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Abstract-The interaction between dissolved organic material (DOM) and water hardness and their effects on the acute toxicity of cadmium (Cd) to Daphnia magna was studied. At an original hardness (0.1 mmol Ca ϩ Mg per liter) of humic lake water (DOC 19.6 mg/L), Cd was significantly less toxic in the humic than in the reference (DOC Ͻ 0.2 mg/L) water. Furthermore, after dilution down to 10% (dissolved organic carbon [DOC] 2.0 mg/L), the humic water still decreased the lethality significantly. The results suggest that the reduced toxicity of Cd in the lake water is due to complexation with DOC. An increase in water hardness (from 0.1 to 0.5 or 2.5 mmol Ca ϩ Mg per liter) decreased the measured binding coefficient of Cd to DOM. In addition, the acute toxicity of Cd decreased, and the difference between the reference and humic water disappeared. As a conclusion, DOM in the soft lake water had a protective effect against Cd toxicity. In hard water, obviously, the added hardness cations, especially Ca 2ϩ , effectively competed with Cd 2ϩ for available binding sites in DOM. Simultaneously, Ca 2ϩ ions interfered also with the uptake of Cd 2ϩ either by competing in transport through cell membranes or by reducing membrane permeability.

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