Enhanced mobility of Pb in the presence of dissolved natural organic matter

The speciation of Pb in batch experiments and its mobility under flowing conditions in column transport experiments were investigated to study Pb behavior in a soil-water system in the presence of dissolved natural organic matter (DOM), peat humic acid (PHA) and peat fulvic acid (PFA). A sandy soil having a significant intraparticle porosity was used as the sorbing media. Batch equilibrium sorption isotherms for single components (Pb, PHA, and PFA) and for Pb in the presence of PHA and PFA were generated. Batch equilibrium experiments were also performed for both PHA and PFA to investigate Pb-DOM binding in the absence of soil. Single component (Pb, PHA, and PFA) and multicomponent (Pb-PHA and Pb-PFA) laboratory-scale column transport experiments were conducted to assess transport behavior of Pb in the presence of DOM. Sorption isotherms indicated that the soil had a higher affinity for PHA than for PFA. However, single component column transport experiments showed that PHA was less retarded than PFA. This anomaly was attributed to the size exclusion of the larger PHA molecules from the intraparticle porosity of the media under the geochemical conditions in the column. Pb retardation predicted by equilibrium equations based upon nonlinear isotherm parameterization agreed well with observed retardation. However, equilibrium retardation equations overpredicted retardation of DOM, indicating sorption kinetic limitations (chemical and/or physical nonequilibrium), molecular size exclusion during column transport, or chemical heterogeneity of the DOM. In multicomponent column transport experiments, Pb retardation decreased by factors of 4–8 in the presence of DOM. Multicomponent batch equilibrium experiments suggested that Pb mobility was governed by speciation of Pb with soluble DOM during transport. Thus, Pb eluted earlier in the presence of PHA than in the presence of PFA because PHA had a higher affinity for Pb binding than PFA.