Occurrence and removal of sulfonamide antibiotics and antibiotic resistance genes in conventional and advanced drinking water treatment processes
Graphical abstract
Introduction
SAs, a class of antimicrobials with amino benzenesulfonyl structure and target dihydropteroate synthetase [1], have larger production and consumption in China than in other countries [2]. With the low octanol-water partition coefficients (logKow) and high emission rates, SAs have high mobility and are frequently detected with high concentrations in the water environment [3,4]. For example, SAs were detected in 83%–94% of the samples in Haihe River in China, with the highest concentrations of 210–385 ng/L [5]. Several studies had also confirmed the existence of SAs in drinking water sources in China (Anhui: 0.2–12.5 ng/L [6]; Nanjing: 4.4–78.8 ng/L [7]), even 1840 ng/L sulfamethoxazole (SMX) was detected in groundwater in Taiwan [8]. What’s more, in the United States, sulfamethazine (SMZ) and SMX were detected in groundwater in 18 states with concentrations ranging from 360 to 1110 ng/L [9]. In Vietnam, the concentration of SMX in the city canal even reached 4330 ng/L [10]. The residual SAs in aquatic environments not only pose a threat to aquatic creatures, but also produce a selective pressure for environmental microorganisms and contribute to the problems of antibiotic resistance in microorganisms [11,12].
As emerging contaminants, ARGs have attracted global attention worldwide. They are usually found coexisting with the mobile genetic elements, such as integrons, plasmids, which would promote the horizontal gene transfer [13,14]. Once these ARGs transmitted into pathogenic bacteria, its expression in harmful pathogens will undermine our ability to treat infectious diseases and exert a serious threat to human health [14]. Among the common four sul ARGs (sul1, sul2, sul3, sulA), sul1 and sul2 are two main sul ARGs reported in recent studies [3,15]. For example, the sul1 and sul2 concentrations in Huangpu River were both more than 105 gene abundance/mL [16]. In Haihe River, sul ARGs were 100% detected in water samples with absolute concentrations of 107–108 gene abundance/mL [3]. Su et al. [17] isolated 3456 E. coli from Dongjiang River (Guangzhou) and 89.1% of them were resistant to at least three antibiotics and the frequency of sul ARGs reached 89.2%. Besides, the pollution of sul ARGs was also detected in Beijiang River, Guangzhou [18], the Manasi River Basin, Xinjiang [19], and Pearl River [15]. All of these studies provided the evidence for the pollution of the aquatic environment by SAs and sul ARGs.
Recently, researchers have started to pay attention to the removal of antibiotics and ARGs in water treatment process [20]. Gaffney et al. [21] found that 65% of SAs could be removed during the chlorination process. The bio-treatment process achieved 77% removal of antibiotics and was efficient in removing ARGs in veterinary hospital wastewater [22]. Other methods, such as ultraviolet (UV) irradiation [23], UV/chlorination [24,25], and ozonation [26] processes were also proved to be effective in eliminating SAs and ARGs (0.80–2.28 log for ARGs). However, these studies only focused on the removal efficiencies of individual water treatment units and the target samples were all wastewater. Therefore, the removal of SAs and ARGs in the whole DWTPs should also be paid high attention to.
In this study, the occurrence and removal of 13 SAs, two sul ARGs (sul1, sul2) and class I integrase gene intI1 were investigated in water samples from the different units of two DWTPs with conventional and advanced treatment processes. The 13 SAs were detected by SPE-HPLC/MS/MS and ARGs were quantified by qualitative polymerase chain reaction (qPCR). The results will provide a basis to understand the prevalence of SAs and ARGs in the DWTPs. The comparison between the conventional and advanced processes will benefit the DWTPs for optimizing the treatment methods to eliminate these contaminants simultaneously.
Section snippets
Standards and reagents
Standards of 13 SAs including trimethoprim (TMP), sulfanilamide (SAM), sulfaguanidine (SG), sulfadiazine (SDZ), sulfamerazine (SMR), SMZ, sulfathiazole (STZ), sulfamethizole (SMT), SMX, sulfisoxazole (SFX), sulfachloropyridazine (SCP), sulfameter (SMD), sulfadimethoxine (SAT), and SMX isotope marker (SMX-13C6) were purchased from Sigma (USA). Hydrochloric acid and edetate disodium (EDTA) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Formic acid, methanol, and acetonitrile
Occurrence of SAs in raw water and finished water
The frequencies and concentrations of 13 SAs in raw water and finished water of two different DWTPs are shown in Table 1. During the conventional process, the frequencies of five SAs (TMP, SAM, SMZ, SMX and SCP) were detected as 100% in raw water. Among these five SAs, two of them (SAM and SMZ) still showed 100% detection frequency in finished water. SMX had the highest concentrations in both raw water (67.27 ng/L) and finished water (22.05 ng/L) in addition to its high frequency of detection.
Conclusion
This study reported the occurrence and removal of 13 SAs, two sul ARGs, and class I integrase gene intI1 in two different DWTPs. The advanced treatment process has a significant advantage over the conventional treatment method in the reduction of SAs, while showed an opposite performance in eliminating sul ARGs and intI1. The oxidation processes including pre-ozonation and post-chlorination could effectively reduce SAs. However, those two processes re-increased the amount of sul1, sul2 and intI
Acknowledgements
This work was supported by the National Water Pollution Control and Management Technology Major Projects (No. 2017ZX07402003), Shanghai Municipal Science and Technology Commission (No. 16DZ1204703) and Open Project of State Key Laboratory of Urban Water Resource and Environment (No. QA201612), China Postdoctoral Science Foundation (No. 2017M621391), the Fundamental Research Funds for the Central Universities (No. 222201814055) and Shanghai Sailing Program (No. 18YF1406000).
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