MODELLING SORPTION OF METAL BINARY MIXTURES IN A MINERAL SOIL

Modeling metal sorption in soils is required to predict the fate of heavy metals and to assess the actual risk derived from a contamination event. The solid liquid distribution coefficient (K-d) of a metal in a soil may be affected by the presence of other sorption competitive metals. To evaluate metal competitive effects, sorption experiments were carried out with Cd, Zn, Cu, and Pb as target metals, with Pb and Cd as competitive metals in a clay soil, and varying concentrations of target and competitive metals. Sorption isotherms, both from single and binary scenarios, were fitted with one-metal, two-metal, and two-site Langmuir equations. When examining binary scenarios, the K-d of the target metal decreased up to three orders of magnitude, depending on the target-competitive metal combination, and on the concentration of the competitive metal. With a few exceptions, the quality of the fitting was excellent in all cases, thus confirming these equations as optimum tools to improve the input data for existing risk assessment models, especially when facing risk predictions in multimetal scenarios.