Effect of kinetics of complexation by humic acid on toxicity of copper to Ceriodaphnia dubia

The rate of reaction of trace metal ions is an important consideration when studying the chemistry of trace metals in natural waters. The application of speciation models to natural water systems requires knowledge of kinetic s if reactions are slow. Most bioassay and toxicity tests conducted in stat ic and flow-through systems have not taken reaction kinetics into account. Therefore. results from these studies may overestimate the toxicity in the receiving waters. In the present study, the kinetics of the interaction of Cu(II) with humic acid (KA) and its influence on the toxicity of copper to Ceriodaphnia dubia were investigated by both chemical kinetic studies using a copper ion selective electrode and bioassay tests using a continuous flo w-through bioassay system. A two-ligand site with first-order rate constant s model gave a very good description of experimental kinetic data of the ch ange of free Cu2+ concentration. Average k(1) was 1.85/h and average k(2) w as 0.094/h. Bioassay tests indicated that different reaction times of coppe r with HA solution produced different toxic effects to organisms. We determ ined the hydrodynamic characteristics of the bioassay chambers to better de scribe the exposure of the organisms to free Cu2+. The bioassays supported the free ion activity model that the bioavailability and therefore toxicity of copper was directly correlated to the free Cu2+ concentration rather th an to the total copper concentration. It was further shown that conventiona l chemical kinetics can be used to predict the toxicity of copper in these bioassays. This study supports the importance of considering reaction kinet ics when studying the chemistry of trace metals in natural waters.