Photodegradation of the azole fungicide climbazole by ultraviolet irradiation under different conditions: Kinetics, mechanism and toxicity evaluation
Graphical abstract
Introduction
The azole fungicide climbazole (CZ) is widely used in shampoos and other personal care products (PCPs) not only as an anti-dandruff active ingredient but also as an antimycotic preservative and an anti-aging agent [1], [2]. The maximum usage in shampoos was up to 2.0%, equivalent to approximately 15 × 103 mg/L [3]. It was estimated that the annual consumptions of CZ in European Union and China were up to 100–1000 tons and 3800 tons, respectively [4], [5]. After use of shampoos and other PCPs, CZ ingredient would be discharged into wastewater treatment plants (WWTPs) along with wastewater [6], [7], [8]. But the conventional WWTPs could not completely remove CZ, with its aqueous removal rates of 34–76%, and the removed CZ was mostly transferred into sludge [7], [8]. Then through sewage effluent discharge and sludge application on land, the remaining CZ ended up in the receiving environments, including water compartment [8], [9], [10] and soil compartment [11], [12] with an approximate distribution ratio of 93% in water and 7% in soil [13]. CZ has been reported to be quite toxic to aquatic and terrestrial organisms [12], [14]. Moreover, azole fungicides like CZ have been regarded as potential endocrine disruptors due to their adverse effects on P450-regulated steroidogenesis [15], [16]. Therefore, it is essential to understand the environmental behaviour and dissipation of CZ and its potential risks to organisms.
The frequent detection of CZ with high levels in various environmental media (influent, effluent, sludge, surface water and sediment as well as soil) indicated its limited dissipation ability or biodegradability in conventional WWTPs and in the natural environment [6], [7], [10]. For example, CZ showed persistence in soil with its half-lives of 175–179 d under the field condition [11]. Therefore, it is vital to remove or reduce this chemical in wastewater stream by selecting a suitable advanced treatment technique. Photolysis under ultraviolet (UV) irradiation may be an option for the removal of CZ in WWTPs, since previous studies have demonstrated that many pharmaceuticals and personal care products (PPCPs) could be degraded effectively under UV irradiation [17], [18], [19]. There are also limited reports about the UV photolysis of other azole fungicides, such as clotrimazole, ketoconazole and miconazole [20], and fluconazole [21]. However, few studies reported the UV photolysis of CZ, which only investigated the kinetics (different pH) and transformation products of CZ [22], [23]. Therefore, a systematic research is clearly needed to explore the influencing factors, degradation by-products, pathways and the toxicity changes during the photolysis of CZ.
The objectives of this study were to investigate the photodegradation kinetics of CZ under the UV-254 irradiation in aqueous solutions under various conditions for elucidating its primary influencing factors, and to identify the photodegradation by-products of CZ and propose its photolysis mechanism. Moreover, the toxicity changes of CZ and its by-products during the photodegradation process were evaluated by duckweed toxicity test and software prediction.
Section snippets
Chemicals and reagents
CZ (99.9%), Furfuryl alcohol (98%) and 1, 4-Benzoquinone (99%) and humic acid (HA, 98%) were obtained from Dr. Ehrenstorfer, Alfa Aesar and J&K Chemical, respectively. Reagents of HPLC grade (methanol, acetonitrile, ethyl acetate, dichloromethane and isopropanol) were purchased from Merck and CNW Technologies. The other chemicals and reagents were of analytical grade. Stock solutions of CZ were prepared in methanol at a concentration of 1 × 103 mg/L and stored in amber bottle and kept at −18 °C.
UV photodegradation compared to chlorination
Given that UV and chlorine oxidation disinfection are applied widely in the advanced treatment for wastewater [28], [29], the degradation behaviour of CZ by UV-254 nm, UV-225–425 nm and chlorine oxidation were compared (Fig. 1). CZ in aqueous solution was found stable in the dark controls (Fig. S2), demonstrating that hydrolysis can be omitted in CZ photodegradation process. As shown in Fig. 1a, CZ was degraded fast under UV-225–425 with the removal rate of 99.32% at 4 min, but the result could
Conclusions
The results from this study revealed the rapid photodegradation of CZ under UV irradiation (UV-225–425 and UV-254), which was more effective than the chlorine oxidation. The pH value of aqueous solutions had almost no influence on the photodegradation of CZ under UV-254. Natural waters was found to have inhibitory effects on the photodegradation of CZ when compared to the ultrapure water, this could be the combined results from the coexisting constituents in natural waters including Fe3+, NO3−
Acknowledgements
The authors would like to acknowledge the financial support from the National Science Foundation of China (NSFC 41473105, U1133005 and 41303077) and National Water Pollution Control Program of China (2014ZX07206-005). Thanks to Agilent Technologies China for assistance in identification of photodegradation by-products. This is a Contribution No. IS-2262 from GIGCAS.
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