CX3R-type Disinfection By-products Research Articles

Identification of key precursors contributing to the formation of CX3R-type disinfection by-products along the typical full-scale drinking water treatment processes

Identification and characterization of disinfection by-product (DBP) precursors could help optimize drinking water treatment processes and improve the quality of finished water. This study comprehensively investigated the characteristics of dissolved organic matter (DOM), the hydrophilicity and molecule weight (MW) of DBP precursor and DBP-associated toxicity along the typical full-scale treatment processes. The results showed that dissolved organic carbon and dissolved organic nitrogen content, the fluorescence intensity and the SUVA254 value in raw water significantly decreased after the whole treatment processes. Conventional treatment processes were in favor of the removal of high-MW and hydrophobic DOM, which are important precursors of trihalomethane and haloacetic acid. Compared with conventional treatment processes, Ozone integrated with biological activated carbon (O3-BAC) processes enhanced the removal efficiencies of DOM with different MW and hydrophobic fractions, leading to a further decrease in almost all DBP formation potential and DBP-associated toxicity. However, almost 50% of the detected DBP precursors in raw water has not been removed after the coagulation-sedimentation-filtration integrated with O3-BAC advanced treatment processes. These remaining precursors were found to be mainly hydrophilic and low-MW (< 1.0 kDa) organics. Moreover, they would largely contribute to the formation of haloacetaldehydes and haloacetonitriles, which dominated the calculated cytotoxicity. Since current drinking water treatment process could not effectively control the highly toxic DBPs, the removal of hydrophilic and low-MW organics in drinking water treatment plants should be focused on in the future.

Elevated risk of haloacetonitrile formation during post-chlorination when applying sulfite/UV advanced reduction technology to eliminate bromate

The formation of haloacetonitriles (HANs) during chlorination after sulfite/ultraviolet (UV) treatment of bromate (BrO3−) in the presence of amino acids (AAs) was investigated. During sulfite/UV treatment, the primary species hydrated electrons (eaq−) and hydrogen atom radicals (H) dominated the reduction of BrO3− to bromide (Br−), whereas the sulfite anion radicals (SO3−) and H degraded AAs to produce the intermediates HN=C(CH3)–COOH, CH3–CH=NH, and CH3–C≡N via α‑hydrogen abstraction and NH2–hydrogen abstraction mechanisms. During post-chlorination, Br− was converted to HBrO/BrO-, and the HN=C(CH3)–COOH, CH3–CH=NH, and CH3–C≡N groups featured higher bromine utilization factor (BUF) and chlorine utilization factor (CUF) values than AAs, enhancing the formation of dibromoacetonitrile (DBAN) and dichloroacetonitrile (DCAN). The energetic feasibility of the transformation pathway, that is, HN=C(CH3)–COOH, CH3–CH=NH, and CH3–C ≡ N formation via hydrogen abstraction by SO3− and H and their further conversion to HANs, was proved by density functional theory calculations, which showed stepwise negative Gibbs free energy changes (ΔG < 0). The effects of pH and water matrices (e.g., HCO3−, Cl−, Fe3+, and natural organic matter) were comprehensively evaluated. Although 72% of BrO3− was removed by sulfite/UV treatment in the presence of AAs, the cytotoxicity index (CTI) and genotoxicity index (GTI) during post-chlorination increased by 213% and 125%, respectively, due to the formation of 24 CX3R-type disinfection by-products (DBPs), especially brominated DBPs. Accordingly, more attention should be given to the formation of brominated DBPs during post-chlorination when using sulfite/UV processes to remove BrO3− in the presence of AAs. As a solution, using monochloramine instead of chlorine as a disinfectant after the sulfite/UV process could significantly lower the CTI and GTI values by alleviating the formation of brominated DBPs.

Effect of oxidation ditch and anaerobic-anoxic-oxic processes on CX3R-type disinfection by-product formation during wastewater treatment

The high chlorine dosages in wastewater treatment plants during the COVID-19 pandemic may result in increased formation of disinfection by-products (DBPs), posing great threat to the aquatic ecosystem of the receiving water body and the public health in the downstream area. However, limited information is available on the effect of biological wastewater treatment processes on the formation of CX3R-type DBPs. This study investigated the effect of oxidation ditch (OD) and anaerobic-anoxic-oxic (AAO), two widely used biological wastewater treatment processes, on the formation of five classes of CX3R-type DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloacetaldehydes (HALs), haloacetonitriles (HANs) and halonitromethanes (HNMs), during chlorination. Experimental results showed that biological treatment effectively reduced the dissolved organic carbon (DOC) and UV254, while it increased the dissolved organic nitrogen (DON), and therefore the ratio of DON/DOC. In addition, increases in the contents of soluble microbial product- and humic acid-like matters, and the transformation of high molecular weight (MW) fractions in the dissolved organic matter into low MW fractions were observed after OD and AAO processes. Although biological treatment effectively decreased the formation of Cl-THMs, Cl-HAAs, Cl-HANs and Cl-HNMs, the formation of DBCM, DBAA, BDCAA, DBCAA, DCAL, TCAL and DBAN (where C = chloro, B = bromo, D = di, T = tri) all increased significantly, due to the increased formation reactivity. Moreover, biological treatment increased the ratio of bromide/DOC and bromine incorporation into THMs, HAAs and DHANs except for HALs and THANs. Different from previous studies, this study revealed that biological treatment increased the formation of some DBPs, especially brominated DBPs, despite the efficient removal of organic matters. It provides insights into the DBP risk control in wastewater treatment, particularly during the COVID-19 pandemic.

Occurrence of CX3R-Type Disinfection Byproducts in Drinking Water Treatment Plants Using DON-Rich Source Water

To understand the occurrence of CX3R-type disinfection byproducts (DBPs) in drinking water treatment plants that use dissolved organic nitrogen (DON)-rich water sources, this study surveyed the occ...

Kinetic degradation of anti-inflammatory drugs in ozone combined free active chlorine system

In this study, free active chlorine (HOCl/OCl-, FAC) was employed as an activator of ozone (O3) to remove typical non-steroidal anti-inflammatory drugs of aspirin (ASA) and phenacetin (PNT). The combination of FAC and O3 significantly improved the removal of pollutants. O3 molecules, hydroxyl radicals (OH) and reactive chlorine species (RCS) contributed to the removal of target contaminants in O3/FAC system. By fitting the pollutant removal pattern with the experiment and model, the contribution of each oxidative species in O3/FAC system was measured. The average removal contribution of O3, OH, and RCS to ASA/PNT was 10.3%/8.4%, 53.8%/10.5%, and 35.9%/81.1%, respectively. Compared with O3 alone system, the average mineralization of pollutant in O3/FAC system increased from 20.6% to 34.0%. The formation of five typical CX3R-type disinfection by-products (DBPs) was evaluated with post-chlorine system. The maximum concentration of total DBPs was achieved at pH 6.0 of 12.5 μM, in which trichloromethane (TCMs) was the main component (accounting for 55.3%). This study provides a new method to remove contaminants based on the combination of O3 and FAC disinfectants.

Anti-inflammatory drugs degradation during LED-UV365 photolysis of free chlorine: roles of reactive oxidative species and formation of disinfection by-products

Light-emitting diode (LED) is environmentally friendly with longer life compared with traditionally mercury lamps. This study investigated the non-steroidal anti-inflammatory drugs (NSAIDs)- phenacetin (PNT) and acetaminophen (ACT)- removal during LED-UV (365 nm) photolysis of free available chlorine (FAC). Degradation of PNT and ACT during LED-UV365/FAC treatment at pH 5.5-8.5 followed the pseudo-first order kinetics. The presence of hydroxyl radicals (·OH), reactive chlorine species (RCS), and ozone (O3, transformed from O (3P)) were screened by using scavengers of ethanol (EtOH), tert-Butanol (TBA), and 3-buten-2ol, and 4-hydroxy-2,2,6,6-tetramethylpiperidine (TEMP), and quantified by competition kinetics with probing compounds of nitrobenzene (NB), benzoate acid (BA), 1,4-dimethoxybenzene (DMOB). Higher pH would lead to decrease of ·OH contribution and an increase of FAC contribution to PNT and ACT degradation. It has been determined that the contribution of O3 to degradation of PNT and ACT was less than 5% for all pHs, and O3(P) reacts toward EtOH with second-order constant of 1.52 × 109 M−1s−1. LED-UV365/FAC system reduced the formation of five typical CX3-R type disinfection by-products (DBPs) as well as the cytotoxicity and genotoxicity of water samples at pH 5.5 and 8.5, compared with FAC alone. The decrease of DBPs formation resulted from fast FAC decomposition upon LED-UV365 irradiation. A feasible reaction pathway of DBPs formation in the LED-UV365/FAC system was examined with density functional theory (DFT). For FAC decay during LED-UV365/FAC with effluent from wastewater, the residual FAC in 15 min was 0.8 mg/L (lower than limit of 0.2 mg/L) once initial FAC was 2.0 mg/L. The results indicate that more tests on the balance of target pollutant removal efficiency, residual FAC and cost should be explored in LED-UV365/FAC system for application.

Removal of CX3R-type disinfection by-product precursors from rainwater with conventional drinking water treatment processes

In addition to surface water and groundwater, rainwater is used as an important drinking water source in many parts of the world, especially in areas with serious water pollution or insufficient water resources. Conventional drinking water treatment technologies can remove dissolved organic matter and therefore reduce the formation of disinfection by-products (DBPs) during subsequent disinfection using surface water or groundwater as drinking water sources. However, little information has been known about the effect of conventional water treatment processes on DBP formation when rainwater is used as drinking water source. This study evaluated CX3R-type DBP precursors removal from rainwater by conventional drinking water treatments and the corresponding decrease of CX3R-type DBP (trihalomethanes (THMs), haloaldehydes (HALs), haloacetonitriles (HANs) and haloacetamides (HAMs)) formation and toxicity during the subsequent chlor(am)ination. The result showed that both sand filtration (SF) and activated carbon filtration (GAC) were able to remove DBP precursors and GAC outperformed SF, but no DBP precursors removal was observed during coagulation-sedimentation treatment. Among all treatments, SF + GAC was the most effective for DBP precursors removal, with removal efficiencies of 64.2% DOC, 98% DON and 76.6% UV254. Correspondingly, both SF and GAC decreased the formation of THMs, HALs, HANs and HAMs, and GAC performed better than SF. The combination of SF and GAC, especially SF + GAC, greatly decreased DBP formation, with average reduction of 79.2% and 85% during chlorination and chloramination respectively. After different treatments, the comprehensive toxicity risk of CX3R-type DBPs was all reduced, among which GAC + SF exhibited superior performance. Generally, the main contribution of integrated toxicity was HANs during chlor(am)ination. The formation potential of THMs, HALs, HANs and HAMs and the corresponding integrated toxicity were greater during chlorination than that during chloramination. Therefore, the combination of GAC and chloramination was promising in mitigating the comprehensive toxicity risk of THMs, HALs, HANs and HAMs for rainwater.

Formation, speciation and toxicity of CX3R-type disinfection by-products (DBPs) from chlor(am)ination of 2,4-diaminobutyric acid (DAB).

2,4-diaminobutyric acid (DAB), a newly identified algal toxins in water, pose a great threat to human health. DAB may react with chlorine or chloramine to produce CX3R-type disinfection by-products (DBPs) during water treatment processes. This study mainly investigated the formation and speciation of DBPs from chlor(am)ination of DAB. The results revealed that haloacetic acids (HAAs), trihalomethanes (THMs) and haloacetonitriles (HANs) were the main kinds of CX3R-type DBPs generated from DAB during chlor(am)ination, of which dichloroacetic acid yielded the highest. The formation and total toxicity of four CX3R-type DBPs from DAB during chloramination was significantly lower than that during chlorination at each Cl2:N molar ratio. However, more formation of Br-THMs and I-THMs were observed during chloramination in the presence of Br-/I-. Futhermore, the effects of chlor(am)ine dosage, solution pH, reaction time, and the concentration of Br- and I- on the formation and speciation of CX3R-type DBPs were also evaluated during chlor(am)ination. The plausible formation pathways of CX3R-type DBPs from DAB were proposed and verified by theoretical calculation. The quantum chemistry calculations indicate that 1N in DAB and 8N in 2,4-diaminochlorobutyric acid (C4H9O2N2Cl) were more likely to be attacked by electrophiles, supporting the proposed pathway schemes.

Characterization of dissolved organic matter derived from atmospheric dry deposition and its DBP formation

Disinfection by-products (DBPs) precursors can be regarded mainly from the drinking water sources and the water treatment processes. A recent study showed that dissolved organic matter (DOM) in atmosphere is an important precursor source of DBPs through atmospheric wet deposition. However, little information is available on the characteristics of DOM derived from dry deposition particulate matter (PM) and the impact of dry deposition on CX3R-type DBP formation. This study determined whether dry deposition directly contributed the production of DBPs during chlor (am)ination and investigated the mechanism behind the contribution based on the combination of the resin and membrane for fractionating DOM fractions. The results showed that the hydrophilic fraction (HPI) contributed the most DOM and low molecular weight DOM (<10 kDa) was the main component of HPI. In addition, aromatic proteins and soluble microbial products-like compounds were the dominant fluorescent species in DOM derived from PM, and <10 kDa transphilic was the most abundant. The concentrations of C-DBPs and N-DBPs in disinfected PM solution were trihalomethanes (THMs) > haloacetic acids (HAAs) > haloaldehydes and haloacetamides > haloacetonitriles > halonitromethanes for both chlorination and chloramination. The main contributors of calculated toxicity are transphilic and hydrophobic in chlorination and chloramination respectively. Dry deposition PM was deduced to contribute DOM and DBP formation after chlorination in surface water, especially THMs and HAAs. These results presented herein provide key information for controlling DBPs from the perspectives of atmospheric dry deposition, especially in the case of heavy air pollution.

Peroxymonosulfate/chloride disinfection versus sodium hypochlorite disinfection in terms of the formation and estimated cytotoxicity of CX3R-type disinfection by-products under the same dose of free chlorine

• CX 3 R-type DBP formation is compared between PMS/Cl − and NaClO disinfection. • CX 3 R-type DBP level in PMS/Cl − is lower than that in NaClO disinfection. • DBP formation pathway during PMS/Cl − disinfection is proposed. • The cytotoxicity from DBPs in PMS/Cl − is lower than that in NaClO disinfection. Peroxymonosulfate (PMS) is receiving increasing interest as an oxidant used during the disinfection of drinking water. However, it does not have continuous disinfection ability as sodium hypochlorite (NaClO) does. PMS/Cl − can produce reactive chlorine species and thus provide a persistent disinfectant residual in distribution systems. This study investigated PMS/Cl − disinfection versus NaClO disinfection in terms of the formation and estimated cytotoxicity of CX 3 R-type disinfection by-products (DBPs) under the same dose of free chorine. Results showed that free chlorine produced by PMS/Cl − reaction can react with organic compounds and the productions of CX 3 R-type DBPs were dependent on the free chlorine dose, pH and reaction time. Specifically, chloroform (CF) and chloral hydrate (CH) generally increased with increasing of free chlorine dose and reaction time. CF increased steadily with increasing pH, while CH firstly increased and then decreased with increasing pH. Haloacetonitriles and haloacetamides firstly increased and then decreased with increasing of free chlorine dose and reaction time, and decreased continuously with increasing pH. The formation pathway of CX 3 R-type DBPs during chlorination (PMS/Cl − ) of tyrosine was proposed, based on the detected intermediates (i.e., Benzyl cyanide, 4-hydroxy-benzyl-cyanide, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol). The estimated cytotoxicity of CX 3 R-type DBPs in chlorinated water firstly increased and then decreased with the increasing of reaction time, free chlorine dose and pH, which highly related to the changes of DCAN (the most toxic CX 3 R-type DBP) levels. Comparing to NaClO disinfection, PMS/Cl − disinfection can better control the formation and cytotoxicity of CX 3 R-type DBPs, which was attributed to the degradation of natural organic matter, CX 3 R-type DBPs intermediates and CX 3 R-type DBPs by the residual PMS. This study demonstrated the superiority of PMS/Cl − as an alternative disinfectant comparing to NaClO.

Using UV/H2O2 pre-oxidation combined with an optimised disinfection scenario to control CX3R-type disinfection by-product formation

The effects of UV/H2O2 pre-oxidation or disinfection methods on the formation of partial disinfection by-products (DBPs) have been studied previously. This study assessed the effect of UV/H2O2 pre-oxidation combined with optimisation of the disinfection method on the formation of six classes of CX3R-type DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloacetaldehydes (HALs), haloacetonitriles (HANs), halonitromethanes (HNMs), and haloacetamides (HAMs). Experimental results showed that a simulated distribution system (SDS) in-situ chloramination or pre-chlorination followed by chloramination effectively decreased total CX3R-type DBP formation by 51.1–63.5% compared to SDS chlorination, but little reduction in DBP-associated toxicity was observed. The dominant contributors to the calculated toxicity were HANs and HALs. UV/H2O2 pre-oxidation was able to destroy the aromatic and dissolved organic nitrogen components of natural organic matter. As a consequence, THM, HAA, and HAL formations increased by 49.5–55.0%, 47.8–61.9%, and 42.0–67.1%, respectively, whereas HAN, HNM, and HAM formations significantly decreased by 52.1–83.6%, 42.9–87.3%, and 74.1–100.0%. UV/H2O2 pre-oxidation increased total CX3R-type DBP formation, during SDS chlorination, whereas SDS in-situ chloramination or pre-chlorination followed by chloramination of UV/H2O2-treated water produced lower total CX3R-type DBPs than water without UV/H2O2 pre-oxidation. Nevertheless, the DBP-associated toxicity of water with UV/H2O2 pre-oxidation was substantially lower than the toxicity for water without UV/H2O2 pre-oxidation, decreased by 24.1–82.7%. HALs followed by HANs contribute to major toxic potencies in UV/H2O2 treated water. The best DBP concentration and DBP-associated toxicity abatement results were achieved for water treated by UV/H2O2 coupled with in-situ chloramination treatment.

Formation of CX3R-type disinfection by-products during the chlorination of protein: The effect of enzymolysis

It is commonly acknowledged that proteins, comprising a large portion of dissolved organic nitrogen pool, are important precursors of disinfection by-products (DBPs). Moreover, proteins can also undergo degradation (e.g. enzymolysis) in aquatic environment or during biological treatment processes. In this study, a well-characterized protein, bovine serum albumin (BSA) was used as model compound to investigate the effect of enzymolysis on the formation of four classes of CX3R-type DBPs (i.e. trihalomethanes, haloacetaldehydes, haloacetonitriles, and haloacetamides) during chlorination. After enzymolysis, the three-dimension structure of BSA was destroyed, increasing the accessibility of inside amino acids to chlorine, thus enzyme-hydrolysed BSA exhibited higher chlorine demand and higher formation of total organic halogen (TOX) than un-hydrolysed BSA. Generally, enzyme-hydrolysed BSA produced more CX3R-type DBPs, which is due to the facilitation of fragmentation of BSA by enzymolysis. In the presence of bromide, enzymolysis promoted the formation of TOX to a greater extent than in the absence of bromide. The effect of enzymolysis on pre-oxidation efficiency in the control of CX3R-type DBPs was also investigated. It was found that the reduction in CX3R-type DBP formation from chlorinated enzyme-hydrolysed BSA was more significant than that from chlorinated un-hydrolysed BSA, but the former still exhibited higher calculated cytotoxicity than the latter, indicating the removal of CX3R-type DBP precursors was indirectly offset by the effect of enzymolysis. This study provides insights into the effect of the structural transformation of protein during water treatment processes on CX3R-type DBP formation.

Effect of reduced sulfur group on the formation of CX3R-type disinfection by-products during chlor(am)ination of reduced sulfur compounds

Previous studies have examined the effects of peptide bond and unsaturated bond on the formation of disinfection by-products (DBPs). However, limited information has been available for the impact of reduced sulfur group on the formation of DBPs. This study investigated the formation of carbonaceous and nitrogenous DBPs (C-DBPs and N-DBPs) with a similar structure of ''CX3R'' (X = H, Cl, Br or I, R = functional group), including trihalomethanes, haloacetaldehydes, haloketones, haloacetonitriles, haloacetamides and halonitromethanes, during chlor(am)ination of three reduced sulfur compounds (RSCs), such as N-acetylcysteine, glutathione and glutathiol. Results showed that all DBPs except dichloroacetonitrile (DCAN) continuously increased with increasing Cl2 or NH2Cl doses in this study. The chlor(am)ination of three RSCs with lower disinfectant doses (the molar ratio of disinfectant to precursor ≤5) generated low DBPs compare to non-RSCs. More chloroform was observed in alkaline condition, while weak acidic condition was in favor of DCAN and dichloroacetamide formation. The results of frontier electron density calculation reported that the much higher reactivity for Cl2 and NH2Cl toward reduced sulfur group in RSCs protects other groups, which account for the formation of CX3R-type DBPs. This phenomenon has important environmental implications. When RSCs are present in the water, Cl2 or NH2Cl will reacts preferably with them rather than non-RSCs to form RSO3H as the major products. Hence, the trade-offs of between the products generated upon S-chlorination, which account for the formation RSO3H, and alkyl halogenation and N-chlorination, which account for the formation of CX3R-type DBPs, influence the formation of CX3R-type DBPs.

The contribution of atmospheric particulate matter to the formation of CX3R-type disinfection by-products in rainwater during chlorination

Atmospheric particulate matter (PM) can be scavenged by rainfall and contribute dissolved organic matter (DOM) to rainwater. Rainwater may serve as a part or the whole of drinking water sources, leading to the introduction of PM-derived DOM into drinking water. However, little information is available on the role of PM-derived DOM as a remarkable precursor of CX3R-type disinfection by-products (DBPs) in rainwater. In this study, samples were collected from ten occurrences of rainfall in Shanghai and batch experiments were executed to explore the contribution of PM-derived DOM to CX3R-type DBP formation in rainwater and to further understand some of unknowns regarding its characteristics. Results revealed that a part of PM was scavenged by rainfall and the scavenge performance was better for smaller PM. The formation potentials (FPs) of individual CX3R-type DBP were similar among size-isolated PM. TCM was predominant (around 0.5–4.5 μg-C/mg-C) and TCAA was the secondary (around 0.6–3.2 μg-C/mg-C) among all detectable CX3R-type DBPs. Based on the PM removal data and DBP FP results, the contribution of PM-derived DOM to CX3R-type DBP formation in rainwater was modeled. Furthermore, aromatic proteins and soluble microbial product-like compounds were found to be significant compositions, which were reported to be DBP precursors. And low molecular weight (< 10 kDa) DOM derived from total PM and rainwater exhibited higher CX3R-type DBP FPs. DOM fractions with higher SUVA254 and SUVA285 values gave relatively higher yields of CX3R-type DBPs, indicating that aromatic compounds played an important role in DBP formation.