A Review of Current Status and Analysis Methods of Antibiotic Contamination in Groundwater in China (2012—2021)

As a form of new emerging pollutant, antibiotics have been detected in soil, surface water, groundwater, sediment and other different environmental media. As a major country in the production and usage of antibiotics, China's production and usage are increasing year by year. However, most antibiotics used for humans or animals cannot be fully absorbed and metabolized and will enter the environment in the form of prototypes or metabolites through waste and wastewater accumulating in soil and leaching into groundwater. Antibiotics entering the environment may affect microbial ecology, produce resistance genes, and even threaten human health. Compared with surface water, polluted groundwater is hidden, lagging and difficult to recover. The pollution of antibiotics in groundwater, as the main source of drinking water, has attracted much attention.So far, the research on antibiotics in China is still mainly on surface water and soil, and there are few observations on antibiotics in groundwater. In order to systematically grasp the current pollution situation of antibiotics in groundwater in China, relevant literature on antibiotics in groundwater from 2012 to 2021 is reviewed in this paper. Twenty-eight antibiotics detected more than 100 times in environmental media in China were selected as target antibiotics, and the detected concentrations were summarized and analyzed. It was found that the concentrations of 28 antibiotics commonly detected in groundwater varied by more than 4 orders of magnitude, from 0.1ng/L to more than 1000ng/L. The most frequently detected antibiotics were norfloxacin, ofloxacin, sulfamethoxazole, sulfadiazine, enrofloxacin, and erythromycin. Through comparative analysis of the detection of antibiotics in various places, it can be seen that the concentration of antibiotics in groundwater is controlled by the properties of antibiotics, the location of pollution sources, hydrogeological structure and the amount of usage and emissions. From the perspective of spatial distribution, sulfonamide antibiotics are the most detected in northeast China, quinolones are the most detected in North and East China, quinolones and tetracyclines are the most detected in southwest China, and the research on antibiotics in groundwater in northwest China is relatively low. So far, restrained by the detection limits and detection types of the analysis methods, a comprehensive investigation and evaluation of antibiotics in groundwater is not possible.Due to the wide variety of antibiotics, their different structures lead to different physical andchemical properties. They exist in trace concentrations in the complex environment media, which also affects the accuracy of their qualitative and quantitative analysis. Therefore, the establishment of a sensitive and specific multi-component simultaneous analysis method has been a key issue for antibiotics research. The analysis methods of antibiotics are summarized, which are divided into qualitative analysis methods and quantitative analysis methods. The principle, advantages, disadvantages and application range of several antibiotic analytical methods are presented. These methods include microbial inhibition method (MIT), thin layer chromatography (TLC), gas chromatogram-mass spectrometry (GC-MS), high-performance liquid chromatogram-nuclear magnetic resonance (HPLC-NMR) and liquid chromatogram-mass spectrometry (LC-MS). Liquid chromatogram-mass spectrometry (LC-MS) is the most commonly used method for antibiotic analysis because of its high sensitivity, low detection limit and simultaneous determination of multiple antibiotics. With the rapid development of antibiotic analysis methods, some antibiotics in groundwater can be accurately quantified by using HPLC-MS/MS and other technologies. However, the number of antibiotics that can be analyzed and identified at one time is still limited. The research group of authors has established the qualitative spectrum library of common drugs by UPLC-MS/MS. In the future, the types of antibiotics that can be qualitatively identified in the spectrum library can be expanded by adding the mass spectrum information of antibiotics. Under specific conditions, the spectrum library can be used to carry out semi-qualitative identification of antibiotics in groundwater. At present, the commonly used quantitative detection methods include enzyme-linked immunoassay, capillary electrophoresis, and liquid chromatography-mass spectrometry. Compared with the other two methods, liquid chromatography-mass spectrometry has the advantages of high sensitivity, good selectivity and accurate quantitative ability. It is commonly used for the detection of trace antibiotics in reported water samples.Antibiotics exist in the environment at trace levels and the matrix of environmental samples is complex, so the pretreatment process, including antibiotic separation, purification and concentration, often becomes the key step of determination. For example, the samples to be tested should be adjusted to an appropriate pH to enhance the enrichment of target antibiotics on HLB columns, and Na2EDTA should be added to inhibit its complexation with calcium and magnesium and other metal ions in groundwater. The accuracy of antibiotic determination will be improved, and the detection limit will be lowered for water samples by solid phase extraction and the subsequent concentration process. In addition to the detection limit and recovery rate of antibiotics affected by the analytical instrument, the presence states of antibiotics in water samples will also affect the accuracy and precision.Antibiotics may exist in the ionized state, complex state, adsorption state and other forms in groundwater. At different pH values, antibiotics may exist in neutral, cationic, anionic, orzwitterionic forms. When it coexists with metal ions, complexation reaction will occur under certain conditions to form antibiotic-metal complex which will reduce the peak area to a certain extent or cause tailing phenomenon on the reverse analytical column. The formation of the complex may also change the environmental behavior (migration, transformation, toxicity, etc.) and ecological effects of antibiotics. In addition, the analysis of antibiotics in different adsorption states can be used to evaluate the differences in microbial killing effects of different adsorption forms, especially the differences in ARG production and spreading. This will be helpful for accurately evaluating the potential effects on the environment or human beings and effectively controlling the risks of antibiotics in environmental media. Therefore, the existing form analysis of antibiotics is of great significance for the further accurate determination of antibiotics and the evaluation of environmental effects.Up to now, limited by the detection limits and detected types of antibiotics in analytical methods, there has not been a comprehensive national-scale investigation and evaluation of antibiotics in groundwater in China. Only by clarifying the concentration level and spatial distribution of antibiotic pollution in China's groundwater can it help to understand the contents of relevant laws and regulations on new emerging pollutants and support the establishment of a regulatory framework for natural resources and the environment. In conclusion, optimizing qualitative and quantitative detection methods, analyzing different existing forms of antibiotics, comprehensively investigating antibiotics in groundwater, and scientifically evaluating the relationship between antibiotic forms and ecotoxicological effects are the main contents of antibiotics research in groundwater in the future.