Combined Toxic Effects Of Lanthanum-cerium-dysprosium Mixtures On Daphnia Magna

Rare earth elements（REEs）are increasingly used in agriculture,livestock,and high-tech industries,and the release of REEs into the environment may have direct or indirect toxic effects on the ecosystem.REEs are usually present in the environment as mixtures,but there is still a lack of quantitative understanding of the mixture toxicity and interaction forms of REEs.In this thesis,the toxic effects of single,binary and ternary mixtures of typical REEs（La,Ce,Dy）on Daphnia magna were investigated based on two dosimetric indicators（dissolved and free ionic state concentrations）.Concentration Addition（CA）and Independent Action（IA）models were used to analyze the combined toxicity of the mixtures and to elucidate the combined toxicity effects and modes of action of the mixtures of REEs.The main results of this study are as follows:（1）The 50%effective concentrations(EC₅₀)of single REEs on Daphnia magna were 0.32μM,0.40μM,0.26μM for La,Ce,Dy,respectively,in the expression of dissolved concentration,and they were 0.12μM,0.14μM and 0.26μM in the expression of free ionic concentration,respectively.The order of toxicity of the three REEs based on EC₅₀ values was Dy>La>Ce.（2）In the combined toxicity prediction of binary mixtures of REEs,the CA model was better than the IA model for toxicity prediction.Additive effects were indicated by CA model for La-Ce mixture and antagonistic effects for the La-Dy and Ce-Dy mixtures were observed based on two dose indices.In the IA model the La-Ce and La-Dy mixtures showed synergistic effects at two dose indices,while the Ce-Dy mixture showed antagonistic effects based on the dissolved concentration index and synergistic effects based on the free ion concentration index.Based on the non-additive interactions of the Mix Tox model with the dissolved state concentration and free ion state concentration data and the deviation parameters,the CA model could explain more than 80%of the toxicity changes and the IA model could explain more than 70%of the toxicity.There were not much different for the prediction results between dissolved concentration and free ion concentration of REEs,probably because the exposed solution components were relatively simple and the REEs did not interact with the exposed solution components in terms of competition and detoxification,thus leading to insignificant geochemical interactions.（3）Based on the CA model and IA model,five rays were designed according to different concentration ratios and the toxic effects of the ternary mixture of La-Ce-Dy on Daphnia magna were evaluated using the UD-Ray experimental design method.With the increase of mixture components,the prediction ability of IA model would be better than that of CA model,and the prediction of mixture toxicity would be more and more accurate,CA model was relatively conservative,which is not affected by the amount of mixture components,and the prediction ability was relatively stable.The results elucidate the combined toxicity characteristics and mode of action of La-Ce-Dy mixtures,and provide a scientific basis for REEs toxicity prediction,water quality criteria,risk assessment and pollution control.