Cadmium in landfill leachates and soils: The application of speciation and surface complexation models

The chemical composition and speciation of four municipal waste landfill leachates was found to be extremely variable. They contained higher than environmentally acceptable levels of some heavy metals, such as Cd, and other toxic elements. Speciation calculations using the model SOILCHEM predicted that alkali and alkaline earth metals existed primarily as uncomplexed ions while organic and inorganic ligands present in the leachates decreased the proportion of uncomplexed ions for most of the heavy metals, including Cd. High concentrations of Cl{dollar}sp-{dollar} and DOC affected the speciation of Cd in landfill leachates.; The adsorption of Cd by nine Ontario soils was found to be pH-dependent and increased with increasing solution Cd concentrations. The experimental data conformed to the Langmuir adsorption isotherm and the calculated adsorption maximas, q{dollar}sb{lcub}rm max{rcub},{dollar} increased the increasing pH. Adsorption maxima ranged from a low of 8.92 mmol kg{dollar}sp{lcub}-1{rcub}{dollar} for a Fox loamy sand to a high of 64.8 mmol kg{dollar}sp{lcub}-1{rcub}{dollar} for a Haldimand heavy clay. Multiple linear regressions of q{dollar}sb{lcub}rm max{rcub}{dollar} against combinations of soil properties showed the importance of clay content and the increasing importance of organic matter content at a soil pH of between approximately 4 and 6.; A chemical equilibrium surface complexation model was written in order to predict the extent of Cd adsorption in the nine soils studied. Data for the surface complexation parameters were taken from the literature. Dissociation constants, K{dollar}sb{lcub}rm a{rcub}{dollar}'s, and a Cd complexation constant, K{dollar}sb{lcub}rm CdL{rcub}{dollar}, with a soil humic acid were determined using potentiometric titrations. Calculated values were found to be pK{dollar}sb{lcub}rm a1{rcub}{dollar} = 4.05, pK{dollar}sb{lcub}rm a2{rcub}{dollar} = 5.85 and pK{dollar}sb{lcub}rm CdL{rcub}{dollar} = 5.02.; In almost every case the model underestimated the amount of adsorption in all the soils studied and suggested that the assumption of non-interactive adsorbing surfaces was reasonable. Humic surfaces were predicted to be extremely important adsorbing sites for Cd in all soils and adsorption onto these surfaces predominated at pH values less than approximately 8. Hydrous ferric oxide surfaces were predicted to be important in all the studied soils that had high extractable Fe contents, but only at pH values of above approximately 7. The edges of clay minerals were not predicted to be important adsorbing surfaces until high pH values.