Factors involved in the retention and release of lead by a Central Italy soil: a stirred-flow approach

To improve the prediction and management of problems associated with toxicity in lead (Pb)-contaminated soils, a better understanding of their sorptive and desorptive behaviors is required. In this research, the sorptive and desorptive behaviors of Pb for a clay loam soil (Fluventic Xerochrept), typical of central Italy, were measured using a stirred-flow reactor, where the products of desorption are continuously removed from solution. In addition, we investigated the effects of the presence of calcium carbonate, organic matter (OM), and clay on Pb sorption and desorption behaviors by successively extracting calcium carbonate, OM, and oxides from the soil sample before the experiments. For all four substrates, Pb sorption was characterized by a fast initial reaction in which all the Pb added to the stirred-flow chamber was sorbed. After this, the sorption continued and appeared to be pH dependent. The presence of calcium carbonate led to a pH in the soil that caused the Pb to be insoluble, this being the main cause of the Pb sorption. When the pH of the original soil was lowered, depriving the soil of calcium carbonate, its Pb-sorptive capacity was significantly reduced and the percentage of Pb desorption increased. The elimination of OM from the soil sample previously deprived of calcium carbonate further increased the percentage of Pb desorption. The Pb sorption characteristics of the clay fraction were fast and nonhysteretic.