Surface response model for prediction of the acute toxicity of Cu(II) and Cr(VI) towardDaphnia magna

A central composite design (CCD) was employed to evaluate the combined effects of pH, hardness and dissolved organic carbon (DOC) on the toxicity of Cu (II) and Cr(VI) towardDaphnia magna. Surface response design (SRD) analysis showed that pH and DOC were found to be most significant for the Cu(II) toxicity (p<0.05), while pH and hardness for the Cr(VI) toxicity. According to the results of chemical speciation using the Visual MINTEQ program, increases in pH and DOC concentration led to decreased toxicity of Cu(II) by reducing the concentration of toxic Cu²⁺ species. For the Cr(VI) toxicity, increases in pH and hardness led to decreased toxicity towardD. magna by reducing the concentrations of toxic HCrO₄ ^? and CrO₄ ^2? species. Based on the SRD analysis, surface response models were developed for prediction of the 24-h and 48-h mortalities ofD. magna in the presence of Cu(II) and Cr(VI) under different pH, hardness and DOC conditions.