Modeling competitive metal sorption 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. Since pollution sources are usually multimetal, the solid–liquid distribution coefficient (Kd) of a target metal must be examined in the presence of other competitive metals. Then, sorption models, such as Freundlich and Langmuir, must be adapted and tested for their use in competitive scenarios. To do this, sorption isotherms were fitted with one-metal and two-metal Freundlich equations, and with one-metal, two-metal, and two-site Langmuir equations, from data originated from sorption experiments with Cd, Zn, Cu, and Pb as target metals, and Pb and Cd as competitive metals in a clay soil. In single scenarios, Pb had the highest ranges of Kd values (127,500–211 L kg), while Cd had the lowest values (2720–11 L kg− 1 ). When examining binary scenarios, the Kd of the target metals decreased up to three orders of magnitude, depending on the target-competitive metal combination, and on the concentration of the competitive metal. The quality of the fitting based on modified Freundlich and Langmuir models was excellent in most 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. Sorption reversibility was also evaluated by the extraction test, and the quantification of a Retention Factor (ratio of Kd and extraction yields) was proposed to evaluate changes in metal mobility in single and multimetallic scenarios.