Influence of dissolved organic matter on the toxicity of copper to Ceriodaphnia dubia: Effect of complexation kinetics

The reaction kinetics of copper interaction with dissolved organic matter ( DOM) in water were studied in order to determine the effect of equilibratio n period on the toxicity of copper to aquatic organisms. The changes in phy sical and chemical forms of the copper during four reaction times were exam ined in four completely mixed reactors in series; the bioavailability of th e copper as a function of these new forms was then determined with a flow-t hrough bioassay system, using Ceriodaphnia dubia as a test organism. This s tudy showed that the toxicity of copper to C. dubia decreased with increasi ng copper-DOM reaction time, which demonstrated that the copper reaction ra te with dissolved organic components in the test water was slow. The toxici ty of copper to C. dubia was closely related to the measured free-copper co ncentration (CU2+) rather than to the total copper concentrations, a fact t hat supports the free ion activity model. We found that the LC50 of copper for C. dubia increased (i.e., toxicity decreased) linearly with increasing total available binding sites. Although a similar trend was observed in bot h natural DOM and commercial humic acid, our results indicated that for a g iven copper-organic carbon ratio, copper binds more strongly to humic acid than to the natural DOM. This difference may be attributed to the greater c opper binding affinity of humic acid (greater than that of other metal-bind ing organic fractions present in DOM, i.e., fulvic acid).