Evaluation of DGT as a metal speciation tool in wastewater

This paper aims to evaluate the performance of the diffusive gradient in thin film technique (DGT) as a speciation tool for metals in wastewater. The validity of metal sampling by DGT in wastewater was checked. DGT was used in parallel with the Daphnia magna acute toxicity test in order to obtain information on the speciation of copper and cadmium in diluted and spiked filtered wastewater (raw and treated) from two treatment plants. Combining the chemical (DGT) and the biological methods (D. magna toxicity test) allowed metal to be fractionated into inorganic, labile organic and inert organic metal. Copper was mainly found as inert organic complexes, whereas the major part of cadmium was found to be labile organic complexes. The proportion of inert organic copper complexes was higher in the presence of treated wastewater than in raw wastewater. The use of restricted gels in DGT devices discriminated more labile organic cadmium than labile organic copper, indicating that cadmium weak ligands have more complex structures than copper weak ligands. In our experimental conditions (i.e. a high metal to ligand ratio), DGT, even equipped with restricted gels, was able to accumulate labile organic complexes. This result highlights that the ecotoxicological interpretation of DGT measurement should be considered carefully. DGT is a reliable tool to assess the chemical characteristics of metals (i.e. reactivity) in wastewater, but it does not ensure that only inorganic metal is measured.