Toxicity of tetramethylammonium hydroxide to aquatic organisms and its synergistic action with potassium iodide Izumi C. Mori a,⇑ , Carlos R. Arias-Barreiro a,1 , Apostolos Koutsaftis a,2 , Atsushi Ogo a , Tomonori Kawano b , Kazuharu Yoshizuka b , Salmaan H. Inayat-Hussain c,3 , Isao Aoyama a a Institute of Plant Science and Resources, Okayama University, Kurashiki 710-0046, Japan b School of International Environmental Science, The University of Kitakyushu, Kitakyushu 808-0135, Japan c Faculty of Health Sciences, Univerisiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia highlights  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. article info Article history: Received 20 August 2013 Received in revised form 2 July 2014 Accepted 3 July 2014 Handling Editor: A. Gies Keywords: Tetramethylammonium hydroxide Potassium iodide Aquatic toxicity Synergism D. magna Semiconductor wastewater abstract 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 tet- ramethylammonium hydroxide (TMAH, CAS No. 75-59-2), a widely utilized etchant. The toxicity of TMAH was low when tested in the 72 h-algal growth inhibition test (Pseudokirchneriellia subcapitata, EC 50 = 360 mg L 1 ) and the Microtox Ò test (Vibrio fischeri, IC 50 = 6.4 g L 1 ). In contrast, the 24 h-micro- crustacean immobilization and the 96 h-fish mortality tests showed relatively higher toxicity (Daphnia magna, EC 50 = 32 mg L 1 and Oryzias latipes, LC 50 = 154 mg L 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 tetrae- thylammonium and tetrabutylammonium were more toxic than TMAH in the D. magna immobilization test. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In the last two decades, the production of thin film transistor liquid crystal displays (TFT-LCDs) has rapidly increased. TFT-LCDs are currently utilized in a myriad of electronic devices, such as televisions, monitor displays, laptop computers, mobile phones and other commonplace mobile gadgets. The recent increase in the demand for TFT-LCDs resulted in a heightened production of indium-tin oxide (ITO) glass wafer (Takano et al., 1992; Katayama, 1999; Service, 2001). A large quantity of organic solvent is used in the manufacturing process of the wafer. Tetramethylammonium hydroxide (TMAH, CAS 75-59-2) is the most common chemical compound used in the anisotropic wet etching of the ITO glass wafer. Anisotropic wet etching is an essential step to make fine patterns of circuit on silicone wafer in an orientation dependent manner. High concentrations of TMAH http://dx.doi.org/10.1016/j.chemosphere.2014.07.011 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved. Abbreviations: TMAH, tetramethylammonium hydroxide; TFT-LCDs, thin film transistor liquid crystal displays; ITO, indium tin oxide; MTI, mixture toxicity index; TU, toxic unit; QAH, quaternary ammonium hydroxide. ⇑ Corresponding author. Address: Institute of Plant Science and Resources, Okayama University, 2-20-1 Chuo, Kurashiki 710-0046, Japan. Tel.: +81 86 434 1215; fax: +81 86 434 1249. E-mail address: imori@okayama-u.ac.jp (I.C. Mori). 1 Present address: P.O. Box 8031, Woolloongabba, Queensland 4102, Australia. 2 Present address: Exponent International Ltd., Harrogate, North Yorkshire, United Kingdom. 3 Present address: Petronas Group HSE Division, Petronas Twin Towers, Kuala Lumpur City Center, 50088 Kuala Lumpur, Malaysia. Chemosphere 120 (2015) 299–304 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere