Transport of Tin and Lead in Soils: Miscible Displacement Experiments and Second-Order Modeling

Understanding the transport and retention of heavy metals is essential in assessing their potential contamination of the ecosystem. In this study, four miscible displacement column experiments were performed to quantify lead (Pb) and tin (Sn) mobility in two loam soils. A pulse of Sn was applied to one set of columns; a second set received a pulse of a mixed solution of Sn and Pb, and a third set received a pulse of Sn followed by a pulse of Pb and vice versa. For all soils columns, Sn was not detected in the effluent which indicates that Sn is immobile and of high affinity. Analysis of the amount of Sn retained revealed that applied Sn was retained in the top 2 cm of the soil columns. For column of a reference sand material, Sn was more mobile with 32.4% recovery in the effluent. Unlike Sn, breakthrough curve results (BTCs) for Pb exhibited extensive mobility with Pb recoveries ranging from 52 to 96.4%. Lead was least mobile in columns where a pulse of Sn was applied before the application of Pb indicative of slowly reversible and irreversible reactions. A snow-plow effect was also observed where Pb concentration in the effluent exceeded that of the input pulse. Such observation suggests that retention processes of Pb should account for competitive sorption. The use of a second-order two site model (SOTS) which accounts for kinetic reversible and irreversible reactions was capable of providing good description of effluent results for Pb for all soil columns.