Abstract: A new and useful way of treating oxidized-contaminant-containing water, soil, rock, other geological or non-geological matrix formation (such as a landfill) is provided. A bioreactor is provided that includes elemental sulfur and a microbial population capable of oxidizing sulfur and reducing pertechnate (TcO4?), arsenate (H2AsO4?), chromate (CrO42?), bromate (BrO3?), chlorite (ClO2?), chlorate (ClO3?), perchlorate (ClO4?), and uranium(VI) oxide, with biological reduction of these oxidized contaminants in the matrix containing the oxidized contaminant performed by the bioreactor, with the elemental sulfur as the electron donor.