Revisiting the transformation of Fe^II-As^V coprecipitates under oxic and anoxic conditions: Insights into the kinetics of formation of parasymplesite and ferrisymplesite

Ferrous (Fe^II) solution is widely used to treat the arsenic (As)-contaminated waters worldwide. However, its long-term effectiveness regarding As fate and its controlling factors remains questionable. In this investigation, the formation kinetics of secondary As-bearing phases (parasymplesite/ferrisymplesite) following Fe^II treatment of As-containing waste waters (from acid leaching of calcium arsenic residues from a copper smelter) was first studied. Key factors including pH, oxic/anoxic conditions, and piling time were considered in laboratory-scale batch experiments. XRD and TEM results indicated that the formation kinetics of the two As-bearing phases followed the same pH-dependent order under both oxic and anoxic conditions: pH 9 > pH 7 > pH 8 > pH 6. Almost all the As in the initial leachates remained in the solid phase throughout the 72-day transformation period. Residual aqueous As showed the highest concentration at pH 6 under oxic conditions, consistent with the transformation rate of the As-bearing phases. By comparison, under anoxic conditions, the formed As-bearing solids had the highest stability at near-neutral pH. Two competing reactions involving parasymplesite and ferrisymplesite (formation vs. stability) controlled the fate of As. The findings are expected to provide valuable insights into the industrial implementation and optimization of this Fe^II-based As treatment technology.