Testing In Situ Sulfate Reduction by H2 injection in a Bench-Scale Column Experiment

Autotrophic microbial sulfate reduction was tested in a fixed-bed bench-scale column experiment to lower sulfate and iron loads in acid-mine-drainage-influenced groundwater. The microbial process was enhanced by injecting H₂ gas as electron donor into the silicate bed. The experiments were performed at 2.5 atm and 10°C. Complete iron removal (3.8?±?0.3 mM) and partial sulfate removal from 17 to 9 mM were achieved at rates of about 0.004–0.019 mmol SO₄ per liter per hour and at hydraulic retention times of 51.5–19.8 days. The tests showed that most microbial activity took place in immobile zones. These zones create stable environmental conditions for the microorganisms leading to constant reduction rate despite possibly unfavorable conditions prevailing in the mobile phase. Diffusion between mobile and the immobile zones was not found to be the limiting factor for sulfate reduction. Rather, low H_2(aq) concentrations due to low H₂ solubility combined with the inhomogeneous distribution of H₂ gas in the pore space limited sulfate reduction. H_2(aq) concentrations in some parts of the sediment body were insufficient to maintain H_2(aq) concentrations in the immobile zones above the level of substrate limitation. Fe and S precipitated mostly as iron monosulfide and accumulated in regions with high H_2(aq) availability. Calculations showed that the deposition of iron sulfide in the pore space does not affect the pore volume significantly.