Next Article in Journal
Formation and Toxicity of Chlorine Species During Zeolite Regeneration by NaCl-NaClO After Stormwater Adsorption
Previous Article in Journal
A Pilot-Scale Study on Cross-Tube Ozone Catalytic Oxidation of Biochemical Tailwater in an Industrial Park in Suzhou (China)
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Water
Volume 17
Issue 13
10.3390/w17131954
water-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

The Formation of Disinfection By-Products in Reactive Chlorine Species (RCS)-Mediated Advanced Oxidation Process

by
Zishao Li
and
Zhong Zhang
*
Guangdong Provincial Key Laboratory of Water Quality Improvement and Ecological Restoration for Watersheds, School of Ecology, Environment and Resources, Guangdong University of Technology, Guangzhou 510006, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(13), 1954; https://doi.org/10.3390/w17131954
Submission received: 26 May 2025 / Revised: 21 June 2025 / Accepted: 25 June 2025 / Published: 30 June 2025
(This article belongs to the Section Wastewater Treatment and Reuse)
Browse Figures
Review Reports Versions Notes

Abstract

Highlights

  • Used laser flash photolysis to selectively generate Cl• and Cl2•−, revealing distinct DBP formation patterns for each RCS.
  • Investigated the formation and toxicities of DBPs from reactions of RCS with DOM during advanced oxidation water treatment.
  • Found a biphasic pattern in DBP formation, with initial increases followed by decreases at higher RCS exposure levels.
  • Highlighted the importance of controlling RCS exposure to minimize DBP formation and toxicities in advanced oxidation processes.

Abstract

This study investigates the formation and toxicity of disinfection by-products (DBPs) arising from the reactions between individual reactive chlorine species (RCS) and dissolved organic matter (DOM) during water treatment. Individual chlorine radicals (Cl•) and dichloride radicals (Cl2•−) were selectively generated with a laser flash photolysis technique, and their interactions with Suwannee River natural organic matter (SRNOM) were analyzed. Results demonstrated a biphasic pattern of DBP formation, where initial increases in RCS exposure enhanced DBP concentrations and toxicities, followed by subsequent decreases at higher RCS exposure. Variations among DBP classes, including trichloromethanes, chloroacetic acids, and chloroacetaldehydes, highlighted the complexity of RCS-DOM interactions. Toxicity assessments further indicated chloroacetonitriles and chloroacetic acids as major toxicity contributors at varying RCS exposures. This study highlights the impact of RCS exposure levels to DBP formation and toxicities, providing mechanistic insights for optimizing parameters in RCS-mediated advanced oxidation processes (AOPs) for safer water treatment.

Graphical Abstract

1. Introduction

Emerging organic contaminants (EOCs) in water/wastewater, such as persistent organic pollutants, endocrine disruptors, and antibiotics, pose significant challenges to ecosystems and human health [1]. Due to their high chemical stability, EOCs are often resistant to removal by traditional biochemical treatment methods [2]. In recent years, advanced oxidation processes (AOPs) have gained widespread attention as effective approaches for eliminating EOCs through the generation of highly reactive oxidizing species [3,4]. Among the various AOPs, UV/chlorine (HOCl) and UV/chloramine (NH2Cl) treatments have attracted considerable attention because of their high efficiency, lower energy consumption, and ability to generate reactive chlorine species (RCS), mainly chlorine radicals (Cl•) and dichloride radicals (Cl2•−) [5,6,7,8].
Current studies have fully demonstrated the high efficiency of the RCS-mediated AOPs for the removal of various EOCs from water/wastewater. However, the tendency of these processes to generate disinfection by-products (DBPs) raises significant concerns regarding water safety. Previous research has reported elevated formation of regulated DBPs, such as trichloromethanes and chloroacetic acids, along with unregulated but potentially hazardous DBPs like chloroacetonitriles and trichloronitromethane during RCS-driven oxidation. Therefore, effective control of DBP formation is critical for the safe application of RCS-mediated techniques. To achieve this, it is essential to first elucidate the underlying mechanisms of DBP formation in RCS-mediated AOPs, which can provide valuable insights for DBP mitigation strategies.
Although several studies have explored the formation of DBPs in RCS-mediated AOPs, there remains substantial uncertainty regarding the mechanistic pathways involved [9,10]. For example, in UV/HOCl processes, some studies attributed DBP generation to HOCl-mediated reactions, while others emphasized radical-specific pathways as dominant drivers [11,12]. These divergent findings are likely due to the complexity of the reaction systems employed in previous studies, which often contained both oxidants (e.g., chlorine and chloramine) and RCS. This mixture makes it difficult to disentangle the specific contributions of each component to DBP formation. Recently, some studies showed that laser flash photolysis can be used as a powerful technique capable of generating individual reactive species under controlled experimental conditions [13,14,15,16]. This technique was successfully applied to investigate the generation of different total organic chlorine (TOCl) concentrations under separate exposures to Cl• and Cl2•− [17]. These pioneering studies offer critical insights into the distinct reactivity profiles of individual RCS and may serve as a valuable foundation for mechanistic investigations into DBP formation in RCS-mediated AOPs. Nevertheless, few studies have investigated the effects of RCS on the formation of different types of DBPs [18,19,20].
This study aimed to investigate the generation of DBPs under exposure to RCSs in order to elucidate the underlying mechanisms of DBP generation during RCS-mediated AOPs. Specifically, Cl• and Cl2•− were individually generated and controlled using the laser flash photolysis technique [5,21]. Under these distinct exposure conditions, the concentrations and types of DBPs, as well as their relationships with radical exposure intensity, were systematically examined. Furthermore, DBP-associated toxicities across different exposure scenarios were clarified. Overall, this study provides new insights into the environmental risks associated with RCS-mediated disinfection processes (e.g., via in vitro cytotoxicity assays and toxicological modeling of DBP exposure pathways) and offers a scientific foundation for developing future strategies to control DBP formation [22,23].

2. Materials and Methods

2.1. Chemicals and Reagents

Chloroacetone (≥98%) was obtained from Adamas Reagent, Ltd. (Shanghai, China). Sodium persulfate (Na2S2O8, ≥98%), phenol (≥99%), and tert-butanol (TBA, HPLC grade ≥ 99.5%) were purchased from Sigma-Aldrich, Ltd. (Livonia, MI, USA). Sodium chloride (NaCl, ≥99%), potassium thiocyanate (KSCN, ≥99%), sodium phosphate monobasic dihydrate (NaH2PO4·2H2O, ≥99%), sodium phosphate dibasic dodecahydrate (Na2HPO4·2H2O, ≥99%), sodium hydrogen carbonate (NaHCO3, ≥99%), and sodium hypochlorite solution (NaOCl, 6–14%) were obtained from Aladdin Bio-Chem Technology, Ltd. (Shanghai, China). HPLC-grade methanol, hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH), and perchloric acid (HClO4, 70%) were purchased from Thermo Fisher Scientific, Ltd. (Waltham, MA, USA). All of the above chemicals were used directly without further purification. All of the solutions were prepared with ultrapure water (≥18.2 MΩ) obtained from an ELGA LabWater system (Ryan End, Buckinghamshire, UK).
A total of 22 chlorinated DBPs in five classes were analyzed, including the four regulated trihalomethanes (trihalomethanes), nine haloacetic acids (haloacetic acids), four haloacetonitriles (haloacetonitriles), four haloacetaldehydes (haloacetaldehydes), and one halonitromethane (HNM: trichloronitro methane (chloropicrin; TCNM)) (Text S1).

2.2. Laser Flash Photolysis Experiments

The laser flash photolysis experiments were conducted with an LKS80 laser flash photolysis system (Applied Photophysics Ltd., Leatherhead, Surrey, UK) equipped with a Nd: YAG laser (266 nm, laser beam cross-section of 0.5 cm2, pulse duration of 4~6 ns). A 150 W xenon lamp was utilized as the detecting light source. The variation in optical signal was recorded by a Philips PM3323B digital oscilloscope and output to a computer. To minimize errors and boost the signal-to-noise (S/N) ratio, the absorption-time kinetic signals for each sample were averaged over 8 observations. All kinetic experiments were conducted under 25 ± 2 °C, and the pH of the solutions was adjusted to 7.0 ± 0.3 with 2 mM or 4 mM phosphate buffer to avoid the dissociation of DOM (Text S2). For all experimental conditions, non-irradiated control samples containing identical concentrations of Cl− (0.04 M or 0.5 M), S2O82− (5 mM), and DOM (2.5 mgC L−1) were prepared in parallel but not subjected to laser irradiation. These controls served to verify the baseline DBP levels without radical generation and validate that observed DBP formation resulted specifically from RCS-mediated reactions.
Cl• and Cl2•− were generated through the reaction between chloride ions (Cl−) and sulfate radicals (SO4•−), with SO4•− produced by the laser photolysis of persulfate ions (S2O82−) [24]. Separate exposures to Cl• or Cl2•− were achieved by adjusting the concentration of Cl−. For single Cl• exposure, a low concentration of Cl− was employed, and the reaction pathway followed Equations (1)–(3). For single Cl2•− exposure, an excess of Cl− was added, and the reaction proceeded according to Equations (1)–(4). In both scenarios, the reaction solutions consisted of either 0.04 M or 0.5 M Cl−, 5 mM S2O82−, and 2.5 mgC L−1 dissolved organic matter (DOM), and were irradiated with a 266 nm pulsed laser. The pH of the solutions was adjusted to 7.0 using 2 mM phosphate buffer. After laser irradiation, residual oxidants were quenched by Na2S2O3 immediately. Control sample (no laser irradiation): A solution containing 0.04 M or 0.5 M Cl−, 5 mM S2O82−, and 2.5 mgC L−1 DOM (identical to experimental samples) was prepared but not subjected to laser irradiation. Recoveries of DBPs, measured via spiked sample analysis, ranged from 75 to 90% (Table S1).
CH3COCH2Cl + hv → CH3COCH2• + Cl•
S2O82− + hv → 2SO4•−
SO4•− + Cl− → SO42− + Cl•
Cl• + Cl− ⇋ Cl2•−
Cl2•− + Cl2•− → Cl2 + 2Cl−

2.3. Determination of DBP Concentrations

After completing the laser flash photolysis reactions, samples were extracted using methyl tert-butyl ether (MTBE) with 1,2-dibromopropane as an internal standard, following US EPA methods 551.1 and 552.3 [25]. The extracted samples were analyzed using gas chromatography–mass spectrometry (GC-MS) employing an SH-I-5il MS column (Shimadzu, Kyoto, Japan) with a 0.25 mm inner diameter and 30 m length. The injection port and detector temperatures were set at 170 °C and 230 °C, respectively, with a non-split injection method and an injection volume of 2 μL. The nitrogen flow rate was maintained at 1.0 mL/min.
For chloroacetonitriles, chloroacetaldehydes, chloronitromethanes, and trichloromethanes, the initial column temperature was set at 35 °C and maintained for 20 min; then, it was increased to 120 °C at a rate of 4 °C/min; finally, it was rapidly increased to 280 °C at 59 °C/min and held for 2 min. For chlorinated acetic acid disinfection by-products (chloroacetic acids), the initial temperature was also 35 °C but held for 10 min. Then, the temperature was increased to 120 °C at 5 °C/min; finally, it was rapidly increased to 280 °C at 59 °C/min and held for 4 min. This temperature program ensured effective separation and detection of the disinfection by-products (Text S3). The chromatograms of DBPs and corresponding calibration curves are provided in the Supplementary Materials (Figure S1 and S2), respectively.
The dual temperature programs were specifically designed to account for the distinct physicochemical properties of the 22 target DBPs. For haloacetonitriles, haloacetaldehydes, trichloronitromethane, and trihalomethanes (lower molecular weight, higher volatility), the initial 35 °C hold (20 min) ensured proper focusing of analytes, while the gradual 4 °C/min ramp to 120 °C resolved co-eluting species. The steeper 59 °C/min final ramp eliminated high-boiling contaminants. For haloacetic acids (higher polarity, lower volatility), the modified program (10 min initial hold, 5 °C/min ramp) prevented peak broadening and maintained resolution of later-eluting congeners. This approach balanced retention time reproducibility with peak sharpness across all analyte classes.

2.4. Toxicity Assessment of Water Samples

The health risks associated with disinfection by-products (DBPs) were evaluated through an integrated approach combining concentration measurements with toxic potency metrics. Our assessment employed two established toxicity thresholds: (1) the cytotoxicity threshold (LC50), representing the concentration causing a 50% reduction in Chinese hamster ovary (CHO) cell viability (acute toxicity), and (2) the lifetime excess cancer risk threshold (LECR50), derived from epidemiological data to estimate carcinogenic potential. For each DBP, we calculated a toxicity-weighted value by dividing its measured concentration by the more stringent of its LC50 or LECR50 values, adopting a conservative risk assessment approach.
The methodology incorporated experimentally determined LC50 values for most DBPs from consistent sources, while LECR50 values were applied for regulated THMs and HAAs (Table S2). Total toxicity was estimated by summing individual toxicity-weighted values, assuming additive effects among DBPs, a well-established approach in water toxicology studies [26,27]. The resulting toxicity index enables comparative evaluation of different treatment processes while supporting evidence-based optimization of disinfection strategies to minimize health risks (Text S4).

3. Results and Discussion

3.1. DBP Formation from the Reaction Between Cl• and DOM

This study investigates the formation of disinfection by-products during reactions between chlorine radicals (Cl•) and dissolved organic matter (DOM), using Suwannee River natural organic matter (SRNOM) as a model substrate. By systematically adjusting the Cl• exposure intensity, we evaluated how the concentrations and compositions of DBPs evolved with increasing radical flux, focusing on five representative DBP classes: chloroacetonitriles, chloroacetaldehydes, chloronitromethanes, chloroacetic acids, and trichloromethanes [28,29].
The concentration profile in Figure 1 illustrates the variations in DBP formation under different Cl• exposure levels, ranging from 0 to 30 × 10−12 M·s [30]. The DBP concentrations initially increased with Cl• exposure, reaching a peak value before gradually declining. Specifically, the DBP concentrations rise sharply at lower Cl• exposure levels (0−10 × 10−12 M·s), achieving a maximum concentration of 0.14 μM. Beyond this point, the DBP concentrations begun to decrease, stabilizing at approximately 0.1 μM at higher Cl• exposure levels (20−30 × 10−12 M·s) (Figure S3). This trend suggested that DBP formation was transient, with initial rapid generation followed by degradation under prolonged Cl• exposure. This biphasic pattern likely stems from two competing mechanisms: (1) the electrophilic substitution of Cl• with DOM to form chlorinated intermediates at lower exposure levels [31], and (2) the subsequent oxidative cleavage of DBP precursors or direct mineralization of existing DBPs by excess Cl• and secondary radicals at elevated concentrations [32]. Such mechanistic consistency underscores the critical role of radical stoichiometry in balancing DBP formation and degradation pathways. Through varying the Cl• exposure, two distinct DBP formation regimes were observed (Figure 1): (1) at low Cl• fluxes (0–10 × 10−12 M·s), DBP formation was dominated by chloronitromethanes and chloroacetic acids via electrophilic substitution, and (2) at high Cl• fluxes (>15 × 10−12 M·s), chloroacetaldehydes and chloronitromethanes became predominant, accompanied by the degradation of previously formed chloroacetic acids [33].
The concentrations of chloroacetonitriles exhibited a consistent decrease as the Cl• exposure rate increased from 2 to 30 × 10−12 M·s. Initially, at 2 × 10−12 M·s, chloroacetonitriles were present at a concentration of approximately 0.07 µM. This concentration gradually diminished, reaching about 0.01 µM at the highest exposure rate of 30 × 10−12 M·s. This trend suggested that higher exposure rates might either promote the degradation of chloroacetonitriles or inhibit its formation. Chloroacetaldehyde concentrations remained relatively stable across the range of exposure rates studied. At 2 × 10−12 M·s, chloroacetaldehyde was present at about 0.02 µM, and this value did not significantly change as the exposure rate increased to 20 × 10−12 M·s. The slight increase observed at the highest exposure rate might indicate a minor enhancement in chloroacetaldehyde formation or a slight decrease in its degradation rate under these conditions. The concentration of chloronitromethanes initially decreased with increasing exposure rates, reaching a minimum at 10 × 10−12 M·s, where it was approximately 0.02 µM. Beyond this point, the concentration of chloronitromethanes began to increase, reaching about 0.05 µM at 30 × 10−12 M·s. This biphasic pattern might reflect changes in the competitive reactions involving chloronitromethane formation and degradation at different exposure rates [34,35]. Chloroacetic acid concentrations initially increased with increasing exposure rates, peaking at around 0.05 µM at 10 × 10−12 M·s. Subsequently, the concentration of chloroacetic acids decreased to zero at 30 × 10−12 M·s. This trend suggested that chloroacetic acid formation was favored at intermediate exposure rates, possibly due to optimal reaction conditions for its formation or reduced degradation at these levels.

3.2. DBP Formation from the Reaction Between Cl2•− and DOM

The formation of disinfection by-products (DBPs) generated through radical-mediated interactions between dichloride radicals (Cl2•−) and Suwannee River natural organic matter (SRNOM) was also investigated [36,37]. The concentration profiled of five DBPs under varying Cl2•− exposure (0−30 × 10−11 M·s) are illustrated in Figure 2. The total DBP concentration initially increased sharply, reaching a peak of approximately 0.43 μM at a Cl2•− exposure of 5 × 10−11 M·s, and then gradually declined to 0.3 μM at 30 × 10−11 M·s (Figure S4). This trend reflected the rapid formation of DBPs at lower exposures, followed by their degradation or transformation at higher Cl2•− concentrations [38]. This trend suggested that DBP formation is transient, with initial rapid generation followed by degradation under prolonged Cl2•− exposure.
DBP formation was dominated by trichloromethanes over the Cl2•− exposure of 2–30 × 10−11 M·s. The concentration of trichloromethanes initially decreased with increasing Cl2•− exposure rates, reaching a minimum at 5 × 10−11 M·s, where it was approximately 0.25 µM. Beyond this point, the concentration of trichloromethanes remained relatively stable, fluctuating slightly around 0.2 µM at higher exposure rates. This trend suggested that trichloromethane formation was initially favored at lower exposure rates, but its concentration became less sensitive to further increases in exposure rate. Similar to trichloromethanes, chloroacetaldehyde concentrations initially decreased with increasing exposure rates, reaching a minimum at 5 × 10−11 M·s, where it was approximately 0.02 µM. Beyond this point, the concentration of chloroacetaldehydes remained relatively stable. This behavior indicated that chloroacetaldehyde formation was also initially favored at lower exposure rates, but its concentration became less sensitive to further increases in exposure rate. The concentration of chloroacetonitriles exhibited a consistent decrease as the Cl2•− exposure rate increased from 2 to 30 × 10−11 M·s. Initially, at 2 × 10−11 M·s, chloroacetonitriles were present at a concentration of approximately 0.02 µM. This concentration gradually diminished, reaching the lowest point at the highest exposure rate of 30 × 10−11 M·s. This trend suggested that higher exposure rates might either promote the degradation of chloroacetonitriles or inhibit their formation. Chloroacetic acid concentrations initially increased with increasing exposure rates, peaking at around 0.06 µM at 5 × 10−11 M·s. Subsequently, the concentration of chloroacetic acids decreased slightly but remained relatively high compared to other DBPs, stabilizing at about 0.04 µM at 30 × 10−11 M·s. This trend suggested that chloroacetic acid formation was favored at intermediate exposure rates, possibly due to optimal reaction conditions for its formation or reduced degradation at these levels [39]. Both trichloromethanes and chloroacetic acids initially increased with the increase in Cl2•− exposure. Subsequently, they both experienced a decrease and eventually tended towards a stable state, suggesting that equilibrium or saturation conditions may be reached within the system under study. The initial rise in trichloromethanes at low-to-moderate Cl2•− exposures (0–5 × 10−11 M·s) likely reflected chlorogenation of hydrophobic precursors via electrophilic substitution. At higher exposures (>10 × 10−11 M·s), oxidative cleavage dominated, fragmenting trichloromethane precursors into smaller carboxylates. Chloroacetonitriles, chloronitromethanes, and chloroacetaldehydes were all relatively low across the entire range, indicating that Cl2•− does not readily form them [40].
By comparing Figure 1 and Figure 2, it can be found that the total amount of DBPs generated by the reaction of Cl2•− was significantly higher than that of Cl•. This might be due to a higher production of Cl2•− radicals. Under equivalent laser excitation conditions, there was a significant difference in the efficiency of free radical generation, with the yield of Cl2•− being about an order of magnitude higher than that of chlorine radicals [17]. This trend might stem from three synergistic factors: (i) The resonance-enhanced multiphoton excitation protocol selectively promoted the simultaneous excitation of two chlorine atoms within precursor clusters, favoring Cl2•− formation over stepwise Cl• generation [41]. (ii) Secondary chain-propagation reactions involving Cl2•− exhibited lower activation barriers than monoradical-mediated pathways, enabling amplified diradical accumulation under continuous irradiation. These observations aligned with recent advances in correlated electron-pair excitation mechanisms and challenged classical single-bond dissociation paradigms in chlorogenic photochemistry [42].
Cl•, characterized by its high oxidizing power and non-selective reactivity, tends to preferentially interact with diverse functional groups in DOM through rapid electrophilic substitution and hydrogen abstraction. These reactions primarily generate short-lived chlorinated intermediates that evolve into more polar and reactive DBPs such as chloroacetonitriles, chloroacetaldehydes, chloroacetic acids, and chloronitromethanes. Trichloromethanes, which typically form via slower, stepwise chlorogenation of less reactive, hydrophobic DOM fractions, are likely suppressed under Cl• exposure due to: (1) competitive consumption of THM precursors by Cl• through oxidative cleavage or mineralization; and (2) the transient nature of Cl•-mediated reactions, which favor the formation of more labile DBPs over the slower THM formation pathway [43].
In contrast, as shown in Figure 2, Cl2•−—with its lower reactivity and greater selectivity—preferentially targets hydrophobic DOM components, enabling the stepwise chlorogenate required for THM accumulation. This mechanistic divergence between Cl• and Cl2•− explains why trichloromethanes were not detected in the Cl•-DOM system but emerged as dominant DBPs in the Cl2•−-DOM system.
The formation of disinfection by-products (DBPs) during water treatment was significantly influenced by the type of disinfectant used, with Cl• and Cl2•− exhibiting distinct DBP formation profiles. At an exposure rate of 10 × 10−12 M·s, which served as a critical point for comparison, the Cl• system showed the ability to form all four types of disinfection by-products (chloroacetonitriles, chloroacetaldehydes, chloronitromethanes, and chloroacetic acids), whereas in the Cl2•− system, trichloromethanes were overwhelmingly the most abundant DBPs. This stark contrast indicated fundamental differences in the reaction mechanisms of the two chlorine species. The Cl• system, known for its strong oxidizing power and high reactivity, led to the formation of a broader range of DBPs, including chloroacetonitriles, chloroacetaldehydes, and chloronitromethanes. This suggested that Cl• readily reacted with a variety of organic compounds, resulting in a complex mixture of DBPs. On the other hand, Cl2•−, with its selective oxidation properties, had lower reactivity with certain organics. This selectivity of Cl2•− could be due to its preference for specific types of chemical bonds or its lower reaction rate with certain precursors. Furthermore, as the exposure increased from 10 to 30 × 10−12 M·s in the Cl• system, there was a noticeable decrease in the concentrations of various DBPs, indicating that the total amount of DBPs could be controlled by adjusting the exposure level. In contrast, the Cl2•− system did not exhibit significant changes in DBP formation with varying exposure levels, suggesting that the Cl2•− system was less responsive to changes in exposure intensity for controlling DBP levels.

3.3. Toxicity Assessment of DBPs During the Reaction Between Cl• and DOM

The formation of disinfection by-products (DBPs) from radical-mediated oxidation processes necessitated concurrent evaluation of their toxicological risks, as structural variations among DBP classes may differentially impact biological systems. The impact of varying exposures of the chlorine radicals (Cl•) on the concentration of toxicity is quantified in units of 10−12 M·s. The toxicity under consideration included chloroacetonitriles, chloroacetaldehydes, chloronitromethanes, chloroacetic acids, and trichloromethanes [44,45].
Figure 3 illustrates the variation in DBP-associated toxicities under different Cl• exposure levels. A marked increase in toxicities was observed when the Cl• exposure was 2 × 10−12 M·s, primarily attributed to the substantial formation of chloroacetonitriles and chloroacetic acids. At this exposure level, chloroacetonitriles exhibited the highest concentrations and thus contributed most significantly to the overall toxicity. As the Cl• exposure further increased, the overall toxicity decreased significantly and stabilized at 20 × 10−12 M·s. At this exposure level, chloroacetic acids contributed most to the toxicities. Conversely, at 30 × 10−12 M·s, chloroacetaldehydes became the most significant contributors to toxicity.
The initial surge in toxicity at 2 × 10−12 M·s exposure was likely due to the rapid formation of chloroacetonitriles, which was a highly reactive and toxic DBP. As the Cl• exposure increases, the concentrations of chloroacetonitriles significantly decreased. The decrease in overall toxicity beyond 2 × 10−12 M·s could be explained by several mechanisms. Firstly, at low Cl• concentrations, the concentrations of chloroacetonitriles and chloroacetic acids were higher and their toxicities were also higher. As Cl• exposure increased, the concentrations of chloroacetonitriles and chloroacetic acids decreased. Therefore, it was inferred that the decreases in toxicities may be due to the decrease in chloroacetonitriles and chloroacetic acid concentrations. Secondly, the competitive reactions among different DBPs might lead to the formation of less harmful compounds or their degradation products (Figure S5). The stabilization of toxicities at 20 × 10−12 M·s suggested that the system might be approaching saturation, where most of the organic precursors had reacted and the formation of new toxic DBPs was limited [46,47].

3.4. Toxicity Assessment of DBPs During the Reaction Between Cl2•− and DOM

Figure 4 illustrates the toxicity levels associated with different dichloride radicals’ (Cl2•−) exposure. At zero exposure, the toxicity was negligible, indicating that there were no significant toxic effects without Cl2•− exposure. Upon exposure to 2 × 10−11 M·s, there was a sharp increase in toxicities, which can be attributed to the substantial generation of chloroacetonitriles and chloroacetaldehydes. At this exposure level, the highest concentrations of chloroacetonitriles were observed, suggesting that chloroacetonitriles contributed most significantly to the observed toxicities. As the exposure rate increased further, the overall toxicities decreased significantly. When exposure reached 20 × 10−11 M·s, the toxicities leveled off and stabilized. At this stage, the contributions of chloroacetic acids to the toxicities became predominant. This trend suggested that chloroacetic acids were more stable or less reactive under higher exposure conditions compared to chloroacetonitriles, which might degrade or transform into less toxic compounds [47].
The observed toxicity trends in response to varying Cl2•− exposure levels can be explained by the differential formation and transformation of DBPs. Initially, at low Cl2•− exposure (2 × 10−11 M·s), there was a sharp increase in toxicities due to the preferential formation of highly toxic chloroacetonitriles and chloroacetaldehydes. As exposure increased, the overall toxicities decreased significantly, likely due to the formation of less toxic DBPs, such as chloroacetic acids, and the potential transformation of chloroacetonitriles into less harmful compounds or their degradation. As the exposure reached 20 × 10−11 M·s, toxicity leveled off and stabilized, with chloroacetic acids becoming the predominant contributors to toxicity, possibly due to their increased stability or slower degradation rate under higher exposure conditions. The stabilization of toxicities might also indicate that the reactive sites on organic precursors were saturated, leading to a plateau in DBP formation (Figure S6).
Overall, DBP concentrations from Cl• were higher than those from Cl2•− under the same exposure level. Conversely, the toxicities under Cl• exposure were lower than those under Cl2•− exposure. This was because chloroacetonitriles, which are more toxic in Cl2•−, have relatively higher concentrations than in Cl•, resulting in higher toxicity of Cl2•− than Cl•.

4. Conclusions

Our findings have practical implications for water treatment processes. Specifically, we demonstrated that under increasing Cl• exposure, chloroacetonitriles and chloroacetic acids are the dominant DBPs formed initially, while higher Cl• levels lead to their degradation. In contrast, Cl2•− preferentially forms trichloromethanes. These results underscore the importance of controlling RCS exposure levels to minimize toxic DBP formation. For instance, optimizing Cl• exposure can reduce the formation of highly toxic chloroacetonitriles, whereas limiting Cl2•− exposure can mitigate trichloromethane production. This advances the field by offering targeted strategies for safer water treatment practices.
This study investigated the formation and toxicities of disinfection by-products (DBPs) from reactions of reactive chlorine species (RCS) with dissolved organic matter (DOM). For Cl• exposure, DBP concentrations initially increased and then decreased with higher exposure levels, showing a biphasic pattern. Toxicities were highest at low exposure levels, mainly due to the formation of chloroacetonitriles and chloroacetic acids, and stabilized at higher exposures. For Cl2•− exposure, DBPs also peaked at low exposure levels before declining, with trichloromethanes being the dominant DBPs. The toxicity trend was similar, driven by chloroacetonitriles and chloroacetaldehydes at low exposures and chloroacetic acids at higher exposures. This research highlighted the need to control RCS exposure levels in AOPs to minimize DBP formation and toxicities, providing insights for safer water treatment processes [48,49,50].
A key limitation of this study is the lack of formal matrix-matched recovery measurements for DBP quantification. Retrospective analysis of spiked blank samples suggests approximate recoveries of 75–90% for major DBP classes, but these values were not validated under real water matrix conditions. Future studies should include rigorous recovery assessment to improve method reliability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17131954/s1, Figure S1: Chromatograms of DBPs; Figure S2: The calibration curves of DBPs; Figure S3: Concentration Variation Trends of DBPs under Different Cl• Exposures; Figure S4: Concentration Variation Trends of DBPs under Different Cl2•− Exposures; Figure S5: Toxicity Variation Trends of DBPs under Different Cl• Exposures; Figure S6: Toxicity Variation Trends of DBPs under Different Cl2•− Exposures; Table S1: The recovery of DBPs; Table S2: Toxic potency metrics for halogenated DBPs; Text S1: DBPs and DBP analytical methods; Text S2: Laser flash photolysis experiments; Test S3: Determination of DBPs mass concentration; Test S4: Toxicity assessment of water samples; Scheme S1 Laser experiments procedures on DBP concentrations.

Author Contributions

Z.L.: Writing—original draft, visualization, methodology, investigation, data curation, and conceptualization. Z.Z.: Conceptualization, writing—reviewing and editing, project administration, and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by funding from the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (2019ZT08L213) and the National Natural Science Foundation of China (52370034).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Badea, S.L.; Niculescu, V.C. Recent progress in the removal of legacy and emerging organic contaminants from wastewater using metal–organic frameworks: An overview on adsorption and catalysis processes. Materials 2022, 15, 3850. [Google Scholar] [CrossRef] [PubMed]
  2. Ahmad, A.L.; Ebenezer, O.I.; Shoparwe, N.F.; Ismail, S. Graphene oxide-doped polymer inclusion membrane for remediation of pharmaceutical contaminant of emerging concerns: Ibuprofen. Membranes 2021, 12, 24. [Google Scholar] [CrossRef]
  3. Deng, Y.; Zhao, R. Advanced oxidation processes (AOPs) in wastewater treatment. Curr. Pollut. Rep. 2015, 1, 167–176. [Google Scholar] [CrossRef]
  4. Pandis, P.K.; Kalogirou, C.; Kanellou, E.; Vaitsis, C.; Savvidou, M.G.; Sourkouni, G.; Zorpas, A.A.; Argirusis, C. Key points of advanced oxidation processes (AOPs) for wastewater, organic pollutants and pharmaceutical waste treatment: A mini review. ChemEngineering 2022, 6, 8. [Google Scholar] [CrossRef]
  5. Zhao, J.; Peng, J.; Yin, R.; Fan, M.; Yang, X.; Shang, C. Multi-angle comparison of UV/chlorine, UV/monochloramine, and UV/chlorine dioxide processes for water treatment and reuse. Water Res. 2022, 217, 118414. [Google Scholar] [CrossRef] [PubMed]
  6. Stasinakis, A.S. Use of selected advanced oxidation processes (AOPs) for wastewater treatment–A mini review. Glob. NEST J. 2008, 10, 376–385. [Google Scholar] [CrossRef]
  7. Kusic, H.; Koprivanac, N.; Bozic, A.L. Minimization of organic pollutant content in aqueous solution by means of AOPs: UV-and ozone-based technologies. Chem. Eng. J. 2006, 123, 127–137. [Google Scholar] [CrossRef]
  8. Wu, Z.; Guo, K.; Fang, J.; Yang, X.; Xiao, H.; Hou, S.; Kong, X.; Shang, C.; Yang, X.; Meng, F.; et al. Factors affecting the roles of reactive species in the degradation of micropollutants by the UV/chlorine process. Water Res. 2017, 126, 351–360. [Google Scholar] [CrossRef]
  9. Zhu, Y.; Wang, C.; Andrews, S.; Hofmann, R. Effect of UV/chlorine oxidation on disinfection byproduct formation from diverse model compounds. ACS EST Water 2022, 2, 573–582. [Google Scholar] [CrossRef]
  10. Wang, D.; Hua, Z.; Cui, Y.; Dong, Z.; Li, C.; Fang, J. Probing into the mechanisms of disinfection by-product formation from natural organic matter and model compounds after UV/chlorine treatment. Environ. Sci. Water Res. Technol. 2023, 9, 1587–1598. [Google Scholar] [CrossRef]
  11. Cai, W.-W.; Peng, T.; Yang, B.; Xu, C.; Liu, Y.-S.; Zhao, J.-L.; Gu, F.-L.; Ying, G.-G. Kinetics and mechanism of reactive radical mediated fluconazole degradation by the UV/chlorine process: Experimental and theoretical studies. Chem. Eng. J. 2020, 402, 126224. [Google Scholar] [CrossRef]
  12. Peng, T.; Xu, C.; Yang, L.; Yang, B.; Cai, W.W.; Gu, F.; Ying, G.G. Kinetics and mechanism of degradation of reactive radical-mediated probe compounds by the UV/Chlorine process: Theoretical calculation and experimental verification. ACS Omega 2022, 7, 5053–5063. [Google Scholar] [CrossRef] [PubMed]
  13. Huang, T.; Deng, L.; Wang, T.; Liao, X.; Hu, J.; Tan, C.; Singh, R.P. Effects of bromide ion on the formation and toxicity alteration of halonitromethanes from nitrate containing humic acid water during UV/chlor (am) ine disinfection. Water Res. 2022, 225, 119175. [Google Scholar] [CrossRef] [PubMed]
  14. Zhang, K.; Parker, K.M. Halogen radical oxidants in natural and engineered aquatic systems. Environ. Sci. Technol. 2018, 52, 9579–9594. [Google Scholar] [CrossRef]
  15. Jasper, J.T.; Shafaat, O.S.; Hoffmann, M.R. Electrochemical transformation of trace organic contaminants in latrine wastewater. Environ. Sci. Technol. 2016, 50, 10198–10208. [Google Scholar] [CrossRef]
  16. Yu, X.Y.; Bao, Z.C.; Barker, J.R. Free radical reactions involving Cl•, Cl2−•, and SO4−• in the 248 nm photolysis of aqueous solutions containing S2O82-and Cl. J. Phys. Chem. A 2004, 108, 295–308. [Google Scholar] [CrossRef]
  17. Lei, Y.; Lei, X.; Westerhoff, P.; Zhang, X.; Yang, X. Reactivity of chlorine radicals (Cl• and Cl2•–) with dissolved organic matter and the formation of chlorinated byproducts. Environ. Sci. Technol. 2020, 55, 689–699. [Google Scholar] [CrossRef] [PubMed]
  18. Yeom, Y.; Han, J.; Zhang, X.; Shang, C.; Zhang, T.; Li, X.; Duan, X.; Dionysiou, D.D. A review on the degradation efficiency, DBP formation, and toxicity variation in the UV/chlorine treatment of micropollutants. Chem. Eng. J. 2021, 424, 130053. [Google Scholar] [CrossRef]
  19. Lian, L.; Yao, B.; Hou, S.; Fang, J.; Yan, S.; Song, W. Kinetic study of hydroxyl and sulfate radical-mediated oxidation of pharmaceuticals in wastewater effluents. Environ. Sci. Technol. 2017, 51, 2954–2962. [Google Scholar] [CrossRef]
  20. Guo, K.; Wu, Z.; Chen, C.; Fang, J. UV/chlorine process: An efficient advanced oxidation process with multiple radicals and functions in water treatment. Acc. Chem. Res. 2022, 55, 286–297. [Google Scholar] [CrossRef]
  21. Chuang, Y.H.; Chen, S.; Chinn, C.J.; Mitch, W.A. Comparing the UV/monochloramine and UV/free chlorine advanced oxidation processes (AOPs) to the UV/hydrogen peroxide AOP under scenarios relevant to potable reuse. Environ. Sci. Technol. 2017, 51, 13859–13868. [Google Scholar] [CrossRef] [PubMed]
  22. Dotson, A.; Westerhoff, P.; Krasner, S.W. Nitrogen enriched dissolved organic matter (DOM) isolates and their affinity to form emerging disinfection by-products. Water Sci. Technol. 2009, 60, 135–143. [Google Scholar] [CrossRef]
  23. Bulman, D.M.; Remucal, C.K. Role of reactive halogen species in disinfection byproduct formation during chlorine photolysis. Environ. Sci. Technol. 2020, 54, 9629–9639. [Google Scholar] [CrossRef]
  24. Kim, C.; Thao, T.T.; Kim, J.-H.; Hwang, I. Effects of the formation of reactive chlorine species on oxidation process using persulfate and nano zero-valent iron. Chemosphere 2020, 250, 126266. [Google Scholar] [CrossRef]
  25. Potter, D.G.; Wiseman, S.; Blitz, M.A.; Seakins, P.W. Laser photolysis kinetic study of OH radical reactions with methyl tert-butyl ether and trimethyl orthoformate under conditions relevant to low temperature combustion: Measurements of rate coefficients and OH recycling. J. Phys. Chem. A 2018, 122, 9701–9711. [Google Scholar] [CrossRef] [PubMed]
  26. Zeng, T.; Mitch, W.A. Impact of nitrification on the formation of N-nitrosamines and halogenated disinfection byproducts within distribution system storage facilities. Environ. Sci. Technol. 2016, 50, 2964–2973. [Google Scholar] [CrossRef]
  27. Chuang, Y.H.; Mitch, W.A. Effect of ozonation and biological activated carbon treatment of wastewater effluents on formation of N-nitrosamines and halogenated disinfection byproducts. Environ. Sci. Technol. 2017, 51, 2329–2338. [Google Scholar] [CrossRef] [PubMed]
  28. Young, T.R.; Li, W.; Guo, A.; Korshin, G.V.; Dodd, M.C. Characterization of disinfection byproduct formation and associated changes to dissolved organic matter during solar photolysis of free available chlorine. Water Res. 2018, 146, 318–327. [Google Scholar] [CrossRef]
  29. Bulman, D.M.; Milstead, R.P.; Remucal, C.K. Formation of targeted and novel disinfection byproducts during chlorine photolysis in the presence of bromide. Environ. Sci. Technol. 2023, 57, 18877–18887. [Google Scholar] [CrossRef]
  30. Lee, J.; Von Gunten, U.; Kim, J.H. Persulfate-based advanced oxidation: Critical assessment of opportunities and roadblocks. Environ. Sci. Technol. 2020, 54, 3064–3081. [Google Scholar] [CrossRef]
  31. Hu, Q.; Wang, R.; Zhang, Y.; Zhou, J.; Qu, G.; Wang, T.; Jia, H. Formation of halogenated macromolecular organics induced by Br− and I− during plasma oxidation/chlorination of DOM: Highlighting competitive mechanisms. Water Res. 2023, 229, 119513. [Google Scholar] [CrossRef] [PubMed]
  32. Gao, Y.Q.; Gao, N.Y.; Chu, W.H.; Zhang, Y.F.; Zhang, J.; Yin, D.Q. UV-activated persulfate oxidation of sulfamethoxypyridazine: Kinetics, degradation pathways and impact on DBP formation during subsequent chlorination. Chem. Eng. J. 2019, 370, 706–715. [Google Scholar] [CrossRef]
  33. Kinani, A.; Sa, H.; Bouchonnet, S.; Kinani, S. Determination of adsorbable organic halogens in surface water samples by combustion–microcoulometry versus combustion–ion chromatography titration. J. Chromatogr. A 2018, 1539, 41–52. [Google Scholar] [CrossRef]
  34. Krasner, S.W.; Mitch, W.A.; Westerhoff, P.; Dotson, A. Formation and control of emerging C-and N-DBPs in drinking water. J. Am. Water Work. Assoc. 2012, 104, E582–E595. [Google Scholar] [CrossRef]
  35. Chang, H.; Chen, C.; Wang, G. Identification of. potential nitrogenous organic precursors for C-, N-DBPs and characterization of their DBPs formation. Water Res. 2011, 45, 3753–3764. [Google Scholar] [CrossRef]
  36. Chen, B.; Bu, Y.; Yang, J.; Nian, W.; Hao, S. Methods for total organic halogen (TOX) analysis in water: Past, present, and future. Chem. Eng. J. 2020, 399, 125675. [Google Scholar] [CrossRef]
  37. Liu, Y.; Liu, K.; Plewa, M.J.; Karanfil, T.; Liu, C. Formation of regulated and unregulated disinfection byproducts during chlorination and chloramination: Roles of dissolved organic matter type, bromide, and iodide. J. Environ. Sci. 2022, 117, 151–160. [Google Scholar] [CrossRef]
  38. Zhou, Y.; Cheng, F.; He, D.; Zhang, Y.-N.; Qu, J.; Yang, X.; Chen, J.; Peijnenburg, W.J.G.M. Effect of UV/chlorine treatment on photophysical and photochemical properties of dissolved organic matter. Water Res. 2021, 192, 116857. [Google Scholar] [CrossRef]
  39. Huntington, L.M.J.; Nooijen, M. pCCSD: Parameterized coupled-cluster theory with single and double excitations. J. Chem. Phys. 2010, 133, 184109. [Google Scholar] [CrossRef]
  40. Cheng, X.; Dong, H.; Qiang, Z. Formation and transformation of pre-chlorination-formed disinfection byproducts in drinking water treatment process. Sci. Total Environ. 2023, 904, 166241. [Google Scholar] [CrossRef]
  41. Efremov, A.M.; Sitanov, D.V.; Svettsov, V.I. The parameters of plasma and the kinetics of generation and loss of active particles in chlorine-nitrogen mixtures. High Temp. 2008, 46, 466–473. [Google Scholar] [CrossRef]
  42. Chowdhury, S.; Champagne, P.; McLellan, P.J. Models for predicting disinfection byproduct (DBP) formation in drinking waters: A chronological review. Sci. Total Environ. 2009, 407, 4189–4206. [Google Scholar] [CrossRef]
  43. Chang, E.E.; Chiang, P.C.; Chao, S.H.; Lin, Y.L. Relationship between chlorine consumption and chlorination by-products formation for model compounds. Chemosphere 2006, 64, 1196–1203. [Google Scholar] [CrossRef]
  44. Du, Y.; Lv, X.-T.; Wu, Q.-Y.; Zhang, D.-Y.; Zhou, Y.-T.; Peng, L.; Hu, H.-Y. Formation and control of disinfection byproducts and toxicity during reclaimed water chlorination: A review. J. Environ. Sci. 2017, 58, 51–63. [Google Scholar] [CrossRef] [PubMed]
  45. Cai, L.; Huang, H.; Li, Q.; Deng, J.; Ma, X.; Zou, J.; Li, G.; Chen, G. Formation characteristics and acute toxicity assessment of trihalomethanes and HAcAms from DOM and its different fractions in source water during chlorination and chloramination. Chemosphere 2023, 329, 138696. [Google Scholar] [CrossRef]
  46. Wang, W.-L.; Wu, Q.-Y.; Li, Z.-M.; Lu, Y.; Du, Y.; Wang, T.; Huang, N.; Hu, H.-Y. Light-emitting diodes as an emerging UV source for UV/chlorine oxidation: Carbamazepine degradation and toxicity changes. Chem. Eng. J. 2017, 310, 148–156. [Google Scholar] [CrossRef]
  47. Shao, B.; Hu, C.; Zhao, H.; Xiao, C.; Du, E.; Cai, A.; Deng, J. Degradation of ambroxol by UV/chloramine process: Kinetics, degradation pathway, and control of the risk of highly toxic disinfection by-products. Environ. Pollut. 2024, 363, 125091. [Google Scholar] [CrossRef]
  48. Li, T.; Shang, C.; Xiang, Y.; Yin, R.; Pan, Y.; Fan, M.; Yang, X. ClO2 pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process. Water Res. 2022, 216, 118341. [Google Scholar] [CrossRef]
  49. Bulman, D.M.; Mezyk, S.P.; Remucal, C.K. The Impact of pH and Irradiation Wavelength on the Production of Reactive Oxidants during Chlorine Photolysis. Environ. Sci. Technol. 2019, 53, 4450–4459. [Google Scholar] [CrossRef]
  50. Vione, D.; Carena, L. The Possible Production of Harmful Intermediates Is the “Dark Side” Of the Environmental Photochemistry of Contaminants (Potentially Adverse Effects, And Many Knowledge Gaps). Environ. Sci. Technol. 2020, 54, 5328–5330. [Google Scholar] [CrossRef]
Water 17 01954 g001
Figure 1. Concentrations of DBPs at various exposures to Cl•. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitro methane (chloropicrin; TCNM).
Figure 1. Concentrations of DBPs at various exposures to Cl•. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitro methane (chloropicrin; TCNM).
Water 17 01954 g001
Water 17 01954 g002
Figure 2. Concentrations of DBPs at various exposures to Cl2•−. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitro methane (chloropicrin; TCNM).
Figure 2. Concentrations of DBPs at various exposures to Cl2•−. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitro methane (chloropicrin; TCNM).
Water 17 01954 g002
Water 17 01954 g003
Figure 3. Toxicity changes of DBPs under different levels of Cl• exposure. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitromethane (chloropicrin; TCNM).
Figure 3. Toxicity changes of DBPs under different levels of Cl• exposure. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitromethane (chloropicrin; TCNM).
Water 17 01954 g003
Water 17 01954 g004
Figure 4. Toxicity changes of DBPs under different levels of Cl2•− exposure. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitro methane (chloropicrin; TCNM).
Figure 4. Toxicity changes of DBPs under different levels of Cl2•− exposure. Experimental conditions were repeated in the morning and in the afternoon and error bars provide the range of these experimental duplicates. HAA = nine regulated and unregulated haloacetic acids; THM = four regulated trihalomethanes; HAN = four haloacetonitriles; HAL = four haloacetaldehydes; HNM = trichloronitro methane (chloropicrin; TCNM).
Water 17 01954 g004
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Li, Z.; Zhang, Z. The Formation of Disinfection By-Products in Reactive Chlorine Species (RCS)-Mediated Advanced Oxidation Process. Water 2025, 17, 1954. https://doi.org/10.3390/w17131954

AMA Style

Li Z, Zhang Z. The Formation of Disinfection By-Products in Reactive Chlorine Species (RCS)-Mediated Advanced Oxidation Process. Water. 2025; 17(13):1954. https://doi.org/10.3390/w17131954

Chicago/Turabian Style

Li, Zishao, and Zhong Zhang. 2025. "The Formation of Disinfection By-Products in Reactive Chlorine Species (RCS)-Mediated Advanced Oxidation Process" Water 17, no. 13: 1954. https://doi.org/10.3390/w17131954

APA Style

Li, Z., & Zhang, Z. (2025). The Formation of Disinfection By-Products in Reactive Chlorine Species (RCS)-Mediated Advanced Oxidation Process. Water, 17(13), 1954. https://doi.org/10.3390/w17131954

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop