As(III) retention kinetics, equilibrium and redox stability on biosynthesized schwertmannite and its fate and control on schwertmannite stability on acidic (pH 3.0) aqueous exposure

High As(III) enrichment in schwertmannite precipitated acid mine impacted areas is a major concern considering its acute toxicity and mobility where the current knowledge of their interaction, redox conditions and schwertmannite metastability is inadequate. In this study we have investigated such aspects through batch isotherm, microscopic and spectroscopic techniques. Schwertmannite produced by biotic process with 14.7 m2 g−1 surface area demonstrated a rapid As(III) uptake followed by slow retention possibly into the internal absorbing sites through multilayer and heterogeneous sorption processes. Chemical, X-ray diffraction, infrared and microscopic examinations revealed that ionic exchange between schwertmannite SO42- and As(III) and surface precipitation governed the total As(III) uptake where lower dissolved SO42- and high sorbent mass enhanced As(III) retention. Redox instability of sorbed As(III) was evidenced from the near-edge spectroscopic analysis at extremely high Fe(III):As(III) ratio (5.5 × 105) leading to surface oxidation to As(V), while As(III) was found as the predominant redox species at high As(III):Fe(III) ratios. Only 0.83% of sorbed As(III) was released which was subsequently re-adsorbed into schwertmannite during 4 months stabilization without any evidence of mineralogical transformation.

► As(III) retention is very rapid.
► High Fe(III)/As(III) ratio can cause oxidation of sorbed As(III).
► Total As(III) uptake is controlled by ligand exchange and possible surface precipitates.
► Sorbed As(III) binds strongly releasing negligible fractions upon re-suspension.
► Retained As(III) restricts mineralogical transformation.