Use of speciation and complexation models to estimate heavy metal sorption in soils

The development of speciation and surface complexation models to calculate partitioning of metals between the soil solid phase and soil solution has made a large progress. We evaluated the use and practical applicability of an assemblage of complexation models to predict sorption of Cd, Cu, Ni, Pb and Zn. We used 353 samples which cover a wide range of soil properties and metal contents. For sorption of cations to solid and dissolved organic matter the NICA-Donnan model was used, to clay minerals a non-specific Donnan type exchange model, and to metal(hydr)oxides a two-site DDL model. Calculated solution concentrations agree well with measurements, except for Pb where calculations overestimate concentrations by one order of magnitude on average. Some highly Zn contaminated samples show relatively large deviations between measured and calculated concentrations. This is most likely caused by Zn containing minerals which are included in the total amount of reactive metals in the soil as determined by acid extraction. Hence, such models can be used for practical applications if one is aware of the possible complications. When the “total reactive metal content” includes a substantial amount of precipitated metals, which may occur for heavily contaminated soils, the model tends to overestimate concentrations in solution.