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 (S degrees). Oxygen limitation was realized by controlling the gas flow and oxygen concentration in the aeration gas. The oxygen supply varied between 150 and 0.5 mol(O2) mol(S degrees)(-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 0.5 mol(O2) mol(S degrees)(-1) resulted in retarding acidification over a period of three weeks and in poor Me solubilization. Oxidation of S degrees occurred even under strong oxygen limitation at a low rate. High surplus of oxygen was necessary for almost complete oxidation of the added S degrees. The maximum Me solubilization was reached at an oxygen supply of 7.5 mol(O2) mol(S degrees)(-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 x 10(-8) to 0.8 x 10(-8) kg(O2) kg(dm)(-1) s(-1), are primarily attributed to high reactivity of the sulfur flower and high tolerance of indigenous autotrophic bacteria to low oxygen concentrations. The S degrees related oxygen consumption was calculated assuming a molar yield coefficient Y-O2/S of 1.21. The oxygen conversion degree, defined as part of oxygen feed consumed by S degrees oxidation, increased from 0.7% to 68% when the oxygen supply was reduced from 150 to 0.5 mol(O2) mol(S degrees)(-1). (c) 2006 Elsevier Ltd. All rights reserved.