Use of metal chelating agents to differentiate among sources of acute aquatic toxicity

Metals are common toxicants found in effluents and other environmental samples. Within Toxicity Identification Evaluation methods proposed by the U.S. Environmental Protection Agency and others, addition of ethylenediaminetetraacetic acid (EDTA) is used as an indicator of metal toxicity. Previous experiments showed that addition of sodium thiosulfate, originally included to detect toxicity due to oxidants, was also effective at reducing toxicity from some common metals. In the present study, we characterized the effectiveness of both EDTA and thiosulfate in removing the toxicity of 16 different metal ions to Ceriodaphnia dubia. Ethylenediaminetetraacetic acid addition removed toxicity associated with all cationic metals tested except for Cr³⁺, Fe²⁺, Al³⁺, and Ag⁺. Thiosulfate addition was less effective than EDTA for Zn²⁺, Mn²⁺, Pb²⁺, and Ni²⁺, but reduced toxicity of both Ag⁺ and selenite (Se[VI]), which EDTA did not. Results of this research can be used to categorize metal toxicity in unknown samples based on the response to additions of EDTA and thiosulfate.