The dissociation kinetics of Cu-dissolved organic matter complexes from soil and soil amendments

Complexes between dissolved organic matter (DOM) and copper (Cu) that dissociate very slowly can theoretically facilitate Cu leaching to the groundwater. Data on dissociation kinetics of Cu–DOM complexes present in soil and in soil amendments are limited. The dissociation kinetics of different Cu–DOM complexes from soil, wastewater, pig manure and sewage sludge was measured with the Competitive Ligand Exchange Method (CLEM) and Diffusive Gradient in Thin films (DGT) technique. The solutions were set at constant pH, Ca concentration and free Cu2+ activity to allow comparison between the different samples. The average dissociation rate constant kd of the complexes, as measured by CLEM, was about 10−3 s−1 and the fractions of dissolved Cu that were undissociated after 8 h ranged from <1% to 25%. These fractions determined by CLEM were significantly correlated with the non-labile fractions (0–82%) determined in the DGT tests and data analysis show that DGT data can be predicted from CLEM data. The dissociation rates decreased when Cu–DOM complexes had been equilibrated at lower Cu2+ activities. Increasing the Cu–DOM contact time (7–297 days) decreased the dissociation rate. The non-labile fractions were positively correlated with the specific UV absorbance suggesting that aromatic moieties in DOM hold non-labile Cu. All natural Cu–DOM complexes contained a detectable fraction with a dissociation rate constant kd lower than 10−5 s−1 which can theoretically lead to non-equilibrium conditions and leaching risks in soil.