Emerging Residual Chlorine Quenchers

Emerging Residual Chlorine Quenchers: Comparison

Please note this is a comparison between Version 2 by Catherine Yang and Version 1 by Jiafu Li.

Disinfection by-products (DBPs) are the most common organic contaminants in tap water and are of wide concern because of their highly developmental toxic, cytotoxic, and carcinogenic properties. Researchers have attempted to find emerging chlorine quenchers. For inorganic DBPs (bromate, chlorate, and chlorite), sodium sulfite has been proven to be the ideal chlorine quencher. For organic DBPs, although ascorbic acid caused the degradation of some DBPs, it remains the ideal quenching agent for most known DBPs. Among the studied emerging chlorine quenchers, n-acetylcysteine (NAC), glutathione (GSH), and 1,3,5-trimethoxybenzene are promising for their application as the ideal chlorine quencher of organic DBPs.

drinking water
disinfection by-products
residual chlorine
chlorine quenchers

1. Introduction

Water plays an important role in our lives, and its quality significantly impacts public health. In order to kill harmful microorganisms in water and prevent the spread of diseases from water, the disinfection of drinking water becomes a necessary process. However, in this process, disinfectants, natural organic matter in water, and inorganic ions (bromine, iodine ions) will react to produce disinfection by-products (DBPs) ^[1][2]. Recent toxicological studies have shown that DBPs are usually cytotoxic, genotoxic, and carcinogenic ^[1][2]. In addition, DBPs have higher biological toxicity and detectable concentrations compared to artificial contaminants in drinking water ^[3]. Therefore, it is essential to control DBPs in drinking water to reduce their risk to human health, which is conducive to the safety of drinking water and to improve the quality of drinking water.

2. N-Acetylcysteine

In the search for new quenchers, reduced sulfur compounds (RSC), including n-acetylcysteine (NAC), and glutathione (GSH), were found to easily react with chlorine and chloramine. Based on the results of the present study, NAC is considered an ideal quencher for most DBP and TOX analyses, except for HNMs. The much higher reactivity of chlorine and chloramine to reducing sulfur groups in NAC protects other functional groups (e.g., alkyls, amines, and amides), thereby avoiding the formation of CX₃R-DBPs and maintaining a stable concentration of DBPs ^[4][5]. Specifically, NAC applies as a quencher for THMs, HAAs, and DCAL, as well as 1,1,1-trichloropropanone (1,1,1-TCP) and TCAN, and can also be used to some extent for analyzing TCAL, DCAN, DBAN, dichloroacetamide (DCAM), and trichloroacetamide (TCAM), but NAC is not applicable to DCNM and TCNM analysis.

Ding et al. (2022) found that the concentration changes of THMs, HAAs, and HKs were negligible within 168 h (almost less than 10.0%) ^[6]. However, TCAL, DCAN, DBAN, DCAM, and TCAM have slight hydrolysis under the same conditions ^{[7][8][9][10][11]}. Although the presence of NAC (20.0 μmol/L) promotes the hydrolysis of TCAL, DCAN, DCAM, and TCAM, the influence of NAC can be ignored due to the comparable rate of reduction and dehalogenation, and the very low molar ratio of NAC to disinfectant in real factory water. Therefore, when NAC acts as a quencher for TCAL, DCAN, DBAN, DCAM, and TCAM before analysis, it is necessary to immediately analyze samples for TCAL, DCAN, DBAN, DCAM, and TCAM to minimize negative interference with hydrolysis. Concurrent experimental results showed an immediate hydrolysis of 1,1,1-TCP and TCAN, resulting in their rapid losses within a few hours at pH 7. However, the effect of NAC (20.0 μmol/L) on 1,1,1-TCP and TCAN stability is negligible. Thus, the addition of NAC as a quencher is suitable for the determination of 1,1,1-TCP and TCAN. Similarly, 1,1,1-TCP and TCAN should be measured as quickly as possible to avoid hydrolysis ^[6].

However, the destruction of DCNM and TCNM by NAC was obvious. In the absence of NAC, DCNM and TCNM were slightly hydrolyzed, but a significant reduction in DCNM and TCNM concentrations in the presence of NAC was observed. In NAC quenched samples, DCNM decomposed a 1.6-fold increase in k_obs. TCNM had completely disappeared after 3 h in the presence of 20.0 μmol/L of NAC, which was also observed when TCNM was resolved by sodium sulfite and sodium thiosulfate ^[12]. The rapid degradation of DCNM and TCNM by NAC limits its application in the DCNM and TCNM analyses. Therefore, such samples should be immediately analyzed without adding any quencher.

In the experiments investigating the effects of various quenchers on TOX assays, a relatively low reduction in TOX was observed in samples quenched with NAC, and GSH for 3 h, ranging from 7.0% to 19.3%. With longer quenching time (24 h), TOX reduction in samples with NAC (8.0%) and glutathione (13.0–19.0%) were lower than those with sodium sulfite (30.0%) and sodium thiosulfate (36.0%) ^[6].

3. Glutathione (GSH)

GSH, which belongs to RSC, was also selected as a quencher. Both NAC and GSH have reduced sulfur groups, therefore, the effect of GSH on chloride and chloramine, organic DBPs, and TOX are the same as NAC.

When the molar ratio of Cl₂ or NH₂Cl to GSH is less than 0.5, the generation of CX₃R-DBPs during the chlorine or chloramine process of GSH will not adversely affect the analysis of CX₃R-DBPs, under which Cl₂ and NH₂Cl can be completely quenched. GSH has an obvious destructive effect on HNMs, and they should be immediately analyzed without adding a quencher, but GSH has little effect on the stability of THMs, HAAs, HALs, HKs, HANs, and HAMs. Among them, 1,1,1-TCP, TCAN, and TCNM should be analyzed as soon as possible to avoid rapid hydrolysis. A comparison of the negative effects of four quenchers on TOX determination: sodium thiosulfate > sodium sulphite > GSH > NAC. GSH is therefore an ideal quencher for THMs, HAAs, HALs, HKs, HANs, HAMs, and TOX analysis ^[6].

4. 1,3,5-Trimethoxybenzene (TMB)

As a new quenching agent, 1,3,5-Trimethoxybenzene (TMB) is used to preserve redox-unstable DBPs. For quenching free chlorine and free bromine, TMB has been proven to be an effective quencher. TMB does not affect the stability of eight known DBPs (TCNM, TCAL, CAN, DCAN, TCAN, BAN, DBAN, and TBAL) ^[13]. TMB does not degrade unstable DBP in the presence of conventional quenchers, and using TMB as a quencher provides the additional benefit of being able to quantify residual free chlorine and free bromine by separately measuring 2-Cl-1,3,5-trimethoxybenzene (Cl-TMB) and 2-Br-1,3,5-trimethoxybenzene (Br-TMB) in quench samples. However, since Cl-TMB and Br-TMB affect the TOX content of the quenched sample, TBM is not a suitable free halogen quencher in samples that are subsequently analyzed for TOX ^[13].

5. Comparison of Traditional and Emerging Residual Chlorine Quencher

Traditional quenching agents are mainly inorganic, but the new ones are organic. The traditional ones are cheap and easy to transport and preserve, but the new ones are relatively expensive and not easy to preserve (Table 1). For traditional quenchers, the quenching mechanism is mainly based on redox reactions, but the new reaction mechanism is more diverse. Among the traditional inorganic quenchers, sodium thiosulfate, sodium sulfite, and ammonium chloride will cause degradation of organic DBPs, and ascorbic acid will cause degradation of chlorate and MX, but the new quenchers are now more friendly to most of the organic DBPs.

Table 1.

Comparison of the advantages and disadvantages of traditional and emerging chlorine quenchers.

Quenching Agents		Advantages	Disadvantages	Applicable DBPs	Not Applicable DBPs
Traditional quenching agents	Ascorbic acid		Best agent for most of known organic DBPs; Reduce cytotoxicity Simple, cheap, and stable			Degradation of inorganic DBPs (chlorate, chlorite, and bromate) Degradation of some of organic DBPs			All of THMs, HAA, and TOX Most of HANs, HKs, HALs, HAMs, and phenolic DBPs	TCNM, TCAN, Chlorite, MX, BDCNM, DBCNM, DBAM, TBAL, inorganic DBPs
	Sodium sulfite		Best agent for inorganic DBPs, MX, THMs, and HAAs Simple, cheap, and stable		Caused degradation of most of priority DBPs and emerging DBPs	THMs, HAAs, chlorate, chlorite, bromate, inorganic DBPs, and MX		Most of HNMs, HANs, HKs, HALs, and HAMs Halopropanes, halopropylenes, halopropanes, and chloropropionitrile TOX
	Ammonium chloride		Good agent for most of organic DBPs Simple, cheap, and stable			Converts free chlorine to combined chlorine, leading to an increase in TOX Reduce MX			All of THMs, HAAs, and TOX Most of HANs, HKs, HALs, HAMs, and phenolic DBPs	TCNM, DBAM, MX
New quenching agents	N-Acetylcysteine		Good agent for most DBPs Simple, cheap, and stable			With a garlic-like odor; Moisture-attracting			All of THMs and HAAs Most of HANs, HALs, HKs, HNMs, HAMs, and TOX	DCNM, TCNM
	Glutathione	Stable for most DBPs		High cost Complex and time-consuming in preparation and operation		Same as n-acetylcysteine	DCNM, TCNM
	1,3,5-Trimethoxybenzene		Good for redox unstable DBPs, free chlorine, and bromine analysis			Can be disturbed by chloramines Impact TOX		TCNM, TCAL, CAN, DCAN, TCAN, BAN, DBAN, and TBAL	TOX

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