The Influence Of PH And DOM On The Acute Toxicity Of Heavy Metals To Daphnia Magna

With the development of industry and agriculture, the problem on heavy metal contamination becomes more and more serious. In the present study, we chose24h-EC50value as the indicator to study the acute toxicity of four heavy metals:zinc sulfat, copper sulfate, cadmium sulfate, lead nitrate to Daphnia magna under the pH ranging from5.0to9.0. In addition, this study researched what changes about toxicity would occur when0.15,0.30,0.60mL of homemade Dissolved Organic Matter was added under different pH environment. Then, with the step-wise regression method of SPSS17.0, we established Regression Equation for each metal and identified the key factors influencing the toxicity in different aquatic environment. The main conclu-sions of this study are as follows:(1) When there is no DOM, the acute toxicity of cadmium sulfate and lead ni-trate to Daphnia magna gradually decreased with increasing pH, while the other two heavy metals showed an opposite tendency. Meanwhile, our study showed that the toxicity of Cu(Ⅱ)and Pb(Ⅱ)was higher than Cd(Ⅱ)was, and the toxicity of Zn(II) was less toxic to Daphnia magna.. In consideration of the the toxicity of Cu(II) and Pb(II), we find that PbOH+and CuOH+were the biologically effective species, just like Pb2+and Cu2+, and we speculated that compared to Pb2+, PbOH+was more toxic and the biological toxicity of CuOH+was slightly smaller than Cu2+(2) At the same pH point, the acute toxicity of zinc sulfate to Daphnia magna decreased with the addition of DOM, and the toxicity reduced more sharply by adding more DOM. In the environment of the same DOM amount, the acute toxicity of zinc sulfate to Daphnia magna decreased with increasing pH. The toxicity changing trends of lead nitrate and copper sulfate were similar to that of zinc, but the changes were much greater than zinc. Compared with pH5.0, adding the same amount of DOM caused the toxicity changing multiples up to22.7(Pb(Ⅱ)) and4.55(Cu(Ⅱ)) under pH9.0. However, this value for Zn(II)was only2.21. To cadmium sulfate, the toxicity increased when the pH increased after adding DOM. In the specific pH, the acute toxicity of cadmium sulfate to Daphnia magna changed very slightly after adding DOM, and the maximum change value was only18%. When adding the same amount of DOM to solution, the toxicity of cadmium sulfate was apparently lower at pH5.0and6than at pH7.0,8.0,9.0:the24h-EC50at pH=9.0was only30%of that at pH=5.0. We found that the complexation order of heavy metals to DOM was:Pb(Ⅱ)> Cu(Ⅱ)>Zn(Ⅱ)>Cd(Ⅱ). In other words, the influence order of DOM on the bio-toxicity of the four heavy metals to Daphnia magna was:Pb(Ⅱ)>Cu(Ⅱ)>Zn(Ⅱ)>Cd(Ⅱ).(3) Finally, we built biological ligands models for each heavy metal by using stepwise regression of SPSS software. According to the previous studies, we chose pH, DOM, pH*DOM, pH2and DOM2as dependent variables. The regression equation for Cu(Ⅱ), Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ)was:24h-EC50=-1.055+0.757pH*DOM+0.191pH,24h-EC50=7.945+5.978pH*DOM,24h-EC50=5.940-0.591pH,24h-EC50=82.988+2.193pH2+10.920pH*DOM-102.178DOM2-27.785pH in turn.The regression analysis showed that toxicity of Pb(Ⅱ)to Daphnia magna was the most significant influenced by water quality, and the influence of DOM on acute toxicity of Cd(Ⅱ) to Daphnia magna was insignificantvariable, which was consistent with the experimental result. The pH had a significant impact on the toxicity of each heavy metal, but in terms of Zn(Ⅱ), this significance was only effective in the pres-ence of both pH and DOM.