Citation: B˛ e´ s, A.; Sikorski, L.; Mikolajczyk, T.; Kuczy´ nski, M.; Luba, M.; Piero ˙ zy´ nski, B.; Jasiecka-Mikolajczyk, A. Electrodegradation of Acid Mixture Dye through the Employment Electrooxidation and Lemnoideae in Na 2 SO 4 Synthetic Wastewater. Appl. Sci. 2022, 12, 8672. https://doi.org/ 10.3390/app12178672 Academic Editors: Wojciech Janczukowicz and Joanna Rodziewicz Received: 25 July 2022 Accepted: 24 August 2022 Published: 29 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Electrodegradation of Acid Mixture Dye through the Employment Electrooxidation and Lemnoideae in Na 2 SO 4 Synthetic Wastewater Agnieszka B ˛ e´ s 1 , Lukasz Sikorski 1 , Tomasz Mikolajczyk 1, * , Mateusz Kuczy ´ nski 1 , Mateusz Luba 1 , Boguslaw Piero ˙ zy ´ nski 1 and Agnieszka Jasiecka-Mikolajczyk 2 1 Department of Chemistry, Faculty of Agriculture and Forestry, University of Warmia and Mazury in Olsztyn, Lódzki Square 4, 10-727 Olsztyn, Poland 2 Department of Pharmacology and Toxicology, Faculty of Veterinary Medicine, University of Warmia and Mazury in Olsztyn, Oczapowskiego 13, 10-719 Olsztyn, Poland * Correspondence: tomasz.mikolajczyk@uwm.edu.pl Abstract: In this study, we report on the effectiveness of electrochemical and biological wastewater treatment for artificially prepared industrial wastewater, comprising small amounts of technologically important dyes, namely Acid Mixture composed of Acid Violet 90 (AV90) and Acid Red 357 (AR357) in Na 2 SO 4 (ESS—electrolyte supporting solution), as well as their impact on the environment, using Lemna minor as a bioindicator. Our study revealed that among the tested dyes, the raw ones (AM in ESS+OM) and those subjected to electrooxidation with the use of an iron anode and a copper cathode [AM Fe/Cuox in ESS+OM (OECD medium is a medium recommended by the Organization for Economic Co-operation and Development for Lemna sp. Growth Inhibition Test)] were the most phytotoxic for L. minor. No phytotoxicity was detected for the tested plants in solution after electrooxidation with graphite anode and cathode (AM Cox in ESS+OM). Quantitative identification of acid mixture removal was carried out by supplementary UPLC/MS-MS (Ultra-Performance Liquid Chromatography/tandem Mass Spectrometry) and UV-VIS (UltraViolet-Visible spectroscopy) instrumental analysis. The final removal after electrochemical and biological treatment of AV90 and AR357 dye components was 98 and over 99%, respectively. The results suggest that it may be a suitable replacement/addition for the generally used wastewater treatment methods. Keywords: electrooxidation; galvanic cell; Lemna minor; wastewater treatment; acid mixture 1. Introduction The deficiency of fresh water has become a significant concern worldwide, as its quality and supply for domestic and industrial purposes are constantly deteriorating due to pollution. Numerous sources cause the above, including almost all industries, agricultural and mining activities, and municipal wastewater [1–3]. However, despite their well-documented hazardous effects on aquatic life and people, dyes are essential substrates in the textile, printing and food industries. The annual production of all commercially available dyes reaches a staggering amount of over 800,000 tons [4,5]. Among the various textile dyes, the most commonly used synthetic pigments are substances known as azo dyes, making up approximately 70% of the world’s dye production. Azo dyes are characterized by the appearance of one or more azo group(s) (-N=N-) in their molecular structure [6–8]. Over the past several years, numerous attempts have been made to find the most suitable and safe methods for dye removal, including physical, chemical and biological techniques. Among these methods, biological processes are often favored due to their low impact on the environment and cost-effectiveness [9,10]. However, the high concentration of azo dyes and the presence of other chemicals (salts or surfactants) in the textile effluent repeatedly inhibited the process of azo dye biodegra- dation [1,9–12]. Hence, to improve the degradation of dyes, researchers have focused on Appl. Sci. 2022, 12, 8672. https://doi.org/10.3390/app12178672 https://www.mdpi.com/journal/applsci