Addressing Speciation in the Effect Factor for Characterisation of Freshwater Ecotoxicity: The case of copper

Purpose: Determination of the ecotoxicity effect factor (EF) in LCIA is based on test data reporting the total dissolved concentration of a substance. In spite of the recognized influence of chemical speciation and physico-chemical characteristics of the aquatic systems on toxicity of dissolved metals, these properties are not considered when calculating characterization factors (CFs) for metals.

It is hypothesized that the main cause of the variation in reported EC50 values of Cu among published test results lies in different speciation patterns for Cu in the test media, and that the toxicity of Cu is predominantly caused by the free Cu2+ ion. Hence, the free Cu2+ ion concentration should substitute the total dissolved metal concentration when determining the EF.

Methods: The study was based on a review of published ecotoxicity studies reporting acute and chronic EC50 data for Cu to Daphnia magna and to different species of fish and algae. The speciation pattern of Cu in the different media applied in the studies was calculated using the Visual Minteq model.

EFs were calculated according to the expression applied in the USEtox™ characterization model. Results and Discussion: Reported EC50 values for Cu show variations of one to several orders of magnitude for the same organism, but the study indicates that the large variation is caused by differences in water chemistry of the test media influencing the metal speciation.

The relationship between the calculated free Cu2+ ion concentration and reported EC50-values indicates that the aquatic ecotoxicity of Cu to D. magna can be predicted from the free ion concentration. Other results confirm that the free Cu2+ ion concentration depends on the [Cu]/[DOC] ratio since the majority of the total dissolved Cu is present as Cu-DOC complexes when the media contains more than 1 mg/L of DOC, and since Cu in such complexes has limited availability to the test organisms.

Conclusions: These results suggest that speciation should be taken into account in the modelling of both EFs and fate factors (FF) for LCIA, and the EF for Cu in the aquatic environment should be based on the concentration of the free Cu2+ ion.