Insights into C-C Bond Cleavage Mechanisms in Dichloroacetonitrile Formation during Chlorination of Long-Chain Primary Amines, Amino Acids, and Dipeptides

A new concern raised from algal bloom: Organic chloramines in chlorination

Algal blooms have become a significant challenge in water treatment all over the world. In chlorination of drinking water, algal organic matter (AOM) leads to the formation of organic chloramines. The objectives of this review are to comprehensively summarize and discuss the up-to-date researches on AOM-derived organic chloramines and their chemical activities and toxicity, thereby drawing attention to the potentially chemical and hygienic risks of organic chloramines. The predominant algal species in water sources varied with location and season. AOM from cyanobacteria, green algae, and diatoms are composed of diverse composition. AOM-derived amino acids take a low portion of the precursors of organic chloramines. Both experimental kinetic data and quantum chemical calculation demonstrate the preferential formation of organic chloramines in the chlorination of model compounds (amino acids and peptides). Organic chloramines are persistent in water and can transform into dichloro- and trichloro-organic chloramines, unknown low-molecular-weight organic chloramines, and nitrogenous disinfection byproducts with the excess of free chlorine. The active chlorine (Cl+) in organic chloramines can lead to the formation of chlorinated phenolic compounds. Organic chloramines influence the generation and species of radicals and subsequent products in UV disinfection. Theoretical predictions and toxicological tests suggest that organic chloramines may cause oxidative or toxic pressure to bacteria or cells. Overall, organic chloramines, as one group of high-molecular-weight disinfection byproducts, have relatively long lifetimes, moderate chemical activities, and high hygienic risks to the public. Future perspectives of organic chloramines are suggested in terms of quantitative detection methods, the precursors from various predominant algal species, chemical activities of organic chloramines, and toxicity/impact.

The ideal model for determination the formation potential of priority DBPs during chlorination of free amino acids

Amino acids (AAs) account for about 15-35% of dissolved organic nitrogen (DON), and are known as the important precursors of nitrogenous disinfection by-products (N-DBPs). Determining the formation potential (FP) of AAs to DBPs is used to reveal the key precursors of DBPs for further control, while the ideal method for N-DBPs FP of AAs during chlorination is not revealed. In this study, the ideal FP test models for five classes of priority DBPs during chlorination of four representative AAs (accounted for about 35% of total AAs) were analyzed. For haloaldehydes (HALs), haloketones (HKs), haloacetonitriles (HANs), haloacetamides (HAMs), and halonitromethanes (HNMs), their FPs during chlorination of four AAs were 0.1-13.0, 0.01-1.1, 0.1-104, not detectable (nd)-173, and nd-0.4 μg/mg, respectively. The FPs of priority DBPs had significant deviations between different FP test models and different tested AAs. For HALs, the model, whose chlorine dosage was determined by 15 × molar concentration of AAs [Cl (mM) = 15 × M](named: model II), was the ideal model. For HKs, model II was also the ideal FP test model for AAs with ≤3 carbons, while for AAs with 4 carbons, the model, whose chlorine dosage was determined by keeping the residual chlorine at 1 ± 0.2 mg/L after 24 h of reaction (named: model 4), was the ideal model. For HANs and HNMs, model 4 was the ideal FP test model for most of the studied AAs. The performance of HAMs during chlorination of amino acids was totally different from other P-DBPs, and model 3 was recommended to be the ideal model, in which chlorine dosage was determined by 3 × mass concentration of AAs [Cl (mg/L) = X × DOC]. This study is a reference that helps researchers select an ideal model for N-DBPs FP study of AAs.

Reaction sites of pyrimidine bases and nucleosides during chlorination: A computational study

As important components of soluble microbial products in water, nucleobases have attracted much attention due to the high toxicity of their direct aromatic halogenated disinfection by-products (AH-DBPs) during chlorination. However, multiple halogenation sites of AH-DBPs pose challenges to identify them. In this study, reaction sites of pyrimidine bases and nucleosides during chlorination were investigated by quantum chemical computational method. The results indicate that the anion salt forms play key roles in chlorination of uracil, thymine, and their nucleosides, while neutral forms make predominant contributions to cytosine and cytidine. In view of both kinetics and thermodynamics, C5 is the most reactive site for uracil and thymine, N3/C5 and N3 for respective uridine and thymidine, N1/C5/N4 and N4 for respective cytosine and cytidine, whose estimated apparent rate constants kobs-est of ∼103, 103/102, 106/102/104, and 103 M-1 s-1, respectively, in consistent with the known experimental results. C6 in all pyrimidine compounds is hardly attacked by Cl+ in HOCl ascribed to its positive charge, but readily attacked by OH‾ in hydrolysis and the N1=C6 bond was found to possess the highest reactivity in hydrolysis among all double bonds. In addition, the structure-kinetic reactivity relationship study reveals a relatively strong correlation between lgkobs-est and APT charge in all pyrimidine compounds rather than FED2 (HOMO). The results are helpful to further understand the reactivity of various reaction sites in aromatic compounds during chlorination.

Comparative formation of chlorinated and brominated disinfection byproducts from chlorination and bromination of amino acids

Amino acids are the main components of dissolved organic nitrogen in algal- and wastewater-impacted waters, which can react with chlorine to form toxic halogenated disinfection by-products (DBPs) in the disinfection process. In the presence of bromide, the reaction between amino acids and secondarily formed hypobromous acid can lead to the formation of brominated DBPs that are more toxic than chlorinated analogues. This study compares the formation of regulated and unregulated DBPs during chlorination and bromination of representative amino acids (AAs) (e.g., aspartic acid, asparagine, tryptophan, tyrosine, and histidine). In general, concentrations of brominated DBPs (trihalomethanes, haloacetonitriles, and haloacetamides, 24.9-5835.0 nM) during bromination were higher than their chlorinated analogues (9.3-3235.3 nM) during chlorination. This indicates the greater efficacy of bromine as a halogenating agent. However, the formation of chlorinated haloacetic acids during chlorination was higher than the corresponding brominated DBPs from bromination. It is likely that an oxidation pathway is required for the formation of haloacetic acids and chlorine is a stronger oxidant than bromine. Moreover, chlorine forms higher levels of haloacetaldehydes (74.4-1077.8 nM) from amino acids than bromine (1.0-480.2 nM) owing to the instability of brominated species. The DBP formation yields depend on the types of functional groups in the side chain of AAs. Eight intermediates resulting from chlorination/bromination of tyrosine were identified by triple quadrupole mass spectrometer, including N-chlorinated/brominated tyrosine, 3-chloro/bromo-tyrosine, and 3,5-dichloro/dibromo-tyrosine. These findings provided new insights into the DBP formation during the chlorination of algal- and wastewater-impacted waters with elevated bromide.