Redox stability of As(III) on schwertmannite surfaces

Journal of Hazardous Materials 265 (2014) 208–216

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Redox stability of As(III) on schwertmannite surfaces Susanta Paikaray a,e,∗ , Joseph Essilfie-Dughan b , Jörg Göttlicher c , Kilian Pollok d , Stefan Peiffer a a

Department of Hydrology, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, 95440 Bayreuth, Germany Department of Geological Sciences, University of Saskatchewan, SK S7N 5E2, Canada Institute for Synchrotron Radiation, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany d Institute of Geosciences, Friedrich Schiller University of Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany e Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research Bhopal, Madhya Pradesh 462023, India b c

h i g h l i g h t s • • • • •

Schwertmannite is a better scavenger than goethite in identical sorbent:solute ratio. Partial As(III) oxidation to As(V) by schwertmannite and goethite in anoxic media. Arsenic-rich surface precipitates form on schwertmannite with decayed morphology. Bidentate binuclear binding of both As(V) and mixed As(III)–As(V) on schwertmannite. Dissolved ferrous iron has negligible control on As(III) uptake and redox stability.

a r t i c l e

i n f o

Article history: Received 1 July 2013 Received in revised form 26 November 2013 Accepted 30 November 2013 Available online 7 December 2013 Keywords: Mine drainage As redox stability As immobilization Surface precipitation Iron oxyhydroxy sulphate

a b s t r a c t As(III)-enriched mine discharge often drains through Fe(III)-mineral abundant land covers which makes the understanding of its fate and redox behaviour extremely important. We therefore conducted batch kinetic and equilibrium studies at pH 3.0 ± 0.05 in anoxic media coupled with spectroscopic and microscopic examinations at variable conditions to understand possible As(III) binding mechanisms and the redox stability of As(III) on schwertmannite, a prominent ferric mineral in acid mine drainage environments. Schwertmannite acted as an efficient scavenger for As(III) compared to goethite at identical sorbent:solute ratios. As K-edge X-ray absorption near-edge structure (XANES) demonstrated partial oxidation of sorbed As(III) to As(V) on both the minerals depending on the Fe(III)/As(III) ratios (goethite acted as a better oxidant than schwertmannite). Sorbed As(III) and As(V) coordinated in a bidentate binuclear binding mechanism with As(III)/As(V)–O and As(III)/As(V)–Fe interatomic distances as 1.78/1.69 ˚ respectively. Scanning (SEM-EDX) and transmission (TEM) electron microscopic, and IR and 3.37/3.31 A, spectroscopic measurements revealed the formation of As-containing surface coatings by sorbed As on schwertmannite. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Oxidation of As-rich mine tailings leads to leaching of high concentrations of As into mine drainage waters. The precipitation of Fe(III)-minerals, e.g., schwertmannite, in such environments has potential control on the mobilization and the redox stability of As. Its poorly ordered structure, its large surface area (≥200 m2 g−1 ) and a large fraction of exchangeable surface adsorbed SO4 2− (33–35%) [1–3] facilitate As uptake from mine water [3–7] by schwertmannite. The predominating As species in environments

∗ Corresponding author at: Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research Bhopal, Madhya Pradesh 462023, India. Tel.: +91 755 4092336; fax: +91 755 4092392. E-mail address: [email protected] (S. Paikaray). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.11.068

where schwertmannite is formed upon oxidation of Fe(II) is As(V) [3,7]. Hence, a large body of studies have been focused on As(V)–schwertmannite interaction (e.g. [4,5,8]). Only little knowledge exists on As(III) interaction, although elevated As(III) concentrations have also been reported from AMD areas, e.g., Carnoulès mine, France (up to 350 mg L−1 As(III) [9,10]). In microbially dominated environments, the mine water enriched As(III) lead to surface precipitation of As(III) rich-schwertmannite or of Fe(III)–As(III)–SO4 2− nanocrystalline phases such as tooeleite [9,10]. The authors have highlighted incomplete oxidation of As(III) in abiotic settings, whereas in presence of microorganisms the formation of As(V)–Fe(III) precipitates on the mineral surfaces could be observed. Recent studies [4,5] suggest that sorbed As(III) on schwertmannite did not undergone oxidation to As(V) under the studied conditions (pH 3, oxic conditions) even though the reaction is

S. Paikaray et al. / Journal of Hazardous Materials 265 (2014) 208–216

thermodynamically favourable [4]. On the other hand, partial As(III) oxidation (10–20%) on goethite at pH 5 has been reported upon 5 days of interaction [11], while Manning et al. [12] could not find oxidation of As(III) on goethite at pH 7.2 for a 16 h reaction time. No redox reaction between As(III) and ferric oxides could be observed in aquatic sediments at circum-neutral pH even after 3 days of interaction [13]. Thus, redox behaviour of sorbed As(III) on Fe(III)minerals is currently a debatable issue. A reason for these partly contradicting results may be the lack of appropriate analytical tools to determine solid-phase As speciation in these studies. We have, therefore, set up batch experiments where we examined the uptake of As(III) by schwertmannite and studied its redox stability using X-ray absorption spectroscopy in combination with microscopic techniques.

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2.4. Aqueous phase analysis A Varian-spectr-AA-20 spectrophotometer was used to determine both total Fe (Fe(T)) and Fe(II) [16] and SO4 2− [17] by 1,10-phenanthrolene and BaCl2 -gelatine method at 512 and 420 nm wavelength, respectively. pH measurements were done by using Mettler Toledo Inlab 412 electrode, precalibrated at pH 4.0 and 7.0 buffers. Total As concentrations (As(T)) were measured by atomic absorption spectrophotometer equipped with graphite furnace (AAS ZEEnit 60, Analytik Jena AG, Jena; Germany). As(III) and As(V) distributions in aqueous phase after each uptake studies were determined within 24 h by using high performance liquid chromatography (HPLC) [18]. 2.5. Solid phase analysis

2. Materials and methods 2.1. Reagents and apparatus Sodium arsenite (NaAsO2 , 99.0%) and hydrated sodium meta arsenate (Na2 HAsO4 ·7H2 O, 99.5%) were used to prepare As(III) and As(V) stock solutions (1 M), respectively. All other reagents were prepared freshly when required out of analytical grade (>99%) chemicals using O2 -free aqua milli-Q water. Laboratory glass and plastic wares were conditioned by 10% HNO3 overnight and rinsed several times by aqua milli-Q water before use. Glove box was preconditioned for at least 2 weeks using ultra pure N2 gas (95% N2 + 5% H2 ) before starting the experiments and flushed 2–3 times daily during the experiments. O2 levels were measured regularly by using gas chromatography (GC 6890, Agilent) and found to be zero. 2.2. Synthesis of schwertmannite and goethite Schwertmannite was obtained from GEOS, Freiberg, Germany which were produced at pH 2.9–3.2 in presence of high concentrations of SO4 [4,14]. The brownish yellow precipitates were air dried at room temperature (20–25 ◦ C) after synthesis. Goethite was synthesized from 100 ml 1 M Fe(NO3 )3 after rapid addition of 180 ml of 5 M KOH as described by Schwertmann and Cornell [15]. 2.3. Uptake experiments Time dependent As(III) uptake on schwertmannite was investigated for 30 days in an anoxic glove box using 0.133 × 10−3 M dissolved As(III) at 10 g L−1 sorbent mass and fixed ionic strength (0.01 M). The pH was adjusted to a value of 3.0 ± 0.05 with diluted NaOH or HCl in a series of glassy crimp vials containing dissolved As(III) before adding schwertmannite. The vials were shaken endover-end in the dark and regularly sampled. Batch equilibrium studies were conducted under similar experimental conditions as a function of dissolved As(III) (0.067 × 10−3 –1.33 × 10−3 M), dissolved Fe(II) (1 × 10−3 and 5 × 10−3 M) and sorbent mass (5–100 g L−1 ) for the predetermined equilibration time. Equilibration time was 2 days. Within this time more than 95% of the As were removed from solution (cf. results). For comparison, mass dependency of sorption capacity was also determined for goethite. As(III) removal potential between the two Fe(III)-minerals were compared using 10, 25, 50, and 100 g L−1 sorbent mass at a constant As(III) concentration (0.13 × 10−3 M) for 48 h equilibrium period. Maximum precautions were taken to maintain darkness and zero O2 level inside the glove box to avoid possible oxidation to As(V) by O2 and light. Sorbents were separated through filtration, dried (22–25 ◦ C) and stored in crimp-sealed serum vials inside the glove box atmosphere until analysis, while the aqueous phase was analysed immediately for Fe (II, III), SO4 2− and As(T).

An X-ray diffraction (XRD) analysis was carried out for pure and As(III) sorbed sorbents using Co-K␣ radiation (D5000, SIEMENS; Germany). Four repeated scans were made from 10 to 80◦ 2 with 0.15 step size and 20 s count time (40 kV, 40 mA) for each sample and the mean of all intensities were reported. Fourier transformed infrared (FTIR) spectra were recorded using a Vektor 22 Bruker FTIR spectrometer equipped with a KBr beam splitter, LADTGS detector, KBr window and siliciumcarbid radiation source. Pellets were prepared from a homogenous mixture of 2 mg samples and 200 mg KBr at 8 kbar pressure and scanned from 350 to 4000 cm−1 with 1 cm−1 resolution in transmission mode after correction for background spectra. For scanning electron microscopic examination the powdered samples were fixed on a carbon tape which were sputtered in a Cressington Sputter Coater 208 HR with 1.3 nm Platin (∼1 min, 40 mA) and imaged at 3 kV by a LEO 1530 SEM equipped with Shotky Cathode and GEMINI column with ZrO2 radiation source (Zeiss, Germany). The specific surface area (SSA) was determined by a five point N2 point adsorption isotherm BET method and found to be 14.7 and 35.3 m2 g−1 for schwertmannite and goethite, respectively. Transmission electron microscopy (TEM) has been performed with a Philips CM20 FEG operating at 200 keV which is equipped with a Ge energy dispersive X-ray (EDX) detector. TEM samples were prepared by dispersing the sample in ethanol and putting a drop of this suspension on a Lacey carbon grid with Cu as grid material. The poorly crystalline nature of schwertmannite can be inferred from the broadened diffractogram patterns with the highest peak at 0.254 nm [1] in contrast to the sharp peaks of goethite (Fig. A1). Schwertmannite was characterized by a spheroidal morphology with spine like surface projections, while goethite consists of needle-like crystals (Fig. A1). Three predominant SO4 2− IR vibrational bands at 1120, 981 and 610 cm−1 corresponding to specifically sorbed ␯3 (SO4 ), outer sphere bound ␯1 (SO4 ) and structural ␯4 (SO4 ), respectively [1,19,20] are observed for schwertmannite. The elemental analysis of schwertmannite yielded the chemical formula Fe8 O8 (OH)4.98 (SO4 )1.51 . The SSA of schwertmannite was smaller (14.7 m2 g−1 than that of goethite (35.3 m2 g−1 ) contrary to as expected. This may be because of inaccessibility of all surfaces by N2 in case of schwertmannite owing to its poor crystallinity. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068. As(III) and As(V) species distribution on schwertmannite and goethite was measured by As K-edge X-ray absorption near-edge structure (XANES) spectra on the SUL-X beam line with wiggler as radiation source at the synchrotron radiation facility, ANKA, KIT, Germany. Energy was monochromatized with a silicon (1 1 1) crystal pair in a double crystal monochromator with fixed exit and calibrated to Au L3 -edge (maximum of the first inflection point

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Fig. 1. (a) Plot showing total amount of arsenic removed by schwertmannite (solid line with filled circles) and goethite (dashed line with open circles) at various sorbent mass. Note the better schwertmannite retention (mmol m−2 ) efficiency compared to goethite. (b) Plot showing Freundlich (solid line) and Langmuir (dashed line) isotherm model fits to the experimental data (filled circles). The increase in As(III) uptake with respect to initial As(III) concentrations from 0.067 to 1.33 mM is shown by the inset.

11.919 eV). Data were recorded from 6 to 8 scans between 11,670 and 12,850 eV with monochromator step size of 0.3 eV in the XANES region (up to 20 eV above the edge), and in k steps of 0.05 A˚ −1 in the EXAFS region up to k = 16. Data was collected both in fluorescence mode using a 7 element Si(Li) solid state detector (Gresham, now e2v) and in transmission mode with one ionization chamber in front of the sample, one behind and a third ionization chamber to record the Au L3 edge parallel to check energy calibration during the measurement of the sample spectra. The flux density on the sample was decreased by collimating the beam and mounting an Al metal sheet (thickness 1 mm) prior to the first ionization chamber to avoid oxidation of As(III) due to the X-ray beam. For each sample a series of up to 6–8 scans (10–16 min each) were performed to assure that no radiation damages occur during data collection and also to average to increase the signal-to-noise ratio. Spectra were pre-edge and post-edge background corrected and normalized to an edge jump of 1 using the Athena programme of the IFFEFIT package [21] and compared with pure salts of NaAsO2 and Na2 HAsO4 ·7H2 O as reference compounds for As(III) and As(V), respectively. Linear combination fit has been carried out also with the Athena program to determine fraction of As(III) and As(V) using NaAsO2 and Na2 HAsO4 ·7Hbur2 O as As(III) and As(V) end members, respectively. The k3 -weighted and Fourier transformed (FT) EXAFS spectra were all fitted in Artemis (IFFEFIT) with ab initio phase and amplitude functions generated with FEFF version 6L included in the IFFEFIT package [21]. 3. Results and discussion 3.1. As(III) removal by schwertmannite As(III) removal by schwertmannite occurred in two steps. A rapid retention removing ∼50% of the initial total aqueous content

(0.133 × 10−3 M) within 1 h of interaction (Fig. A2) was followed by a slower uptake removing an additional amount of ∼0.06 × 10−3 M in ∼10 h. After this time more than 95% of the As were taken up, the residual dissolved As concentrations probably reflecting an adsorption equilibrium with the solid phase. A slight drop of concentration between 10 h and 30 days from 0.036 × 10−3 to 0.003 × 10−3 M was still observable indicating some very slow further uptake. As(III) sorption kinetics in the second phase are best explained by a parabolic model (see Table A1), which implies that diffusion into internal absorbing sites is the rate controlling process for the uptake of As(III) by schwertmannite under these conditions. Similar observations were made in a study on As(III) partitioning between the aqueous solution and ferrihydrite [22]. The initial rapid uptake is likely due to easily accessible surface sites as reflected by the high amount of exchangeable sulphate in SHM [2,4,23,24]. This assumption is supported by a comparison of the sorption properties between goethite and schwertmannite (Fig. 1). Once normalized to the surface area the specific uptake exceeds that of goethite by a factor of 2.5 (Fig. 1). The higher As(III) uptake by schwertmannite may be caused by exchange of AsO3 − versus sulphate [4–6], and/or surface precipitations (cf. below). As(III) sorption on schwertmannite increased directly as dissolved As(III) concentrations increased from 0.13 × 10−3 to 1.33 × 10−3 M. Equilibrium sorption data modelled using IsoFit v1.2 [25] suggests Freundlich model better explains the experimental data (Fig. 1A) with narrower confidence intervals compared to Langmuir model (Table 1), hence, indicating involvement of multiple sites with heterogeneous sorption energies for As(III) uptake [4]. Addition of Fe(II) to the aqueous phase in this study did not enhance the sorption capacity of schwertmannite significantly (0.0126 mmol g−1 in the absence compared to 0.0128 mmol g−1 in the presence of 5 mM Fe(II)aq ). The uptake was almost identical at both the studied Fe(II)aq concentrations (data not shown). In a control experiment (without As(III)) no

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Table 1 A. Langmuir and Freundlich model fits to the experimental sorption isotherm data with 95% confidence intervals of model fits. B. Parameters obtained from EXAFS data fitting of As K-edge spectra for As(III) and As(V) sorption onto schwertmannite. K

n Freundlich model cs = KF · Ceq

Langmuir model cs = c(stot ) 1+KAds

ceq

Ads ceq

A. KF (Ln mmoln−1 g−1 ) n

0.49 0.76

KAds (L mmol−1 ) c(stot ) (mmol g−1 )

0.46–0.52 0.73–0.79

As–O shell

Ba . As(III) As(V)

4.04 0.31

1.31–6.78 0.17–0.45

As–Fe shell

N

˚ R (A)



3 4

1.78 1.69

0.0036 0.0032

2

E0

N

˚ R (A)

2

E0

3.77 5.74

2 2

3.37 3.31

0.0059 0.0058

3.77 5.74

a The fitting uncertainties for the interatomic distances (R) were ±0.01 A˚ and ±0.02 A˚ for the As–O and As–Fe shells, respectively. The coordination numbers (N) for the both the As–O shell as well as the As–Fe shell were fixed during the fitting process.

change in dissolved Fe(II) concentrations could be observed indicating no oxidation to Fe(III) or adsorption on schwertmannite which would have otherwise caused higher As(III) retention. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068.

respectively (Fig. 2, see also Fig. A3). Noticeable broadening of the signal occurred at a mass of 100 g L−1 with both species being overlapping (the adsorption edge of As(V) is 11874.9 eV). The As(V) fraction increased from 17.8 to 44.5% (Table 2). The increase in schwertmannite mass lead to a decrease in As(III) loading from

3.2. Redox stability of As(III) In order to assure no beam damage the flux density was decreased by collimating the beam and mounting a 1 mm thick Al metal sheet prior to the first ionization chamber. Multiple scans with approximately 10–16 min each performed on each sample were reproducible and showed no deviation from each other evidencing photooxidation by X-ray beam is most unlikely. Hence, all the observed redox changes are considered to be happened by Fe(III). Our XANES measurements clearly indicate oxidation of As(III) in the presence of both minerals (Fig. 2, Table 2, see also Fig. A3, Table A3). As(III) is found as the dominant surface species with 78.8% As(III) and 21.2% As(V) after 1 h of reaction. This observation contradicts results obtained from comparable experiments that performed in the presence of O2 , i.e., even at higher redox potential [4]. No evidence for the occurrence of As(V) could be derived from XANES measurements even though oxidation of As(III) was thermodynamically feasible. A reason for this contradiction could be that no LC fitting was undertaken in that study so that the amount of As(V) was too small to be detected from spectral signatures only. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068. After 30 days the fraction of the surface As(V) species decreased to 15.5% although the total As loading of schwertmannite increased from 0.74 × 10−3 to 1.6 × 10−3 mol As/mol Fe during that time period. The decrease in As(V) fraction after 30 d is because of constant schwertmannite mass used in this experiment which might have saturated with its efficiency to oxidize As(III) to As(V). A similar trend was observed in batch equilibrium experiments. The fraction of surface As(V) decreased from 70.2 to 8.5% when the initial dissolved As(III) concentration was increased from 0.067 × 10−3 to 1.33 × 10−3 M at a constant schwertmannite mass (8.19 × 10−3 mol Fe g−1 ). The addition of Fe(II) had no major effect on As redox speciation (Table 2). The active role of Fe(III) for As(III) oxidation was highlighted by schwertmannite mass variation. The increase of mass shifted the absorption edge (energy of the peak maximum) from 11871.3 eV at 10 g L−1 corresponding to As(III) to higher values indicating higher oxidation state (11871.5 eV at 25 g L−1 and 11871.6 eV at 50 g L−1 ,

Fig. 2. As K-edge X-ray absorption near-edge structure (XANES) spectroscopic spectra for time dependent As(III) retention (kinetic 1 h and kinetic 30 d) and 48 h equilibrium studied samples. The As(III) (as NaAsO2 ) and As(V) (as Na2 HAsO4 ·7H2 O) reference spectra are shown at the bottom.

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Table 2 As(III) and As(V) fractions determined by linear combination fit using ATHENA at variable experimental conditions. Final pH denotes the pH value measured after 48 h equilibrium study where the study was conducted at an initial pH of 3.0 ± 0.05. Experimental conditiona

Mass oxide (g L−1 )

c(As(III)) added (M)

Duration of exp (h)

% As(III)

% As(V)

Final pH

10 g GT 25 g GT 50 g GT 100 g GT 10 g SHM 25 g SHM 50 g SHM 100 g SHM 1 mM Fe(II) 5 mM Fe(II) Kinetic 1h Kinetic 30d Eq 0.067 Eq 0.133 Eq 0.334 Eq 0.667 Eq 1.33

10 25 50 100 10 25 50 100 10 10 10 10 10 10 10 10 10

0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.133 × 10−3 0.067 × 10−3 0.133 × 10−3 0.334 × 10−3 0.667 × 10−3 1.33 × 10−3

24 24 24 24 24 24 24 24 24 24 1 720 24 24 24 24 24

70.3 66.3 58.0 57.9 82.2 75.6 67.5 55.5 81.8 82.7 78.8 84.5 29.8 82.2 85.6 90.5 91.5

29.7 33.7 42.0 42.1 17.8 24.4 32.5 44.5 18.2 17.3 21.2 15.5 70.2 17.8 14.4 9.5 8.5

8.8 9.6 9.9 10.0 2.5 2.4 2.3 2.2 2.6 2.6 2.6 2.5 2.5 2.5 2.5 2.6 2.7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.27 0.07 0.06 0.05 0.01 0.00 0.01 0.01 0.00 0.01 0.06 0.01 0.02 0.01 0.00 0.01 0.01

a SHM = schwertmannite, GT = goethite; 10–100 g and Eq 0.067–Eq 1.33 represents equilibrium studies at respective mass and As(III) concentrations, respectively; kinetic 1h and 30d represents kinetic studies for respective time duration, i.e., 1 h and 30 days.

1.54 × 10−3 to 0.16 × 10−3 mol As/mol Fe (cf. Fig. 1) and hence to a larger availability of reactive Fe(III) surface sites. Goethite even acted as a better As(III) oxidant although it showed slightly lower As(III) uptake under identical experimental conditions. A shifting in peak position towards As(V) could be observed even at 50 g L−1 and was persistent at higher mass resulting 29.7 to 42.1% surface As(V), a fraction that was higher than that at schwertmannite at any identical mass.

As can be noticed from Table 2 the final pH in case of goethite raised to ∼10.0 (directly proportional to mass) although the experiments were started with pH of 3.0 ± 0.05. We estimated the thermodynamic feasibility of As(III) oxidation by goethite between pH 3.0 and 10.0 (Table A2) using conventional standard free energies. The calculated free energies are found to be positive in the entire pH range (+76.1 to +114.3 kJ mol−1 ) suggesting most As(V) formation likely occurred in aqueous phase especially at low pH

Fig. 3. Distribution of As(III) and As(V) in aqueous phase in the kinetic study (a), with respect to initial As(III) concentration variations (b), in presence of 1 and 5 mM Fe(II)aq (c), with respect to mass of goethite (d) and schwertmannite (e).

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conditions and subsequently adsorbed (see discussion below). Nevertheless, solid phase oxidation might also have contributed to the overall As(V) fraction according to earlier findings [11]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068. Increase of schwertmannite mass from 50 to 100 g L−1 caused an increase in As(III) surface oxidation (As(V) = 32.5–44.5%), while goethite seemed to reach an optimum oxidation potential already after 50 g L−1 (As(V) = 42.0%) even though there was higher retention. This observation is also partly supported by As(III)–As(V) distribution is aqueous phase (see below). Nevertheless, a complete oxidation to As(V) did not occur on none of the two mineral surfaces within the time frame of the experiments. The distribution between As(III) and As(V) in aqueous phase measured after completion of each uptake experiments well align with the XANES observation on solid phases (Fig. 3) evidencing that part of the initially used As(III) also oxidized to As(V). As(III) remained to be the predominant species in all experiments except at 100 g L−1 schwertmannite. Based on our previous examination of likely oxidation of As(III) (data not shown) by dissolved Fe(III) and the role of SO4 2− , we found that ∼95% of initially used As(III) remained as As(III) (94.4 ± 4.6 ␮g L−1 ), while only ∼5% As(V) was detected (6.1 ± 0.3 ␮g L−1 ) in control reactors. Almost 100% oxidation of As(III) (0.3 ± 0.1 ␮g L−1 ) was found in presence of 1 mM Fe(III). The partial oxidation of As(III) in control reactors may be caused due to presence of atmospheric O2 as the experiments were conducted in ambient conditions in sealed PE reactor vials. Hence we assume that similar non-oxidation or negligible oxidation in absence of schwertmannite or goethite can happen in absence of O2 during the present experiments and all As(III) oxidation was caused by schwertmannite or goethite only. Under the given experimental conditions, oxidation of As(III) by O2 as previously reported for other Fe(III) oxides [11,26–28] can be ruled out. Hence, we propose that Fe(III)-mineral surfaces are the only oxidants. Such oxidation effects of Fe(III) were reported by Greenleaf et al. [29] by hydrated ferric oxide (HFO) in anoxic plug flow reactor systems. The authors, based on aqueous As species measurements, have noticed almost complete As(III) oxidation within ∼6 days in plug-flow columns and postulated that oxidation efficiency is proportional to density of >Fe(III) sites. Sun and Doner [11] reported partial oxidation of sorbed As(III) at 150 ␮mol g−1 after 100 h at pH 5 on goethite surfaces. Our observations demonstrate that oxidation of sorbed As(III) can occur on goethite even at lower As loading (13.2 ␮mol g−1 ) under anoxic conditions. Our observations demonstrate that oxidation of As(III) can occur on surfaces of iron minerals under anoxic conditions. At pH 3 still significant amounts of Fe(III) are dissolved in the presence of schwertmannite (∼0.1 mM) opening the possibility of Fe(III) being an oxidant for As(III) in solution. However, at pH >7 the solubility of Fe(III) is so low that this species can be ruled out as an oxidant and the observed reaction needs to take place at the mineral surface. Certain amount of aqueous phase Fe(II) was detected in all experiments, e.g., up to 10 mg L−1 in kinetic experiments after 10 days. This indicates likely reduction of Fe(III). Oxidation of As(III) by Fe(III), of course, requires the generation of Fe(II) which may be evidenced from these findings (Fig. 4) further supporting As(III) oxidation by Fe(III) mineral surfaces. Our results conflict with previous research of As(III) sorption onto schwertmannite which report non-oxidative sorption of As(III) [4,5,26] although oxidation of As(III) is thermodynamically feasible at pH 3.0 on Fe(III)-minerals [4,13]. A reason for this may be the relatively small fraction of As(V) compared to As(III) (see Table 2) which may have remained undetected in these studies. Our current XANES observation, however, highlights that sorbed

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Fig. 4. Plots showing aqueous phase Fe(III) and Fe(II) concentration variations (mg L−1 ) with respect to experimental reaction time (a), at different initial As(III) concentrations (b), and at different schwertmannite mass (c).

As(III) on schwertmannite undergoes partial oxidation in O2 -free environment at least at the given current experimental conditions. It seems that there are Fe(III) surface sites to which As(III) rapidly (after 1 h) and preferentially (at low As(III) concentrations) adsorb that are triggering the oxidation. 3.3. Surface signatures of sorbed As and effect on schwertmannite stability Microscopic images evidenced degradation of schwertmannite morphology marked by uneven porous surfaces with corroded boundary. Higher degree of degradation was noticed upon more As loading showing destruction of surface spines and formation of gel like surface features (Fig. 5a, Fig. A4). The SEM-EDX spectrum collected on these gel-like features confirmed the presence of As entities detectable by EDX. Bright field TEM images of schwertmannite samples loaded with 0.006 × 10−3 mol As g−1 and 0.118 × 10−3 mol As g−1 were examined for signatures of possible surface precipitates. Surface coatings were clearly noticed even at 0.006 × 10−3 mol g−1 As loading (data not shown), but the thickness of these surface coating increased as As loading increased to 0.118 × 10−3 mol g−1 (Fig. 5b). TEM-EDX spectra confirm the presence of traces of As in such surface coatings especially in experiment with a loading of 0.118 × 10−3 mol g−1 . Here the As peak intensity was better visible for the spectrum obtained from the outer rosette (spectrum 2) compared to that from the interior (spectrum 1). The sulphur EDX peak is very weak (spectrum 2)

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Fig. 5. (a) Scanning electron microscopic (SEM) images of 0.118 mmol g−1 sorbed As. Note the gel like surface features. The EDX spectrum (marked as SEM-EDX) was obtained on the surface precipitates and the image above the SEM-EDX shows the position at which the spectrum was obtained. Note the appearance of As peak. SEM image of the control experiment (without As) is shown (marked as control) for comparison; (b) Bright field TEM images of 0.118 mmol g−1 loaded As(III) showing the growth of surface precipitates. The EDX spectra on the right top of the image (marked as TEM-EDX) were obtained from the As-containing surface precipitates. Spectrum 2 (relatively intense As peak) was taken from the outer part of the rosettes, while spectrum 1 (relatively poor As peak) was taken from the center (represented by arrows).

which may be due to the occurrence of mixed As(III)/As(V)–Fe(III) type surface precipitates together with As(III)/As(V)–Fe(III)–SO4 2− type. Similar As(III)-containing amorphous surface coatings were observed on magnetite [30]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068. In consistency with our earlier reports [4,24], slight shifting in diffractogram peaks at ∼0.332 nm towards higher d-spacing after As loading was noticed from Fig. 6a. The level of shifting was found to be directly influenced by amount of As loading, e.g., more shifting in case of 30 days and 0.118 × 10−3 mol g−1 As compared to their respective counter parts of 1 day and 0.006 × 10−3 mol g−1 As, respectively (Fig. A5). An absolute distinction between the two Fe(II)aq is not visible possibly owing to its identical As loadings. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068. The microscopical observations are supported by FTIR measurements. A low intensity new IR band was observed consistently at ∼1385 cm−1 after incorporation of As(III), the intensity of which increased with As loading (Fig. 6b, Fig. A6), while other IR bands were identical to original schwertmannite. This band is also noticed

Fig. 6. X-ray diffractograms (a) and FT-infrared spectra (b) of As sorbed schwertmannite (i) 10 g L−1 SHM loaded with 0.013 mmol g−1 As (0.133 mM initial As(III)) after 30 days (labelled “kinetic 30 d”), (ii) 10 g L−1 SHM loaded with 0.118 mmol g−1 As (1.33 mM initial As(III)) after 48 h (labelled “1.33 mM As(III)”), (iii) 100 g L−1 schwertmannite loaded with 0.013 mmol g−1 As (0.133 mM initial As(III)) after x days (“10 g SHM”). Additionally, in Fig. 4a the XRD of 10 g L−1 SHM loaded with 0.013 mmol g−1 As (0.133 mM initial As(III)) after 48 h in the presence of 5 mM Fe(II)aq (“5 mM Fe(II)” is presented. The original diffractogram and FTIR spectrum (marked as original) are shown for comparison (“original”). The two vertical lines in (a) mark the shifting in peak positions after As loading and in (b) mark the appearance of new vibration band at ∼1385 cm−1 .

on As(III) loaded schwertmannites under oxic conditions [4] indicating a possible As-containing surface precipitate. An identical IR peak was also observed for As(III) loaded goethite (see Fig. A6), however a similar peak was already present in the original goethite sample demonstrating it did not arise from As(III) loading in case of goethite. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 11.068. Degradation of schwertmannite morphology and possible formation of Fe(III)–As(V) surface precipitates during As(V) sorption on schwertmannite has been previously reported [7]. Similar phenomena can be inferred from the current study during As(III) sorption possibly in the form of Fe(III)–As(III)/As(V)–SO4 2− type. Raven et al. [22] based on studies of As(III) sorption on ferrihydrite at pH 4.6 reported ferric arsenite type surface precipitates. 3.4. Nature of bonding Arsenic K-edge EXAFS spectroscopic measurements were conducted on schwertmannites loaded with 1.26 × 10−5 mol g−1 As(III) (82% As(III) + 18% As(V)) and the other with 1.33 × 10−5 mol g−1 As(V) (100% As(V)) to understand the nature of As(III) and As(V)

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Fig. 7. k3 -weighted (k) EXAFS (a) and Fourier transform magnitude (b) of 1.26 × 10−5 mol g−1 As(III) (lower) and 1.33 × 10−5 mol g−1 As(V) loaded (upper) schwertmannites. The solid lines represent experimental data and the dashed lines represent the theoretical modelled best fit line.

bonding differences on schwertmannite. Since the above findings demonstrate partial oxidation of As(III) to As(V), this EXAFS examinations may best suggest the bonding nature of partly oxidized As(III) on schwertmannite (combination of As(III) and As(V)) with that of only As(V). In the As(III) doped schwertmannite bonding length of As with both O and Fe could be identified that corresponded to both species As(III) and As(V). In contrast, in the As(V) doped schwertmannite only the As(V)–O and As(V)–Fe length were observable. The k3 -weighted (k) spectra and the corre˚ for the sponding Fourier transformed magnitude in R-space (A) As(III) and As(V) loaded schwertmannites are shown in Fig. 7. Parameters derived after best fitting between the theoretical model and the measured data (dotted lines in Fig. 7) are given in Table 1B. ˚ between the two Shoulders and smaller peaks (R = 2–3 A) confirmed As(III)/As(V)–O and As(III)/As(V)–Fe are not considered here for detailed bonding mechanism analysis and taken as present due to contributions from noise and multiple scattering. The theoretical EXAFS fit analysis (conducted with Artemis in the IFFEFIT package [21]) of the measured experimental data shows an interatomic As(III)–O first-shell backscatter peak at 1.78 A˚ and that of As(V)–O at 1.69 A˚ with coordination numbers (N) 3 and 4, and with As(III)–Fe and As(V)–Fe distances from a second backscatter peak at 3.37 and 3.31 A˚ with N = 2, respectively. Three models generally describe AsO4 tetrahedra and FeO6 polyhedra coordination, i.e., bidentate corner-sharing (2 C, RAs–Fe = 3.0), bidentate edge-sharing (2 E, RAs–Fe = 2.3), monodentate corner-sharing (1 V, RAs–Fe = 3.6) and Sherman and Randall [31] proposed a bidentate corner-sharing mechanism between As(V) and Fe(III)-(hydr)oxides (ferrihydrite, goethite, lepidocrocite, hematite) based on RAs–Fe = 3.2–3.3 A˚ where other two coordination were energetically unfavourable. Previous researchers demonstrated almost similar R-values for the adsorption of As-species on other minerals, e.g., Manning et al. [12] based on As(III)-goethite sorp˚ As–Fe = 3.38 A), ˚ Ona-Nguema et al. [28] from tion (As–O = 1.79 A, As(III) interaction on 2L ferrihydrite, hematite, goethite and lep˚ As–Fe = 3.3–3.4 A), ˚ Farquhar et al. idocrocite (As–O = 1.76–1.78 A; [32] from As(III) and As(V) sorption on goethite and lepidocrocite ˚ As(III)–Fe = 2.97–3.41 A˚ (As(III)–O = 1.78 A˚ and As(V)–O = 1.69 A; ˚ attributing to a bidentate binuclear and As(V)–Fe = 2.93–3.31 A) bonding mechanism. As(V)–schwertmannite EXAFS investigations so far [33,34] demonstrated inner sphere bidentate corner-sharing mechanism with an identical As(V)–Fe and As(V)–O bonding distance (e.g., As(V)–Fe = 3.3, As(V)–O = 1.67 [33]). Comparative knowledge about As K-edge EXAFS on As(III)–schwertmannite interaction is, to the best of our knowledge, not available. However, the comparative As–O and As–Fe distances between As(V) and As(III) sorbed schwertmannite indicates As(III) also might have co-ordinated in a similar bonding mechanism. The slightly

higher As(III)–Fe bonding distance compared to that of As(V)–Fe is believed to be due to a relatively larger size of As(III) than As(V) [32]. The larger As–Fe bond distance in As(III) sorbed schwertmannite compared to that of As(V) (both 0.133 mM) (see Table 1B) supports the incomplete oxidation of As(III) to As(V) on schwertmannite surfaces which aligns the XANES observation (see Section 3.2, Table 2). SEM and TEM finding of As-rich surface precipitate with high Fe(III) cannot completely rule out Fe(III)–As(III)–As(V) combined binding is partly contributed from this surface precipitates.

4. Environmental implications and conclusions This study shows schwertmannite act as an efficient As(III) scavenger compared to goethite in anoxic acidic environment where the later seems to act as a better As(III) oxidant under identical Fe(III)/As(III) ratios. As(III) uptake by schwertmannite is rapid and proceeds in a two-step process. The presence of dissolved Fe(II) seems to neither effect the retention nor the redox stability of As(III) significantly. Both As(III) and As(V) binds by a bidentate binuclear coordination on schwertmannite characterized by Fe–As and As–O interatomic distances almost identical to other Fe(III)–(hydr)oxide minerals. Sorption of As(III) leads to the development of As(III)/As(V)–Fe(III)–SO4 2− or As(III)/As(V)–Fe(III) type surface precipitates and causes significant morphological degradation without forming of new mineral phases. These observations have severe implications for environments affected by acid mine drainage, since schwertmannite is the predominant Fe(III) precipitate in AMD [1,8–10,35,36]. Considering increasing global threats from AMD pollutions with As, the role of schwertmannite for As(III) immobilization and its redox behaviour deserves attention. Schwertmannite is frequently exposed to anoxic set ups (e.g. upon burial in mining lake sediments [36]) where elevated As(III) concentrations may occur [9,10]. Schwertmannite is metastable with respect to goethite and tends to transform into goethite in such systems [36–38]. In this study, both minerals are found to cause partial oxidation of As(III) to As(V) which in one hand detoxify As toxicity and partly can restrict As mobility in acidic anoxic environments.

Acknowledgements The work was supported by the German Academic Exchange Service (DAAD) and the Geotechnologien programme (BMBF, No03G0714A). Dr. Janneck (GEOS, Freiburg) is acknowledged to provide the schwertmannite sample.

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