COMBINED EFFECTS OF DISSOLVED ORGANIC MATERIAL AND WATER HARDNESS ON TOXICITY OF CADMIUM TO DAPHNIA-MAGNA

The interaction between dissolved organic material (DOM) and water har dness and their effects on the acute toxicity of cadmium (Cd) to Daphn ia magna was studied. At an original hardness (0.1 mmol Ca + Mg per li ter) of humic lake water (DOC 19.6 mg/L), Cd was significantly less to xic in the humic than in the reference (DOC < 0.2 mg/L) water. Further more, after dilution down to 10% (dissolved organic carbon [DOC] 2.0 m g/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 DOG. An increase in water hardness (from 0.1 to 0.5 or 2.5 mmol Ca + Mg per liter) decreased the measured binding c oefficient of Cd to DOM. In addition, the acute toxicity of Cd decreas ed, and the difference between the reference and humic water disappear ed. As a conclusion, DOM in the soft lake water had a protective effec t against Cd toxicity. In hard water, obviously, the added hardness ca tions, especially Ca2+, effectively competed with Cd2+ for available b inding sites in DOM. Simultaneously, Ca2+ ions interfered also with th e uptake of Cd2+ either by competing in transport through cell membran es or by reducing membrane permeability.