Toxicity of tetramethylammonium hydroxide to aquatic organisms and its synergistic action with potassium iodide

•Daphnia magna is sensitive to TMAH.•TMAH and KI have synergistic toxic effects in D. magna.•Larger quaternary ammonium compounds are more toxic to D. magna than TMAH. The aquatic ecotoxicity of chemicals involved in the manufacturing process of thin film transistor liquid crystal displays was assessed with a battery of four selected acute toxicity bioassays. We focused on tetramethylammonium hydroxide (TMAH, CAS No. 75-59-2), a widely utilized etchant. The toxicity of TMAH was low when tested in the 72h-algal growth inhibition test (Pseudokirchneriellia subcapitata, EC50=360mgL−1) and the Microtox® test (Vibrio fischeri, IC50=6.4gL−1). In contrast, the 24h-microcrustacean immobilization and the 96h-fish mortality tests showed relatively higher toxicity (Daphnia magna, EC50=32mgL−1 and Oryzias latipes, LC50=154mgL−1). Isobologram and mixture toxicity index analyses revealed apparent synergism of the mixture of TMAH and potassium iodide when examined with the D. magna immobilization test. The synergistic action was unique to iodide over other halide salts i.e. fluoride, chloride and bromide. Quaternary ammonium ions with longer alkyl chains such as tetraethylammonium and tetrabutylammonium were more toxic than TMAH in the D. magna immobilization test.