Formation of odorous aldehydes, nitriles and N-chloroaldimines from free and combined leucine during chloramination

Amino acids (AAs) are a major group of odorous disinfection by-product (O-DBP) precursors. O-DBPs formations during free chlorine disinfection has been previously investigated. However, knowledge regarding the O-DBP formation mechanism and kinetics under chloramination of AAs is very limited. In this study, the generation of odorous isovaleraldehyde, isovaleronitrile and N-chloroisovaleraldimine from leucine (Leu), a typical and abundant AA in many drinking water sources, in its free and combined forms during chloramination under several typical addition schemes of disinfectants was investigated. Free Leu and glycylleucine (Gly-Leu) were chosen as model compounds since they have been indicated to be O-DBP precursors during chlorination. In-termediate product analysis and kinetics studies were conducted to study the reaction mechanisms. Impacts of disinfectants dosages and pH were also investigated in experiments and simulations. The results indicated that comparing with chlorination, chloramination of Leu has its uniqueness by participating in reacting with iso-valeraldehyde to form N-chloroisovaleraldimine. And all the three O-DBPs formations from free Leu and Gly-Leu during chloramination (with preformed NH2Cl) were less than those during chlorination, indicating that using NH2Cl for disinfection ensures control over the off-flavor problems to some degree. When chloramination was realized by adding chlorine and ammonia separately, a longer pre-chlorination time led to greater yields of the O-DBPs from both precursors, whereas adding ammonia before chlorine promoted more isovaleraldehyde forma-tion from free Leu. Under alkaline conditions, more isovaleronitrile and N-chloroisovaleraldimine were pro-duced, and acidic conditions led to more isovaleraldehyde formation during chloramination. Notably, O-DBPs yields from free Leu were approximately 1000 times greater than those from Gly-Leu during chloramination under all the schemes. In addition, chlor(am)ination experiments with real water from Taihu Lake (the third largest freshwater lake and water source for twenty million people in China) indicated the formation of N- chloroisovaleraldimine and isovaleraldehyde was highly likely to cause odorous problems in drinking water. This study facilitates further understanding of the causes of off-flavor issues in drinking water and can help control the odorous problems by optimizing the operating parameters of drinking water treatment plants.

Automated summary

This study investigates the mechanisms and yields of odorous disinfection by-products (O-DBPs) formation from leucine and its derivatives during chloramination. The results show that chloramination has its unique characteristics compared to chlorination, with leucine participating in the reaction with isovaleraldehyde to form N-chloroisovaleraldimine. The yields of O-DBPs from leucine and glycylleucine during chloramination are lower than those during chlorination. The production of isovaleronitrile and N-chloroisovaleraldimine is enhanced under alkaline conditions, while acidic conditions promote the formation of isovaleraldehyde. The study contributes to a better understanding of off-flavor issues in drinking water and provides insights for controlling odorous problems in water treatment plants.