Effects of dissolved organic matter derived from freshwater and seawater on photodegradation of three antiviral drugs

Highlights

• Seawater DOM has low rate of light absorption compared with freshwater DOM.

• Triplet-excited DOM is largely responsible for photodegradation of acyclovir.

• Seawater DOM shows weak inhibitory effect on photodegradation of zidovudine.

• Photodegradation half-lives of antiviral drugs are more than 20 days.

Abstract

Dissolved organic matter (DOM) is the most important light absorber that may induce indirect photolytic transformation of organic pollutants in natural waters. In this study, effects of DOM derived from freshwater and seawater on the photodegradation of three antiviral drugs acyclovir, lamivudine and zidovudine were investigated. Results show that the photodegradation of acyclovir is promoted mainly by excited triplet states DOM (³DOM*), and the photodegradation of lamivudine is accelerated by ³DOM*, •OH and ¹O₂ together; however, the photodegradation of zidovudine is inhibited by DOM mainly via light screening. Compared with DOM from freshwater, promotion effect of DOM extracted from seawater (SDOM) on the photodegradation of acyclovir and lamivudine is weaker, which is attributed to lower productivity of reactive intermediates. On the other hand, inhibitory effect of SDOM on the photodegradation of zidovudine is also weaker, which is due to weaker light screening caused by lower light absorption. Photodegradation half-lives of the three antiviral drugs are predicted to be all more than 20 days in freshwater and seawater bodies of the Yellow River estuarine region. These findings are significant for understanding the phototransformation processes of antiviral drugs and other organic pollutants in estuarine and coastal regions.

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.