Research on Generate Change of 4 Trihalomethanes in Drinking Water after Disinfection with Single/Combined Disinfectants Based on Gas Chromatography-Mass Spectrometry with Purge-and-Trap

The title study was conducted to investigate the changes in the generation amount and proportion of 4 trihalomethanes (THMs, including TCM, DCBM, DBCM and TBM) in drinking water after disinfection with sodium hypochlorite and chlorine dioxide+sodium hypochlorite. Water sample (stabilized by ascorbic acid) was taken and filled into a 40 mL-purge-and-trap bottle, and 10.0 μL of 10.0 mg·L^-1 internal standard (fluorobenzene) solution was added. After tightening the bottle cap, the mixture was shaken for 1 min, and an aliquot (5 mL) was taken for purge-and-trap with purge temperature of 30℃, purge time of 9.0 min, purge flow of 40 mL·min^-1, desorption temperature of 225℃, and desorption time of 2.0 min. The resulting gas was introduced into the gas chromatograph-mass spectrometer, and the 4 THMs were separated on DB-5MS quartz capillary column (60 m×0.32 mm, 1.0 μm) by temperature program with initial column temperature of 45℃, ionized by EI source, detected by SIM mode, and quantified by internal standard method. It was shown that linear relationships between values of the mass concentration ratio and the corresponding peak area ratio of the 4 THMs to the internal standard were kept in definite ranges, with detection limits (3s) in the range of 0.027-0.030 μg·L^-1. The spiked recovery test was made at the 3 concentration levels using baseless water and pipeline water as substrates, giving recoveries in the range of 88.1%-105%, and RSDs (n=6) of the determined values in the range of 1.7%-4.7%. The proposed method was used for analyzing the 4 THMs in 76 drinking water samples from 3 water plants in Shenzhen. The detection amounts of 4 THMs and total amount in water samples from each water plant did not exceed the limits specified by GB 5749-2022. The sum (THMs^* value) of the ratios of the detection amount to the limit of each THMs in the water samples disinfected with a single disinfectant of sodium hypochlorite posed a risk of exceeding the limit. The detection amount of TCM and the total amount of THMs in the water samples disinfected by chlorine dioxide+sodium hypochlorite decreased by nearly 50%, the ratio of TCM detection amount to the total amount of THMs decreased, the ratio of the other three bromotrichloromethanes increased, and the amount and frequency of sodium hypochlorite dropped significantly, reducing the risk of THMs^* value exceeding the standard. It was suggested that relevant water plants focused on changes in water quality during hot and rainy summer, and used the combined disinfection method of chlorine dioxide and sodium hypochlorite if necessary.