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

This study investigates the effect of various SO₄²⁻ concentrations (0, 50, and 100 mM) on the phase transformation of hydrous ferric arsenate (HFA) and partitioning behaviors of As(V), Fe(III), and SO₄²⁻ 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 SO₄²⁻ concentration played a secondary, pH-dependent role. More specifically, at pH 4 and under ambient temperature, SO₄²⁻ enhanced the release of As(V) and Fe(III) into the solution. By contrast, at pH 6 and 8, SO₄²⁻ promoted the formation of basic ferric arsenate sulfate, which immobilized As(V), and later dissolved upon heating. SO₄²⁻ incorporation into the solid phase occurred across all pH levels and was enhanced at higher concentrations and temperatures. Thus, SO₄²⁻ 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.