Elimination kinetics and detoxification mechanisms of microcystin-LR during UV/Chlorine process
Graphical abstract
Introduction
The increasing occurrence of harmful algal blooms in the surface drinking water resource (lakes, reservoirs, and rivers) arouses a worldwide public concern (Preece et al., 2017; Bullerjahn et al., 2016). One of the most widespread algal toxins is microcystin-LR (MC-LR), accounting for 46.0%99.8% of the total microcystins in cyanobacterial blooms (Sharma et al., 2012). MC-LR is a cyclic heptapeptide containing a large amino acid moiety (Adda) and two variable amino acid moieties-Leucine and aRginine. MC-LR is a potent hepatoxin that induces chronic or acute liver injury by inhibiting activity of protein phosphatases 1 and 2A (Harke et al., 2016). The World Health Organization has proposed a provisional guideline concentration of no more than 1.0 μg L−1 for MC-LR in drinking water.
However, conventional water treatment processes, including coagulation, flocculation and filtration, are not effective for MC-LR removal (Westrick et al., 2010). Ultraviolet irradiation at 254 nm (UV254) is able to reduce MC-LR, but only at UV doses higher than 1500 mJ cm−2, which are 1–3 orders of magnitude greater than those applied for disinfection (40–100 mJ cm−2) (Tsuji et al., 1995). Chlorination is capable for MC-LR degradation (Zhang et al., 2016b). Previous study indicates that hypochlorous acid (HOCl) is the major reactive species for MC-LR degradation, with a reaction rate constant of 1.16 × 102 M−1s−1 (Acero et al., 2005). It has been reported MC-LR removal by chlorination needs a molar ratio of chlorine and MC-LR higher than 12:1 (Merel et al., 2009). Notably, the removal of MC-LR by chlorination was highly dependent on the chlorine dose and exposure time (CT value) (Tsuji et al., 1997). Therefore, a relative high CT value was applied in chlorination to ensure MC-LR degrade thoroughly. But this generally promotes the generation of disinfection by products (DBPs) (Chu et al., 2017).
The UV/chlorine process, an advanced oxidation process (AOP), has been demonstrated to be more effective for degradation of emerging contaminants, such as ibuprofen, carbamazepine, caffeine and other pharmaceuticals than UV irradiation alone or dark chlorination alone (Pan et al., 2017; Yang et al., 2016). The UV/chlorine process also represents an alternative for ammonia control, with less chlorine dosage and less DBPs formation compared with breakpoint chlorination (Zhang et al., 2015). The hydroxyl radicals (HO) and reactive chlorine species (RCS, including Cl, Cl2- and ClO) generated from photolysis of chlorine also contribute to contaminants removal (Eqs. (1), (2), (3), (4), (5), (6)) (Mártire et al., 2001):
HO is a non-selective radical and has a high oxidation potential of 2.73 eV. The reaction rate of MC-LR with HO was determined to be 1.1 × 1010 M−1s−1 (He et al., 2014). The oxidation potential for Cl, Cl2- and ClO are also high at 2.43, 2.13, and 1.39 eV, respectively (Jin et al., 2011). RCS are selective radicals and preferentially react with electron-rich moieties, such as phenols, anilines, olefins, and amines, through either electron transfer or H-abstraction (Minakata et al., 2017). MC-LR contains both a conjugated diene and an aromatic ring on Adda moiety, which are all potential electron donating groups. Thus, RCS may also contribute to MC-LR degradation when applying UV/chlorine treatment. Though the removal of MC-LR under UV/chlorine AOP process is not well understood yet, a previous study has reported better MC-LR removal and more cytotoxicity reduction under sequential chlorination followed by UV254 irradiation than UV and chlorination alone (Zhang et al., 2016a). The simultaneous exposure to UV and chlorination under UV/chlorine treatment may take additional advantages of the HO and RCS for more effective removal of MC-LR removal. The elimination kinetics and the associated toxicity of MC-LR degradation during the UV/chlorine process are not clear and need investigation.
The objectives of this study are to examine the kinetics, mechanisms and toxicity of MC-LR degradation during UV/chlorine treatment. The elimination kinetics of MC-LR were evaluated in ultrapure water and real waters and the impacts of operational factors (chlorine does, pH) and water constituents (alkalinity, natural organic matter, bromide ions) were also investigated. High resolution mass spectrometry was used to identify the MC-LR transformation products and the degradation pathways were proposed. A protein phosphatase (PP2A) inhibition assay was used to evaluate the changes of cytotoxicity before and after UV/chlorine treatment of MC-LR.
Section snippets
Chemicals and water samples
MC-LR were obtained from Taiwan Algal Science Inc. Nitrobenzene (NB), benzoic acid (BA) and acetonitrile were obtained from Aladdin Industrial Corporation (Beijing, China). Sodium bicarbonate and potassium bromide were obtained from J&K Scientific (Beijing, China). A stock solution of free chlorine (HOCl) was prepared by diluting 5% sodium hypochlorite (Sigma-Aldrich, USA) to 800 mg L−1 as Cl2. Chlorine concentrations were analyzed using diethyl-p-phenylene diamine (DPD) ferrous titration.
MC-LR degradation kinetics in ultrapure water
Fig. 1 shows the observed first-order degradation rate constants (kobs) of MC-LR in UV irradiation alone, chlorination alone and the UV/chlorine process at various chlorine dosage (1.0–4.0 mg L−1 Cl2). UV irradiation alone resulted in MC-LR degradation with a degradation rate constant of 8.8 × 10−4 s−1. MC-LR oxidation by chlorination was dose-dependent. The kobs of MC-LR in chlorination increased from 1.2 × 10−4 to 3.8 × 10−3 s−1 with chlorine dosage increasing from 1.0 to 4.0 mg L−1. About
Conclusions
UV/chlorine process can degrade MC-LR effectively. Around 92.5% MC-LR was removed in 10 min under 3 mg L−1 of chlorine dosage and 125 mJ cm−2 of UV254 exposure. MC-LR degradation rate increased from 1.6 × 10−3 to 7.2 × 10−3 s−1 with chlorine dosage increasing from 1.0 to 4.0 mg L−1. Faster MC-LR degradation was achieved at pH 6.0 than pH 9.0. UV irradiation, free chlorine, HO and RCS each contributed to 21.1%, 47.3%, 10.3% and 21.3% of MC-LR degradation at pH 7.0 and a chlorine dosage of
Acknowledgements
We thank the National Science Foundation of China (grants 51708562 and 21622706), Natural Science Funds for Distinguished Young Scholars (2015A030306017), Guangdong Province Science and Technology Planning Project (2017B020216005), and the Fundamental Research Funds for the Central Universities (grant 17lgpy93 and 17lgjc16) for their financial support of this study.
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