The mutual promotion of photolysis and laccase-catalysis on removal of dichlorophen from water under simulated sunlight irradiation
Graphical abstract
Introduction
Release of organic contaminants with wastewater effluents discharge into surface water is a widespread problem threatening aquatic environment [1], [2], [3], [4]. Dichlorophen (DDM, 2,2′-Dihydroxy-5,5′-Dichlorodiphenylmethane), an emerging broad-spectrum antimicrobial which is generally used as bacteriocide and fungicide in cosmetics [5] and as anthelmintics in pesticide products [6], was detected at levels of 10–450 ng/L in wastewater effluents [7]. The hydrophobic property of DDM (LogKow = 4.3)[8] allows it to be easily accumulated in aquatic organisms [9], and eventually in human bodies through biomagnification. DDM has been proved to be highly toxic to aquatic organisms in laboratory experiments, and once released, may have long-term adverse effects on aquatic life and/or human health (EC Directive 2001/58/EC). Knowledge of the fate of DDM is critical to properly evaluate their potential environmental risks in aquatic environment.
Laccases, the common blue multi-copper phenoloxidases [10], are endogenously produced in extracellular forms by a wide variety of fungi and higher plants [11], [12], which catalyze the oxidation of organic contaminants in the presence of oxygen [10], The removal of aromatic contaminants by laccase-catalysis has been widely investigated [13], [14], [15]. Laccases are capable of mediating oxidative coupling reaction of organic contaminants, and thus forming polymerization products as the primary reaction pathway [14], [16], [17], [18]. In our previous work, it has been demonstrated that DDM could be transformed and removed from water by laccase-catalyzed reaction process, and the coupling reaction of DDM via C–C or C–O–C pattern to generate polyhalogenated diphenyl ethers and polyhalogenated biphenyls was the most important mechanism [19]. The resulting polyhalogenated products are generally considered to be harmful to human health and ecosystem [16], [20]. And those oligomers are not synthetic chemicals, and their toxicity is higher than that of DDM based on the estimation with ECOSAR Program (Ecological Structure Activity Relationships, USEPA) [21]. Therefore, laccase-catalyzed transformation of DDM needs to be improved to eliminate the potential risk. Yin et al. [22] reported that laccase catalysis and visible light-driven photolysis supported by photocatalyst NTB (nanoporous Ti-doped β-Bi2O3) can complement each other for the degradation of chlorophenols, in which the degradation efficiencies of pentachlorophenol (PCP) and 2,4-dichlorophenol (2,4-DCP) can be effectively enhanced. Peralta-Zamora et al. [23] also found that the degradation efficiency of laccase can be enhanced under the condition of photochemical pre-treatment (TiO2 as photocatalyst). However, the fate of coupling products has not yet been followed. Consequently, the ubiquitous solar light-driven photolysis is chosen as a priority to supplement the potential defects of enzyme degradation of organic contaminants.
Meanwhile, the photolysis of light-absorbing contaminants in natural waters is of great importance to understand their environmental fate and ecological risk [24], [25], [26]. Photodegradation of organic contaminants with absorption spectra overlap with the solar irradiation spectrum may involve direct photolysis caused by energy excitation and self-sensitized reaction mediated by reactive oxygen species (ROS) [27], [28], [29], [30], [31]. Photolysis is a facile treatment method to remove various organic contaminants from water [32], [33], [34], and may have potential to complement with laccase-catalysis for DDM degradation. In addition, solar light-driven photolysis may also occur in a wide range of organic contaminants, and these processes often occur simultaneously or sequentially in practice with other processes such as enzyme-catalyzed degradation [35], [36]. For instance, if the partially transformed intermediates of a contaminant in one process have high reactivity in the other process, then the interaction of two degradation processes may greatly change the degradation rate of the contaminant and the distribution of the reaction products. Therefore, the investigation of combined photolysis and enzyme-catalyzed degradation process of organic contaminants is of great importance.
The objective of this work is to explore the photolysis of DDM under simulated sunlight irradiation and its complementary role for laccase-catalyzed degradation of DDM in water. The experiments were performed by combining degradation kinetics, product analysis and toxicity evaluation. Theoretical computation based on density functional theory (DFT) was employed to explain the possible photolysis mechanism of DDM. The possible mechanisms of laccase inactivation induced by simulated sunlight were also investigated.
Section snippets
Chemicals and pretreatment
DDM, 2,2-azinobis-3-ethylbenzthiazoline-6-sulfonic acid (ABTS), laccase (from Trametes versicolor), 2-propanol, furfuryl alcohol (FFA), sorbic acid (SA) were purchased from Sigma-Aldrich (St. Louis, MO). Stock solution (2 mM) of DDM was prepared in 0.01 mM NaOH solution and stored at 4 °C. The freshwater green algae Scenedesmus obliquus (FACHB-14) were provided by the Institute of Hydrobiology, Chinese Academy of Science (Wuhan, China). Furthermore, a surface water sample collected in Taihu
Photodegradation and laccase-catalyzed removal of DDM at different pH values
Direct photolysis of DDM under different pH values revealed appreciable removal of aqueous DDM, and followed pseudo-first-order kinetics (Fig. 1a). It was apparent that increase of pH can lead to the increase of degradation rate. DDM, with pKa1 of 7.66 and pKa2 of 11.60 [42], has three different species under different pH: neutral (DDM0), monoanion (DDM-H), and dianion (DDM-2H) (Fig. S1). Seen from Fig. S2, a redshift of UV–Vis absorption spectra of DDM solution was evidently observed with the
Conclusions
This study highlights the mutual promotion of enzyme- and photo-transformation reaction of DDM under simulated sunlight irradiation. The photolysis of DDM obeyed pseudo-first-order kinetics and increase of pH can lead to the increase of degradation rate. According to the experimental measurements and DFT calculation, we found that 1O2 and 3DDM∗ played a key role in DDM photolysis. Meanwhile, our study demonstrated that photolysis and laccase-catalysis of DDM can promote each other. It was also
Acknowledgements
This research was financially supported by the National Natural Science Foundation of China (No. 21577059) and the Commonwealth and Environmental protection project for the MEP grant (201509053).
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