Inorganic Thiolated Arsenic in Mine Wastewater: Sources, Stability, and Potential Environmental Impacts

Abstract

Aqueous inorganic thiolated As species (thio-As) can be essential for the biogeochemical cycling of As in both natural and engineered environments. Mine waste systems such as tailings ponds and waste rock piles may have the sub-oxic and pH conditions that allow for the formation and mobilization of thio-As, but there remains a paucity of quantitative data on the presence and kinetic stability of these forms of As. This thesis investigated the presence, behaviour, and stability of thio-As in mine waste systems across various environmental conditions and evaluated geochemical controls on their formation and degradation (oxidation kinetics). Laboratory column experiments were conducted with mine waste rocks from the Antamina mine, Peru, and processed tailings from Montague, Nova Scotia, to assess the production of thio-As. Drainage from column experiments contained thio-As concentrations up to 13 µg/L, primarily as monothioarsenate. Thio-As abundances were notably higher (<5% of total dissolved As) in drainages from enargite-rich materials compared to those of arsenopyrite-bearing materials (<0.5%). Investigation of legacy tailings in Ontario and Nova Scotia revealed significant thio-As in porewaters (up to 5 mg/L; 17% of total dissolved As), primarily as monothioarsenate, as well as lesser amounts of di- and tri-thioarsenates and methylated thioarsenates. Thio-As was most abundant in sub-oxic porewaters and tailings across the studied sites, and strongly related to the prevailing redox conditions and porewater hydrochemistry, less to the As-bearing mineralogy. The oxidation kinetics of thio-As at varying pH, dissolved Fe, and (thio-)As concentration were also examined. Thio-As oxidation rates increased at lower pH, reaching several μmoles/L/d at pH 3. Trithioarsenate oxidation was about two orders-of-magnitude faster than di- and mono-thioarsenate. Experimental data was used to parameterize rate equations and calibrate a kinetic model for rate constants, offering insights into thio-As mobility and its potential environmental impacts. This thesis demonstrates that thio-As can exist in mine waste systems and potentially influence As mobility in these and receiving downstream environments. This highlights the need for including thio-As in mine waste management, environmental assessment, and remediation strategies, particularly at high-As sites.