Resume/ Abstract
Organic matter (OM) plays a significant role in influencing metal and metalloid speciation, mobility, and fate, particularly by altering geochemical stability and complexation with organic ligands. The main objective of this study was to evaluate the potential effect of tailings enriched in OM originating from surface plant recolonization on the speciation, mobility, and fate of As in underlying As‑rich aged mine tailings. Tailings samples were collected from 2 superficial and deeper horizons, at a legacy silver mine site, including oxidized tailings enriched with OM and non-oxidized tailings. Then, 12 individual cell tests were conducted to simulate the weathering of the tailings under four different treatments, each replicated three times: (1) non-oxidized tailings; (2) OM-rich oxidized tailings overlying non-oxidized tailings; (3) a mixture of OM-rich oxidized and non-oxidized tailings; and (4) OM-rich oxidized tailings alone. Each cell was rinsed twice a week over a 16-week period, with a one-month no-rinse pause following the 10th week. Drainage waters were collected and analyzed for pH, EC, ORP, metal/metalloid concentrations, and total organic carbon (TOC). Arsenic speciation was also analyzed on combined replicate samples from every two consecutive rinses using ion-exchange cartridges. The results showed that compared to the treatment with non-oxidized tailings alone, the mobility of As was reduced when a layer of OM-rich tailings was applied over the non-oxidized tailings or when the two were mixed (approximately twice less cumulative arsenic), which might be explained by the higher concentration of As in the non-oxidized tailings.