The removal of heavy metals from contaminated river sediments was studied using suspension leaching under laboratory conditions and percolation leaching in a pilot plant. The leaching potential of indigenous sulfur-oxidizing bacteria was compared with acid treatment. Bioleaching with elemental sulfur as substrate was found to be better than treatment with sulfuric acid for the solubilization of all metals tested. The physical and chemical properties of the sediments used in this study did not affect leaching capacity under optimum conditions in the laboratory.

Under the practical conditions in the pilot plant, the redox state of sludge had a considerable influence on leaching efficiency. In a deposited oxic sediment with good permeability, about 62% of the metals tested were removed by percolation leaching after 120 days. Zn, Cd, Ni, Co and Mn were sufficiently leached to enable treated sediments to be reused as soil. In a freshly dredged anoxic sediment, only a total of 9 % of metals were removed. The results indicate that freshly dredged sediments need to undergo pretreatment before percolation leaching to improve mass transfer and to activate the leaching active bacteria.