Modeling of Cd and Pb speciation in soil solutions by WinHumicV and NICA-Donnan model

Speciation modeling is an essential tool to estimate the chemical composition of an aqueous sample and to assess the bioavailability and potential risks associated with metal contamination. It is also an indispensable alternative for quantifying the metal speciation when analytical techniques are not available. In this study, we tested the effectiveness of two models, WinHumicV and the Non-Ideal Competitive Adsorption (NICA)-Donnan model, in predicting the amount of Cd2+ and Pb2+, which were originally measured by anodic stripping voltammetry (ASV). In general, with its default parameters, WinHumicV satisfactorily estimated the free Cd concentrations. For Pb, at high pH the model predictions were lower than the ASV measurements. The discrepancy between the measured and predicted free Pb concentrations may be attributed to the uncertainty of stability constants in WHAM model V for metal-DOC interactions, or the possible overestimation of free metals by ASV at high pH. Using the published generic parameters for fulvic acid (FA), the consistent NICA-Donnan model also made predictions comparable to the experimental data when Al and Fe competition was taken into account in the model. The agreement of model predictions and experimental data demonstrates that both models provide reasonable predictions of the metal speciation in soil solutions. However, the results stress the requirement of estimates or measurements of Al and Fe concentrations in soil solution speciation to provide reasonable estimates of trace element speciation.