Evaluation of the performance and limitations of empirical partition-relations and process based multisurface models to predict trace element solubility in soils

Here we evaluate the performance and limitations of two frequently used model-types to predict trace element solubility in soils: regression based “partition-relations” and thermodynamically based “multisurface models”, for a large set of elements. For this purpose partition-relations were derived for As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Se, V, Zn. The multi-surface model included aqueous speciation, mineral equilibria, sorption to organic matter, Fe/Al-(hydr)oxides and clay. Both approaches were evaluated by their application to independent data for a wide variety of conditions. We conclude that Freundlich-based partition-relations are robust predictors for most cations and can be used for independent soils, but within the environmental conditions of the data used for their derivation. The multisurface model is shown to be able to successfully predict solution concentrations over a wide range of conditions. Predicted trends for oxy-anions agree well for both approaches but with larger (random) deviations than for cations.

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► Partition relations are robust predictors for metal cations between pH 3–8.
► Oxy-anions and DOC have a very similar pH dependent solubility.
►  The DOC coefficient accounts for the pH dependence of oxy anion concentrations.
►  Multisurface models can predict element solubility for a wide range of conditions.