Enhancement of Arsenic Adsorption during Mineral Transformation from Siderite to Goethite: Mechanism and Application

Synthesized siderite was used to remove As­(III) and As­(V) from water solutions under anoxic conditions and oxic conditions. Results showed that As adsorption on synthetic siderite under anoxic conditions was around 10 mg/g calculated with Langmuir isotherm. However, the calculated As adsorption on synthetic siderite under oxic conditions ranged between 115 and 121 mg/g, which was around 11 times higher than that under anoxic conditions. It was found that 75% siderite was transformed into goethite during oxic adsorption. However, synthetic goethite had lower As adsorption capacity than siderite under oxic conditions, although its adsorption capacity was a little higher than siderite under anoxic conditions. It suggested that the coexistence of goethite and siderite bimineral during mineral transformation probably contributed to the robust adsorption capacity of siderite under oxic conditions. Results of extended X-ray absorption fine structure (EXAF) spectroscopy indicated both As­(III) and As­(V) formed inner-sphere complexes on the surface of As-treated solid regardless of substrates, including the bidentate binuclear corner-sharing (²C) complexes and the monodentate mononuclear corner-sharing (¹V) complexes. Monodenate (¹V) and bidentate (²C) complexes would be related to high As adsorption capacity of siderite under oxic conditions. It showed that more Fe atoms were coordinated with As atom in the monodentate complexes and the bidentate complexes of As­(V)/As­(III)-treated siderite under oxic conditions, in comparison with As­(V)/As­(III)-treated siderite under anoxic conditions and As­(V)/As­(III)-treated goethite. Calcinations of natural siderite resulting in the coexistence of goethite and siderite greatly increased As adsorption on the solid, which confirmed that the coexistence of bimineral during mineral transformation from siderite to goethite greatly enhanced As adsorption capacity of siderite adsorbent. The observation can be applied for modification of natural siderite for As removal from high As waters

Stimulation of Fe(II) Oxidation, Biogenic Lepidocrocite Formation, and Arsenic Immobilization by <i>Pseudogulbenkiania</i> Sp. Strain 2002

An anaerobic nitrate-reducing Fe­(II)-oxidizing bacterium, <i>Pseudogulbenkiania</i> sp. strain 2002, was used to investigate As immobilization by biogenic Fe oxyhydroxides under different initial molar ratios of Fe/As in solutions. Results showed that Fe­(II) was effectively oxidized, mainly forming lepidocrocite, which immobilized more As­(III) than As­(V) without changing the redox state of As. When the initial Fe/As ratios were kept constant, higher initial Fe­(II) concentrations immobilized more As with higher As_immobilized/Fe_precipitated in biogenic lepidocrocite. EXAFS analysis showed that variations of initial Fe­(II) concentrations did not change the As–Fe complexes (bidentate binuclear complexes (²<i>C</i>)) with a fixed As­(III) or As­(V) initial concentration of 13.3 μM. On the other hand, variations in initial As concentrations but fixed Fe­(II) initial concentration induced the co-occurrence of bidentate binuclear and bidentate mononuclear complexes (²<i>E</i>) and bidentate binuclear and monodentate mononuclear complexes (¹<i>V</i>) for As­(III) and As­(V)-treated series, respectively. The coexistence of ²<i>C</i> and ²<i>E</i> complexes (or ²<i>C</i> and ¹<i>V</i> complexes) could contribute to higher As removal in experimental series with higher initial Fe­(II) concentrations at the same initial Fe/As ratio. Simultaneous removal of soluble As and nitrate by anaerobic nitrate-reducing Fe­(II)-oxidizing bacteria provides a feasible approach for in situ remediation of As-nitrate cocontaminated groundwater

Sulfur Cycling-Related Biogeochemical Processes of Arsenic Mobilization in the Western Hetao Basin, China: Evidence from Multiple Isotope Approaches

The role of sulfur cycling in arsenic behavior under reducing conditions is not well-understood in previous investigations. This study provides observations of sulfur and oxygen isotope fractionation in sulfate and evaluation of sulfur cycling-related biogeochemical processes controlling dissolved arsenic groundwater concentrations using multiple isotope approaches. As a typical basin hosting high arsenic groundwater, the western Hetao basin was selected as the study area. Results showed that, along the groundwater flow paths, groundwater δ³⁴S_SO4, δ¹⁸O_SO4, and δ¹³C_DOC increased with increases in arsenic, dissolved iron, hydrogen sulfide and ammonium concentrations, while δ¹³C_DIC decreased with decreasing Eh and sulfate/chloride. Bacterial sulfate reduction (BSR) was responsible for many of these observed changes. The δ³⁴S_SO4 indicated that dissolved sulfate was mainly sourced from oxidative weathering of sulfides in upgradient alluvial fans. The high oxygen–sulfur isotope fractionation ratio (0.60) may result from both slow sulfate reduction rates and bacterial disproportionation of sulfur intermediates (BDSI). Data indicate that both the sulfide produced by BSR and the overall BDSI reduce arsenic-bearing iron­(III) oxyhydroxides, leading to the release of arsenic into groundwater. These results suggest that sulfur-related biogeochemical processes are important in mobilizing arsenic in aquifer systems