Removal of sulfate from extremely acidic mine waters using low pH sulfidogenic bioreactors

A low pH sulfidogenic bioreactor, maintained at between pH 2.8 and pH 4.5, was used to remove sulfate from two extremely acidic (pH 1.3 to 3.0) synthetic mine waters that contained small amounts of ferrous iron, but no other chalcophilic metals. By changing the concentration of glycerol added as an electron donor to fuel sulfate reduction, and lowering its pH, 98% of sulfate was removed from a synthetic ground water with a chemical composition based on that at a lignite mine in Germany. While more sulfate (up to 35 mmoles L^{- 1}) was removed with synthetic raffinate (with a chemical composition based on that at a mine site in northern Chile), this accounted for a smaller fraction (between 50 and 60%) of the total sulfate present. There was a good correlation between glycerol oxidized and sulfate reduced, with the relationship being close to that predicted by the stoichiometry of the reaction in the case of the synthetic ground water. Acetic acid was detected in varying concentrations in the bioreactor, due to some of the glycerol being incompletely oxidized, but lowering the pH of the bioreactor caused the concentration of acetic acid to decrease. Lowering the operating pH of the bioreactor from 4.5 to ∼ 3 caused the composition of the microbial consortium to change, and a known acetogenic sulfidogen (Desulfosporosinus M1) to become less dominant.