Influence of pH and sulfate concentration on hydrous ferric arsenate transformation behavior and As(<scp>v</scp>) 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 <i>via</i> structural incorporation and ion competition, with distinct behaviors at acidic <i>versus</i> circumneutral pH. These findings offer guidance for risk assessment and design of sulfate-rich, mining-impacted remediation systems.