Chemical Behavior of Lead in a Sandy Soil Long-Term Irrigated With Sewage Effluent

Abstract

Three soil profiles were dug in Abuo-Rawash farm, Giza, Egypt; two of them were nearly highway and subjected to continuous irrigation with effluent up to 40 years, the other one was not contaminated. The collected soil samples under contamination condition were investigated for their total content Pb content, availability and distributions among the different chemical fractions,. The non contaminated one was subjected to adsorption-desorption reaction to asses the ability of soil for Pb retention.The obtained results showed that the level of both total and DTPAextractable Pb as well as those of some detected heavy metals, i.e, Fe, Mn , Zn, Cu, Ni and Cd were, to some extent higher, particularly in the surface layers of the contaminated soils. However, the soil samples (0-10cm) taken 5m from road side contained markedly higher total Pb amounts reaching 20 and 28 folds those of the non contaminated one and are considered phytotoxic. Likewise, the corresponding values of the chemically available Pb were 13.7 and 23 fold, respectively and exceeded the maximum background level. Sequential extraction procedure results showed that , although the exchangeable Pb fraction increased due to increasing soil contamination , most of the added Pb to the studied soils was associated with the residual and/or the organically bound fraction indicating the expected high efficiency of soil organic matter and clay in immobilizing Pb. Thus, the tested soil has relative ability to minimize the immediate risk of the added Pb.Lead adsorption was confirmed to the Langmuir isotherm and the calculated adsorption maximum value (b) revealed that Pb saturated up to 54.5% of the cation exchange capacity of the non contaminated soil. Desorption of the adsorbed Pb represented about 57.1%. This means that more than 40% of the adsorbed Pb was considered not chemically available (retained).