Sorption and Desorption of Lead (Pb) and Effect of Cyclic Wetting-Drying on Metal Distribution in Two Soils with Different Properties

Heavy metals added to soils are partitioned between solid and solution phases and then slowly redistribute among solid-phase components. Heavy metal distribution has an important effect on its toxicity and bioavailability in soil. The aim of this study was to evaluate the sorption and retention of lead (Pb) in two calcareous soils with different properties (a saline-sodic and a nonsaline-nonsodic soil) and to evaluate the effects of metal loading quantity and wetting-drying (WD) cycles on distribution of different loadings of Pb to these soils. To quantify the binding intensity, the metal sorption capacity of soils, and the mobility and availability of loaded Pb, the reduced partition index (IR), the sorption intensity factor (SI), and the percentage or absolute concentration of Pb in soluble + exchangeable (S+E) fractions were used. A small amount of sorbed Pb was desorbed by soils, indicating a strong and irreversible binding of Pb in the studied soils. In terms of SI values, more than 97% of the added Pb was sorbed by the soils. The SI factor, how ever, exhibited no significant change (p≤0.05) with increased Pb loading quantities, probably demonstrating the precipitation of Pb in the soils. While, the first cycle of WD showed a significant effect (p≤0.05) on the values of IR index, the subsequent cycles exhibited no significant effect (p≤0.05). The IR values increased when Pb loading quantities to soils increased, demonstrating an increase in the mobility of Pb with increased soil pollution level. Wetting-drying of soils had a significant effect (p≤0.05) on mobility and availability of soil Pb. Comparing the values of S+E indicated that the mobility and availability of Pb decreased by WD and increased as the Pb loading quantities to soils increased.