The dissolution of scorodite in gypsum-saturated waters: Evidence of Ca–Fe–AsO4 mineral formation and its impact on arsenic retention

This paper reports the results of an investigation on scorodite behavior in a gypsum-saturated aqueous environment in terms of arsenic release and solid phase transformation. The experiments were conducted at fixed pH (5–9) and temperature (22 °C and 75 °C). The apparent equilibrium arsenic concentrations obtained in a gypsum-saturated system were lower than in a pure (gypsum-free) system. At pH 7 and 22 °C, the apparent equilibrium arsenic concentration was 3.6 mg/L in the presence of gypsum compared to 5.9 mg/L in a pure system. At higher temperatures and pH values, this effect was even more significant. One possible explanation for this reduced arsenic concentration level in the presence of gypsum may be the formation of calcium–iron(III)–arsenate compounds. Experimental evidence for this was obtained at 75 °C and pH 7 and 9 where scorodite was found to partly transform to yukonite (Ca2Fe3(AsO4)4(OH)·12H2O).