Assessment of toxic interactions of heavy metals in binary mixtures: A statistical approach

Toxicity of zinc, copper, cobalt, and chromium ions and their binary intera ctions were studied at varying test levels by using a battery of two tests, Microtox and duckweed with Vibrio fisheri and Lemna minor as test organism s, respectively. The type of toxic interaction at each test combination was assessed by a statistical approach based on testing the null hypothesis of "additive toxicity" at 95% confidence level. The interactions were called "antagonistic," "additive," or "synergistic" in accordance with the statist ical significance and the sign of the difference between the tested hypothe sis and the value of the observed toxicity at the binary test level concern ed. In the majority of the combinations studied by the two bioassays, the i nteractions were of antagonistic nature. Additive toxicity was the next fre quently predicted interaction in both test results, the frequency being muc h higher in Microtox responses than in those of duckweed. Finally, synergis m was found to be a rare interaction in Microtox results, but totally unlik ely in duckweed within the selected test combinations.