Study On The Occurrence And Thermolysis Of Chlorinated Acetaldehydes In Drinking Water

The widespread use of disinfection technology in drinking water has greatly reduced the outbreaks of aquatic diseases.However,the reaction of disinfectants with natural organic matters in source water often results in an unavoidable result,i.e.,the formation of disinfection by-products in drinking water.Halogenated acetaldehydes are the third kind of disinfection by-products in drinking water.Compared with halogenated methane and halogenated acetic acid,the cytotoxicity and genotoxicity of halogenated acetaldehyde are about 1-3 orders of magnitude higher.Therefore,it is necessary to understand their occurrence in drinking water and control them.This study selected an office building of an university as the sampling point,and carried out a year-long intensive water quality survey to know the concentration of halogenated acetaldehydes in this area.This paper collected a total of 50 water samples and detected the contents of halogenated acetaldehydes.The results showed that dichloroacetaldehyde and trichloroacetaldehyde were the most common halogenated aldehydes in this area,and their concentration were ranged within 0.26-12.05 ug/L and 2.77-22.36 ug/L,respectively.They showed obvious seasonal distribution characteristics as summer > autumn > winter > spring.The maximum occurrrences of the two compounds appeared in July.The water quality parameters included water temperature,residual chlorine,pH value,dissolved organic matter(NPOC)and total soluble solids(TDS).Results showed that the concentration of dichloroacetaldehyde and trichloroacetaldehyde is moderately positively correlated.It means that the formation mechanism of these two halogenated acetaldehydes may be similar.As an electron donor,NPOC reacts with chlorine to form disinfection by-products,so that there is a negative correlation between NPOC with HALs in tap.The water temperature is also positively correlated with TDS,that is,increase of water temperature led to increase of TDS in water.In contrast,dichloroacetaldehyde and trichloroacetaldehyde have no significant correlations with the abovementioned water quality parameters.That is to say,the occurrence of halogenated acetaldehyde in this area is not related to those water quality parameters.In order to remove halogenated acetaldehyde from tap,boiling and microwave techniques were used to simulate the heating process for tap water in people's lives,and the removal performances and mechanisms of halogenated acetaldehyde were investigated.In this paper,the application water containing monochloro,dichloro and Trichloroacetaldehyde was boiled by electric kettle.The results show that the removal rate of three halogenated acetaldehydes can be effectively increased by reducing the amount of boiling water and prolonging the boiling time under the action of turbulence and bubbles generated during the boiling process.In addition,the removal efficiency of three halogenated acetaldehydes can be effectively increased by increasing the microwave power in the microwave boiling process.The removal mechanism of halogenated acetaldehyde in the boiling process was further explored.The results showed that in the presence of residual chlorine,the degradation rate of chloroacetaldehyde would accelerate with the increase of water temperature.At the same time,the halogenated acetaldehyde would be transformed from low halogenated to high halogenated,and finally the carbon-carbon bond could be removed by high temperature hydrolysis or cracking.When residual chlorine does not exist,the removal of halogenated acetaldehyde mainly depends on its own high-temperature volatilization,and its volatilization degree increases with the increase of boiling time.