Effects of dissolved organic matter derived from freshwater and seawater on photodegradation of three antiviral drugs☆
Graphical abstract
Introduction
Antiviral drugs, as a class of pharmaceuticals, have been recently detected in natural waters. In Germany Hessian Ried rivers and streams, concentrations of nine antiviral drugs are detected with ng L−1 level (Prasse et al., 2010). In South African surface water, average concentrations of twelve antiviral drugs are in the range of 26.5–430 ng L−1 (Wood et al., 2015). In China Pearl River Delta, concentrations of six antiviral drugs range from not detected to 113 ng L−1 (Peng et al., 2014). Although concentrations of the antiviral drugs are at trace levels in natural waters, their continuous release has raised serious concerns such as potential ecosystem alteration and development of viral resistance (Sanderson et al., 2004; Singer et al., 2008). Moreover, the antiviral drugs are defined as “extremely hazardous therapeutic class” (Swanepoel et al., 2015), and part of them are found to possess carcinogenic potential (Bottoni et al., 2010). Consequently, it is of great significance to understand their transformation behavior in aquatic environment.
Photochemical transformation is an important elimination pathway of organic pollutants in sunlit surface waters (Boreen et al., 2003; Zeng and Arnold, 2013; Su et al., 2014). Dissolved organic matter (DOM), ubiquitous in natural waters, plays an important role in the photochemical transformation of organic pollutants (Schwarzenbach et al., 2003; Guerard et al., 2009a; Sharpless and Blough, 2014; Li et al., 2016a). Under solar irradiation, DOM can be excited to a singlet state and rapidly undergo intersystem crossing to the excited triplet states (3DOM*) (Wenk et al., 2015). The 3DOM* may react directly with organic pollutants through energy transfer or oxidation (McNeill and Canonica, 2016). Meanwhile, the 3DOM* is considered as an important precursor for single oxygen (1O2) and hydroxyl radical (•OH) (Haag et al., 1984; Vaughan and Blough, 1998). These reactive intermediates (RIs) can react with organic pollutants and promote their photodegradation (Wenk et al., 2011; Xu et al., 2011; Xie et al., 2013). On the other hand, DOM can inhibit the photodegradation of organic pollutants through light screening, static quenching by combining with pollutants and decreasing their light absorption, and dynamic quenching by quenching the excited state of pollutants (Walse et al., 2004; Wenk et al., 2013). Meanwhile, DOM is able to scavenge some RIs such as •OH and CO3−• (Vione et al., 2014). The specific effect of DOM on the photodegradation of organic pollutants is closely related with the source and composition of DOM (Niu et al., 2016; Zhang et al., 2018).
Previous studies on DOM affecting photodegradation of organic pollutants mainly focus on DOM derived from freshwater. The freshwater DOM can be divided into allochthonous source and autochthonous source. The allochthonous DOM is mainly derived from higher plants, whereas the autochthonous DOM is mainly derived from cellular excretions of phytoplankton and bacteria (Guerard et al., 2009b). Different sources can lead to the distinction of DOM in structure and further affect their photophysical and photochemical behavior. For example, the higher aromatic content in allochthonous DOM allows it to absorb more light per unit carbon compared with autochthonous DOM (McKnight et al., 2001). Zhang et al. (2014) reported that autochthonous DOM (with lower aromaticity and much higher nitrogen content) shows a higher •OH productivity than allochthonous DOM. Consequently, DOM from allochthonous and autochthonous sources may show different effects on the photodegradation of antiviral drugs.
Although effects of freshwater DOM on photodegradation of organic pollutants have been widely studied (Boreen et al., 2005; Guerard et al., 2009a), the role of DOM extracted from seawater (SDOM) has been seldom investigated. Compared with freshwater DOM, SDOM has more branched aliphatic structures, less aromatic structures and lower chromophore and fluorophore contents (Esteves et al., 2009). SDOM with different structures and contents may have disparate photochemical reactivities, thereby exerting different influences on the photodegradation of organic pollutants. In addition, the area of ocean covering about 71% of the earth’s surface is much larger than that of freshwater, and DOM concentration in some coastal seawater is comparable or even higher than that in freshwater (Del Vecchio and Blough, 2004b; Housari et al., 2010). Therefore, it is particularly important to investigate the effect of SDOM on the photodegradation of antiviral drugs.
In this study, acyclovir, lamivudine and zidovudine were selected as model antiviral drugs. The acyclovir is one of the oldest antiviral drugs for treating herpes virus (Bryan-Marrugo et al., 2015), and the lamivudine is the most widely used nucleoside reverse transcriptase inhibitors in China (Yan et al., 2016). The zidovudine, commonly used for the treatment of human immunodeficiency virus, is not removed by wastewater treatment plant (Prasse et al., 2010). The three antiviral drugs are frequently detected in wastewaters and rivers with concentrations up to 0.1–9 μg L−1 (Prasse et al., 2010; K’Oreje et al., 2012). Suwannee River fulvic acid (SRFA), Suwannee River humic acid (SRHA), Suwannee River natural organic matter (SRNOM), Pony Lake fulvic acid (PLFA), Nordic Aquatic fulvic acid (NAFA), Mississippi River natural organic matter (MRNOM) and SDOM were selected as representative DOM. Photophysical and photochemical properties of the seven DOM were characterized. Effects of DOM derived from freshwater and seawater on the photodegradation of the three antiviral drugs were investigated. Photodegradation half-lives of the three antiviral drugs were predicted considering the diurnal variation of sunlight intensity.
Section snippets
Chemicals and reagents
Acyclovir (98% purity) was obtained from Shanghai TCI Development Co., Ltd. Lamivudine (98% purity) and zidovudine (99% purity) were provided from Beijing J&K Scientific Ltd. SRFA, SRHA, SRNOM, PLFA, NAFA and MRNOM were purchased from International Humic Substances Society. SDOM was extracted from Dalian coastal seawater and isolated with the coupled reverse osmosis/electrodialysis method (Wang et al., 2016). Sorbic acid (99%) was obtained from Tokyo Chemical Industry. Acetonitrile, methanol
Light absorption of DOM
As shown in Fig. 1, the UV–vis absorption spectra of SRFA, SRHA, SRNOM, PLFA, NAFA, MRNOM and SDOM solution were measured at the initial concentration of 5 mg C L−1. The light absorption of allochthonous DOM (e.g. SRHA, SRFA and SRNOM) is significant stronger than that of autochthonous DOM (PLFA), which is related with the higher aromatic content in allochthonous DOM (McKnight et al., 2001). At the same time, freshwater DOM absorbs more light compared with SDOM. This phenomenon can be explained
Conclusion
In this study, disparate effects of DOM derived from freshwater and seawater on the photodegradation of antiviral drugs acyclovir, lamivudine and zidovudine were investigated. Compared with freshwater DOM, the SDOM has lower rates of light absorption and steady-state concentrations of RIs, leading to weak promotion effect on the photodegradation of acyclovir and lamivudine mainly transformed via indirect photolysis. Meanwhile, the SDOM shows a weak inhibitory effect on the photodegradation of
Acknowledgements
This study was financially supported by the National Science Fund for Distinguished Young Scholars (No. 51625801), the China Postdoctoral Science Foundation (2018M643671), the Guangdong Innovation Team Project for Colleges and Universities (No. 2016KCXTD023), and Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017).
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This paper has been recommended for acceptance by Heidelore Fiedler.