Suppression of arsenic trioxide dissolution by antimony substitution

Roasting arsenopyrite during gold production generates hazardous arsenic trioxide roaster waste (ATRW). At the Giant Mine, Canada, > 20,000 tonnes ATRW were aerially dispersed while another 237,000 tonnes are buried. Arsenolite [As₂O₃] largely comprises ATRW, but minor elements may have substantial geochemical influence (e.g. Fe, Sb, Ca). To investigate controls on ATRW dissolution, we performed batch experiments on variable-composition Giant Mine samples. Following two weeks of anoxic reaction (21 and 4 ˚C), As₂O₃ dissolution (17.9 and 12.3 g L^-1) only reached 72% and 73% of As₂O₃ solubility Up to a 27% decrease in dissolution was attributed to increased Sb:As weight ratios. Ionic strength (0.02 M NaCl, 0.20 M NaCl) and starting pH (4, 6, 8) differences resulted in < 5% variation in dissolution. X-ray absorption spectroscopy (XAS) identified no substantial As/Sb bonding differences between consistently trivalent pre- and post-dissolution solids. Pair distribution function (PDF) and automated mineralogy results support XAS modelling, indicating an increased abundance of As-Sb bonding compared to initial solids. Therefore, Sb may be predominantly locked in a persistent As₂O₃-Sb₂O₃ solid-solution, where residue may remain stable following management-related dissolution of ATRW. Selective As dissolution increases relative concentrations of residual Au and Sb, and redistribute the risks associated with wastes.