The behavior of chromium and arsenic associated with redox transformation of schwertmannite in AMD environment

Chemosphere 222 (2019) 945e953

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The behavior of chromium and arsenic associated with redox transformation of schwertmannite in AMD environment Cong Fan a, Chuling Guo a, b, *, Yufei Zeng a, Zhihong Tu c, Yanping Ji a, John R. Reinfelder d, Meiqin Chen e, Weilin Huang d, Guining Lu a, b, Xiaoyun Yi a, b, Zhi Dang a, b, * a

School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, South China University of Technology, Guangzhou, 510006, PR China c College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004, PR China d Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, 08901, USA e School of Environmental and Biological Engineering, Guangdong University of Petrochemical Technology, Maoming, 525000, PR China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Adsorbed CrO2-4 and AsO3-4 both inhibited the abiotic Fe(II)-induced transformation of schwertmannite.
Strong reduction of Cr(VI) to Cr(III) occurred, but As(V) did not convert to As(III).
CrO2-4 -schwertmannite persisted for up to 30 d, AsO3-4schwertmannite transformed to goethite via an intermediate lepidocrocite.
Both sorption and redox of Cr(VI) imprisoned chromium; arsenic was persistently locked up in schwertmannite and its transformation products.

a r t i c l e i n f o Article history: Received 15 April 2018 Received in revised form 17 January 2019 Accepted 24 January 2019 Available online 24 January 2019 Handling Editor: X. Cao

Keywords: Chromate Arsenate Schwertmannite

a b s t r a c t Schwertmannites are metastable and often contaminated by chromium and arsenic. However, when schwertmannite is subjected to reducing conditions, the effect of reductive dissolution of schwertmannite coupled to the potential reduction of Cr(VI) or As(V) on the behavior of chromium and arsenic are less known. The present study systematically explored Fe(II)-induced transformation of schwertmannite with adsorbed CrO2-4 or AsO3-4 at pH 6.5, and the consequent redistribution of chromium and arsenic. The results show that adsorption of oxyanions obviously facilitated the release of 2schwertmannite-bound SO2 4 from both the mineral surface and the lattice structure via SO4 -exchange. Adsorbed chromate and arsenate both inhibited the Fe(II)-induced transformation of schwertmannite. CrO2-4 -schwertmannite persisted as the dominant mineral phase for up to 30 d as a consequence of Fe(II) consumption and coverage of Cr(III)-Fe(III) hydroxide. In contrast, AsO3-4-schwertmannite transformed to poorly crystalline goethite via an intermediate lepidocrocite in 4 mM Fe(II) treatment. Both sorption and redox processes of Cr(VI) imprisoned chromium on schwertmannite. Transformation of AsO3-4-schwertmannite caused no re-mobilization of arsenic into solution, indicating the repartition

* Corresponding authors. School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China. E-mail addresses: [email protected] (C. Guo), [email protected] (Z. Dang). https://doi.org/10.1016/j.chemosphere.2019.01.142 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

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to secondary minerals. Reduction of Cr(VI) to Cr(III) by Fe(II) would greatly lower the environmental hazard of chromium, and the persistence of As(V) would limit the toxicity of arsenic. These findings will help to understand the geochemical behavior of the important toxic Cr(VI) and As(V) specifically in the environmental systems with cyclic redox conditions. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Schwertmannite (Fe8O8(OH)x(SO4)8-2x, 1 < x < 1.75) is an Fe(III)oxyhydroxysulfate mineral precipitated over the pH range of 2.8e4.5 in acid mine drainage (AMD) and acid rock drainage (ARD) (Bigham et al., 1996). Extremely high levels of chromium (Cr) (812 mg kg1) and arsenic (As) (6740 mg kg1) were enriched in AMD sediments where schwertmannite formed (Chen et al., 2018; Regenspurg and Peiffer, 2005). The ionic radii of CrO2-4 (r ¼ 0.24 nm) and AsO3-4 (r ¼ 0.248 nm) allows for their exchange with sulfate (SO2 4 ) (r ¼ 0.23 nm) in schwertmannite. Therefore, metastable schwertmannite is especially prone to accumulate Cr(VI) and As(V), and as a result, may control the fate of these trace elements in the environment. The environmental risk and bio-availability of chromium and arsenic correlate with their redox speciation (Cr(VI), Cr(III), As(V), As(III)) (Amstaetter et al., 2010; Ding et al., 2018; Fendorf, 1995; Liao et al., 2011). Cr(VI) species are quite toxic and mainly occur as 2 mobile forms (HCrO 4 and Cr2O7 ) at low pH, while Cr(III) may be immobilized as solid phase Cr(OH)3 at pH ~6e11.5 (Kotas and Stasicka, 2000). Both As(III) and As(V) compounds are harmful to organisms, but As(III) is much more poisonous and more mobile than As(V). As(III) generally exists as H3AsO3 at pH < 9, and As(V) as 2 the oxyanions H2AsO over a pH range of ~2e12 4 and HAsO4 (Cerkez et al., 2015). Reduction of Cr(VI) to Cr(III) can therefore detoxify contaminated waters, while conversion of As(V) to As(III) may increase their toxicity. Given the widespread occurrence and potentially carcinogenicity of chromium and arsenic, redistribution of Cr and As with respect to the mineralogical changes of schwertmannite in AMD systems have received extensive attention (Cutting et al., 2012; Karimian et al., 2017; Paikaray et al., 2012; Pettine et al., 1998; Xie et al., 2017). To some degree, adsorbed or substituted arsenic and chromium stabilize schwertmannite and delay subsequent transformation (Regenspurg and Peiffer, 2005; Zhang et al., 2016). As suggested by Paikaray et al. (2012), only 0.83% of pre-sorbed As(III) was re-mobilized and subsequently re-adsorbed to the host mineral over 120 d. In addition, biotic reduction of Cr(VI)-loaded schwertmannite by Shewanella oneidensis MR-1 showed retention of chromium by the bulk minerals during the transformation to goethite (Wan et al., 2018). Zhou et al. (2012) found that schwertmannite could serve as a bridge during the electron transport between sulfide and Cr(VI), and thus accelerated the reduction of Cr(VI). Acid mine drainage is Fe-rich, and Fe(II)/Fe(III) cycling is an important component of Fe(III)-oxide evolution (Peine et al., 2000). Fe(II) can lead to accelerated transformation of schwertmannite to thermodynamically favourable crystals (e.g., goethite) (Burton et al., 2008; Fan et al., 2019). Fe(II) also acts as a natural reductant for redox active chromate and arsenate, labilizing Cr(VI) and As(V) loaded in schwertmannite (Buerge and Hug, 1997; Karimian et al., 2017; Richard and Bourg, 1991). Nevertheless, the underlying mechanism of how the reductive dissolution of schwertmannite in conjunction with the potential reduction of mineral-bound Cr(VI) and As(V) affects mineral recrystallization under variable redox

potential conditions have not been examined. Moreover, in such multi-ion coexistence system, potential interactions between Fe(II) and the oxyanions on the speciation and behavior of chromium and arsenic need to be addressed. Here, we hypothesized that reductive transformation of schwertmannite is critical in releasing absorbed oxyanions from the minerals, and thus controlling the behavior of chromium and arsenic in the environmental systems. To test this hypothesis, we examine (1) the effects of adsorbed Cr(VI) and As(V) on schwertmannite recrystallization in redox fluctuated systems; (2) the speciation and behavior of chromium and arsenic accompanied with the transformation of schwertmannite; and (3) the underlying mechanism controlling the phase partitioning of chromium and arsenic. 2. Materials and methods 2.1. Schwertmannite synthesis Schwertmannite (denoted as “Sch”) was prepared following the method according to Regenspurg et al. (2004) by first dissolving 10.0 g of FeSO4 ,7H2O in 1 L of water, and then adding 5 mL H2O2 (30%) slowly and uniformly while stirring over 2 min. The ocherous precipitate was incubated for 24 h, then washed, centrifuged, freeze-dried and kept in ziplock bags. Schwertmannite loaded Cr(VI) and As(V) (denoted as “SchCr” and “SchAs”) were prepared via mixing schwertmannite with 1 mM of K2CrO4 or Na2HAsO4,7H2O for 24 h to reach adsorption equilibrium. The composition of synthesized schwertmannite was measured by digesting 0.05 g of mineral in 6 M HCl for 6 h. Concentrations of Fe were determined by the ferrozine photometry method (UVeVis, UV2550, Shimadzu) at l ¼ 510 nm and SO2 4 content by ion chromatography (IC, ICS-90, DIONEX). 2.2. Transformation experiments To investigate the effects of reductive dissolution of schwertmannite coupled to the potential reduction of Cr(VI) and As(V) on Fe(II)-induced recrystallization of schwertmannite, anoxic conditions were maintained for all incubations. Batch experiments were performed in a buffer solution containing 0.05 M MES, 0.05 M MOPS, and 0.1 M NaCl, that was adjusted to target pH with 6 M NaOH (Burton et al., 2008). Depending on the experimental treatments, the buffer solution contained no oxyanions, or 1 mM Cr(VI) or As(V) respectively with K2CrO4 or Na2HAsO4,7H2O. A 1 M Fe(II) stock solution was freshly prepared by dissolving FeCl2$4H2O in deoxygenated distilled water. Schwertmannite was dispersed with buffer solution (2 g/L) in serum bottles. The experiments were conducted with (0.4 & 4 mM) or without aqueous Fe(II) at pH 6.5 based on the field data of subsurface sediments in acid sulfate environments (Burton et al., 2006). Before adding aqueous Fe(II), schwertmannite suspensions were equilibrated for 24 h, and then deoxidized, sealed, and transferred to glove box (YQX-II, CIMO). All of the injections and sampling operations were proceeded in glove box. The transformation

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experiments were triggered by adding Fe(II)(aq) to attain the designed Fe(II) concentrations of 0.4 and 4 mM, and the bottles were then sealed with butyl rubber stoppers and aluminium covers. Thereafter, samples were transferred to an orbital shaker and incubated for 30 d (25  C, 150 rpm). The pH variation was no more than ± 0.2 throughout the reaction. At particular time intervals following injection of Fe(II)(aq), triplicate samples for each treatment were collected sacrificially. The suspension was filtered using 0.45-mm filters and acidified to preservation; the solid was separated from the suspension through centrifugation and freezedried for mineralogical characterization. 2.3. Analytical methods Aqueous chromium and arsenic were determined using atomic absorption spectroscopy (AAS, Z-2000, Hitachi) and atomic fluorescence spectrometer (AFS, 9130, Titan), respectively. Mineralogical composition was determined by X-ray diffraction (XRD, D8 Advance, Bruker) performed using Cu Ka radiation (40 kV, 40 mA) from 10 to 80 2q with per step size of 0.02 and 0.1 sec/step. The functional groups on mineral surface were examined by Fourier Transform Infrared (VERTEX 70, Bruker) within the wavelength range of 400 e 4000 cm1. Morphology of these oxides was observed by scanning electron microscopy (SEM, Merlin, ZEISS). Composition and oxidation state of solid-phase elements were detected by X-ray photoelectron spectrometer techniques (XPS, Axis Ultra DLD, Kratos) with high-resolution spectra binding en€ssbauer spectra were detected with a Wissel MS-500 ergies. Mo spectrometer (Germany). The measurement was performed with a57Co/Rh source at 295 K in transmission mode. Spectra were fitted with Lorentzian-shaped doublets and/or sextets using the Recoil software. 3. Results and discussion 3.1. Initial schwertmannite characterization The synthetic mineral was verified as typical schwertmannite by XRD and SEM patterns (Fig. 1 and Supplemental Fig. S1) (Bigham et al., 1990). The schwertmannite contained 7.6 mmol g1 Fe and 2.2 mmol g1 SO4, leading to a formula of Fe8O8(OH)3.36(SO4)2.32. The morphology of schwertmannite was very similar, showing clusters of irregular smooth spherical with a diameter around 1 mm determined by SEM (Fig. S1). The surface area of schwertmannite determined by the BET method was 7.5 m2/g. After pre-equilibrated 3 period with CrO2 4 and AsO4 , no discernible changes in XRD peak positions and morphology of schwertmannite were observed, suggesting the absorption of oxyanions did not structurally modify schwertmannite. 3.2. Uptake of arsenate and chromate by schwertmannite The pre-equilibrated period removed all the added arsenate and >81% of the added chromate via sorptive interactions with schwertmannite (Fig. 2). At the same time, about 40%, 68%, and 66% of schwertmannite-bound SO2 4 were released in incubations of Sch, SchCr, and SchAs (Fig. 2), indicating that the capture for CrO2 4 2 and AsO3 4 enhanced SO4 detachment. Therefore, the mechanisms 3 of CrO2 4 and AsO4 adsorption on schwertmannite should involve anion exchange (Antelo et al., 2012; Fukushi et al., 2003). Since the majority of SO2 4 was held within schwertmannite structure, the 2 released percentage of SO2 4 implied the dissolved SO4 was not solely detached from mineral surface but involved tunnel-SO2 4 3 €nsson et al., 2005). After adsorption with CrO2 (Jo 4 and AsO4 , we observed the decrease in intensity of v1(SO4) (979 cm1) due to

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2 1 outer-sphere SO2 4 , v4(SO4) (609 cm ) due to structural SO4 , and the split of v3(SO4) (1214, 1124 and 1085 cm1) (Fig. 3) (Paikaray and Peiffer, 2010). Meanwhile, As-O absorption band at 829 cm1 and CrO2-4 absorption bands at 828, 874, and 914 cm1 appeared (Fig. 3a) (Regenspurg and Peiffer, 2005). This further indicated a simultaneously ligand exchange of superficial and structural SO2 4 by arsenate and chromate during adsorption, consistent with SO2 4 release data. The released percentage of SO2 4 also showed that 2 3 CrO2 4 was more capable for replacing SO4 than AsO4 , and the 2 3 exchange coefficient of SO2 for CrO and AsO obtained were 4 4 4 SO4/CrO4 z 1.45,SO4/AsO4 z 1. Nevertheless, the content of chromate and arsenate adsorbed on schwertmannite were 54 mmolCr mol Fe1 and 65 mmolAs mol Fe1, showing better adsorption property for As(V) than Cr(VI), which was consistent with the report of Antelo et al. (2012). The possible principle to explain the adsorption was the higher surface complexation constants of arsenate than chromate on hydrous ferric oxide (Dzombak and Morel, 1990), hence, the stronger affinity between arsenate and Fe(III)-OH groups on schwertmannite resulted in more adsorption. After the pre-equilibration period with CrO2-4 and AsO3-4, the OH absorption bands (3358 cm1) belonging to schwertmannite were diminished in intensity and drifted to high absorption bands of 3379 cm1 and 3380 cm1, respectively (Fig. 3a). That further indicated the adsorption mechanism may involve complexation with surface hydroxyl groups. Due to the surface complexation constant for chromate is very small, adsorption of Cr(VI) occurred mainly through SO24 -exchange. In addition, the point of zero charge (pHpzc) of synthetic €nsson et al., 2005), thus under schwertmannite was around 7.0 (Jo the experimental conditions (pH 6.5), positively charged schwertmannite surfaces facilitated abundant retention of oxyanions via electrostatic attraction. Above all, the combination mechanisms of arsenate and chromate on schwertmannite involved electrostatic adsorption, complexation with surface hydroxyl groups, and SO24 -exchange. All these mechanisms contributed differently to the oxyanions retention on schwertmannite. When chromate was present, adsorption mainly occurred through anion exchange with SO2 4 groups, while for arsenate, adsorption involved both complexation with mineral surface hydroxyl groups and SO24 -exchange.

3.3. SO2 4 release The drastic increase of SO2 4 in incubations of Sch after reacting with Fe(II) demonstrated the extent and accelerated rates of Fe(II)driven transformation (Fig. 2c) (Bigham et al., 1996). The concentrations of SO2 4 reached a peak after 1 d or 6 h, respectively, in incubations with 0.4 or 4 mM Fe(II), showing more complete and faster transformation compared to SchCr and SchAs. With 0.4 mM Fe(II), 100%, 79%, and 87% of solid-SO24 were released in the case of Sch, SchCr and SchAs over 30 d (Fig. 2), suggesting chromium and arsenic stabilized mineral structure and inhibited phase transformation. Surface coverage with Fe(II) is considered to be a critical step in controlling reductive dissolution of schwertmannite. In incubations with CrO2 4 , initial 4 mM Fe(II) decreased dramatically with chromium content within 10 min and then maintained at extremely low levels (Fig. S2), indicating the Fe(II) applied for later catalysis of schwertmannite could be affected by redox activity of 3 the added oxyanions. In incubations with CrO2 4 and AsO4 , only 2 SchAs released all the SO4 over 30 d with 4 mM Fe(II), indicating that higher concentration of Fe(II) could shield the inhibitory effect of As(V) and promote the transformation. Concentrations of SO2 4 in incubations of SchCr and SchAs may not correctly revealed the transformation rates, but represented the strong ability of CrO3 4 2 and AsO3 4 exchanging SO4 (Burton et al., 2010). Therefore, XRD

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Fig. 1. Comparison of XRD diffractograms of Sch, SchCr, and SchAs during transformation at pH 6.5 with (a) 0.4 and (bed) 4 mM Fe(II).

Fig. 2. Concentrations of dissolved (a) chromium, (b) arsenic and (c) sulfate in incubations with Sch, SchCr and SchAs during the 30-d reaction period. The bars donate S.D..

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Fig. 3. Comparison of infrared spectra of (a) original schwertmannite (Sch), SchCr and SchAs, transformation products over 30 d with (b) 0.4 mM and (c) 4 mM Fe(II). The y-axis represents transmission.

and FTIR spectra were necessarily used to further identify the mineral phase and transformation degree. Besides, concentrations of Fe(III) was below the detection limit during the transformation, which might be attributed to its rapid hydrolysis and precipitation of secondary minerals (Paikaray and Peiffer, 2015).

3.4. Characterization of Fe phases The XRD diffractograms obtained at different sampling time are shown in Fig. 1. Discrepancies between XRD diffractograms suggested that the transformation rates were: control (no oxyanions) > SchAs > SchCr, and goethite was the dominant endproduct. The evidence of characteristic peaks of goethite in incubations of Sch appeared within 6 h with 0.4 mM Fe(II) (data not shown), and complete transformation to goethite accomplished within only 1 d. Time-dependent characteristic peaks represented the structural arrangement during the phase transformation. However, the transition from schwertmannite to goethite was 3 suppressed by the presence of CrO2 4 and AsO4 . For SchCr, the main broad peak of schwertmannite at 35.2 persisted for up to 30 d (Fig. 1a and c). For SchAs, transformation products were a mixture of goethite and lepidocrocite over 8 d with 4 mM Fe(II) (Fig. 1d). Then the lepidocrocite transformed to goethite finally, revealing lepidocrocite was an intermediate product resulted from the inhibited transformation. The lepidocrocite mainly precipitated via Fe(OH)2þ or Fe(OH)þ 2 generated from Fe(II) that released from the crystal lattice (Paikaray and Peiffer, 2015). According to the standard free energy (DG) of formation of goethite (496 J/mol) and lepidocrocite (471 J/mol), lepidocrocite is a precursor and has a tendency to transform into thermodynamically stable goethite (Ishikawa et al., 2004; Misawa, 1973). Furthermore, Fe(II) could facilitate this process by increasing the dissolution rate of lepidocrocite (van Oosterhout, 1967). Compared with the goethite in incubations of Sch, transformation products of SchAs over 8 d and 30 d showed much weaker intensity in the diffractogram and were surrounded by unshaped goethite (Fig. S1), indicating that AsO3 4 interfered with nucleation of secondary minerals. Changes in reactive functional groups and mineral phase were also detected by FTIR spectra (Fig. 3). The IR spectra of SchCr retained CrO2-4 absorption bands (828, 874, and 914 cm1) and FeO absorption band (701 cm1) belonging to original SchCr for up to 30 d in the presence of 4 mM Fe(II), indicating the negligible transformation of SchCr. The intensity of absorption bands for secondary minerals in incubations of SchAs was much weaker than that of Sch, implying a poor crystallinity of newly formed minerals. All of the initial SO4 absorption bands damaged after 30 d in incubations of Sch, suggesting the collapse of schwertmannite

structure and entirely release of SO2 4 during the transformation. The emerging absorption bands at 3407, 3138 cm1 were identified as OH stretching vibration, and at 1047, 888, 794 cm1 were due to Fe-OH, d-OH, g-OH bending vibration, respectively. All of these bands belonged to iron hydroxyl compound, suggesting the recrystallization of schwertmannite to goethite (Gagliano et al., 2004; Lewis and Farmer, 1986). Hence, the IR spectra analysis further identified the inhibitory effect of CrO2-4 and AsO3-4 on transformation of schwertmannite as well as the XRD patterns. The XPS spectra was employed to explore the Fe speciation before and after Fe(II)-induced transformation of schwertmannite by measuring the binding energy of Fe 2p (Fig. 4). The binding energy of 724.8 and 710.8 eV represented Fe2p 1/2 and Fe2p 3/2 of Fe(III). Fe2p 3/2 spectra was composed of four peaks at binding energy of 713.3, 711.8, 710.8, and 709.9 eV, which were interpreted as Fe(III)-SO4, Fe-(OH-O) and Fe-O, respectively (BonnisselGissinger et al., 1998; Derycke et al., 2013) (Fig. 4a). Meanwhile, the peak at 719.2 eV was the Fe 2p satellite. The above XPS peaks were belonged to the samples at 0 d and the sample of SchCr at 30 d. Besides, an additional peak of FeOOH belonging to goethite occurred with the sample of SchAs with 4 mM Fe(II) at 30 d (Fig. 4b).

3.5. Fate and speciation of chromium and arsenic With extremely slow transformation (Fe(II) 0 mM), chromium maintained the concentration obtained after pre-equilibration period (Fig. 2a). When Fe(II):Cr(VI) molar ratio was 0.4, Fe(II)activated desorption of previously-bound chromium occurred within 10 min, suggesting the competition with surface sites or labilization of schwertmannite under the attack of Fe(II). When Fe(II):Cr(VI) molar ratio was 4, no residual chromium was detected in solution, which was due to Fe(II)-CrO2-4 redox reactions and the uptake by schwertmannite. That indicated reduction process of Cr(VI) in the system was conducive to reducing the migration and toxicity of chromium. Cr 2p spectra at 0 d displayed four major peaks at 587, 579.4, 577.5, and 576.3 eV which attributed to Cr(VI), CrO2 4 , Fe-CrO4, and Cr-O (Biesinger et al., 2011) (Fig. 4c). This indicated that adsorption to schwertmannite did not alter the valence of chromate, and the occurrence of Fe-CrO4 verified that adsorption mechanism involved anion exchange. Deconvolution of Cr 2p peak for samples incubated with 4 mM Fe(II) over 30 d showed the presence of five peaks at 587.3, 586.4, 577.7, 576.8, and 576 eV, corresponding to Cr(III), Cr(VI), Cr(III)-hydroxite, Fe-CrO4, and Cr-O (Fig. 4d). In this case, the Cr(III) and Cr(III)-hydroxite were generated from chromate reduction. Concentrations of aqueous arsenic decreased and then

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Fig. 4. The XPS spectra of (aeb) Fe 2p for SchCr and SchAs, (c-d) Cr 2p for SchCr, and (e-f) As 3d for SchAs before and after reactions with 4 mM Fe(II).

remained stable at an extremely low value (<0.4 mg L1) in all cases during 30 d incubations (Fig. 2b), reflecting arsenic was persistently locked up in schwertmannite and its transformation products. This is consistent with the report by Cutting et al. (2012) that microbial reduction of As(V)-bearing schwertmannite caused no remobilization of arsenic into aqueous phase, but repartitioned on or incorporated into the bulk minerals. Chromate and arsenate could both form monodentate and bidentate complexes with iron hydroxyl groups on schwertmannite, previously report suggested that chromate mainly forms weakly bound outer-sphere complexes, however, arsenate generally presents as firm inner-sphere complex (Antelo et al., 2012; Fendorf et al., 1997). This could explain the extreme impaired mobility of arsenic in comparison with chromium during the aging process. The XPS spectra of arsenic were compared to insure whether the adsorbed arsenate was reduced considering the potential reduction of As(V) by co-existed Fe(II). The spin-orbit doublets As 3d5/2 and 3d3/2 presented splitting energy of 45.7, 45 eV before reaction and 45.6, 44.9 eV after transformation (Fig. 4e and f). These peaks were all assigned to be As(V)-O bond (Nesbitt et al., 1995), indicating that no redox interaction occurred between arsenate and 4 mM Fe(II),

and As(V) refused to become more hypertoxic during the Fe(II)induced recrystallization of schwertmannite. This was similar with the findings of Burton et al. (2010) that no reduction of As(V) occurred when schwertmannite transformation induced by 10 mM Fe(II) with the presence of 1 mM As(V). Furthermore, XPS results revealed that As(V) might be the dominant redox species during the whole ripening process at As(V):Fe(II) moler ratios 1:4. To further investigate the impacts of crystallinity of secondary € ssbauer spectra of the minerals on the behavior of arsenic, Mo original schwertmannite, SchAs, and their transformation products over 30 d at room temperature were compared (Fig. 5). The € ssbauer parameters are shown in Table 1. The isomer shift (d) of Mo transformation products were attributed to the IS range of Fe(III), implying the products were dominantly Fe(III)-hydroxide (Fysh and Clark, 1982). The quadrupole splitting (DEQ) provided information of site distortion, which was increased with As(V) presence in the sample of SchAs. The magnetic field strength of transformed SchAs over 30 d was 362.4 koe, confirming the composition of Fe(III)hydroxide products involved goethite (Murad and Schwertmann, €ssbauer spectra obtained at room temperature also 1983). Mo contained information about the crystalline degree and impurity

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€ssbauer spectra obtained at room temperature for (a) original schwertmannite, (b) SchAs and (d) transformation products of SchAs with 4 mM Fe(II) over 30 d. Fig. 5. Mo

Table 1 € ssbauer parameters of Sch, SchAs and transformation prodRoom-temperature Mo ucts with 4 mM Fe(II) for 30 d used for model fitting. Mineral

da (mm/s)

DEQb (mm/s)

Pure Sch 0.39 0.76 Sch aged for 30 d goethite 0.37 0.26 AsO3-4-adsorbed schwertmannite Sch 0.38 0.75 SchAs 0.41 1.85 SchAs aged for 30d goethite 0.49 0.42 a b c d

Hc (koe)

Lorentzian HWHMd (mm/s)

e

0.14

354.2

0.17 0.12 0.43

362.4

0.42

Isomer shift. Quadrupole splitting. Effective magnetic field strength. Half width half maximum of Lorentzian lineshape.

€ssbauer containing of the iron minerals. At room temperature, Mo spectra of schwertmannite presents as a paramagnetic doublet. In addition, pure and well-crystalline goethite is magnetically ordered and exhibits as a sextet (Cornell and Schwertmann, 1996). Whereas € ssbauer spectra of ageing SchAs showed no sextets, which the Mo was contributed by the impurities in goethite such as incorporatedAs during the reprecipitation and/or a low degree of crystallinity (Paikaray et al., 2017). Above all, the different behavior of CrO2-4 and AsO3-4 in the experiment was controlled by their affinities to solid-Fe(III), the potential redox reactions with Fe(II), and the transformation degree of schwertmannite. 3.6. Schwertmannite transformation and Cr, As repartitioning mechanisms According to the above results, a comprehensive schematic of

the transformation mechanisms is proposed and shown in Fig. 6. The participation of Fe(II) had remarkably accelerated reductive transformation of schwertmannite and AsO3-4-schwertmannite. The catalytic mechanism of Fe(II) on schwertmannite transformation was considered to involve ETAE (electron transfer and atom exchange) between adsorbed-Fe(II) and solid-phase Fe(III) at the mineral-water interface (Handler et al., 2014; Williams and Scherer, 2004). The presence of labile Fe(II)-O bond led to crystal dissolution and the subsequent homoepitaxial mineral growth of the dissolved Fe(III) (Eq. (1)) (Bigham et al., 1996; Latta et al., 2012). þ Fe8O8(OH)8-2x(SO4)x(s) þ 2xH2O / 8FeOOH(s) þ xSO2 4 þ 2xH (1)

However, results showed that CrO2-4 and AsO3-4 attenuated the reactivity of schwertmannite via replacing SO2 4 due to their stronger binding capacity to mineral-Fe(III) than SO2 4 . It has been proposed that the substitution of SO2 4 by chromate and arsenate could lower the solubility of schwertmannite (logIAP ¼ 18.5) to 16.2 and 13.5 respectively (Bigham et al., 1996), indicating that the mineral structure was reinforced after substitution and the crystal phase transformation of schwertmannite would be delayed accordingly. Despite the coexisting Fe(II) assisting the dissolution, inhibitory effect of CrO2-4 and AsO3-4 caused by interfering with schwertmannite dissolution and goethite formation surpassed the catalytic effect of Fe(II) on mineral transformation (Ishikawa et al., 2004). It was noteworthy that arsenate showed stronger inhibition than chromate on schwertmannite transformation in the absence of Fe(II) (Regenspurg and Peiffer, 2005), while in this study, the transformation rate of SchCr was much slower than SchAs with cooccurring 4 mM Fe(II). The anaerobic reduction of Cr(VI) by Fe(II) was instantaneous (Salem et al., 1989). The consequences of Cr(VI) reduction on schwertmannite transformation was consumption of catalyst Fe(II) and generation of Cr(III) which would form a mixed

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formation of an amorphous Fe(III)-As(V) hydroxysulfate also interfered in the nucleation of secondary minerals (Maillot et al., 2013). Although Fe(II) was a potential reductant of arsenate, As(III) was not detected during the whole incubations according to XPS spectra, indicating that arsenate reduction might not happen under the given experimental conditions. Coincidently, Amstaetter et al. (2010) demonstrated that Fe(II)-goethite system did not cause As(V) reduction with 1 mM Fe(II) at pH 7. However, as reported by Karimian et al. (2017), some As(V) in jarosite was reduced to As(III) in the presence of a high concentration of Fe(II) (10 mM) with a Fe(II):As(V) molar ratio of 125. The transformation processes of schwertmannite could be described as: (1) adsorption of CrO2-4 and AsO3-4 onto schwertmannite surfaces; (2) abiotic reductive transformation of schwertmannite through Fe(II)-catalyzed pathway, which process might be inhibited via two types: simultaneous consumption of Cr(VI) and catalyst Fe(II), and the coverage of Cr(III)-Fe(III) hydroxide synergistically promoted mineral persistence; the strong bonding between mineral-Fe(III) with arsenate led to retardation of transformation and enduring entrapment of arsenic. 4. Conclusions

Fig. 6. The proposed transformation mechanisms. (a) The possible redox reactions during the Fe(II)-induced transformation of CrO24 -schwertmannite. (1) reduction of Cr(VI) by aqueous Fe(II), (2) adsorption of Fe(II) by schwertmannite, (3) electron transfer and atom exchange into the bulk schwertmannite forming the secondary minerals, (4) reduction of adsorbed Cr(VI) by Fe(II), (5) coverage of Cr(III)-Fe(III) hydroxide precipitate. (b) Redox transformation of AsO34 -schwertmannite. (1) adsorption of Fe(II) by schwertmannite, (2) electron transfer and atom exchange into the bulk schwertmannite, (3) formation of Fe(III)-As(V) hydroxysulfate, (4) precipitation of the secondary minerals.

Cr(III)-Fe(III) hydroxide (Eq. (2)) (Chang et al., 2012; Sass and Rai, 1986). xCr3þ þ (1-x)Fe3þ þ 3H2O 4 (CrxFe1-x) (OH)3(s) þ 3Hþ

(2)

Surface coverage with Fe(II) is considered to be a critical step during the catalytic transformation of schwertmannite. Reductive dissolution of schwertmannite coupled to the potential reduction of Cr(VI) facilitated electron transfer between added Fe(II), solidFe(III) and chromate. Decrease of Fe(II) content limited the Fe(II)Fe(III) ETAE and then decelerated the transformation rates and secondary mineralization. Despite transformation retardant of Cr(VI) was consumed and decreased to extremely low concentrations, the coverage of Cr(III) precipitates still blocked the Fe(II)Fe(III) ETAE. As a result, phase transformation in incubations of SchCr was negligible. Such processes transferred Cr(VI) to benign Cr(III) and simultaneously made chromium negatively transported. In addition, once the mineral structure degraded, chromium would dissolve again into solution and affect the particle growth of newly formed minerals. In summary, both sorption and redox processes of Cr(VI) imprisoned chromium and delayed the transformation. For incubations of SchAs, the strong retardation of transformation was attributed to the tight bound between As(V) and Fe(III), and contributed to accumulation of lepidocrocite prior to subsequent goethite precipitation. Meanwhile, the possible

This study illustrated reductive dissolution of schwertmannite coupled to the potential reduction of Cr(VI) and As(V) inhibited the abiotic Fe(II)-induced transformation of schwertmannite, and helped to immobilize chromium and arsenic. XRD diffractograms suggested that the Fe(II)-induced transformation rates were: control (no oxyanions) >SchAs > SchCr. Strong reduction of Cr(VI) to Cr(III) occurred, but As(V) did not convert to As(III). Chromium and arsenic pollution in AMD is an urgent environmental issue. This study reveals the application possibility of schwertmannite for trapping and immobilizing Cr and As in regions contaminated by AMD. Reduction of Cr(VI) by Fe(II) can lower aqueous chromium concentrations and generate hypotoxicity Cr(III), however, the remobilization of chromium accompanying with schwertmannite transformation will result in secondary pollution. In contrast, both persistence and transformation of schwertmannite will not cause re-migration of arsenic, implying the entrapment of arsenic is firm and schwertmannite can act as a long-term effective pool of arsenic. The results are helpful for understanding the roles of reductive transformation of schwertmannite in geochemical redistribution and toxicity of chromium and arsenic in AMD systems. Acknowledgements This study was supported by the National Natural Science Foundation of China (Nos. 41720104004, 41673090, and 41330639), the National Key Research and Development Program of China (2017YFD0801000), the Fund of Science and Technology Bureau of Shaoguan City (No. 2017SGTYFZ201), and the Fundamental Research Funds for the Central Universities (No. 2015ZZ113). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.01.142. References Amstaetter, K., Borch, T., Larese-Casanova, P., Kappler, A., 2010. Redox transformation of arsenic by Fe(II)-Activated goethite (alpha-FeOOH). Environ. Sci. Technol. 44, 102e108. Antelo, J., Fiol, S., Gondar, D., Lopez, R., Arce, F., 2012. Comparison of arsenate, chromate and molybdate binding on schwertmannite: surface adsorption vs

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