Biotic and Abiotic Schwertmannites as Scavengers for As(III): Mechanisms and Effects

Arsenic mobility around mining districts is primarily controlled by distribution and abundance of iron minerals. Arsenite-rich mine waters although frequently reported, the interaction of which with schwertmannite is poorly understood despite its high toxicity and mobility. We examined three synthetic schwertmannite types distinguished by surface area (19.9-227.5 m(2)/g), Fe/S molar fractions (4.7-6.6), and saturation index (-1.6-0.8) towards arsenite retention through controlled batch equilibrium studies at 22 +/- 2A degrees C and 1 atmospheric pressure in oxic conditions. Sorption isotherms were investigated as a function of dissolved arsenite concentrations (0.13-1.33 mmol/L) at constant sediment load (10 g/L) and pH (3.0) in order to understand the role of synthesis pathway and physicochemical properties on arsenite immobilization. Multilayer surface coverage with more than one process governs arsenite uptake. X-ray diffractograms, infrared spectroscopy, and high resolution electron microscopic examination revealed new phase formation where schwertmannite underwent morphological and structural degradation. Ionic exchange between schwertmannite SO (4) (2-) and aqueous arsenite has resulted in an elevated aqueous SO (4) (2-) that varied according to dissolved arsenite concentrations. Stoichiometric calculations showed that 1 mol of dissolved arsenite can effectively replace 0.12-0.19 mol of schwertmannite SO (4) (2-) . This study implies that schwertmannites can be used as potential adsorbents for arsenite treatment where the total uptake will be strongly controlled by both ion exchange and surface precipitation.