Microbial Sulfur Pathways and Outcomes in Tailings Impoundments: A Mesocosm Study

Abstract

In mine wastewaters, three microbial sulfur oxidation pathways have the potential to cause different water quality outcomes that can differ from abiotic model sulfate and acidity predictions, currently used to monitor potential sulfur risks. However, studies integrating microbiology and geochemistry in active mine tailings impoundments, are very limited. Here, we developed a novel diagnostic approach to detect microbially driven sulfur pathways. Within this 28-day study, eight, on-site, 500 L mesocosms were filled with water extracted directly from the water cap of an active Ni/Cu mine tailings impoundment. Diverse combinations of tailings, sulfur compound and nitrate amendments were added to the environmental mesocosms simulating common operational variations experienced by active tailings impoundments. Mesocosm results linked SOx, S4I, and incomplete SOx + rDSR pathway occurrence (metagenomes, inferred from the identity, i.e. 16S rRNA) and activity (mRNA) to physico-chemistry and S geochemistry. By integrating the three lines of evidence, the diagnostic approach was able to identify which S pathways were active under varying physico-chemical conditions and how sulfur geochemical outcomes were affected. A relationship emerged between acid generation and SoxCD occurrence, indicating complete SOx pathway activity. However, observed proton yields and sulfate concentrations were lower than those predicted by SOx pathway activity alone, indicating other pathways, e.g., SOx and the partial S4I pathways (within Thiomonas and Halothiobacillus), and/or activity of the incomplete SOx pathway (within Thiobacillus and Desulfurivibrio), when not coupled to rDSR and/or paired with use of nitrate, were influencing overall S outcomes.