Importance of water chemistry for the toxicity of uranium on an aquatic organism

Abstract : Uranium (U) is a radioactive element found at trace concentrations throughout the natural environment. Its level can increase in association with human activities associated with nuclear fuel cycle. The aim of our studies was first to investigate the influence of water chemistry on acute toxicity of uranium on Daphnia magna. Acute uranium toxicity for Daphnia magna was determined in two different exposure media, differing in pH and alkalinity. LC50 varied strongly between media, from 390+-40 g.L-1 U at pH 7 to 7.8+-3.2 mg.L-1 U at pH 8. According to the free ion activity model uranium toxicity varies as a function of free uranyl activity. This assumption was examined by calculating uranium speciation in our water conditions and in those reported in the literature. Predicted changes in free uranyl concentration could not solely explain observed differences in toxicity, which might be due to a competition or a non-competitive inhibition of H+ for uranium transport and/or the involvement of other bioavailable chemical species of uranium. Uranium toxicity was also compared to the toxicity of other alpha-emitting radionuclides and stable trace metals. Our results confirmed the general assumption that uranium chemical toxicity predominates over its radiotoxicity.