inner-sphere complexations. Researchers have proposed these mechanisms based on transmission Fourier transform infrared, attenuated total internal reflectance-Fourier transform infrared, and extended X-ray adsorption fine structure analyses (Jia et al. 2007; Neupane et al. 2014; O’Reilly et al. 2001; Waychunas et al. 1996). However, arsenite can form both inner-sphere and outer-sphere complexes on iron hydroxides, which has been reported by a few researchers (Catalano et al. 2008). In addition, arsenite associated with Fe and Mn oxyhydroxides in aqueous systems can be oxidized to arsenate in the presence of dissolved O2 (Zhao et al. 2011). Therefore, we focused on the characterization of the samples after arsenate adsorption and studied the mechanism of arsenate adsorption onto Fe(III)-exchanged molecular sieves. In our case, the uptake of Fe(III) by 13X molecular sieves could be attributed to cation exchange and the formation of iron hydroxides (Figs. 1 and 2). ATR-FTIR spectroscopy suggested that arsenate forms inner-sphere complexations on the surface of the Fe(III)-modified molecular sieves. The pH of the solution increased after arsenate adsorption in the batch experiments. Based on the pKa values of arsenate acid, the dominant arsenate species would be H2AsO4− and HAsO42− under most of

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the experimental conditions. Thus, the surface complexation reactions for the arsenate removal mechanisms would be as follows: ≡½ Fe – OHðsÞ þ H2 AsO4 – →≡ Fe – OAsO3 H þ OH–

ð1Þ

≡½ Fe – OHðsÞ þ H2 AsO4 – →≡ Fe – O2 AsO2 H þ OH–

ð2Þ

modified molecular sieves clearly show that the molecular sieves were physically stable after arsenate adsorption and are suitable for arsenic removal and separation. Moreover, the enhancement of arsenic removal could be attributed to the formation of inner-sphere complexations between Fe hydroxide on the surface of the molecular sieves and arsenate, as indicated by ATR-FTIR spectroscopy. Acknowledgments This work was supported by the National Natural Science Fundation of China (Nos. 41530643, 41273133) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB14020203).

≡½ Fe – OHðsÞ þ HAsO4 2– →≡Fe – OAsO3 H– þ OH–

ð3Þ

–

≡½ Fe – OHðsÞ þ HAsO4 →≡Fe – O2 AsO2 H 2–

þ OH–

ð4Þ

≡2½ Fe – OHðsÞ þ HAsO4 2– →≡2FeO – AsO2 H þ 2OH–

ð5Þ

The removal of arsenate by Fe(III)-exchanged molecular sieves resulted in the release of hydroxide and increased the solution pH. The formation of innersphere complexations between Fe hydroxide on the surface of the molecular sieves and arsenate would cause the enhanced As removal.

4 Conclusions In summary, the synthetic 13X molecular sieves were exchanged and modified with Fe(III), and the properties related to the adsorption of arsenic by the molecular sieves were investigated in batch experiments. The uptake of Fe(III) by the molecular sieves reached 357.14 mmol/kg, and the loadings of arsenate and arsenite on the Fe-modified molecular sieves were 1167.79 and 731.56 mg/kg, respectively. The adsorption efficiency of arsenic was highly pH dependent due to the surface properties and different adsorption behaviors of arsenate and arsenite. The results of the powder X-ray diffraction analyses, SEM images, and EDX surface analysis for the adsorption of arsenate onto the Fe-

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