Influence of pH and sulfate concentration on hydrous ferric arsenate transformation behavior and As(v) mobilization.

This study investigates the effect of various SO4 2- concentrations (0, 50, and 100 mM) on the phase transformation of hydrous ferric arsenate (HFA) and partitioning behaviors of As(v), Fe(iii), and SO4 2- under ambient (25 °C, 15 d) and subsequent elevated temperature (80 °C, 35 d) conditions. The results revealed that the primary factor controlling the transformation of HFA into crystalline scorodite was the pH, whereas the SO4 2- concentration played a secondary, pH-dependent role. More specifically, at pH 4 and under ambient temperature, SO4 2- enhanced the release of As(v) and Fe(iii) into the solution. By contrast, at pH 6 and 8, SO4 2- promoted the formation of basic ferric arsenate sulfate, which immobilized As(v), and later dissolved upon heating. SO4 2- incorporation into the solid phase occurred across all pH levels and was enhanced at higher concentrations and temperatures. Thus, SO4 2- modulates As(v) mobility via structural incorporation and ion competition, with distinct behaviors at acidic versus circumneutral pH. These findings offer guidance for risk assessment and design of sulfate-rich, mining-impacted remediation systems.