Abstract

The effectiveness of natural clay, calcium phosphate, poultry manure and rice husks as cheap and ecologically non-invasive amendments for immobilizing Cu and Pb in contaminated soil was assessed. A moderately contaminated soil was sampled from a cultivated field in the vicinity of an active waste dump, characterized and amended with the immobilizing agents at 5 to 20% w/w. Single (calcium nitrate), sequential (optimized European Communities Bureau of Reference, BCR method) and kinetic (ethylenediaminetetraacetic acid, EDTA) leaching tests were performed before and after soil amendments. Single extractions showed that metal immobilization efficiencies, E were generally high (20 ≤ E ≤ 81%), increased as the agent dose and varied in the order: phosphate > natural clay > poultry manure > rice husks. BCR extractions showed that the agents attenuated Cu and Pb mobilities and induced changes in their fractionation patterns as most of the hitherto mobile fractions became associated with the less bioavailable forms. Two metal forms: readily labile and less labile, with apparent leaching rates k₁ (10^-2 min^-1) and k₂ (10^-3 min^-1), respectively were identified by fitting EDTA leaching data into a two-first-order model. The immobilizing agents suppressed both k₁ and k₂ for Cu and only k₂ for Pb. These agents may reduce risks resulting from Cu and Pb contamination in soil since metal mobility is related to its bioavailability.

Key words: Bioavailability, chemical extraction, contaminated soil, immobilizing agent