Thermodynamic Modeling of Zinc, Cadmium, and Copper Solubilities in a Manured, Acidic Loamy‐Sand Topsoil

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Abstract
Soil solution samples collected during a 14‐mo period from manured, loamy‐sand soil profiles in the Netherlands showed variations in dissolved Zn, Cd, and Cu concentrations of up to two orders of magnitude. To try to account for variations in the dissolved metals, a thermodynamic model was developed for the chemical‐equilibrium computer program CHARON. In addition to solution complexation and solid‐phase precipitation of inorganic components, the model accounted for metal complexation with dissolved organic carbon (DOC) ligands and with a solid organic matter (OM) exchanger phase. Both dissolved and solid organic materials were assumed to behave like fulvic acid having a complexing capacity for Zn2+, Cd2+, Cu2+, Ca2+, Mg2+, and Al3+ of 2 tool kg−1 of C. To obtain a single (pH‐dependent) stability constant for each metal‐organic ligand complex, stoichiometries of 1:1:n metal/organic‐ligand/OH− complexes were determined from published linear relationships between pH and average equilibrium quotients normalized for complexing capacity of a polyfunctional complexer. Exchange in the exchanger phase included monovalent cations and anions to maintain phase electroneutrality. Model predictions of dissolved Zn, Cd, and Cu were calculated from relevant soil properties and macrochemical concentrations in each of 44 soil‐solution samples collected from three manured field plots. Model‐predicted Zn and Cd concentrations deviated from measured concentrations on the average 1.4‐ and 2‐fold for measured concentration ranges of 120‐ and 34‐fold (respectively). Copper was typically underpredicted by the model. Model‐predicted speciation between the two principal dissolved metal species, free‐ionic and DOC ligand‐complexed, varied depending on soil solution macrochemistry. Soil solution chemical conditions ranged from pH 4.5 to 6.7, 30 to 260 g DOC m−3, and electrical conductivities (EC) of 15 to 510 mS m−1.
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