Arsenic associated with gypsum produced from Fe(III)-As(V) coprecipitation: Implications for the stability of industrial As-bearing waste

Gypsum is a high-volume by-product of hydrometallurgical arsenic removal using the Fe(III)-As(V) coprecipitation technique. However, the role of gypsum in As fixation during the Fe(III)-As(V) coprecipitation process and the potential risk of As-bearing gypsum are still not well known. In this study, the fixation of As by gypsum was quantitatively investigated as a function of the initial As and H2SO4 concentrations, Fe/As molar ratio, pH, and neutralization mode. Column leaching tests were also performed to evaluate the potential risk of Fe(III)-As(V) coprecipitates. The results showed that the gypsum isolated from Fe(III)-As(V) coprecipitates by ascorbic acid (VC) treatment could fix up to 4.5% of the total As in the system at pH 8 in fixed-pH mode. The gypsum produced in fixed-pH mode contained a significantly higher concentration of As than that produced in acid-alkaline neutralization mode. The column leaching tests showed that a faster leaching rate could enhance the release of As from Fe(III)-As(V) coprecipitates. It seems that the presence of gypsum could prevent the release of As from Fe(III)-As(V) coprecipitates. Linear combination fits of the As and Fe K-edge XANES spectra suggested that As was likely to occur as adsorbed As and yukonite in VC-isolated gypsum.