Investigating the geochemistry of selenium in the residual from biologically treated mine-impacted water

Bacterially-mediated wastewater treatment is commonly used to prevent selenium (Se) release to the environment. This research focuses on the characterization of a selenium-rich residual, the by-product of industrial wastewater treatment. Solid-phase selenium speciation was investigated through sequential extraction procedures (SEP) and X-ray Absorption near Edge Spectroscopy (XANES). Selenium mobility and changes in Se speciation were tested through batch experiments under varying redox and pH conditions. The residual exhibits some heterogeneity, and is dominated by metal selenide species, with some evidence for less-reduced Se species. The greatest Se mobility was in mildly oxidizing conditions at neutral pH which also showed the least speciation change in the residual. Under highly-oxidizing and/or low-pH conditions Se mobilization was lower and reaction was dominated by oxidation of the metal-selenides to Se0 and/or SeS2. Additionally, unexpected speciation changes in the solid phase during the SEP were observed