Effects of dissolved organic matter on the mobility of copper in a contaminated sandy soil

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Abstract
Summary: Changes in land use can result in increased soil organic matter content, and decreases in Ca and pH which will affect the mobility of Cu in soil. We studied how the mobility and coagulation of dissolved organic matter and pH affected the mobility of Cu in contaminated sandy soil by batch and column experiments in the laboratory. The soil, with pH ranging from 3.8 to 5.7, had been polluted with Cu in the range 0.13–1.9 mmol kg−1 more than a decade ago. Calcium and Cu bound by dissolved organic matter (purified humic acid) was determined in the pH range 4–8; Cu2+ concentration ranged from 10−4 to 10−12M and Ca2+ concentration from 10−3 to 10−6M. Binding of Cu by dissolved organic matter as affected by Ca and pH could be predicted well with the non‐ideal competitive adsorption (NICA) model. Coagulation of dissolved organic matter was affected by the amount of trivalent (Al3+) and divalent (Ca2+ and Cu2+) cations in solution. There was little effect of pH on coagulation between pH 4 and 6. The concentration of the divalent cations, Ca2+ and Cu2+, at which coagulation of dissolved organic matter appeared could be explained by differences in the binding of Ca and Cu by dissolved organic matter. Binding of Cu by dissolved organic matter as well as by solid organic matter, both affected by Ca and pH, could be described well with the NICA model. We investigated the coagulation and mobility of dissolved organic matter in column experiments and found that they enhanced Cu mobility. Three processes, Cu desorption by soil, dissolved organic matter coagulation and Cu complexation by dissolved organic matter, act simultaneously in the soil columns. All three with counteracting effects on Cu mobility are influenced by Ca and pH and could be adequately represented by the multicomponent NICA model.