Review of Free Amino Acids in Source Water (River, Lake, and Reservoir): Occurrence, Composition, Molar Yields, Formation Potential, and Contribution to N-DBPs

Although free amino acids (FAAs) are known as an important precursor of nitrogenous disinfection byproducts (N-DBPs), their levels and composition in source water as well as their contributions to drinking water N-DBPs are not clear. This review provides a summary of occurrence and compositions of FAAs in different water sources as well as their molar yields and contributions to N-DBPs formation. Moreover, the impacts of advanced oxidation processes (AOPs) on N-DBPs formation are also summarized. The average concentrations of FAAs in rivers, lakes, and reservoirs were 439, 402, and 370 nM (about 56.2, 51.5, and 47.4 μg/L), in which cysteine, ornithine, alanine, glutamic acid, and serine were dominant among individual FAAs, with an average level of 25.6, 8.6, 6.2, 6.0, and 5.3 μg/L, respectively. During the chlorination process, the molar yields of FAA for dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), dichloroacetamide (DCAM), and trichloronitromethane (TCNM) were not detectable (ND)-7.1, ND-3.55, ND-0.93, and ND-1.99 μmol/mmol, respectively, contributing 7.0%, 11.9%, 0.3%, and 10.3%, on average, to drinking water N-DBPs. During chloramination, the molar yields of FAA fall within ND-5.55, ND-3.55, 0.4-176, and ND-1.52 μmol/mmol, constituting on average 5.3%, 18.4%, 0.8%, and 3.0% of DBPs’ formation in drinking water. The information provided may help enrich the knowledge of FAAs and gain insights toward the importance of FAAs in forming N-DBPs.