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Equilibria, kinetics, and spectroscopic analyses on the uptake of aqueous arsenite by two-line ferrihydrite a

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Soon-Oh Kim , Woo Chun Lee , Hyen Goo Cho , Byung-Tae Lee , Pyeong-Koo Lee & Sun Hee Choi

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Department of Earth and Environmental Sciences and Research Institute of Natural Science (RINS) , Gyeongsang National University , Jinju , 660-701 , Republic of Korea b

School of Environmental Science and Engineering, Gwangju Institute of Science and Technology(GIST) , Gwangju , 500-712 , Republic of Korea c

Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources , Daejeon , 305-350 , Republic of Korea d

Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH) , Pohang , 790-784 , Republic of Korea Accepted author version posted online: 18 Jul 2013.Published online: 20 Aug 2013.

To cite this article: Soon-Oh Kim , Woo Chun Lee , Hyen Goo Cho , Byung-Tae Lee , Pyeong-Koo Lee & Sun Hee Choi , Environmental Technology (2013): Equilibria, kinetics, and spectroscopic analyses on the uptake of aqueous arsenite by twoline ferrihydrite, Environmental Technology, DOI: 10.1080/09593330.2013.824508 To link to this article: http://dx.doi.org/10.1080/09593330.2013.824508

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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.824508

Equilibria, kinetics, and spectroscopic analyses on the uptake of aqueous arsenite by two-line ferrihydrite Soon-Oh Kima , Woo Chun Leea , Hyen Goo Choa , Byung-Tae Leeb , Pyeong-Koo Leec and Sun Hee Choid∗ a Department

of Earth and Environmental Sciences and Research Institute of Natural Science (RINS), Gyeongsang National University, Jinju 660-701, Republic of Korea; b School of Environmental Science and Engineering, Gwangju Institute of Science and Technology(GIST), Gwangju 500-712, Republic of Korea; c Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea; d Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

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(Received 23 October 2012; final version received 21 June 2013 ) Arsenite sorption from aqueous solutions was investigated using two-line ferrihydrite at room temperature, as a function of solution pH and arsenite loading. The isotherms, pH envelopes, and kinetics of arsenite sorption were characterized and its mechanism was elucidated via X-ray absorption spectroscopic studies. Arsenite sorption showed only slight pH dependence with a sorption maximum centered around pH 8.0. The Langmuir isotherm is most appropriate for arsenite sorption over the wide range of pH, indicating the homogenous and monolayer sorption of arsenite. The kinetic study demonstrated that arsenite sorption onto two-line ferrihydrite is considerably fast and the equilibrium is achieved within the reaction time of 3 h. X-ray absorption near-edge structure spectroscopy elucidated a slight change in oxidation state of arsenite for the initial concentration of 13.35 mM at pH 4. The extended X-ray absorption fine structure (EXAFS) spectroscopy results indicate that types of surface complexes of arsenite appeared to be very similar to those proposed by the previous studies in that the bidentate binuclear corner-sharing (2 C) complex is predominant at all the surface loadings. However, our EXAFS results suggest that regardless of pH, the mixed complexes of 2 C and bidentate mononuclear edge-sharing surface complex (2 E) as well as the 2 C complex are favoured at low and intermediate surface loadings, but only the 2 C complex is dominant at high surface loading. Overall, the EXAFS results support the efficient removal of arsenite by the two-line ferrihydrite through the formation of highly stable inner-sphere surface complexes, such as 2 C complex. Keywords: arsenite; two-line ferrihydrite; sorption equilibria; sorption kinetics; X-ray absorption spectroscopy

1. Introduction Arsenic (As) has been introduced into surrounding environments, resulting in significantly adverse effects. The major sources of As contamination have been reported mining and smelting operations, As-containing pesticides and herbicides, and various industrial activities.[1,2] Concern has been raised, particularly in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh, and Vietnam.[1] In the most of those regions, it has been reported that As-contaminated groundwater has a great potential for immediate human exposure.[1] The scale of As problem in terms of population exposed to high As concentrations is greatest in the Bengal Basin with more than 40 million people drinking water containing excessive As.[1,3,4] The range of As concentrations found in natural waters is large, ranging from less than 0.5 μg l−1 to more than 5000 μg l−1 and baseline concentrations of As in soils are generally of the order of 5–10 mg kg−1 .[1] The predominant forms of As in the environment, such as soils, groundwaters, and surface waters, are the inorganic species ∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

arsenate [As(V)] and arsenite [As(III)]. It is known that the arsenite is 25–60 times more toxic than arsenate.[5] As a result of slow oxidation of As(III) and microbial reduction of As(V), those two species of As can commonly coexist in nonequilibrium state.[6,7] For these reasons, arsenite was studied in this study. Arsenite remains protonated as arsenious acid (H3 AsO03 ) at acidic and neutral pHs less than 2− 9.22, whereas exists as oxyanions (H2 AsO− 3 and HAsO3 , with pKa1 = 9.22 and pKa2 = 12.13, respectively) at alkaline pH.[8,9] The strong sorption of As onto the surfaces of iron oxides such as goethite, ferrihydrite, lepidocrocite, maghemite, and hematite has been invoked as an important mechanism of natural attenuation of As pollution in soil and groundwater and lacustrine sediments.[8,10–19] Ferrihydrite is one of the most ubiquitous iron (oxyhydr)oxides and identified by poorly crystalline to amorphous phase with stoichiometry near 5Fe2 O3 ·9H2 O.[9] It is the solid formed upon rapid hydrolysis of ferric iron solutions at 20–30◦ C.[20] Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial

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sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order.[21] Recent X-ray spectroscopic studies, however, have indicated that the ferrihydrite structure has close similarities to those of the FeOOH-type minerals, specifically goethite and akaganéite, rather than to hematite.[22] It is a strong scavenger of metal ions from aqueous solutions. Ferrihydrite that is effectively amorphous, yielding only two broad peaks in a powder X-ray pattern, has often been called ‘two-line ferrihydrite’. With ageing, two-line ferrihydrite orders into structures having progressively more long-range order, giving rise to three-, four-, five-, and six-line powder X-ray diffraction (XRD) patterns.[23,24] Because of reactivity and large specific surface area of ferrihydrite, it has been known to be one of the most important scavengers of minor elements in surface and groundwater systems.[10] This premise is based on studies of its adsorptivity in natural environments as well as numerous laboratory studies of cation and/or anion adsorption.[5,8,10–12,15,16,19,25] Recently, there have been researches on a facile one-step method for synthesis of mesoporous two-line ferrihydrite for the efficient removal of As from natural water.[26,27] Besides, the naturally formed two-line ferrihydrite could be used in the state-of-the-art water treatment technologies as an effective catalyst for hydrogen peroxide decomposition and as a suitable precursor for synthesis of passivated zerovalent iron nanoparticles.[28] For other available researches on As removal, one can refer to Mohan and Pittman.[29] Until recently, the structures of As sorption complexes on the surfaces of various iron (oxyhydr)oxide minerals have been extensively studied using X-ray absorption spectroscopy (XAS).[10–19] However, most of these studies concerned As(V), and only a few of them have focused on As(III) [8,14,17]. In addition, it has been reported that the environmental fate of As in subsurface environments is highly dependent on the As speciation, pH, ionic strength, and the presence of adsorbents such as metal oxides and phyllosilicates.[30] Up to date, there have been a number studies on arsenite adsorption onto ferrihydrite as well.[5,8,14] However, there has been a lack of comprehensive study encompassing macroscopic and microscopic aspects. In particular, crucial system variables such as pH and arsenite loading have not been taken into account simultaneously in the previous studies. The main objective of the present study was to investigate the uptake of arsenite from aqueous solution through interaction with two-line ferrihydrite. For the sake of accomplishing the objective, this study was designed to embrace equilibrium, kinetics, and spectroscopic analyses of arsenite adsorption onto two-ferrihydrite. The effect of pH and initial loading on the removal of arsenite was elucidated within the pH range of 2–14 at different As concentrations. Particularly, XAS data on arsenite sorbed onto two-line ferrihyrite are reported to investigate the mechanism of arsenite sorption onto two-line ferrihydrite at

pHs 4, 7, and 10. The XAS results were interpreted to identify the dominant surface structure of arsenite on two-line ferrihydrite.

2. Materials and methods 2.1. Synthesis and characterization of two-line ferrihydrite Two-line ferrihydrite (5Fe2 O3 ·9H2 O) used as a sorbent for arsenite uptake was synthesized in the laboratory as described by Schwertmann and Cornell,[24] with slight modifications.[5,8] Forty gram of Fe(NO3 )3 · 9H2 O was dissolved in 500 mL distilled water and 310 mL of 1 M KOH was added at a constant rate of the addition of approximately 100 mL min−1 , during the vigorous stirring with a magnetic stirrer. Subsequently, the pH of the suspension was adjusted to 7.5 by the drop-wise addition of 1 M KOH. Once the pH was stabilized at 7.5, the solution was centrifuged to separate the suspension. And then, the Milli-Q water was mixed with the suspension in the ratio of 1:10 (solid:water) and subsequently was vigorously mixed by hand and then again shaken in the reciprocal shaker at about 350 ∼ 400 rpm for 15 min. After shaking, the suspension was again separated using high-speed centrifuge at 12,000 g for 15 min. This was the first washing step and it was repeated three times. After washing, the solid was finally separated by centrifugation at 12,000 g for 15 min. The solid was freeze-dried and stored at 2◦ C. The mineralogical and morphological features of the two-line ferrrihydrite synthesized were observed using XRD, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). The two-line ferrihydrite was confirmed by powder XRD using Siemens D5005 X-ray diffractometer (40 kV, 35 mA, 0.4◦ min−1 , 3 h 30 min). CoKα radiation was used for XRD analysis, since CuKα radiation which is usually used is less suitable because it is strongly absorbed by Fe rich phases leading to loss of X-ray intensity and a high background due to fluorescence radiation.[24] The morphology and chemical composition of the two-line ferrihydrite were characterized by SEM (Philips XL30S FEG, the Netherlands) and EDS (Jeol JSM-6380LV, Japan), respectively. In addition, the surface area of a freeze-dried sample was determined with micro pore physisorption analyzer (ASAP-2020M, Micromeritics, USA) using the three-point Brunauer–Emett–Teller (BET) method of N2 gas adsorption at liquid N2 temperature. The point of zero charge (PZC) for the two-line ferrihydrite was evaluated using potentiometric titration.[15,31, 32] The stock solutions containing 0.001, 0.01, and 0.1 M NaCl were prepared. Each titration point (30–40 per curve) was obtained by pre-equilibrating 20 mL of a 2.5 g L−1 twoline ferrihydrite suspension in a capped 50-mL polyethylene centrifuge tube for 4 h. After pre-equilibration, a given amount of 0.1 M HCl or NaOH was added to each centrifuge tube to give final supernatant pH values ranging from 3 to

Environmental Technology 11. All sample manipulations were carried out under N2 purge. The centrifuge tubes were capped and placed on a reciprocating shaker for 24 h. After equilibration, the suspension was centrifuged, the pH of the supernatant was measured. Surface charge calculations were carried out for the two-line ferrihydrite. The amount of H+ or OH− required to achieve the specified pH from the PZC was determined, and the surface charge was calculated with the assumption that 1 mol of H+ or OH− added was equivalent to 1 mol of positive or negative charge gained, respectively. To calculate the surface charge for pure two-line ferrihydrite, the following equation was used:

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− σ0 = F({≡ FeOH+ 2 } − {≡ FeO })

=F

(CA − CB ) − ([H+ ] − [OH− ]) , a

where σ0 (mmolc g−1 ) is the surface charge, F (96,485 − C/mol) is the Faraday constant, {≡ FeOH+ 2 } and {≡FeO } −1 (mmol g ) are the concentrations of surface species, a (g L−1 ) is the two-line ferrihydrite concentration, CA and CB (mM) represent the total concentrations of acid (HCl) and base (NaOH) added to suspension, respectively, and [H+ ] (mM) and [OH− ] (mM) are 10−pH and 10−(pKw−pH) , respectively.

2.2.

Equilibria and kinetics

In order to evaluate equilibria and kinetics on uptake of aqueous arsenite by two-line ferrihydrite, three different experiments were conducted. Arsenite sorption isotherms and pH-edges (envelopes) were obtained from two types of equilibrium experiments. The third one was kinetic experiment to investigate the removal of arsenite from the aspect of kinetics. The common conditions applied to those three experiments were: laboratory-synthesized two-line ferrihydrite (-200 mesh) was used as a sorbent, all the stock solutions containing arsenite were prepared using NaAsO2 (Aldrich) in 0.01 M NaCl electrolyte, and the pHs of arsenite solutions were adjusted by adding a given amount of 0.01, 0.1, 1.0 M HCl or NaOH. All chemicals used were reagent grade or better. Water used was triple-deionized water passed through a MILLI-Q system. All glassware and plastic ware were cleaned using longer than 24 h phosphorus-free detergent soak, 24 h soak in 0.1 M HCl, and at least four rinses with MILLI-Q water. Aqueous concentrations of arsenite were measured by inductively coupled plasma-optical emission spectrometry (OPTIMA 5300DV, PerkinElmer). Total dissolved concentrations of arsenite were also determined for samples of < 0.05 mg L−1 by graphite furnace atomic absorption spectrometry (GFAAS, AA-6800, Shimadzu). Duplicate (equilibria experiments) or triplicate (kinetic experiment) samples were taken and measured for the sake of checking reproductivity and precision of experiments and analyses. Particularly, the relative

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standard deviations of the analytical data were measured to less than 5%. Sorption isotherms were obtained at pHs 2.0, 4.0, 7.0, 9.2, and 12.2 using initial arsenite solution concentrations of 0.133, 0.400, 0.667, 1.068, 1.335, 4.004, 6.674, 13.35, 40.04, 66.74, 133.47, 266.95, 400.4, 533.9, and 667.4 mM. It has been well known that the As of lower concentrations (few ppbs ∼ few ppms) can be effectively removed by a number of sorbents, but there has a lack of study applied to the extremely higher concentrations of As which may be generated by a number of physico-chemical treatments of contaminated soils and sediments. For this reason, the present study tested the arsenite concentration ranging from ∼10 ppm (0.133 mM) to ∼50,000 ppm (667.4 mM). A series of 50-mL polyethylene tubes containing 0.5 g twoline ferrihydrite and each arsenite stock solution of 20 mL were prepared and shaken at 200 rpm on a reciprocal shaker. After 24 h, the suspension was then centrifuged at 12,000 g for 15 min, and the supernatant was passed through a membrane filter (0.45 μm nominal pore size) and stored at 2◦ C until analysed. The adherence of the sorption isotherms at 5 pH points to Freundlich, Langmuir, BET, and Temkin equations was fitted graphically. To clearly evaluate the effect of pH on arsenite removal, the experiments on sorption envelopes (edges) were investigated at two initial arsenite solution concentrations, i.e. low (13.35 mM) and high (133.47 mM), based on the results of sorption isotherms. The pHs of the solutions were adjusted to 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.2, 10.0, 11.0, and 12.2. The other experimental operations and conditions were equivalent to those used in the sorption isotherm experiments. Finally, kinetics of arsenite removal was examined at pH 8.0, based on the results of sorption envelopes. The initial arsenite concentration of 133.47 mM was used because preliminary sorption experiments showed that this concentration resulted in maximum arsenite removal. The kinetic experiments were conducted for 4 h and an aliquot of solutions was sampled after 5, 10, 15, 20, 25, 30, 40, 50, 60, 90, 120, 150, 180, 210, and 240 min of reaction. The sampled suspension was immediately filtered using a membrane filter (0.45 μm nominal pore size) and stored at 2◦ C until analysed. The consistence of the sorption kinetic data to pseudo-first order,[5,33,34] pseudo-second order,[35– 37] power function,[5,34,38] parabolic diffusion,[5,34,38] and simple Elovich [5,34,38] equations was evaluated graphically.

2.3. X-ray absorption spectroscopic analysis To elucidate the dependence of structure of As–Fe complexes on pH and surface coverage (moles of As per mole of surface sites), three kinds of arsenite stock solutions were used in sorption experiments for XAS study, in which solutions final aqueous arsenite concentrations

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were low (13.35 mM), intermediate (66.74 mm), and high (133.5 mM). Fifty-milli litre polyethylene tubes containing 0.5 g two-line ferrihydrite and each arsenite stock solution of 20 mL were prepared. The pHs of each suspension were adjusted to 4.0, 7.0, and 10.0. After 24 h shaking at 200 rpm, the suspension was then centrifuged at 12,000 g for 15 min, and the filtrate was rinsed with MILLI-Q water three times to remove excess salts, and then freeze-dried prior to XAS analysis. For analysis of aqueous arsenite concentrations remaining after reaction, in addition, the supernatant obtained after centrifugation was passed through a membrane filter (0.45 μm nominal pore size). Aliquots of supernatant were then stored at 2◦ C until analysed. Synchrotron X-ray measurements were conducted on wiggle beamline 5A (2.5 GeV; 150–180 mA) at Pohang Accelerator Laboratory, Korea. The incident beam was monochromatized with a Si(111) double crystal monochromator and detuned by 30% to minimize the contamination from higher harmonics, particularly, the third-order reflection of the silicon crystals.[39] The spectra for K-edge of As (E0 = 11867 eV) were taken in a fluorescence mode with a He-filled IC Spec ionization chamber and a joint detector of Lytle and Passivated Implanted Planar Silicon (PIPS) detectors for incident beam and fluorescent signal from the sample, respectively. Helium gas flowed through the Lytle– PIPS detector to prevent air scattering of X-ray fluorescence during the measurements. Since As foil was not available due to its toxicity, energy was calibrated by using of Au foil of which LIII -edge energy (11919 eV) is very close to that of As K-edge. The energy scan was performed in five regions for good energy resolution in a steep absorption and measurement of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra at a time, 5 eV-step in region of 11,667– 11,817 eV, 1 eV-step in 11,817–11,857 eV, 0.25 eV-step in 11,857–11,897 eV, 0.03 k-step in 11,897–12,217 eV, and 0.04 k-step in 12,217–12,767 eV. The obtained data were processed in the usual way to obtain the absorbance and analysed with ATHENA and ARTEMIS in the suite of IFEFFIT software programs.[40] Pre-edge absorption due to the background and detector was subtracted using a linear fit to the data in the range of −200 to −50 eV relative to E0 . E0 was defined as the first inflection point on the rising absorption edge.[39] Each spectrum was then normalized by a constant, extrapolated value to E0 of third-order polynomial fit over absorption at 150–850 eV relative to E0 . The resulting normalized spectra presented As K-edge XANES for the samples. To isolate EXAFS signal, the post-edged background function was approximated with a piece-wise spline that could be adjusted so that the low-R component of pre-Fourier transformed data was minimized.[39] After calculation of EXAFS function χ (k), k 3 -weighted EXAFS function in momentum (k) space was Fourier transformed to reveal the neighbouring atoms arranged according to distance from a

central As atom in R-space.[39] The k range of the transform varied between a kmin of 2.8–3.0 Å−1 and a kmax of 12.2– 13.0 Å−1 . Kaiser-Bessel function was adopted as a window function and the windowsill of dk = 2.0 was also used in the transform.[39] A shell of interest in R-space was backtransformed into the momentum space with Kaiser-Bessel window function and windowsill of dR = 0.2. Fourierfiltered spectra derived from the experiments were fitted by using of the theoretical standards generated with the ab initio FEFF 8.2 code.[41] The standard As–O and As–Fe phase-shift and amplitude functions were extracted from the structure of the iron arsenite fetiasite.[8,42]

3. Results and discussion 3.1. Characteristics of synthetic two-line ferrihydrite The identity of two-line ferrihydrite was confirmed by powder XRD analysis, and the result is presented in Figure S1 (Supplementary material). The X-ray diffractogram of our laboratory-synthesized two-line ferrihydrite was in agreement with that given by Schwertmann and Cornell.[24] In particular, the XRD well shows two very broad peaks and amorphous nature of two-line ferrihydrite, and its peculiar non-crystallinity is a crucial feature when used as a sorbent. The morphology and chemical composition of two-line ferrihydrite was observed prior to and after reactions with arsenite by SEM and EDS analyses, and the results are given in Figure S2 (Supplementary material). The SEM image suggests that the average diameter of two-line ferrihydrite aggregates ranges 10–50 nm and particles are aggregated. From the results of EDS analyses, it is confirmed that only Fe peaks existed before reaction, whereas As peaks appeared after reaction. The surface area of two-line ferrihydrite measured by the three-point BET method of N2 gas adsorption was 247 ± 11 m2 g−1 . The PZC obtained by potentiometric titration appeared to be approximately 8.2, in good agreement with the experimental result (8.5) of Jain et al.[15]

3.2. Sorption isotherms Arsenite sorption isotherms onto two-line ferrihyrite are given in Figure 1. Overall, except for pH 12.2, arsenite sorption showed only slight pH dependence. The maximum uptake of arsenite was achieved at pH 7.0, and it is well consistent with the fact that maxima in the sorption envelopes of oxyanions appear at pH values close to their pKa .[5] The sorption isotherms have been considered to establish the most suitable correlation for the sorption equilibrium isotherms for the sake of optimizing the design of processing systems. Until recently, a variety of isotherm models have been proposed, and among them Langmuir,[43–45] Feundlich,[43–45] BET,[43–46] and Temkin [43,47–49] isotherms have been most frequently used. In this study, our sorption equilibrium data are

Environmental Technology

Adsorbed concentration (mmol/g)

(a)

3

of pH, and it indicates that arsenite sorption preferentially takes place at relatively homogenous and monolayer sites rather than heterogeneous and multilayer surfaces. In particular, the Q0 values (adsorption maxima) calculated in the Langmuir isotherm increase up to pH 7.0 and then significantly decrease with increasing pH, and it is consistent with the results mentioned previously. This pattern is also shown in the KF values (adsorption capacity) in the Freundlich isotherm, although its R2 values are relatively smaller than those of Langmuir isotherm. Additionally, the largest Q0 value (2.627 mmol/g) at pH 7.0 in the Langmuir shows an excellent agreement with experimental data (2.611–2.727 mmol/g) shown in Figure 1.

2

1 pH 2.0

pH 4.0

pH 7.0

pH 9.2

pH 12.2

0 0

100

200

300

400

500

600

5

700

3.3. (b) 100

Removal Efficiency (%)

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Equilbrium arsenite solution concentration (mM)

pH 2.0

pH 4.0

pH 7.0

pH 9.2

Sorption of arsenite on two-line ferrihydrite for total arsenite concentrations ranging from 13.35 to 667.36 mM is shown in Figure S3 (Supplementary material). The data in Figure S3 were obtained by rearranging the results shown in Figure 1. The sorption envelope of arsenite appeared to be centred at pH 7 and the arsenite removal was significantly decreased in alkaline pHs (Figure S3). At pH 7.0, the arsenite removal was higher than 90%, about 77%, and lower than 35% for total arsenite concentrations of lower than 40.04 mM, 66.74 mM, and higher than 133.47 mM, respectively. The maximum uptake concentration was observed to be 2.727 mmol/g (0.262 molAs /molFe ) for total arsenite concentration of 667.36 mM at pH 7.0, and this great removal can be contributed to a much greater specific area of two-line ferrihydrite. To clearly evaluate the effect of pH and initial loading on the removal of arsenite by sorption, two initial arsenite solution concentrations, i.e. 13.35 mM and 133.5 mM, were tested at 11 pH points (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.2, 10.0, 11.0, and 12.2.), and the results are presented in Figure 2. In the pH range of the experiments, arsenite

pH 12.2

80 60 40 20 0

0

100

200

300

400

500

600

Sorption edges

700

Equilbrium arsenite solution concentration (mM)

Figure 1. (a) Sorption isotherms and (b) removal efficiency for arsenite by two-line ferrihydrite at different pHs.

fitted to those four isotherms. The values of R2 (goodnessof-fit) and calculated constants for each isotherm are presented in Table 1. Overall, the Langmuir isotherm is most appropriate for sorption of arsenite over the wide range

Table 1. Determination coefficients (R2 ) and calculated constants for the fit of arsenite sorption data on two-line ferrihydrite at each pH to several isotherm equations. pH

Langmuir

Freundlich BET Temkin

R2 KL Q0 RL R2 KF n R2 Kb qm R2 A B

2.0

4.0

7.0

9.2

12.2

0.975 0.040 2.539 0.908 0.961 0.540 3.888 0.905 −13.20 0.310 0.891 2392 0.145

0.976 0.044 2.597 0.898 0.917 0.530 3.789 0.924 −13.98 0.322 0.931 1655 0.159

0.984 0.065 2.627 0.853 0.917 0.590 3.681 0.923 −16.69 0.345 0.983 4228 0.174

0.985 0.063 2.332 0.871 0.903 0.550 3.854 0.879 −11.61 0.272 0.964 1836 0.151

0.988 0.057 1.220 0.935 0.903 0.331 4.498 0.713 −6.382 0.085 0.947 2375 0.077

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Figure 3. Sorption kinetics and removal efficiency for arsenite by two-line ferrihydrite at pH 4.0 and a total arsenite concentration of 133.47 mM. Table 2. Determination coefficients (R2 ) for the fit of arsenite sorption data on two-line ferrihydrite at pH 8.0 to several kinetic models. Kinetic modela Pseudo-first order Pseudo-second order Power function Parabolic diffusion Elovich

Linear plotb

R2

ln(1-[As]t /[As]∞ ) vs t 1/(1-[As]t /[As]∞ ) vs t ln[As]t vs lnt 1/t([As]t /[As]∞ ) vs 1/t1/2 [As]t vs lnt

0.9807 0.9595 0.9762 0.9908 0.9796

a Raven

Figure 2. (a) Sorption edges and the removal efficiency for arsenite by two-line (open and close symbols represent adsorbed concentration and the removal efficiency, respectively) and (b) comparison with chemical speciation of arsenite in solution. Total arsenite concentrations were 13.35 mM and 133.47 mM.

sorption increases with increase in pH (up to 8.0), gradually decreases up to pH 11.0, and then significantly decreases at pH 12.2. The sorption envelope for arsenite indicates that the amount of arsenite sorbed onto two-line ferrihydrite is greater than 90% at most of pH conditions, except for 76% at pH 12.2 for the lower initial concentration (13.35 mM). In the case of the higher arsenite loading (133.5 mM), the removal of arsenite significantly decreases throughout the pH range: greater than 29% at pHs 2.0–10.0 and 18–20% at pHs 11.0–12.2. The trend of arsenite sorption might be attributable to the pH dependence of arsenite speciation as well as the surface charge of two-line ferrihydrite. Overall, there was a significant increase in sorption at pH 8.0, which coincided approximately with the PZC (8.2) of two-line ferrihydrite and the first pKa (9.22) of arsenite, as shown in Figure 2. Particularly, the lower sorption of arsenite at high pH values is attributable to an increased repulsion between the more negatively charged arsenite species and negatively charged surface sites of two-line ferrihydrite. Accordingly,

et al. [5]; Sparks [34,38]; Ho and Mckay [35]; Arami et al. [43]. b [As] , adsorbed arsenate at time t; [As] , adsorbed arsenate at t ∞ equilibrium; t, reaction time.

the sorption behaviour of arsenite is affected by interaction between arsenite speciation and the surface charge of two-line ferrihydrite.

3.4. Sorption kinetics The variation of the arsenite sorption with the time of contact on two-line ferrihydrite is shown in Figure 3, and it indicates that arsenite sorption was considerably faster, i.e. sorption equilibrium was approximately achieved within the reaction time of 3 h. This fast sorption of arsenite on iron oxides has been observed in numerous researches, and the reactions are complete within 4 h.[5,14,18] To determine the kinetic model which is most suitable to simulate sorption of arsenite onto two-line ferrihydrite, our kinetic data were fitted to several kinetic models proposed so far. The results of regression are summarized in Table 2, and it indicates that the arsenite sorption kinetic data in this study were generally best described by the parabolic diffusion equation, and this result is similar to that obtained by Raven et al.[5] The parabolic diffusion equation is often used to suggest that diffusion-controlled phenomena are rate

Environmental Technology limiting and has successfully described metal reactions on soils and soil constituents.[38] Therefore, our fitting results suggest that the reactions between arsenite and two-line ferrihydrite were diffusion controlled. It is reported that the time dependence of arsenite sorption on two-line ferrihydrite could be by a general model for diffusion into a sphere if a subset of surface sites located near the exterior of aggregates is assumed to attain equilibrium rapidly.[50] The other kinetic models evaluated also show a relatively good consistence with our empirical data (R2 > 0.9595).

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3.5.

X-ray absorption spectroscopic analyses of arsenite sorption on two-line ferrihydrite The results of the fitting of the EXAFS data for the As K-edge spectra for the interaction of arsenite in solution with the two-line ferrihydrite surfaces are given in Table 3. Representative XANES spectra and the Fourier transforms of EXAFS spectra are shown in Figures 4 and S4 (Supplementary material), respectively. In addition, the Table 3.

pH 4

7

10

7

Fourier-filtered EXAFS functions of the first and second shells for arsenite complex sorbed on two-line ferrihydrite are given in Figures 5 and 6, respectively. The first coordination shell surrounding the As in the arsenite-sorbed two-line ferrihydrite involves 3.1–3.3 oxygen atoms in most of cases, except for 3.8 for initial arsenite concentration of 13.35 mM at pH 4. The As–O distance of 1.74–1.79 Å, and those distances are consistent with other results reported so far.[8,13,14,17,19,30,51] The As–O first shell (N = 3.1–3.3) at distances 1.74–1.79 Å is correspondent to As trioxide. However, the As–O coordination number of 3.8 for the lowest concentration at pH 4.0 indicates partial oxidation of arsenite to arsenate,[8,51] as supported by an examination of the XANES spectra for the same samples (Figure 4). The XANES spectrum given was obtained from the analysis of the As(III)-sorbed twoline ferrihydrite sample. In addition to the peak for As(III), however, it also showed As(V) peak, as shown in Figure 4. The As(V) peak can be attributed to the partial oxidation of As(III) either during the sorption experiment or under the

EXAFS fit parameters of arsenite complexes sorbed on two-line ferrihydrite at different pHs 4, 7, and 10.

Initial solution concen.a

Sorption densityb

Surface coveragec

13.35

0.047

0.51

66.74

0.146

1.60

133.47

0.151

1.65

13.35 66.74

0.050 0.161

0.55 1.77

133.47

0.170

1.87

13.35

0.048

0.53

66.74

0.148

1.62

133.47

0.152

1.67

Shell O Fe Fe O Fe Fe O Fe O Fe Fe O Fe O Fe Fe O Fe Fe O Fe Fe

Nd (±0.5)

R (Å)e (±0.01)

σ 2 (Å2 )f (±0.002)

E0 (eV)g (±1)

3.8 1.1 1.6 3.1 0.8 0.9 3.2 1.5 NDi 3.2 0.9 1.4 3.1 1.5 3.2 1.0 1.6 3.3 1.0 1.0 3.2 0.9 1.1

1.74 3.08 3.23 1.79 3.14 3.28 1.79 3.26 ND 1.78 3.15 3.29 1.79 3.26 1.74 3.06 3.21 1.77 3.14 3.28 1.78 3.14 3.27

0.0052 0.0045 – 0.0031 0.0021 – 0.0037 0.0085 ND 0.0038 0.0056 – 0.0031 0.0089 0.0026 0.0033 – 0.0043 0.0039 – 0.0037 0.0045 –

4.5 −19.4 – 7.2 −10.0 – 7.1 −9.0 ND 5.4 −12.7 – 6.4 −9.1 4.6 −19.6 – 6.5 −14.2 – 6.4 −13.5

R − factorh 0.031 0.003 0.003 0.031 0.003 0.003 0.016 0.019 ND 0.015 0.005 0.005 0.014 0.00 0.030 0.007 0.007 0.017 0.005 0.005 0.019 0.003 0.003

Note: Site density (sites nm−2 ) [8,18] was assumed to be 2.31 and experimentally determined surface area of 247 (m2 g−1 ) was used for calculation of surface coverage. a Expressed in mM. −1 b Expressed in mol As molFe . −1 c Expressed in mol As molsurface site . d Coordination number. e Interatomic distance. f Debye–Wallar factor. g Energy-shift. h Goodness-of-fit which gives a sum-of-squares measure of the factional misfit. i Not determined. During the fits, all parameter values indicated by (–) were linked to the parameter value placed above in the table.

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S.-O. Kim et al.

Figure 4. As K-edge XANES spectra of arsenite-sorbed two-line ferrihydrite samples at different pHs. The sample sorbed at pHs 4 (red), 7 (black), and 10 (blue) is arranged from top to bottom and arsenite reference NaAsO2 is also displayed at the bottom (gray). At pH 4, the samples in upper, middle and lower spectra have the sorption density of 0.047, 0.146, and 0.151 molAs mol−1 Fe , respectively. At pH 7, the samples in upper, middle, and lower spectra have the sorption density of 0.050, 0.161, and 0.170 molAs mol−1 Fe , respectively. And, at pH 10, the samples in upper, middle, and lower spectra have the sorption density of 0.048, 0.148, and 0.152 molAs mol−1 Fe , respectively.

X-ray beam. Ona-Nguema et al. reported that As(III) was not oxidized under the X-ray beam in two-line ferrihydrite sample prepared under anoxic conditions.[8] Because the two-line ferrihydrite sorption sample was prepared in oxic condition, accordingly, the As(III) partial oxidation occurring in the study is likely due to Fenton reactions during the sorption experiment, rather than due to beam exposure during the X-ray measurement. According to Hug and Leupin, the oxidation of As(III) can occur under oxic and slightly oxidizing (microaerophilic) conditions when Fenton reactions take place via reactive oxygen species (e.g. O− 2 , H2 O2 , or ·OH) formed as intermediate species during the oxidation of Fe(II) by dissolved O2 .[52] Furthermore, it was recently reported that ferrihydrite has the catalytic effect on oxidation of As(III) and the major factors influencing on this catalytic oxidation of As(III) are Fe/As ratio, ageing of ferrihydrite, media pH, and coexisting ions.[53] Therefore, we speculated that the partial oxidation of As(III) is thought to be due to a Fenton reaction involving Fe(II) originating from photoreduction of Fe(III). This partial oxidation of arsenite is thought to be due to a Fenton reaction involving Fe(II) originating from photoreduction of Fe(III). This small amount

Figure 5. Fourier-filtered k 3 -weighted χ(k) EXAFS functions of the first shell for As3+ complex sorbed on two-line ferrihydrite at (a) pH 4, (b) pH 7, and (c) pH 10. Experimental and calculated spectra are displayed as solid line and dots, respectively.

of dissolved Fe(II) could react with dissolved oxygen to form active radical species able to oxidize arsenite.[8,51] The second coordination shell of arsenite complexes were fitted using As–Fe pairs at different distances. An As– Fe distance of 3.21–3.29 Å dominates the second-neighbour contribution, regardless of pH, in the arsenite-sorbed twoline ferrihydrite (Table 3). Some As–Fe coordination numbers, such as 1.6 for R = 3.23 and 1.5 for R = 3.26

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Environmental Technology

Figure 6. Fourier-filtered k 3 -weighted χ (k) EXAFS functions of the second shell for As3+ complex sorbed on two-line ferrihydrite at (a) pH 4, (b) pH 7, and (c) pH 10. Experimental and calculated spectra are displayed as solid line and dots, respectively.

(Table 3), at the distance of 3.21–3.29 Å were calculated to be larger than one, because multiple scattering of As–O–O was not considered in the fits. The number of independent measurements in a spectrum for the second shell was 6–7 and thus, the use of too many variables might lead to an unreasonable fit. It is worth noting that multiple scattering weakly affects EXAFS signals of nanoparticles and surface species compared with usual bulk particles.[54] Another

9

shorter As–Fe distance of 3.06–3.15 Å was observed to have a strong influence on the second-neighbour contribution. Finally, the last As–Fe distance of 3.32 at pH 4 was also measured in the second coordination shell. The dominant As–Fe distance at 3.21–3.29 Å can be interpreted as a bidentate binuclear corner-sharing surface complex (2 C), which distance is known to be 3.24–3.28 Å.[8,10,12,17,19,51] However, the another distance of 3.06–3.15 Å is slightly shorter than the 2 C complex, and is slightly longer than that of a bidentate mononuclear edge-sharing surface complex (2 E), that is, 2.80–2.85 Å.[8,12,17,19,51] Accordingly, this As–Fe distance at 3.06–3.15 Å could be due to a mixture of contributions from both 2 C and 2 E complexes. Meanwhile, a monodentate mononuclear corner-sharing surface complex (1 V ) at an As–Fe distance of near 3.50– 3.60 Å [8,10,12,19,51] was not found to be dominant in our results. Sherman and Randall evaluated the relative energies of surface complexes of As to iron(III) (hydr)oxides using density-functional theory calculations and founded that the 2 C complex is more stable than the 2 E and 1 V complexes and in particular, the 1 V complex is energetically unfavourable.[19] For this reason, the 1 V complex did not appear to be dominant in this study. Fendorf et al. conducted EXAFS analysis to elucidate the surface complexes of arsenate on goethite and concluded the monodentate (1 V ) complex at low surface coverage (0.005 mole of arsenate/mole of Fe), both monodentate (1 V ) and bidentate (2 C and 2 E) complexes at intermediate surface coverage (0.007 mole of arsenate/mole of Fe), and predominantly the bidentate (2 C and 2 E) complexes at very high surface coverage (0.009 mole of arsenate/mole of Fe).[13] In addition, Ona-Nguema et al. reported that the dominant complex types of arsenite on two-line ferrihydrite are 2 E and 2 C at both high (0.24 molAs /molsurface site ) and low (0.06 molAs /molsurface site ) surface coverage.[8] Overall, the types of surface complexes of arsenite in our EXAFS results seem to be very similar to those proposed by Ona-Nguema et al.[8] in that the 2 C complex is predominant at all the surface loadings. However, our EXAFS results indicate that regardless of pH, the mixed complexes of 2 C and 2 E as well as the 2 C complex are favoured at low and intermediate surface loadings, but only the 2 C complex is dominant at high surface loading. Compared with the data reported by the previous studies, the difference in the surface complexes obtained from our EXAFS data might be caused by overwhelmingly higher surface loadings used in this study. Furthermore, the 2 C complex observed more dominantly at pHs 4 and 7 than at pH 10, and it is consistent with the fact that arsenite sorption is more effectively achieved at acidic and neutral pHs than at basic pH, because it has been reported that the 2 C complex has the highest stability among a variety of surface complexes.[19] Consequently, the EXAFS data support that the two-line ferrihydrite can efficiently remove arsenite from aqueous solution on the mechanism of the strong inner-sphere surface complexation (2 C binding) of arsenite.

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4. Conclusions In this study, we demonstrated the effectiveness of two-line ferrihydrite in scavenging arsenite from aqueous solutions using sorption equilibria and kinetics and spectroscopic studies (XANES and EXAFS). The two-line ferrihydrite is an efficient sorbent for arsenite due to its peculiar amorphous characteristic, large surface area, and high PZC. The results of sorption equilibria indicate that arsenite has a strong affinity for two-line ferrihydrite and, in particular, the pH is a crucial factor in sequestrating arsenite from aqueous solutions using two-line ferrihyrite. The kinetic study indicates that arsenite uptake by two-line ferrihydrite is a fast reaction almost completed within the duration of 3 h. The mechanism for this high retention of arsenite by twoline ferrihydrite was elucidated by spectral evidence and structural data obtained by X-ray absorption spectroscopic measurements. There was a slight change in oxidation state of arsenite for the initial concentration of 13.35 mM at pH 4 and it might be attributed to a Fenton reaction involving Fe(II) produced from photoreduction of dissolved Fe(III). The EXAFS data verify that energetically stable inner-sphere surface complexes, such as bidentate binuclear corner-sharing (2 C) complex, are formed in the arsenitesorbed two-line ferrihyrite and the efficient removal of arsenite by the two-line ferrihydrite can be contributed to this strong inner-sphere surface complexation.

Acknowledgements This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-313-C00755). S. H. Choi acknowledges Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant NRF-2010-0011397).

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