SORPTION OF CD AND PB BY SELECTED TEMPERATE AND SEMIARID SOILS - EFFECTS OF SLUDGE APPLICATION AND AGING OF SLUDGED SOILS

Five soils from semi-arid regions of India and 5 soils from England we re compared with regard to their ability to sorb Cd and Pb when mixed with sewage sludge and as unsludged controls. The application of sewag e sludge at 150 t ha-1 significantly increased the amount of Cd retain ed by the soils. The sorption data were statistically best-fit to the linearized Freundlich equation and the slopes of the isotherms were st eeper with sludge application, indicating an increased affinity for Cd in the soil- sludge mixtures. A similar trend in Pb sorption was also observed for the English soils following the sludge treatment. In con trast, most of the Indian soils showed a decline in Pb sorption follow ing the sludge application. Liming an acidic English soil to pH 7.0 wa s shown to increase its metal sorption capacity. The longer-term persi stence of these observed effects of sewage sludge addition on metal so rption by soils was investigated in two sludge-soil mixtures maintaine d under experimental conditions for up to 450 days. Samples of these s oils taken 1, 60 and 450 days after the mixing with the sludge were ba tch equilibrated with Cd and Pb and it was found that the metal retent ion in both soils decreased significantly over this time period.