Ultraviolet/peroxydisulfate degradation of ofloxacin in seawater: Kinetics, mechanism and toxicity of products

Highlights

• OFL was degraded by UV/PDS in the synthetic seawater.

• The degradation rates of OFL were affected by the halogen ions.

• The active sites of OFL were calculated to infer the reaction pathway.

• The degradation of OFL by UV/PDS is safer than NaClO.

Abstract

The ultraviolet/peroxydisulfate (UV/PDS) system was used to degrade ofloxacin (OFL) in fresh water, synthetic marine aquaculture water and synthetic seawater. The comparison of the reaction degradation rate constants proved that the order of reaction rate was the following: synthetic seawater (0.77 min⁻¹) > synthetic marine aquaculture water (0.74 min⁻¹) > freshwater (0.30 min⁻¹). Bromide (Br⁻) and bicarbonate (HCO₃⁻) promote the degradation of OFL, whereas chloride (Cl⁻) inhibits the degradation. The piperazine ring of OFL was the main reactive group, and atoms N1, C6, C7 and N2 were identified as the reaction sites. Based on the intermediate and final products, the possible degradation pathways of OFL in the three kinds of water were proposed. Additionally, during the UV/PDS treatment of synthetic marine aquaculture water containing Cl⁻ and Br⁻, the oxidation products of OFL showed a slight toxicity to Chlorella pyrenoidosa (C. pyrenoidosa) and Priacanthus tayenus (P. tayenus). The maximum growth inhibition rate of the products to C. pyrenoidosa was 9.72%. The products also caused liver cells of P. tayenus to be damaged and reduced the species richness and diversity of intestinal microorganism. Nevertheless, compared with the products degraded by traditional disinfection methods using NaClO, the biological toxicities were much lower. UV/PDS can be used for seawater as a new alternative disinfection method.

Introduction

Nowadays, antibiotics have brought serious threats to the ecological environment due to their high chemical stability and low rate of biodegradation (Xie et al., 2019; Zhou et al., 2019). With the increasing demand for seafood, antibiotics have been extensively used in Chinese marine aquaculture industry to promote the growth and clear infections of sea animals (Du et al., 2019; Suzuki et al., 2019; Bu et al., 2020; Xu et al., 2020). There are mainly eight classes of antibiotics in the market, including β-lactams, macrolides antibiotics, lincomycin antibiotics, polypeptide antibiotics, tetracyclines, quinolones and sulfa antibiotics (Reis et al., 2020a; Reis et al., 2020b; Tran et al., 2019). As a class of antibiotics, fluoroquinolones (FQs) have been extensively used to cure infections caused by various bacterial species in marine aquaculture (Rong et al., 2018; Wang et al., 2010; Zhang et al., 2017). As a kind of FQ, ofloxacin (OFL) has been widely used and frequently detected in marine aquaculture water and coastal waters (Sun et al., 2019; Wang et al., 2019). For example, OFL was reported at concentrations of 26.9 ng/L in coastal water adjacent to marine aquaculture areas in the Bohai Sea (Du et al., 2019). Therefore, it is necessary to find an effective method to remove residual OFL from the marine aquaculture water.

Advanced oxidation processes have been effectively applied for the degradation of antibiotics including OFL. Yu et al. found that the removal efficiency of OFL by the Fenton process reached 91.5% at pH 6 (Yu et al., 2019). Gao et al. found that the degradation efficiency of OFL can reach 93% in only 10 min in the presence of peroxymonosulfate (PMS) and LaNiO₃ (Gao et al., 2019). A study indicated that the OFL efficiency removal reached 95.8% after 60 min of catalytic oxidation by the microwave-assisted persulfate process (Liu et al., 2019). Among these studies, UV/PDS has gained increasing attention because UV irradiation can effectively activate PDS to generate highly reactive sulfate radicals (•SO₄⁻, E₀ = 2.5–3.10 V) (Tang et al., 2019). Wang et al. found that 96.2% of OFL was degraded by Cu-Fe/persulfate at a reaction time of 30 min (Wang et al., 2018). A study indicated that the removal rate of OFL by CuO/persulfate can reach 92% in 60 min (Li et al., 2019a, Li et al., 2019b). Previous studies have been carried out to prove that adding persulfate to drinking water is not toxic to human beings. For example, sulfate radical-based technology was used to remove 2-methylisoborneol and 2-methylisoorneol-producing algae in drinking water sources (Li et al., 2019a, Li et al., 2019b). A study indicated that UV and sodium persulfate were used to jointly inactivate microorganisms in drinking water, which is expected to be a high efficient method to disinfect bacteria in drinking water (Bekris et al., 2017; Lei et al., 2017).

However, the studies were usually carried out in fresh water, and few studies have focused on marine aquaculture water. In general, marine aquaculture water is a mixture of fresh water and seawater at a ratio of 2:1 (Pan et al., 2019; Shao et al., 2018; Zhang et al., 2019; Zhang et al., 2017). It contains a large number of inorganic anions including bromide (Br⁻: 22 mg/L), chloride (Cl⁻: 6.6 g/L), sulfate (SO₄²⁻: 0.903 g/L) and bicarbonate (HCO₃⁻: 0.047 g/L) etc. The reaction processes and products may be different due to the presence of the inorganic anions. The above anions, especially the halogen ions, could participate in the reaction of UV/PDS with FQs. For example, Lutze et al. studied the formation of ClO₃⁻ in bromide-containing water by the oxidation of UV/PDS. The results demonstrated that ClO₃⁻ and •OH were formed by the oxidation of Cl⁻ by •SO₄⁻ under acidic conditions and neutral/alkaline conditions, respectively (Lutze et al., 2015). A recent study by Lu et al. reported that •SO₄⁻ could oxidize Br⁻ to BrO₃⁻ and that HBrO was the key intermediate (Lu et al., 2015). Fang et al. analysed the transformation of Cl⁻ and Br⁻ in the UV/PDS oxidation system. The results indicated that Cl⁻ and Br⁻ were first oxidized by •SO₄⁻, •OH and halogen radicals to form intermediate products such as ClO⁻/HClO and HBrO/BrO⁻, and finally, ClO₃⁻ and BrO₃⁻ were formed (Fang and Shang, 2012; Fang et al., 2017). Our previous studies proved that the oxidation rates of antibiotics by sodium hypochlorite in seawater and marine aquaculture water were faster than that in fresh water. Moreover, several brominated products were found except for the chlorinated products (Pan et al., 2019; Shao et al., 2018). The brominated substances were proposed to be far more harmful than antibiotics and chlorinated products to the growth of seafood and may even affect human health by transmission through food chains. However, neither the inorganic nor organic brominated products of OFL in the UV/PDS oxidation process have been reported. In addition, studies on the toxicity of OFL degradation products have rarely been reported.

Taking the abovementioned into consideration, the objectives of this study are (1) to investigate the UV/PDS degradation of OFL in three kinds of water matrices, especially in marine aquaculture water and synthetic seawater; (2) to investigate the reaction kinetics between OFL and UV/PDS in marine aquaculture water, transformation pathways of OFL and halogen ions, and the identification of oxidation products; and (3) to analyse the toxicity of the reaction solutions via liver injury of P. tayenus and the growth inhibition rate of C. pyrenoidosa.