Effect of oxygen limitation on solid-bed bioleaching of heavy metals from contaminated sediments

The effects of oxygen limitation on solid-bed bioleaching of heavy metals (Me) were studied in a laboratory percolator system using contaminated sediment supplemented with 2% elemental sulfur (So). Oxygen limitation was realized by controlling the gas flow and oxygen concentration in the aeration gas. The oxygen supply varied between 150 and 0.5molO2molSo-1 over 28 d of leaching. Moderate oxygen limitation led to temporarily suppression of acidification, rate of sulfate generation and Me solubilization. Lowering the oxygen supply to 0.5molO2molSo-1 resulted in retarding acidification over a period of three weeks and in poor Me solubilization. Oxidation of So occurred even under strong oxygen limitation at a low rate. High surplus of oxygen was necessary for almost complete oxidation of the added So. The maximum Me solubilization was reached at an oxygen supply of 7.5molO2molSo-1. Thus, the oxygen input during solid-bed bioleaching can be reduced considerably by controlling the gas flow without loss of metal removal efficiency. Oxygen consumption rates, ranging from 0.4 × 10−8 to 0.8×10-8kgO2kgdm-1s-1, are primarily attributed to high reactivity of the sulfur flower and high tolerance of indigenous autotrophic bacteria to low oxygen concentrations. The So related oxygen consumption was calculated assuming a molar yield coefficient YO2/SYO2/S of 1.21. The oxygen conversion degree, defined as part of oxygen feed consumed by So oxidation, increased from 0.7% to 68% when the oxygen supply was reduced from 150 to 0.5molO2molSo-1.