Photoreductive dissolution of schwertmannite induced by oxalate and the mobilization of adsorbed As(V)
Graphical abstract
Introduction
Arsenic (As) is a toxic contaminant in natural aquatic environments and originates from both natural processes, such as geothermal sources, weathering of As-bearing minerals, microbial activities, and anthropogenic activities (Smedley and Kinniburgh, 2002). Concentrations of arsenic in drinking water in many regions worldwide are higher than the World Health Organization (WHO) recommended safety limit of 10 μg L−1 (WHO, 1993), which puts a severe threat to the health of millions of people. In natural environments, As exists in a variety of valent states and in numerous organic and inorganic forms. Arsenate [As(V)] is a predominant inorganic specie in oxygen-rich environments and primarily exists as H2AsO4− and HAsO42− in natural aquatic environments since the pKa values for arsenic acid are pKa1 = 2.3, pKa2 = 6.8, and pKa3 = 11.6 (Rasheed et al., 2017; Mohan and Pittman, 2007).
Iron (hydr)oxides are present in the environment as a wide range of minerals, most commonly goethite (α-FeOOH), ferrihydrite (Fe5HO8·4H2O), lepidocrocite (γ-FeOOH) and schwertmannite [Fe8O8(OH)8-2x(SO4)x, 1.0 ≤ x ≤ 1.75] (Smedley and Kinniburgh, 2002; Cornell and Schwertmann, 2003). Owing to their great abundance and strong binding affinities to arsenic, iron (hydr)oxides are probably the most important adsorbents for the immobilization of arsenic in natural environments (Cornell and Schwertmann, 2003). Sunlight irradiation can lead to the photoreductive dissolution of iron minerals in photic water and soil surfaces (Sulzberger and Laubscher, 1995; Borer et al., 2005, 2009a, 2009b; Banwart et al., 1989). Two reaction steps are involved in this photoreductive dissolution process: (i) photoreduction of Fe(III) at the (hydr)oxides surface and (ii) subsequent release of surface-bound Fe(II) into solution (Banwart et al., 1989). Previous study has demonstrated that two mechanisms are known to potentially account for the formation of surface Fe(II) in natural waters: (i) electrons and holes are generated under irradiation via the charge transfer between lattice O(-II) and Fe(III) in iron (hydr)oxides, and then the photo-induced electrons will result in surface Fe(III) reduction (that is, the mechanism of semiconductor); (ii) ligand-to-metal charge transfer (LMCT) in photo-active surface Fe(III) complexes contribute to the formation of surface Fe(II) (Borer et al., 2005). The detachment of surface Fe(II) which determines the overall dissolution rate is of great importance in the photoreductive dissolution reaction of iron (hydr)oxides (Waite and Morel, 1984).
Dissolved organic matter (DOM) can affect the fate of As via different mechanisms (Sundman et al., 2014). Interactions between As and DOM are believed to occur mainly through Fe-bridges in ternary DOM-iron-As complexes (Lin et al., 2004; Redman et al., 2002; Sharma et al., 2010). DOM could effectively promote the reductive dissolution of iron minerals (Borer et al., 2005, 2009a, 2009b; Banwart et al., 1989). Siderophores [Desferrioxamine B (DFOB) and Aerobactin], for example, can accelerate the photoreductive dissolution of lepidorocite. At pH = 3, lepidocrocite dissolved 91 times faster in the presence than in the absence of DFOB (Borer et al., 2009b). DOM also competes with As(III/V) for binding sites on mineral surface (Redman et al., 2002). Oxalate is commonly found in natural environments (concentration varies from 2.5 × 10−5 to 4.0 × 10−3 M) and has a great contribution to photoreductive dissolution of iron (hydr)oxides (Reichard et al., 2007). Fe(III)/oxalate systems in atmospheric waters, in iron-rich surface waters, and possibly on soil surfaces are of interest in the degradation of pollutants. Wu et al. (2012) revealed that after 5 min UV irradiation 25 mg L−1 dissolved Fe(II) was yielded in the system of Sch (0.2 g L−1) and oxalate (2 mM) at pH 4.0. In addition, 97% of methyl orange with the initial concentration of 50 mg L−1 was removed after 40 min UV irradiation. However, little attention has been paid to the photochemical reactions of the ternary As(V)-iron (hydr)oxides-oxalate system. Under light illumination, the effects of oxalate on the mobilization of As(V) adsorbed on iron minerals have not been studied, and the mechanism involved in the immobilization of the released As(V) by the newly-formed secondary iron minerals is also not clearly understood.
Acid-mine drainage (AMD) is polluted water with low pH and high concentrations of heavy metals and other toxic elements (Cheng et al., 2009). Nordstrom and Alpers (1999) reported that As(V) concentration could even reach 850 mg L−1 in an acid seep at Iron Mountain, California. Sch, a poorly crystalline Fe(III)-hydroxy-sulfate mineral, commonly found in AMD, is a good adsorbent for As(III) and As(V) (Bigham et al., 1994; Jönsson et al., 2005; Regenspurg et al., 2004). Our recent study suggested that the maximal adsorption capacity of As(V) on Sch could reach 182.86 mg g−1 Sch (Song et al., 2015). In the present study, we will further investigate oxalate and the amount of As(V) loading on the photoreductive dissolution of Sch with (and without) adsorbed As(V) and the mobilization of As(V).
Section snippets
Materials
All laboratory glasswares were repeatedly rinsed with deionized water before use. All reagents were of analytical grade, and all solutions were prepared with deionized water. NaH2AsO4·7H2O (Merck, purity > 99.0%) and oxalate (Tianjin Bodi Chemical Co., Ltd., Tianjin, P.R. China, purity>98.0%) were used as As(V) and oxalate sources, respectively.
Synthesis of Sch and Sch*-As(V)
Synthetic schwertmannite was prepared according to the method by Kumpulainen et al. (2008). In this method, 500 mL pre-heated deionized water was mixed
Characterization of synthetic Sch and Sch*-As(V)
In our study, Sch*-As(V)-i (i = 1, 2 or 3) represented Sch with various amount of pre-loaded As(V). Specifically, the adsorbed As(V) on Sch was 97.0, 128.7 and 147.9 mg g−1 for Sch*-As(V)-1, Sch*-As(V)-2 and Sch*-As(V)-3, respectively (Table 1).
The curve a in Fig. 1 showed the XRD pattern of synthetic Sch, in which all diffraction peaks are in good agreement with rhomb-centered hexagonal (rch) Sch (JCPDS Card No. 47–1775). Specifically, the diffraction peaks at 2θ of 26.3°, 35.2° and 61.3° were
Conclusion
The present study reveals the effects of UV irradiation and oxalate on the dissolution of Sch and Sch*-As(V). Oxalate play different roles in the dissolution of Sch in the dark and under UV irradiation. In the dark, dissolution of Sch by oxalate was primarily controlled by the ligand-promoted dissolution. Under UV irradiation, Fe(III)-oxalate complexes formed at the surface could be converted into Fe(II)-oxalate. UV irradiation has almost no effect on the mobilization of As(V) in Sch*-As(V) in
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgments
We greatly acknowledge the financial support from the Science and Technology Plans of Tianjin (No. 15PTSYJC00230), Tianjin Research Program of Application Foundation and Advanced Technology (No. 17JCQNJC08000) and the National Natural Science Foundation of China (No. 41373114 and No. 41201487).
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2022, Geochimica et Cosmochimica ActaCitation Excerpt :The mineral morphology was close to the original Sch in the Fe/C = 0.7 group (Fig. 3a, c and d). Goethite peaks with a small amount of Sch peaks were observed in the Fe/C = 0.35 treatment (Fig. 3a), which was consistent with previous observations under similar reaction conditions (Ren et al., 2018). The SEM images also showed the appearance of stick-shaped goethite (Fig. 3e).