UV-induced activation of organic chloramine: Radicals generation, transformation pathway and DBP formation

Highlights

• UV photolysis of CDMA can effectively degrade BPA.

• Cleavage of N-Cl in CDMA by UV resulted in the formation of chloride radical.

• Chloride and hydroxyl radicals were the dominant contributors to BPA degradation.

• Chlorination and chloramination reduce TCNM and NDMA formation, respectively.

Abstract

Organic chloramines of little disinfection efficacy commonly exist in disinfection process (chlor(am)ination) due to the wide presence of organic amines in water, of which N-chlorodimethylamine (CDMA) is a typical one. For the first time, UV photolysis for the activation of CDMA was investigated. UV photolysis caused the cleavage of N-Cl bond in CDMA to form Cl^• and subsequently HO^•, both of which are dominant contributors to the destruction of model contaminant bisphenol A (BPA). Typical spectra of HO^• were detected by electron paramagnetic resonance (EPR) experiments, while spectra of reactive nitrogen species (RNS) were not detected during UV photolysis of CDMA. The increase of pH (6.0–8.0), HCO₃⁻/CO₃²⁻, Cl⁻ and nature organic matter inhibited the degradation of BPA. We proposed pathways of CDMA and BPA degradation based on the identified transformation products. UV photolysis of CDMA and BPA reduced the formation of N-nitrosodimethylamine (NDMA) at pH 8.0, but increased the formation of trichloronitromethane (TCNM) at pH 7.0 and 8.0. The increasing toxicity and the formation of TCNM and NDMA gave us a hint that formation of organic chloramines should be concerned.

Introduction

Chlorination is the most widely used disinfection process in water treatment plants to kill the pathogenic microorganisms in water and prevent the regeneration of bacteria by keeping the sufficient residual chlorine (Jonkergouw et al., 2009, Li et al., 2011, Gopal et al., 2007). However, due to the strong oxidation ability of chlorine, some toxic disinfection byproducts (DBPs) are generated in the chlorination process. For example, trichloromethane (TCM) was first detected and identified in the chlorinated drinking water in 1974 (Bellar et al., 1974). TCM belongs to one of the unintended DBPs that may pose risk on human health (Richardson et al., 2007, Hu et al., 2018). Therefore, it is critical to control the formation of DBPs in the chlorination process. In recent years, chloramine has been considered as an alternative disinfectant because (1) chloramine is stable and its disinfection in the distribution system is long-lasting; (2) it has been reported that haloacetic acids (HAAs) and trihalomethanes (THMs) are less formed in the chloramine disinfection process (Y. Wu et al., 2019). However, it is worth noting that the chloramine disinfection process may produce nitrogenous DBPs with the higher genotoxicity and cytotoxicity, such as N-nitrosodimethylamine (NDMA) and trichloronitromethane (TCNM) (Mitch et al., 2003, Plewa et al., 2008, Le Roux et al., 2011).

In the practical application, the most common components in real water matrix include inorganic ammonia nitrogen and organic nitrogen compounds. Therefore, chlorine or chloramine may react with inorganic ammonia nitrogen and organic nitrogen compounds to generate chloramine and organic chloramine (Zhang et al., 2016, Zhang et al., 2018, Deborde and von Gunten, 2008, Zhang et al., 2016). For example, the concentration of organic chloramine reached over 0.15 and 0.25 mg-Cl₂ L⁻¹ per DOC (mg-C L⁻¹) during chlorination and chloramination in Yangtze water (Zhang et al., 2016a). Besides, the reaction rates between chlorine and organic amines are much faster than that between chlorine and ammonia (Deborde and Von Gunten, 2008, Zhang et al., 2016). As reported, the disinfection efficacy of organic chloramine (<0.09 L mg⁻¹ min⁻¹) was much inferior to that of chlorine (2.56 L mg⁻¹ min⁻¹) and monochloramine (0.72 L mg⁻¹ min⁻¹) (Donnermair and Blatchley, 2003). Moreover, organic chloramine can be stable in water due to the slow hydrolysis rate (<10⁻⁵ M⁻¹ s⁻¹) (Yoon and Jensen, 1993) and long half-life (>34 h) (McCormick et al., 1993, Conyers and Scully, 1993), and become nitrogenous DBPs precursors. Organic chloramine itself, as a chlorinated nitrogenous organic substance, is much more toxic than the parent substance (Li et al., 2017). Therefore, to control the organic chloramine is of great importance on keeping drinking water safety.

Currently, the research related to UV-based advanced oxidation processes (AOPs) including the UV/chlorine process and the UV/chloramine process are very popular (Duan et al., 2018, Fang et al., 2014, Wu et al., 2019, Zhou et al., 2020). Through the UV photolysis of chlorine or chloramine, the UV/chlorine process and UV/chloramine process can generate various reactive radicals, such as hydroxyl radical (HO^•, 2.7 V) and chloride radical (Cl^•, 2.4 V) (Y. Wu et al., 2019). Theses reactive radicals have high redox potentials, and hence result in effective destruction of emerging contaminants (ECs) (Bu et al., 2018, Guo et al., 2017). As reported by Chen et al. that the increased sensitivity to direct photolysis of chlorinated organic amines compared with the original un-chlorinated compound was due to the photolabile N-Cl bond (Chen and Blatchley, 2020a). Besides, referring to UV/chlorine and UV/chloramine processes, the UV photolysis of organic chloramine has been proposed to generate Cl^• (Soltermann et al., 2013), HO^• and reactive chlorine species (RCS; e.g., Cl₂^•− and Cl^•) through a series of chain reactions (Zhou et al., 2019). Meanwhile, the cleavage of N-Cl bond generates R^• radical (where R represent the nitrogen-containing part after N-Cl bond fission), which is also involved in transformation of free radical in the system. Consequently, it is necessary and important to investigate the ECs degradation, radical contribution, and DBP formation during UV photolysis of organic chloramine.

In this study, the simplest secondary amine dimethylamine (DMA) was chosen to provide the unique substitution site on the nitrogen atom of DMA to form organic chloramine (i.e., N-chlorodimethylamine (CDMA)). Bisphenol A (BPA), a typical endocrine disrupting chemical and containing no nitrogen, was chosen as the target contaminant in this study. This work aims to achieve the following objectives: (1) investigating the degradation mechanism of BPA and the transformation fate of CDMA during UV photolysis; (2) evaluating the impacts of solution pH, inorganic anions and nature organic matter (NOM) on BPA destruction; (3) identifying the types of free radicals formed through UV photolysis of CDMA and their roles in the oxidization of BPA; (4) proposing a transformation pathway of CDMA and BPA based on the determined transformation products, and assessing the acute toxicity of reacted solution; (5) clarifying the effect of UV photolysis on controlling the formation potential of NDMA and TCNM.