Effect of UV/chlorine treatment on photophysical and photochemical properties of dissolved organic matter
Graphical abstract
Introduction
Dissolved organic matter (DOM), which is among the most complex molecular mixtures of biotically and abiotically degraded biomolecules known, plays amongst others an important role in the photochemical processes in the aquatic environment (Zark and Dittmar, 2018; Zhang et al., 2018a). The photophysical and photochemical properties of DOM isolated from natural waters and their roles in the photodegradation of emerging pollutions are of great interest and have been investigated in many previously (Leresche et al., 2019; Mostafa and Rosario-Ortiz, 2013; Wang et al., 2018). In recent years, effluent organic matter (EfOM) isolated from the effluent of wastewater treatment plants (WWTPs), which refers to NOM that has been modified during wastewater treatment, also received much attention as EfOM can alter the photoreactivity of natural DOM in the receiving waters (Zhang et al., 2014).
The photophysical properties of DOM are mainly assessed using the absorption ability of light (McKay et al., 2018), including specific ultraviolet absorbance (e.g., at 254 nm, SUVA254), the ratio of DOM absorbance at 254 to 365 nm (E2/E3 ratio), and slope characteristics (S, nm−1) of the absorbance spectrum (Twardowski et al., 2004; Weishaar et al., 2003). It was reported that the optical properties of EfOM differ when compared with natural DOM (Bodhipaksha et al., 2015), and of seawater DOM when compared with freshwater DOM (Wang et al., 2019) due to the differences in chemical composition were reported. Thus, the DOM with different sources or undergo any chemical treatment that may induce the degradation of DOM are with different photophysical properties (Laszakovits et al., 2020; Wang et al., 2019).
Upon irradiation, the ground state DOM reaches its excited singlet state (1DOM*), and then undergoes intersystem crossings (ISC) via spin orbital couplings to the triplet excited state (3DOM*) (McNeill and Canonica, 2016). Both 1DOM* and 3DOM* can fall back to the ground state of DOM via fluorescence/phosphorescence emission and nonradiative transition, which are the photophysical processes of DOM (McNeill and Canonica, 2016). Besides, 3DOM* can take part in the redox reaction of organic pollutants via energy transfer or electron transfer, and is an important precursor of reactive oxygen species (ROS) such as singlet oxygen (1O2) and the hydroxyl radical (·OH) (Dalrymple et al., 2010, Dong and Rosario-Ortiz, 2012). These reactive species can initiate the indirect photolysis of various emerging pollutants in natural water (Apell et al., 2019; Ge et al., 2019; Zhou et al., 2019c). Furthermore, DOM can also inhibit the photodegradation of micropollutants via either light screening or quenching effect (Janssen et al., 2014).
DOM from different sources has been proven to possess different photochemical properties (Lee et al., 2013; Wang et al., 2020). The 3DOM*, 1O2, and ·OH formation quantum yields of EfOM are higher compared with natural DOM (Bodhipaksha et al., 2015; Zhang et al., 2014), and the 3DOM* formation quantum yields of seawater DOM are higher compared with freshwater DOM (Wang et al., 2019). This leads to different effects of DOM isolated from different waters on the photodegradation of micropollutants (Wang et al., 2019). Although many studies have investigated the properties of different DOM and their effects on the photodegradation of micropollutants, little is known about the effects of sewage treatment technologies on the properties of DOM of the effluents.
UV irradiation, and sometimes UV/chlorine, is frequently used for the disinfection in WWTPs, and UV/chlorine based advanced oxidation processes (AOPs) were recently developed for the removal of recalcitrant micropollutants in wastewater (Fang et al., 2014). Various highly reactive electrophilic radicals, such as ·OH and reactive chlorine species (RCS) are produced during the UV/chlorine treatment (Guo et al., 2017, Zhou et al., 2019b), and they play important roles in the degradation of micropollutants and in the inactivation of water-borne pathogens (Duan et al., 2018; Xiang et al., 2016).
UV/chlorine treatment was proven to degrade humin acid (Gao et al., 2019), and induced reactions in the aliphatic, olefinic and aromatic components of DOM (Varanasi et al., 2018; Zhang and Parker, 2018). Among which, the reaction rate constant of chlorine radial (Cl·) with aromatic structures reaches up to 1010 M−1 s−1 level via single electron transfer or chlorine addition (Varanasi et al., 2018). Furthermore, it has been demonstrated that ozone treatment can change the molecular structure of DOM, thereby significantly affecting the photophysical and photochemical properties of DOM (Leresche et al., 2019). For instance, an increase in the quantum yields of 1O2 and fluorescence after ozone treatment were observed (Leresche et al., 2019). Thus, it can be hypothesized that the synergistic effect of UV irradiation and reactive species induced changes in the composition of DOM can lead to a significant variation of its properties. However, the influence of UV/chlorine treatment on the properties of DOM is still unclear.
In this study, the effect of UV/chlorine treatment, a common step in sewage treatment, on the photophysical and photochemical properties of DOM was investigated. Suwanee River fulvic acid (SRFA) and Suwanee River natural organic matter (SRNOM) were selected as DOM isolates. The optical properties and the apparent quantum yields of 3DOM*, 1O2, and ·OH of the two types of DOM were determined during the UV/chlorine treatment. Besides, to investigate the effect of treated DOM on the photodegradation of micropollutants, bezafibrate (BZF) was selected as a target contaminant, which is a widely used lipid regulator and is frequently detected in wastewater, in sewage effluents and in natural water (Zhou et al., 2019c). The contribution of different reactive species generated from DOM treated by the UV/chlorine was evaluated.
Section snippets
Chemicals
BZF (98%), sorbic acid (SA, 99%), sodium azide (NaN3, 98%), 2,4,6-trimethylphenol (TMP, 98%), furfuryl alcohol (FFA, 98%), isopropanol (IPA, 99%), benzene (99%), phenol (PhOH, 99%), nitrobenzene (NB, 98%), benzoic acid (BA, 98%), benzophenone (BP, 98%), hydroquinone (HQ, 98%), 4-hydroxybenzophenone (4-hBP, 98%), 4‑chloro-4′-hydroxybenzophenone (4-Cl-4hBP, 98%), 2-chlorohydroquinone (2-Cl-HQ, 95%), p-benzoquinone (p-BQ, 99%), sodium perchlorate (NaClO4) and sodium hypochlorite solution
Photophysical properties of SRFA and SRNOM before and after UV/chlorine treatment
UV–vis spectra of SRFA and SRNOM before and after UV/chlorine treatment with different chlorine dosages (0–200 μM) were determined, and the results are shown in Fig. 1. The absorbance intensity of SRFA and SRNOM decreased after UV irradiation (Fig. 1A and C, Cl-0 vs. initial), indicating that UV-induced degradation can weakens the light absorption ability of the two DOM. The addition of chlorine further weakened the absorption ability and the absorbance intensity of SRFA and SRNOM decreased
Conclusion
This study evaluated the effect of a practical wastewater treatment process, UV/chlorine treatment, on the photophysical and photochemistry properties of DOM. The reactive species generated in the UV/chlorine system were found to be able to significantly decrease the aromaticity, molecular weight, and electron-donating capacity of SRFA and SRNOM. This leads to the inhibitory effects on the absorbance abilities of the two DOM. At low chlorine dosages, the ΦRIs of the two DOM was decreased due to
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (21707017, 41877364, 21976027), the Fundamental Research Funds for the Central Universities (2412019FZ019), and the Jilin Province Science and Technology Development Projects (20190303068SF, 20200301012RQ).
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Yangjian Zhou and Fangyuan Cheng contributed equally to this work.