Reduction of bioaccessibility and leachability of Pb and Cd in soils using sludge from water treatment plant

Abstract

The adsorption properties of sludge from drinking water treatment station make it an option to remediate soils contaminated with toxic metals. This paper describes the reuse of this sludge to immobilize Pb and Cd from contaminated soils, as well as the efficiency of this process applying bioaccessibility assays to evaluate how this sludge/soil mixture can mitigate the exposition that could potentially affect the health of individuals. The adsorption test revealed that Pb and Cd bound five and eight times more strongly to the sludge as compared to soil, respectively. The bioaccessibility assay showed that the best proportion of sludge/soil was 1:1 for both metals, with the reduction of 28.8% and 34.5% for Pb and Cd bioaccessibility, respectively. The high amount of organic matter of sludge could underlie the Pb and Cd stabilization and the decrease in the bioaccessibility. Chemical fractioning revealed that Pb stabilized better on Fe, Al, and Mn oxides. After 4 months, the exchangeable fractions together were less than 3% in samples with sludge/soil ratio 1:1, indicating that Pb is less available for leaching and became more stable. Cd was less leachable and reached stable fractions faster in samples with sludge/soil ratio 1:1. Based on these results, the sludge represents a potential adsorbent for simple Pb and Cd remediation in soils.