Effects of dissolved organic matter on photochemical transformation of organic micropollutants in natural waters

Organic micropollutants (OMPs) am ubiquitous in natural water bodies and can have negative impacts to health of aquatic ecosystems and human beings. It is of importance to investigate fate of the OMPs so as to assess their environmental risks. Photodegradation is an important pathway to transform the OMPs in natural water bodies, including direct photolysis and indirect photodegradation induced by photosensitizers. Dissolved organic matter (DOM) is an important photosensitizer that is ubiquitous in water bodies. DOM plays an important role in photodegradation of the OMPs especially for those recalcitrant to direct photolysis. On one hand, DOM can produce photochemically produced reactive intermediates (PPRIs), e.g., hydroxyl radical (HO center dot), singlet state oxygen (O-1(2)), and excited triplet-state DOM ((DOM)-D-3*), after absorbing solar irradiation. The PPRIs are capable of inducing indirect photodegradation of the various OMPs. On the other hand, DOM can exhibit inhibition effects (light screening effects and quenching effects) on photodegradation of the OMPs. Chromophores in DOM can compete photons with the OMPs and thus inhibit direct photolysis of some OMPs, exhibiting the light screening effects. Antioxidants like phenolic moieties in DOM can quench the formed PPRIs and reduce the reactive intermediates of OMPs to the parent OMPs, manifesting the quenching effects. Generally, the effects of DOM on photodegradation of the OMPs depend on sources of DOM. DOM can derive from autochthonous (microbial) and allochthonous (terrestrial) sources based on its forming processes. Many previous studies on DOM mainly focused on formation pathways of PPRIs, reactivity of (DOM)-D-3* from different sources, and effects of different DOM on photodegradation of the OMPs. These efforts in early phase laid a foundation for emerge of the discipline of environmental (aquatic) photochemistry. In recent decades, many studies focused on photochemical properties of autochthonous and allochthonous DOM extracted from freshwater bodies. Very limited studies focused on seawater DOM (S-DOM). S-DOM may undergo longer period of photobleaching than freshwater DOM. Due to sources of riverine inputs, atmospheric depositions, wastewater discharges and mariculture activities, S-DOM in coastal seawaters can be quite different from S-DOM in distant seas or DOM in inland freshwater bodies. Therefore, S-DOM may potentially exhibit photochemical properties different from that of freshwater DOM. As seawater is a final sink of many OMPs, it is necessary to investigate photochemical property of SDOM and its effects on photodegradation of the OMPs. Some recent studies did prove that S-DOM and freshwater DOM has different photochemical reactivities towards some OMPs. Data on environmental photodegradation kinetics are necessary for assessing fates, persistence and ecological risks of the OMPs. However, translation and extrapolation of reaction rates from environment to environment and laboratory to environment is a challenging task. Underwater downward sunlight attenuation in field water bodies should be taken into consideration when estimating photolytic persistence of the OMPs. More research efforts are needed to develop prediction models on photodegradation kinetics of OMPs in different field water bodies, and to develop prediction models (such as quantitative structure-activity relationship models that can predict second-order reaction rate constants between OMPs and PPRIs) on photodegradation kinetics parameters of the vast and ever-increasing number of OMPs (or environmental organic chemicals). In this review, the recent progresses in aquatic photochemistry of DOM were summarized into three aspects, mechanisms for photogeneration of PPRIs from DOM of different sources, effects of DOM from different sources on photodegradation of OMPs, and photodegradation kinetics prediction models of OMPs in natural water bodies. In addition, potential differences in the effects of freshwater DOM and S-DOM on photodegradation of OMPs were discussed. Based on the research progresses, future efforts in the field of aquatic environmental photochemistry were suggested.