Removal of kathon by UV-C activated hydrogen peroxide: Kinetics, mechanisms, and enhanced biodegradability assessment

Abstract

Kathon (CMI-MI), a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI), was extensively used in industry as a nonoxidizing biocide or disinfectant. However, it would show adverse effects on aquatic life when it is discharged into surface water. In this study, the removal performance, parameter influence, degradation products and enhancement of subsequent biodegradation of CMI-MI in UV/H2O2 system were systematically investigated. The degradation rate of CMI-MI could reach 90% under UV irradiation for 20 min when the dosage of H2O2 was 0.3 mmol·L–1. The DOC (dissolved organic carbon) mineralization rate of CMI-MI could reach 35% under certain conditions ([H2O2] = 0.3 mmol·L–1, UV irradiation for 40 min). kobs was inversely proportional to the concentration of CMI-MI and proportional to the concentration of H2O2. The degradation rate of CMI-MI was almost unchanged in the pH range from 4 to 10. Except the presence of CO3^2- inhibited the removal rate of CMI-MI, SO4^2-, Cl-, NO3-, and NH4+ did not interfere with the degradation of CMI-MI in the system. It was found that UV/H2O2 system had lower energy consumption and more economic advantage compared with UV/PS system by comparing the EEO (electric energy per order) values under the same conditions. Two main organic products were identified, namely HCOOH and CH3NH2. There's also the formation of Cl- and SO4^2-. After UV and UV/H2O2 photolysis, the biochemical properties of CMI-MI solution were obviously improved, especially the UV/H2O2 treatment effect was better, indicating that UV/H2O2 technology is expected to combine with biotechnology to remove CMI-MI effectively and environmentally friendly from wastewater.

1. Introduction

Kathon (CMI-MI), a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI), was widely used in industry as a nonoxidizing biocide or disinfectant [1,2]. Both in circulating cooling water systems and in cosmetic personal care products (moisturizers, eye shadows, shampoos), CMI-MI is an indispensable ingredient because of its excellent inhibition and killing effect on molds, bacteria and other microorganisms [3–5]. In order to inhibit the breeding of microorganisms and prevent the generation of biological sludge, the added concentration of CMI-MI in the reverse osmosis system and circulating cooling water system is 0.04–2.6 mmol·L–1, 100–300 mg·L–1, respectively [6]. The concentration varies with different adding methods. MI concentration in cosmetics is required to be within 0.01 percent in China, which is attribute to its strong allergenic properties [6]. The structures of CMI and MI were shown in Supplementary Material (Table S1).

With the rapid development of industry, the water consumption and displacement of circulating cooling water are increasing, contributing to the mass release of the residual isothiazolones in wastewater into the natural environment and or sewage treatment plants. Several studies have reported the presence of CMI-MI in natural water bodies or wastewater treatment plants [5,7,8]. The existence of CMI-MI in wastewater treatment plants increased the complexity of water treatment, because of its negative impact on activated sludge activity and nitrification process [9,10]. It is known as pseudo-persistent compounds because CMI-MI was continuously released into the environment with circulating cooling wastewater [11,12].

CMI-MI exposed to the environment poses a potential threat to ecology and human health [13,14]. Several studies have shown that CMI or MI has irreversible toxic effects on many organisms [15–20]. For example, long-term exposure to low levels of MI and related compounds damaged irreparably the immune and nervous systems of mice [15]. The hydrophilic substances MI and CMI exhibited pronounced high cytotoxic effects on the human hepatoblastoma cell line Hep G2, the marine bacterium Vibrio fischeri, and the limnic green alga Scenedesmus vacuolatus [20]. Tang et al. [16] discovered that kathon was highly genotoxic in vitro, which contributed to an increase in the genotoxicity of the reverse osmosis concentrate. In addition, due to the high toxicity of MI/CMI, bactericides lingering in the environment for a long time could induce the development of resistance in bacteria [21]. Thus, it is of great significance to take effective measures to remove CMI-MI from wastewater.

Biodegradation is considered as an efficient, environmentally friendly and low-cost approach to eliminate organic contaminants from the aqueous environment [22]. Microbial me...