Radical Chemistry and Structural Relationships of PPCP Degradation by UV/Chlorine Treatment in Simulated Drinking Water

Generation of DBPs from dissolved organic matter by solar photolysis of chlorine: Associated changes of cytotoxicity and reactive species

Since elevated amounts of chlorine disinfectant were discharged into surface water, more attention should be paid to the reactions between dissolved organic matter (DOM) and chlorine under sunlight. However, disinfection byproducts (DBPs) formed from DOM by solar photolysis of chlorine, and changes of cytotoxicity during this process remain unclear. In this study, it was found that solar photolysis of chlorine significantly promoted the formation of aliphatic chlorinated DBPs and aromatic chlorinated DBPs (including chlorobenzoquinone) by 44.7-109 % and 81.7-121 %, respectively compared with dark chlorination. Unknown total organic chlorine contained in low molecular weight fraction (<1 kD) significantly positively correlated to the cytotoxicity of water samples. Several factors (bicarbonate, dissolved oxygen, pH, nitrate, ammonia, bromide, and iodide) affecting the radical chemistry, and the formation of DBPs under solar photolysis of chlorine were also investigated. Reactive species including HO•, Cl•, O3, and reactive nitrogen species (RNS) were responsible for forming different DBPs. Especially O3 increased the formation of most categories of DBPs tested in this study, and RNS contributed to the formation of nitrogenous DBPs. This study provided more understanding of the adverse impact of overused chlorine, and reaction mechanisms between reactive species and DOM.

Environmental fate and ecotoxicity of diclofenac degradation products generated by photo-assisted advanced oxidation processes

Diclofenac (DCF), a widely used non-steroidal anti-inflammatory drug (NSAID), poses environmental concerns due to its persistence, bioaccumulation potential, and transformation into toxic byproducts during oxidative and chlorination processes. This study investigated the photodegradation of DCF, both directly and in the presence of oxidants, to characterize the resulting degradation products and assess their potential environmental impact. The highest efficiency for direct UV photodegradation of DCF was observed at pH 5, while the addition of oxidants significantly accelerated the degradation rate. Among the advanced oxidation processes (AOPs) examined, the H₂O₂/UV system, with a DCF:H₂O₂ molar ratio of 1:30, exhibited the most effective performance in terms of DCF removal and total organic carbon (TOC) reduction. However, ecotoxicity assessments using Alivibrio fischeri, Daphnia magna, and Lemna minor revealed that AOPs generally increased the toxicity of the resulting solutions compared to untreated DCF. Toxicity analyses showed that post-reaction mixtures from AOPs involving NaOCl exhibited the highest toxic effects, consistent with forming specific transformation products identified as highly toxic by ECOSAR modeling. Additionally, the analysis of the physicochemical properties of DCF and its transformation products, including solubility and organic matter affinity, suggests a limited potential for long-range transport. These compounds are more likely to bind to sediments, reducing their mobility in groundwater.

Insight into homogeneous activation of sodium hypochlorite by dithionite coupled with dissolved oxygen (DO@NaClO/DTN) for carbamazepine degradation

Emerging contaminants (ECs) including carbamazepine (CBZ) in aquatic systems pose non-target risks to wildlife. We introduce an innovative advanced oxidation process (AOP) utilizing sodium hypochlorite (NaClO), which achieved 45.3 % degradation and mineralization of CBZ within 60 mins. Natural saturated state dissolved oxygen (DO, ∼7.5 mg·L-1) played a crucial role in synergistically activating NaClO with dithionite (DTN) without extra energy consumption. In DO@NaClO/DTN system, scavenging tests and electron spin resonance (ESR) analysis confirmed that ·OH and Cl· were dominant for CBZ degradation. The critical DO was responsible for the direct simultaneous production of ·OH and Cl·, confirmed by the greater thermodynamic data ΔG from density functional theory (DFT) calculation. These reactive species participate in subsequent transformations of SO4·-, O2·-, and 1O2. Preferential hydroxylation of CBZ first occurred due to the attacking at the reactive sites of C(21) and C(22) atoms. LC-MS/MS detection and DFT theoretical calculations also verified the sequent mechanisms of Meinwald rearrangement, deamidation and hydroxylation, cyclized hydroxylated and dehydrated with the decreasing ΔG. Ubiquitous Cl- accelerated CBZ degradation remarkably, regardless of its concentration. The significant enhancement of Cl- for CBZ degradation in DO@NaClO/DTN system suggest its promising application for ECs degradation in high-chloride seawater including offshore wastewater and tailwater in mariculture.

Transformation behavior and toxicity assessment of beaytlmethodeyammonNium chbride (BAC-12) disinfectant during hospital wastewater treatment

This work focused on the transformation behavior of the emerging beaytlmethodeyammonium chbride (BAC-12) disinfectant existed in the treatment of medical sewage during its disinfection treatment. The degradation ability of ozone (O3) to BAC-12 was the best, followed by UV/NaOCl, UV, and NaOCl. The enhancement of BAC-12 in UV/NaOCl system is caused by the combined effect of UV photolysis, reactive chlorine species (RCS), and •OH. The transformation products of BAC-12 in the disinfection treatment were detected, and the chemical structure of products was rationalized by frontier molecular orbital and transition state theory methodologies. According to the ecological structure-activity relationship (ECOSAR) assessment, the intermediates of BAC-12 in UV, NaOCl, and UV/NaOCl treatments had lower half lethal concentration (LC50) and chronic toxicity (ChV) values with a higher ecotoxicity than BAC-12. O3 disinfection treatment of these toxic intermediates can significantly reduce the toxicity of the BAC-12 solution. This work provides necessary information on the potential environmental risks of BAC-12 arising from different disinfection methods in the treatment of medical wastewater.

The effect of UV365/Fenton process on the removal of gaseous ethylbenzene in a bubble column reactor

As volatile organic compounds (VOCs), gaseous ethylbenzene has adverse effects on human health and ecology. Therefore, an effective degradation process is highly desirable. The Fenton process under UV 365 nm was selected as the first option to remove gaseous ethylbenzene in a bubble column reactor. The main parameters for the batch experiments were systematically studied, including H2O2 concentration, [H2O2]/[Fe2+], pH, UV wavelength, UV intensity, gaseous ethylbenzene concentration, gas flow rate, and process stability towards removal efficiency. The optimum conditions were found to be H2O2 concentration of 100 mmol·L-1, [H2O2]/[Fe2+] of 4, pH of 3.0, UV wavelength of 365 nm, UV power of 5 W, gas flow rate of 900 mL·min-1, and gaseous ethylbenzene concentration of 30 ppm, resulting in a removal efficiency of 76.3%. The study found that the Fenton process, when coupled with UV 365 nm, was highly effective in removing gaseous ethylbenzene. The degradation mechanism of gaseous ethylbenzene was proposed in the UV365/Fenton process based on EPR, radical quenching experiments, iron analysis, carbon balance, and GC-MS analysis. The results indicated that •OH played a crucial role in the process.

Involvement of inorganic nitrogen species (NOX- (x = 2, 3)) in the degradation of organic contaminants in environmental waters via UV irradiation or chemical oxidation: A dual-edged approach

OH-mediated advanced oxidation processes (AOPs) are widely used in wastewater treatment and drinking water purification. Recently, an increasing number of studies have indicated that common inorganic nitrogen ions can efficiently generate •OH under UV irradiation, demonstrating strong performance in the degradation of various contaminants. Conversely, the presence of inorganic nitrogen ions in UV or other oxidation processes dramatically increases the yield of toxic nitro (so)-aromatic products and the formation potential of nitrogenous disinfection by-products with high genotoxicity and cytotoxicity. This suggests that the presence of inorganic nitrogen ions in water and wastewater treatment is a 'double-edged sword', offering both benefits and potential harms. Herein, we systematically review the dual roles of inorganic nitrogen ions in contaminant degradation and nitrogenous by-product formation. First, the degradation kinetics of the UV/NOx- (x = 2, 3) and oxidant/NO2- processes are summarized for various contaminants. The pseudo-first-order rate constants (kpfo) of contaminant degradation in the UV/NO3- system range from 10-3 to 10-1min-1, while those in the UV/NO2- and peracetic acid/NO2- system vary from 10-3 to 102min-1 and 10-2 to 10-1min-1, respectively. Moreover, the properties of the water matrix (i.e., pH and O2) play a crucial role in the degradation kinetics by influencing the concentrations and distribution of reactive nitrogen species (RNS), as well as the morphology of the contaminants. Second, this review provides a general overview of the sources and properties of key RNS, including •NO2, ONOO-/ONOOH, and free nitrous acid (FNA), which are closely associated with the formation of nitrogenous by-products. Finally, the formation pathways of nitro (so)-aromatic products and nitrogenous disinfection by-products are discussed. These pathways are driven either by RNS alone or by the combination of RNS with reactive oxygen species (ROS).

Nanoconfinement-Enabled Resistance to Water Matrix Components for Selective Degradation of Micropollutants

Despite recent substantial advances in water treatment, the ability to selectively degrade trace micropollutants in real waters with complex matrix components remains a grand challenge. Here we report rational crafting of graphene oxide (GO)-wrapped defective TiO2 composite catalysts that creates nanoscopic confinement over the TiO2 surface within GO, thereby enabling the selective degradation of micropollutants through effectively excluding natural organic matter (NOM) and anions from the nanoconfined catalytic sites. In contrast to unconfined counterparts, the nanoconfined composite catalysts retain high degradation efficiency when exposed to various concentrations of NOM and anions, even in real water samples. Oxygen vacancies in TiO2 promote electron separation and oxygen adsorption, leading to a 4.9-fold increase in hydroxyl radical concentration within the nanoconfined space compared to the bulk solution. In situ X-ray photoemission spectroscopy and Raman spectroscopy unveil the nanoconfined catalytic sites on the TiO2 surface, where micropollutants smaller than the pores of GO are effectively degraded, while NOM with a larger size is excluded by GO. Furthermore, the formation of toxic disinfection byproducts is well controlled due to the exclusion of NOM. This work provides a simple yet viable strategy for designing 2D material-wrapped catalysts to selectively degrade target micropollutants in complex real waters.

Highly Efficient Removal of Organic Pollutants with HCO3--Enhanced Ru(III)/NaClO Process

The design of efficient advanced oxidation processes (AOPs) in the presence of bicarbonate has long attracted considerable attention in the field of environmental catalysis. In this study, sodium bicarbonate (NaHCO3) as one of the most abundant substances in actual water, was introduced to a NaClO/Ru(III) system to enhance the removal of acid orange 7(AO7). NaHCO3 could significantly improve the removal efficiency of the Ru(III)/NaClO process in HCO3- at a pH range of 6.9-10.0. Ru(V)=O was identified as a dominant reactive species involved in the degradation of pollutants in the NaHCO3/NaClO/Ru(III) system. HCO3- interacts with Ru(III) to generate Ru(III)-HCO3-, which enhances the activation performance of Ru(III) under neutral or alkaline conditions. The removal of AO7 was significantly enhanced with increasing NaHCO3 concentration, and the rate constant increased more than 2-fold to 4-fold as NaHCO3 concentrations increased from 0 to 100 mM at pH 6.9 and 8.5. This study proposed a novel strategy to enhance the Ru(III)/NaClO process with environmentally friendly inorganic ligands and highlights its potential applications in the removal of pollutants.

Clearing the path: Unraveling bisphenol a removal and degradation mechanisms for a cleaner future

Bisphenol A (BPA) is a prevalent chemical found in a range of consumer goods, which has raised worries about its possible health hazards. Comprehending the breakdown pathways of BPA is essential for evaluating its environmental consequences and addressing associated concerns. This review emphasizes the significance of studying the degradation/removal of BPA, with a specific focus on both natural and artificial routes. It explores natural processes such as photolysis, hydrolysis, and biodegradation, as well as manmade methods including advanced oxidation processes (AOPs) and enzymatic degradation. Examining the decomposition of BPA helps to understand how it behaves in the environment, providing valuable information for managing risks and addressing pollution. Furthermore, comprehending degradation mechanisms aids in the creation of more secure substitutes and regulatory actions to reduce BPA exposure and safeguard human health. This review emphasizes the need of promptly addressing this environmental and public health concern through the research of BPA degradation.

Overlooked Generation of Reactive Oxidative Species from Water and Dioxygen by Far UV Light

Far UV light at 222 nm (UV222) is gaining much attention for efficient water purification in UV222 irradiation and UV222-based advanced oxidation processes (AOPs). The direct photolysis of pollutants is regraded to be their major removal mechanism by a sole UV222 treatment. However, this paper reports the important roles of reactive oxidative species (ROS) generated from dioxygen and water under only UV222 radiation. Multiple ROSs are identified, including hydroxyl radical (HO·), singlet oxygen (1O2), superoxide radical anion (·O2-), and ozone (O3). HO· is the major ROS for the degradation of 18 organic micropollutants under UV222 radiation, with an observed quantum yield of 0.447 and the concentration of 10-13 M at pH 7. Dioxygen is the initial source of ROS, while water mainly serves as a medium to react with the photolytic intermediate of O3 (i.e., O(1D)) to form HO·. Water matrix components of HCO3- and natural organic matter can inhibit the HO· concentration, whereas NO3- significantly enhances it. In drinking water, UV222 alone removes 18 micropollutants more efficiently than the typical UV254/H2O2 AOP (150 μM), with reduced energy consumption. This study discloses a novel mechanism of ROS generation in UV222 irradiation and underscores UV222 as an emerging chemical-free AOP for water purification.

Assessing the discrepant role of anions in the transformation of reactive oxygen species in H2O2 and PDS system: A comparative kinetic analysis

Clarifying reactive oxygen species (ROS) variation in the presence of co-existing anions is significant for understanding the catalytic effect of magnetite (Fe3O4)-induced advanced oxidation processes (AOPs) in natural environment, yet this remains controversial. Herein, we compare the specific impacts of NO3-, SO42-, and Cl- on ROS (•OH, SO4•-, O2•-, and 1O2) exposure concentration in H2O2 and peroxydisulfate (PDS) systems catalyzed by Fe3O4, as well as how these variations affect the catalytic efficiency by developing kinetic model. In both two systems, NO3- demonstrates no discernible effect on ROS, whereas SO42- inhibits the exposure of all ROS and thus micropollutants degradation. Through theoretical calculation, it is proposed that SO42- primarily exerts its influence through affecting the electronic structure over catalyst surface. Regarding Cl-, it affects ROS exposure mainly by reacting with ROS. It shows inhibitory effect on 1O2 in both systems, but its suppressive impact on •OH is markedly more pronounced in H2O2 system compared to PDS system, which may be related to its rapid reactivity with SO4•-. Besides, the chlorine radicals (mainly ClO•) generated through the reaction of Cl- may exert a selective influence on micropollutants degradation. This study can help to re-understand the influence behavior of co-existing anions during AOPs.

Transformation of dissolved organic matter leached from biodegradable and conventional microplastics under UV/chlorine treatment and the subsequent effect on contaminant removal

The ultraviolet (UV)/chlorine process has been widely applied for water treatment. However, the transformation of microplastic-leached dissolved organic matter (MP-DOM) in advanced treatment of real wastewater remains unclear. Here, we investigated alterations in the photoproperties of MP-DOM leached from biodegradable and conventional microplastics (MPs) and their subsequent effects on the degradation of sulfamethazine (SMT) by the UV/chlorine process. Spectroscopy was used to assess photophysical properties, focusing on changes in light absorption capacity, functional groups, and fluorescence components, while photochemical properties were determined by calculating the apparent quantum yields of reactive intermediates (ΦRIs). For photophysical properties, our findings revealed that the degree of molecular structure modification, functional group changes, and fluorescence characteristics during UV/chlorine treatment are closely linked to the type of MPs. For photochemical properties, the ΦRIs increased with higher chlorine dosages due to the formation of new functionalities. Both singlet oxygen (1O2) and hydroxyl radicals (•OH) formation were strongly correlated with excited triplet state of DOM (3DOM*) in the UV/chlorine treatment. Additionally, we found that the four types of MP-DOM inhibit the degradation of SMT and elucidated the mechanisms behind this inhibition. We also proposed degradation pathways for SMT and assessed the ecotoxicity of the resulting intermediates. This study provides important insights into how the characteristics and transformation of MP-DOM affect contaminant degradation, which is critical for evaluating the practical application of UV-based advanced oxidation processes (UV-AOPs).

Unveiling the optical and molecular characteristics of aging microplastics derived dissolved organic matter transformed by UV/chlor(am)ine oxidation and its potential for disinfection byproducts formation

The investigations into the existence and behavior of microplastics (MPs) in water environment were widely conducted, while the characteristics of dissolved organic matter derived from MPs (MPs-DOM) during advanced oxidation have garnered comparatively little attention. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was employed along with multiple statistical analyses to gain a deeper understanding of the conversion of MPs-DOM in UV/chlor(am)ine advanced oxidation processes (AOPs). The diverse treatments exhibited varying degrees of augmentation in both aging and fragmentation of MPs with the order of UV/Cl2 > UV > UV/NH2Cl. The fragmentation degree of MPs upon two UV-based AOPs (UV-AOPs) was dependent on their monomer chemical structure. The highest TOC values of three MPs-DOM were observed after UV/Cl2 AOP and the lowest after UV/NH2Cl AOP. Polyvinyl chloride (PVC) displayed a greater release of MPs-DOM under varying leaching conditions. UV/Cl2 AOP favored the reaction with saturated MPs-DOM, while UV/NH₂Cl AOP reduced unsaturated MPs-DOM, alleviating disinfection byproducts (DBPs) formation after chlorination. The precursors generated by UV/Cl₂ AOP owned lower H/C, higher modified aromatic index (AImod), and lower molecular weight (MW) products after chlorination. PVC-DOM with fewer CH₂ groups was more reactive. -H₂O, +O and -CH₂ reactions dominated in PVC-DOM (CHO compounds), while -2H, +O, -CH₂ did in PVC-DOM (CHON compounds). The dominant chlorine addition/substitution reactions occurred in PVC-DOM treated by UV/Cl₂ AOP, identifying 195 Cl-DBPs with 220 precursor-product pairs. Mass difference analyses showed that +2H and +O reactions were the most frequent of the 24 reaction types.

Far-UVC direct photolysis of iohexol and acetochlor: an experimental and mechanism study

Recently, the emission of 222 nm Far-UVC krypton chloride (KrCl*) excimer lamps, has gained widespread attention in the field of water treatment. This study compared the degradation kinetics of IOX and ACE under UV222 and UV254 irradiation. The results demonstrated that UV222 irradiation exhibited higher efficiency, increasing the removal rates of IOX and ACE from 72.46% and 19.31% to 100%, respectively. Probe experiments and electron paramagnetic resonance (EPR) spectroscopy were used to identify the major active species generated during UV222 irradiation ([HO•]ss = 2.74 × 10-13 M). In addition, the effect of pH, pollutant concentration, anions, and natural organic matter (NOM) on the photolysis of IOX and ACE was investigated. The results indicated that IOX and ACE exhibited minimal dependence on pH, and IOX showed low sensitivity to water matrix components. Finally, the electrical energy consumption of the IOX and ACE photolysis by UV222 and UV254 irradiation was evaluated. The results revealed that UV222 irradiation demonstrated superior economic benefits (EE/OUV222/IOX = 0.59951 KWh/L, EE/OUV222/ACE = 0.25443 KWh/L), effectively reducing treatment costs. This study elucidated the photolysis characteristics of IOX and ACE under Far-UVC irradiation, providing a reference for the selection of process conditions in practical applications.

Unveiling the Critical Pathways of Hydroxyl Radical Formation in Breakpoint Chlorination: The Role of Trichloramine and Dichloramine Interactions

Chlorination of ammonia or chloramine-containing waters induces breakpoint chlorination reactions, producing a hydroxyl radical (•OH), but enhances the formation of undesirable N-nitrosamines. The prevailing view attributes •OH formation to a nitrosyl intermediate derived from the hydrolysis of dichloramine, but this pathway is unlikely at neutral or acidic pH. This study reveals a novel mechanism where •OH is generated via interactions between trichloramine (NCl3) and dichloramine (NHCl2), which also form nitrosation agents. Our experiments demonstrated that the NCl3-NHCl2 interaction degrades micropollutants with kinetics 2-3 times faster than breakpoint chlorination. Using electron paramagnetic resonance, we detected •OH in the NCl3-NHCl2 reaction. Micropollutant removal was unimpaired under low dissolved oxygen (O2(aq)) conditions, aligning with negligible O2(aq) changes during the NCl3-NHCl2 reaction and suggesting O2(aq) does not participate in •OH formation. Using benzene as a probe in 18O-labeled H2O, we confirmed water contributes to the oxygen source of •OH in NCl3-NHCl2 interactions, through which parallel reactions occur, leading to the formation of one mole of •OH alongside 1.92 mol of N2. A kinetic model developed in this study accurately predicted •OH and N2 and demonstrated the NCl3-NHCl2 interaction as the primary pathway for •OH formation in breakpoint chlorination, providing new insights into breakpoint chemistry.

Combination of oxidative and reductive effects of phenolic compounds on the degradation of aniline disinfection by-products by free radicals

In this study, we selected 13 phenolic compounds containing -COOH, -CHO, -OH, and -COCH3 functional groups as model compounds for dissolved organic matter (DOM), and explored the redox reactions during the co-degradation of phenolic compounds with aniline disinfection by-products (DBPs) at the molecular level. When phenolic compounds and aniline DBPs were degraded, phenoxy radicals and aniline radicals were the most important intermediates. Phenoxy radicals can degrade aniline DBPs via hydrogen atom abstraction (HAA) reactions, and the reaction rates were related to the reduction potentials of the compounds. Compounds containing electron-withdrawing groups were more likely to oxidize aniline DBPs. Aniline DBPs were more easily degraded by phenoxy radicals when they contained electron-donating groups, and the increase in the number of chlorine atoms inhibited the reaction rates of aniline DBPs degradation by phenoxy radicals. Although phenolic compounds can reduce aniline DBPs, there was no significant correlation between the reaction rates and the reduction potentials of the compounds. Considering the redox effects of phenolic compounds on aniline DBPs, co-degradation simulations showed that phenolics inhibited the degradation efficiency of aniline DBPs. This work provided new insights into the transformation mechanisms and degradation efficiencies of DOM and aniline DBPs when they were co-degraded.

Degradation kinetics and disinfection by-products formation of benzophenone-4 during UV/persulfate process

The degradation kinetics, reaction pathways, and disinfection by-products formation of an organic UV filter, benzophenone-4 (BP4) during UV/persulfate oxidation were investigated. BP4 can hardly be degraded by UV alone, but can be effectively decomposed by UV/persulfate following pseudo-first order kinetics. BP4 degradation rate was enhanced with increasing persulfate dosage and decreasing pH from 8 to 5. However, the degradation rate of BP4 at pH 9 was higher than that at pH 8 because of the presence of phenolic group in BP4 structure. and SO 4 - ⋅ were confirmed as the major contributors to BP4 decomposition in radical scavenging experiments, and the second-order rate constants between HO ⋅ and BP4 as well as those between SO 4 - ⋅ and BP4 were estimated by establishing and solving a kinetic model. The presence of B r - and humic acid inhibited the decomposition of BP4, while N O 3 - promoted it. The mineralisation of BP4 was only 9.1% at the persulfate concentration of 50 μM. Six degradation intermediates were identified for the promulgation of the reaction pathways of BP4 during UV/persulfate oxidation were proposed as a result. In addition, the formation of DBP in the sequential chlorination was evaluated at different persulfate dosages, pH values, and water matrix. The results of this study can provide essential knowledge for the effective control of DBP formation with reducing potential hazard to provide safe drinking water to the public.

Removal of antibiotic resistant bacteria and antibiotic resistance genes by an electrochemically driven UV/chlorine process for decentralized water treatment

The UV/chlorine (UV/Cl2) process is a developing advanced oxidation process and can efficiently remove antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). However, the transportation and storage of chlorine solutions limit the application of the UV/Cl2 process, especially for decentralized water treatment. To overcome the limitation, an electrochemically driven UV/Cl2 process (E-UV/Cl2) where Cl2 can be electrochemically produced in situ from anodic oxidation of chloride (Cl-) ubiquitously present in various water matrices was evaluated in this study. >5-log inactivation of the ARB (E. coli) was achieved within 5 s of the E-UV/Cl2 process, and no photoreactivation of the ARB was observed after the treatment. In addition to the ARB, intracellular and extracellular ARGs (tetA, sul1, sul2, and ermB) could be effectively degraded (e.g., log(C0/C) > 4 for i-ARGs) within 5 min of the E-UV/Cl2 process. Atomic force microscopy showed that the most of the i-ARGs were interrupted into short fragments (< 30 nm) during the E-UV/Cl2 process, which can thus effectively prevent the self-repair of i-ARGs and the horizontal gene transfer. Modelling results showed that the abatement efficiencies of i-ARG correlated positively with the exposures of •OH, Cl2-•, and ClO• during the E-UV/Cl2 process. Due to the short treatment time (5 min) required for ARB and ARG removal, insignificant concentrations of trihalomethanes (THMs) were generated during of the E-UV/Cl2 process, and the energy consumption (EEO) of ARG removal was ∼0.20‒0.27 kWh/m3-log, which is generally comparable to that of the UV/Cl2 process (0.18-0.23 kWh/m3-log). These results demonstrate that the E-UV/Cl2 process can provide a feasible and attractive alternative to the UV/Cl2 process for ARB and ARG removal in decentralized water treatment system.

Kinetics and mechanisms of non-radically and radically induced degradation of bisphenol A in a peroxymonosulfate-chloride system

Bisphenol A, a hazardous endocrine disruptor, poses significant environmental and human health threats, demanding efficient removal approaches. Traditional biological methods struggle to treat BPA wastewater with high chloride (Cl-) levels due to the toxicity of high Cl- to microorganisms. While persulfate-based advanced oxidation processes (PS-AOPs) have shown promise in removing BPA from high Cl- wastewater, their widespread application is always limited by the high energy and chemical usage costs. Here we show that peroxymonosulfate (PMS) degrades BPA in situ under high Cl- concentrations. BPA was completely removed in 30 min with 0.3 mM PMS and 60 mM Cl-. Non-radical reactive species, notably free chlorine species, including dissolved Cl2(l), HClO, and ClO- dominate the removal of BPA at temperatures ranging from 15 to 60 °C. Besides, free radicals, including •OH and Cl2 •-, contribute minimally to BPA removal at 60 °C. Based on the elementary kinetic models, the production rate constant of Cl2(l) (32.5 M-1 s-1) is much higher than HClO (6.5 × 10-4 M-1 s-1), and its degradation rate with BPA (2 × 107 M-1 s-1) is also much faster than HClO (18 M-1 s-1). Furthermore, the degradation of BPA by Cl2(l) and HClO were enlarged by 10- and 18-fold at 60 °C compared to room temperature, suggesting waste heat utilization can enhance treatment performance. Overall, this research provides valuable insights into the effectiveness of direct PMS introduction for removing organic micropollutants from high Cl- wastewater. It further underscores the critical kinetics and mechanisms within the PMS/Cl⁻ system, presenting a cost-effective and environmentally sustainable alternative for wastewater treatment.

Ethylene Glycol (EG)-Derived Chlorine-Resistant Cu0/TiO2-x for Efficient Photocatalytic Degradation of Nitrate to N2 without Sacrificial Agents at Near-Neutral pH Conditions: The Synergistic Effects of Cu0 and EG Radicals

The selective photoreduction of nitrate to nontoxic nitrogen gas has emerged as an energy-efficient and environmentally friendly route for nitrate removal. However, the coexisting high-concentration chloride ions in wastewater can exert a significant influence on nitrate reduction due to the competitive adsorption and corrosion of Cl- on photocatalysts. Herein, we prepared ethylene glycol-Cu/TiO2-x (EG-Cu/TiO2-x) through a solvothermal reaction of Cu-doped TiO2 in an EG solution. The photodegradation of nitrate using EG-Cu/TiO2-x without adding sacrificial agents can efficiently occur in near-neutral pH solutions containing 50 mM Cl- with 95.26% of NO3- removal and 76.52% of N2 selectivity. Moreover, the photocatalyst performance remained at a high level after 8 cycles. In this work, NO3- was first converted to NH4+ by Cu0 and Ti3+, followed by the NH4+-to-N2 conversion by photogenerated chlorine free radicals. Compared to HO•, Cl•, and Cl2•-, ClO• is proved to play the predominant role in transforming NH4+ to N2. The EG radicals produced by UV light impede Cl- adsorption on Cu, protecting Cu0 from being corroded. What's more, photoelectrons can reduce Ti4+ to Ti3+ and protect Cu0 from being oxidized, enabling the stability of reactive sites. This work provides novel insights and understanding on designing photocatalysts for NO3- removal in solutions containing chloride ions, highlighting the significance of eliminating Cl- by EG radicals and adjusting the conversion process of NO3- for the efficient removal of NO3-.

UV-induced reactive species dynamics and product formation by chlorite

Chlorite (ClO2-) is a regulated byproduct of chlorine dioxide water treatment processes. The transformation of chlorite under UV irradiation into chloride (Cl-) and chlorate (ClO3-) involves reactive species chain reactions that could enhance chlorine dioxide water treatment efficiency while reducing residual chlorite levels. This study conducted a mechanistic investigation of chlorite phototransformation by analyzing reaction intermediates and stable end products, including chlorine dioxide (ClO2), free chlorine (HOCl/OCl-), hydroxyl‑radical (•OH), Cl-, and ClO3- through combined experimental and modeling approaches. Experiments were performed at UV254 irradiation in pure buffered water within the pH range of 6 to 8. Results indicated that the apparent quantum yields for chlorite phototransformation increased from 0.86 to 1.45, and steady-state •OH concentrations at 1 mM initial chlorite concentration rose from 8.16 × 10-14 M - 16.1 × 10-14 M with decreasing pH values. It was observed that under UV irradiation, chlorite acts as both a significant producer and consumer of reactive species through three distinct reaction pathways. The developed kinetic model, which incorporates optimized intrinsic chlorite quantum yields Φchloritein ranging from 0.33 to 0.39, effectively simulated the loss of oxidants and the formation of major products. It also accurately predicted steady-state concentrations of various species, including •OH, •ClO, Cl• and O3. For the first time, this study provides a comprehensive transformation pathway scheme for chlorite phototransformation. The findings offer important insights into the mechanistic aspects of product and oxidizing species formation during chlorite phototransformation.

Recent progress in electro-Fenton technology for the remediation of pharmaceutical compounds in aqueous environments

The global focus on wastewater treatment has intensified in the contemporary era due to its significant environmental and human health impacts. Pharmaceutical compounds (PCs) have become an emerging concern among various pollutants, as they resist conventional treatment methods and pose a severe environmental threat. Advanced oxidation processes (AOPs) emerge as a potent and environmentally benign approach for treating recalcitrant pharmaceuticals. To address the shortcomings of traditional treatment methods, a technology known as the electro-Fenton (EF) method has been developed more recently as an electrochemical advanced oxidation process (EAOP) that connects electrochemistry to the chemical Fenton process. It has shown effective in treating a variety of pharmaceutically active compounds and actual wastewaters. By producing H2O2 in situ through a two-electron reduction of dissolved O2 on an appropriate cathode, the EF process maximizes the benefits of electrochemistry. Herein, we have critically reviewed the application of the EF process, encompassing diverse reactor types and configurations, the underlying mechanisms involved in the degradation of pharmaceuticals and other emerging contaminants (ECs), and the impact of electrode materials on the process. The review also addresses the factors influencing the efficiency of the EF process, such as (i) pH, (ii) current density, (iii) H2O2 concentration, (iv) and others, while providing insight into the scalability potential of EF technology and its commercialization on a global scale. The review delves into future perspectives and implications concerning the ongoing challenges encountered in the operation of the electro-Fenton process for the treatment of PCs and other ECs.

A critical review of ozone-based electrochemical advanced oxidation processes for water treatment: Fundamentals, stability evaluation, and application

In recent years, electrochemical advanced oxidation processes (EAOPs) combined with ozonation have been widely utilized in water/wastewater treatment due to their excellent synergistic effect, high treatment efficiency, and low energy consumption. A comprehensive summary of these ozone-based EAOPs is still insufficient, though some reviews have covered these topics but either focused on a specific integrated process or provided synopses of EAOPs or ozone-based AOPs. This review presents an overview of the fundamentals of several ozone-based EAOPs, focusing on process optimization, electrode selection, and typical reactor designs. Additionally, the service life of electrodes and improvement strategies for the stability of ozone-based EAOPs that are ignored by previous reviews are discussed. Furthermore, four main application fields are summarized, including disinfection, emerging contaminants treatment, industrial wastewater treatment, and resource recovery. Finally, the summary and perspective on ozone-based EAOPs are proposed. This review provides an overall summary that would help to gain insight into the ozone-based EAOPs to improve their environmental applications.

A critical review of persulfate-based electrochemical advanced oxidation processes for the degradation of emerging contaminants: From mechanisms and electrode materials to applications

The persulfate-based electrochemical advanced oxidation processes (PS-EAOPs) exhibit distinctive advantages in the degradation of emerging contaminants (ECs) and have garnered significant attention among researchers, leading to a consistent surge in related research publications over the past decade. Regrettably, there is still a lack of a critical review gaining deep into understanding of ECs degradation by PS-EAOPs. To address the knowledge gaps, in this review, the mechanism of electro-activated PS at the interface of the electrodes (anode, cathode and particle electrodes) is elaborated. The correlation between these electrode materials and the activation mechanism of PS is systematically discussed. The strategies for improving the performance of electrode material that determining the efficiency of PS-EAOPs are also summarized. Then, the applications of PS-EAOPs for the degradation of ECs are described. Finally, the challenges and outlook of PS-EAOPs are discussed. In summary, this review offers valuable guidance for the degradation of ECs by PS-EAOPs.

Intensifying the sonochemical degradation of hydrophilic organic contaminants by organic and inorganic additives

The sonochemical system is highly effective at degrading hydrophobic substances but has limitations when it comes to eliminating hydrophilic compounds. This study examines the impact of organic and inorganic additives on improving the sonochemical degradation of hydrophilic pollutants in water. The effects of adding an organic substance (CCl4) and two inorganic ions (Fe2+ and HCO3-) were tested. The treatment was focused on a representative hydrophilic antibiotic, cefadroxil (CDX). Initially, the sonodegradation of CDX without additives was assessed and compared with two reference pollutants more hydrophobic than CDX: dicloxacillin (DCX) and methyl orange (MO). The results highlighted the limitations of ultrasound alone in degrading CDX. Subsequently, the impact of the additives on enhancing the removal of this recalcitrant pollutant was evaluated at two frequencies (375 and 990 kHz). A significant improvement in the CDX degradation was observed with the presence of CCl4 and Fe2+ at both frequencies. Increasing CCl4 concentration led to greater CDX elimination, whereas a high Fe2+ concentration had detrimental effects. To identify the reactive sites on CDX towards the species generated with the additives, theoretical calculations (i.e. Fukui indices and HOMO-LUMO gaps) were performed. These analyses indicated that the β-lactam and dihydrothiazine rings on CDX are highly reactive towards oxidizing species. This research enhances our understanding of the relationship between the structural characteristics of contaminants and the sonochemical frequency in the action of additives having diverse nature.

Effects of Fe(III) on the formation and toxicity alteration of halonitromethanes, dichloroacetonitrile, and dichloroacetamide from polyethyleneimine during UV/chlorine disinfection

Trace iron ions (Fe(III)) are commonly found in water and wastewater, where free chlorine is very likely to coexist with Fe(III) affecting the disinfectant's stability and N-DBPs' fate during UV/chlorine disinfection, and yet current understanding of these mechanisms is limited. This study investigates the effects of Fe(III) on the formation and toxicity alteration of halonitromethanes (HNMs), dichloroacetonitrile (DCAN), and dichloroacetamide (DCAcAm) from polyethyleneimine (PEI) during UV/chlorine disinfection. Results reveal that the maxima concentrations of HNMs, DCAN, and DCAcAm during UV/chlorine disinfection with additional Fe(III) were 1.39, 1.38, and 1.29 times higher than those without additional Fe(III), instead of being similar to those of Fe(III) inhibited the formation of HNMs, DCAN and DCAcAm during chlorination disinfection. Meanwhile, higher Fe(III) concentration, acidic pH, and higher chlorine dose were more favorable for forming HNMs, DCAN, and DCAcAm during UV/chlorine disinfection, which were highly dependent on the involvement of HO· and Cl·. Fe(III) in the aquatic environment partially hydrolyzed to the photoactive Fe(III)‑hydroxyl complexes Fe(OH)2+ and [Fe(H2O)6]3+, which undergone UV photoactivation and coupling reactions with HOCl to achieve effective Fe(III)/Fe(II) interconversion, a process that facilitated the sustainable production of HO·. Extensive product analysis and comparison verified that the HO· production enhanced by the Fe(III)/Fe(II) internal cycle played a primary role in increasing HNMs, DCAN, and DCAcAm productions during UV/chlorine disinfection. Note that the incorporation of Fe(III) increased the cytotoxicity and genotoxicity of HNMs, DCAN, and DCAcAm formed during UV/chlorine disinfection, and yet Fe(III) did not have a significant effect on the acute toxicity of water samples before, during, and after UV/chlorine disinfection. The new findings broaden the knowledge of Fe(III) affecting HNMs, DCAN, and DCAcAm formation and toxicity alteration during UV/chlorine disinfection.

Enhanced oxidation of parabens in an aqueous solution by air-assisted cold plasma

Various contaminants of emerging concern (CECs) including pharmaceuticals and personal care products (PPCPs) have been known to threaten the aquatic ecosystem and human health even at low levels in surface water. Among them, the wide variety use of parabens as preservatives may pose potential threat to human because parabens may present estrogenic activity. Various advanced oxidation processes have been attempted to reduce parabens, but challenges using cold plasma (CP) are very rare. CP is worth paying attention to in reducing parabens because it has the advantage of generating radical ions, including reactive oxygen/nitrogen species and various ions. Accordingly, this study demonstrates how CP can be utilized and how CP competes with other advanced oxidation processes in energy requirements. Quantified ethyl-, propyl-, and butyl-paraben indicate that CP can effectively degrade them up to 99.1% within 3 h. Regression reveals that the kinetic coefficients of degradation can be increased to as high as 0.0328 min-1, comparable to other advanced oxidation processes. Many by-products generated from the oxidation of parabens provide evidence of the potential degradation pathway through CP treatment. In addition, we found that the electrical energy consumption per order of CP (39-95 kWh/m3/order) is superior to other advanced oxidation processes (69∼31,716 kWh/m3/order). Overall, these results suggest that CP may be a viable option to prevent adverse health-related consequences associated with parabens in receiving water.

Solar/ClO2 system inactivates fungal spores in drinking water: Synergy, efficiency and mechanisms

The risk of fungal pollution in drinking water has been paid attention. Solar/chlorine dioxide (ClO2) combined system is an environment-friendly, economical and efficient disinfection method, especially for countries and regions that are economically backward and still exposed to unsafe drinking water. In this paper, the kinetics, influencing factors, mechanism and regrowth potential of inactivated Aspergillus niger (A. niger) spores by solar/ClO2 were reported for the first time. The inactivation curve can be divided into three stages: instant inactivation within 1-2 min, slow linear inactivation and finally a tail. The synergistic factors produced by solar/ClO2 in terms of log reduction and maximum inactivation rate were 1.194 and 1.112, respectively. The inhibitory effect on the regrowth of A. niger spores inactivated by solar/ClO2 was also stronger than that by ClO2 alone. Strongly oxidizing reactive species produced by solar/ClO2 accelerated the accumulation of endogenic reactive oxygen species (ROS) caused by oxidation stress of A. niger spores, improving the inactivation ability of the system. The inactivation order of A. niger spores was: loss of culturability, accumulation of intracellular ROS, loss of membrane integrity, leakage of intracellular species and change of morphology. The inactivation performance of solar/ClO2 was better than solar/chlor(am)ine according to the comparison of inactivation efficiency and regrowth potential. Results also suggested that solar/ClO2 process was more suitable for the treatment of ground water sources.

Unveiling the Mechanism and Kinetics of Pollutant Attenuation by Free Radicals Triggered from Goethite in Water Distribution Systems

Investigating the fate of persistent organic pollutants in water distribution systems (WDSs) is of great significance for preventing human health risks. The role of iron corrosion scales in the migration and transformation of organics in such systems remains unclear. Herein, we determined that hydroxyl (•OH), chlorine, and chlorine oxide radicals are generated by Fenton-like reactions due to the coexistence of oxygen vacancy-related Fe(II) on goethite (a major constituent of iron corrosion scales) and hypochlorous acid (HClO, the main reactive chlorine species of residual chlorine at pH ∼ 7.0). •OH contributed mostly to the decomposition of atrazine (ATZ, model compound) more than other radicals, producing a series of relatively low-toxicity small molecular intermediates. A simplified kinetic model consisting of mass transfer of ATZ and HClO, •OH generation, and ATZ oxidation by •OH on the goethite surface was developed to simulate iron corrosion scale-triggered residual chlorine oxidation of organic compounds in a WDS. The model was validated by comparing the fitting results to the experimental data. Moreover, the model was comprehensively applicable to cases in which various inorganic ions (Ca2+, Na+, HCO3-, and SO42-) and natural organic matter were present. With further optimization, the model may be employed to predict the migration and accumulation of persistent organic pollutants under real environmental conditions in the WDSs.

Dissolved oxygen facilitates efficiency of chlorine disinfection for antibiotic resistance

Controlling the dissemination of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) is a global concern. While commonly used chlorine disinfectants can damage or even kill ARB, dissolved oxygen (DO) may affect the formation of reactive chlorine species. This leads to the hypothesis that DO may play roles in mediating the effectiveness of chlorine disinfection for antibiotic resistance. To this end, this study investigated the impacts of DO on the efficiency of chlorine disinfection for antibiotic resistance. The results revealed that DO could increase the inactivation efficiency of ARB under chloramine and free chlorine exposure at practically relevant concentrations. Reactive species induced by DO, including H2O2, O2-, and OH, inactivated ARB strains by triggering oxidative stress response and cell membrane damage. In addition, the removal efficiency of extracellular ARGs (i.e. tetA and blaTEM) was enhanced with increasing dosage of free chlorine or chloramine under aerobic conditions. DO facilitated the fragmentation of plasmids, contributing to the degradation of extracellular ARGs under exposure to chlorine disinfectants. The findings suggested that DO facilitates disinfection efficiency for antibiotic resistance in water treatment systems.

Machine learning for predicting halogen radical reactivity toward aqueous organic chemicals

Rapid advances in machine learning (ML) provide fast, accurate, and widely applicable methods for predicting free radical-mediated organic pollutant reactivity. In this study, the rate constants (logk) of four halogen radicals were predicted using Morgan fingerprint (MF) and Mordred descriptor (MD) in combination with a series of ML models. The findings highlighted that making accurate predictions for various datasets depended on an effective combination of descriptors and algorithms. To further alleviate the challenge of limited sample size, we introduced a data combination strategy that improved prediction accuracy and mitigated overfitting by combining different datasets. The Light Gradient Boosting Machine (LightGBM) with MF and Random Forest (RF) with MD models based on the unified dataset were finally selected as the optimal models. The SHapley Additive exPlanations revealed insights: the MF-LightGBM model successfully captured the influence of electron-withdrawing/donating groups, while autocorrelation, walk count and information content descriptors in the MD-RF model were identified as key features. Furthermore, the important contribution of pH was emphasized. The results of the applicability domain analysis further supported that the developed model can make reliable predictions for query compounds across a broader range. Finally, a practical web application for logk calculations was built.

Degradation and detoxification of ribavirin by UV/chlorine/Fe(II) process in water treatment system

Ribavirin (RBV), which is extensively used to treat viral diseases such as COVID-19, is considered one of the major emerging contaminants due to its long-term existence and health risk in the aqueous environmental system. However, research on effective removal of RBV still remains insufficient. In this study, we investigated the RBV degradation kinetics and mechanism in UV/chlorine/Fe(II) process. The degradation rate constant kobs-RBV of RBV was 2.52 × 10-4 s-1 in UV/chlorine/Fe(II) process, which increased by 1.6 times and 1.3 times than that in chlorine alone and UV/chlorine process, respectively. Notably, trace amount Fe(II) promoted RBV degradation in UV/chlorine system through Fe2+/Fe3+ cycles, enhancing the yield of reactive species such as HO· and certain species reactive chlorine radicals (RCS). The contributions of HO· and RCS toward RBV degradation were 53.91% and 16.11%, respectively. Specifically, Cl·, ClO·, and Cl2·- were responsible for 8.59%, 2.69%, and 4.83% of RBV removal. The RBV degradation pathway indicated that the reactive species preferentially attacked the amide moiety of RBV, which cleaved the ether bond and the hydroxyl group. The toxicity evaluation of RBV degradation products elucidated that UV/chlorine/Fe(II) process was beneficial for RBV detoxification.

Theoretical studies on the aqueous phase and graphene heterogeneous degradation of acrylamide and acrylonitrile by HO, ClO, and BrO radicals

The newly discovered ClO• and BrO• contribute to pollutant degradation in advanced oxidation processes, while acrylamide (AM) and acrylonitrile (ACN) are always the focus of scientists concerned due to their continuous production and highly toxic effects. Moreover, various particles with a graphene-like structure are the companions of AM/ACN in dry/wet sedimentation or aqueous phase existence, which play an important role in heterogeneous oxidation. Thus, this work focuses on the reaction mechanism and environmental effect of AM/ACN with ClO•/BrO•/HO• in the water environment under the influence of graphene (GP). The results show that although the reactivity sequence of AM and ACN takes the order of with HO• > with BrO• > with ClO•, the easiest channel always occurs at the same C-position of the two reactants. The reaction rate constants (k) of AM with three radicals are 2 times larger than that with ACN, and amide groups have a better ability to activate CC bonds than cyanide groups. The existence of GP can accelerate the target reaction, and the k increased by 9-13 orders of magnitude. The toxicity assessment results show that the toxic effect of most products is lower than that of parent compounds, but the environmental risk of products from ClO•/BrO•-adducts is higher than those from HO•-adducts. The oxidative degradation process based on ClO• and BrO• deserves special attention, and the catalytic effect of GP and its derivatives on the oxidation process is non-negligible.

Unveiling the activity difference cause and ring-opening reaction routes of typical radicals induced degradation of toluene

This study employs five UV-AOPs (PMS, PDS, H2O2, NaClO and NaClO2) to produce radicals (•OH, SO4•-, ClO•, O2•- and 1O2) and further comparatively studies their activity sequence and activity difference cause in toluene degradation. The toluene mineralization efficiency as a descending order is 73 % (UV-PMS) > 71 % (UV-PDS) > 70 % (acidified-UV-NaClO) > 55 % (UV-H2O2) > 36 % (UV-NaClO) > 35 % (UV-NaClO2); that of conversion efficiency is 99 % (acidified-UV-NaClO) > 95 % (UV-PMS) > 90 % (UV-PDS) > 74 % (UV-H2O2) > 44 % (UV-NaClO) > 41 % (UV-NaClO2). Acidic pretreatment significantly boosts the reactivity of UV-NaClO. ESR combined with radical quenching tests reveals the radicals' generation and evolution, and their contribution rates to toluene conversion, i.e. ClO• > SO4•- > O2•- > 1O2 > •OH. Theoretical calculations further unveil the ring-opening reaction routes and the nature of the activity difference of different radicals. The minimum energy required for ring-opening reaction is 116.77, 150.63, 168.29 and 191.92 kJ/mol with respect to ClO•, SO4•-, 1O2 and •OH, and finding that the ClO•-HO• pair is the best for toluene mineralization. The difficulty for eliminating typical VOCs by using UV-AOPs method is determined as toluene > chlorobenzene > benzene > ethyl acetate.

Enhanced ammonia oxidation by a photoelectrocatalysis‑chlorine system: The role of ClO• and free chlorine

The decomposition of ammonia-N to environmental-friendly N2 remains a fundamental problem for water treatment. We proposed a way to selectively and efficiently oxidize ammonia to N2 through an integrated photoeletrocatalysis‑chlorine reactions (PECCl) system based on a bifunctional TiO2 nanotube photoanode. The ·OH and HClO can be simultaneously generated on the TiO2 nanotube photoanode in this system, which can in situ form ClO· for efficient ammonia removal. Compared with electrochemical‑chlorine (EC-Cl), photocatalysis‑chlorine (PC-Cl) and photoelectrocatalysis (PEC) systems, the PEC-Cl system exhibited much higher electrocatalytic activity due to the synergetic effect of photoelectrocatalyst and electrocatalyst in bifunctional TiO2 nanotube electrode. The removal efficiency of ammonia-N and total-N reached 100.0 % and 93.3 % at 0.3 V (vs Ag/AgCl) in the PEC-Cl system. Moreover, the system was efficient under various pH conditions. The reactions between ClO-/ClO· and the N-containing intermediates contributed to the high performance of the system, which expanded the reactions from the electrode surface to the electrolyte. Furthermore, radical scavenging and free chlorine determination experiments confirmed that ClO· and free chlorine were the main active species that enabled the ammonia oxidation. This study presents new understanding on the role of active species for ammonia removal in wastewater.

Ultraviolet activation of monochloramine to treat contaminants of emerging concern: reactions, operating parameters, byproducts, and opportunities

The presence of CECs in aquatic systems has raised significant concern since they are potentially harmful to the environment and human health. Eliminating CECs has led to the development of alternatives to treat wastewater, such as advanced oxidation processes (AOPs). The ultraviolet-mediated activation of monochloramine (UV/NH2Cl) is a novel and relatively unexplored AOPs for treating pollutants in wastewater systems. This process involves the production of amino radicals (•NH2) and chlorine radicals (Cl•) from the UV irradiation of NH2Cl. Studies have demonstrated its effectiveness in mitigating various CECs, exhibiting advantages, such as the potential to control the amount of toxic disinfection byproducts (TDBPs) formed, low costs of reagents, and low energy consumption. However, the strong influence of operating parameters in the degradation efficiency and existence of NH2Cl, the lack of studies of its use in real matrices and techno-economic assessments, low selectivity, and prolonged treatment periods must be overcome to make this technology more competitive with more mature AOPs. This review article revisits the state-of-the-art of the UV/NH2Cl technology to eliminate pharmaceutical and personal care products (PPCPs), micropollutants from the food industry, pesticides, and industrial products in aqueous media. The reactions involved in the production of radicals and the influence of operating parameters are covered to understand the formation of TDBPs and the main challenges and limitations of the UV/NH2Cl to degrade CECs. This review article generates critical knowledge about the UV/NH2Cl process, expanding the horizon for a better application of this technology in treating water contaminated with CECs.

Using UV/peracetic acid as pretreatment for subsequent bio-treatment of antibiotic-containing wastewater treatment: Mitigating microbial inhibition and antibiotic resistance genes proliferation

UV/peracetic acid (PAA) treatment presents a promising approach for antibiotic removal, but its effects on microbial community and proliferation of antibiotic resistance genes (ARGs) during the subsequent bio-treatment remain unclear. Thus, we evaluated the effects of the UV/PAA on tetracycline (TTC) degradation, followed by introduction of the treated wastewater into the bio-treatment system to monitor changes in ARG expression and biodegradability. Results demonstrated effective TTC elimination by the UV/PAA system, with carbon-centered radicals playing a significant role. Crucially, the UV/PAA system not only eliminated antibacterial activity but also inhibited potential ARG host growth, thereby minimizing the emergence and dissemination of ARGs during subsequent bio-treatment. Additionally, the UV/PAA system efficiently removed multi-antibiotic resistant bacteria and ARGs from the bio-treatment effluent, preventing ARGs from being released into the environment. Hence, we propose a multi-barrier strategy for treating antibiotic-containing wastewater, integrating UV/PAA pre-treatment and post-disinfection with bio-treatment. The inhibition of ARGs transmission by the integrated system was verified through actual soil testing, confirming its effectiveness in preventing ARGs dissemination in the surrounding natural ecosystem. Overall, the UV/PAA treatment system offers a promising solution for tackling ARGs challenges by controlling ARGs proliferation at the source and minimizing their release at the end of the treatment process.

Unraveling the Role of Humic Acid in the Oxidation of Phenolic Contaminants by Soluble Manganese Oxo-Anions

Humic acid (HA) is ubiquitous in natural aquatic environments and effectively accelerates decontamination by permanganate (Mn(VII)). However, the detailed mechanism remains uncertain. Herein, the intrinsic mechanisms of HA's impact on phenolics oxidation by Mn(VII) and its intermediate manganese oxo-anions were systematically studied. Results suggested that HA facilitated the transfer of a single electron from Mn(VII), resulting in the sequential formation of Mn(VI) and Mn(V). The formed Mn(V) was further reduced to Mn(III) through a double electron transfer process by HA. Mn(III) was responsible for the HA-boosted oxidation as the active species attacking pollutants, while Mn(VI) and Mn(V) tended to act as intermediate species due to their own instability. In addition, HA could serve as a stabilizer to form a complex with produced Mn(III) and retard the disproportionation of Mn(III). Notably, manganese oxo-anions did not mineralize HA but essentially changed its composition. According to the results of Fourier-transform ion cyclotron resonance mass spectrometry and the second derivative analysis of Fourier-transform infrared spectroscopy, we found that manganese oxo-anions triggered the decomposition of C-H bonds on HA and subsequently produced oxygen-containing functional groups (i.e., C-O). This study might shed new light on the HA/manganese oxo-anion process.

Kinetics of chlorine and chloramine reactions in reverse osmosis permeate and their impact on radical formation during UV/chlorine advanced oxidation for potable reuse

Knowledge of the speciation of chlorine and chloramines in reverse osmosis (RO) permeate is needed to estimate the performance (i.e., pollutant log reduction) of subsequent UV/chlorine advanced oxidation processes (AOPs). To accurately predict the speciation, a previously reported breakpoint chlorination kinetic model was experimentally validated for pH 5.5 and reaction times < 3 min and used to predict the kinetics of breakpoint chlorination in RO permeate. The predictions showed that eliminating chloramines by adding chlorine at a dose beyond the chlorine-to-nitrogen (Cl/N) breakpoint ratio is not practical due to the high breakpoint Cl/N ratio for RO permeate (∼3.0 molar ratio) and an estimated > 40 min reaction time. The conversion from monochloramine (NH2Cl) to dichloramine (NHCl2) is the major process involved, and either or both free chlorine and chloramines may be the major species present, depending on the Cl/N ratio. Model simulations showed that increasing the oxidant dose may not always enhance the performance of UV/chlor(am)ine in RO permeate, due to the need for a low free chlorine dose for optimal •OH exposure in RO permeate. Further UV/AOPs modelling showed that it is important to control the NH2Cl concentration to improve the UV/AOP performance in RO permeate, which may be achieved by extending the reaction time after chlorine is added or increasing the applied Cl/N ratio (e.g., increasing chlorine dose). However, these measures only enhance the pollutant percentage removal by about 5 % under the conditions modelled. A simulation tool was developed and is provided to predict the speciation of chlor(am)ine in RO permeate.

Experimental and theoretical insight into carbamazepine degradation by chlorine-based advanced oxidation processes: Efficiency, energy consumption, mechanism and DBPs formation

Chlorine has been widely used in different advanced oxidation processes (AOPs) for micropollutants removal. In this study, different chlorine-based AOPs, namely medium pressure (MP) UV/chlorine, low pressure (LP) UV/chlorine, and in-situ chlorination, were compared for carbamazepine (CBZ) removal efficiency, energy consumption, and disinfection by-products (DBPs) formation. All three processes could achieve nearly 100% CBZ removal, while the reaction time needed by in-situ chlorination was double the time required by UV/chlorine processes. The energy consumed per magnitude of CBZ removed (EE/O) of MP UV/chlorine was 13 times higher than that of LP UV/chlorine, and relative to that of in-situ chlorination process. Accordingly, MP and LP UV/chlorine processes generated one to two orders of magnitude more hydroxyl radicals (•OH) and reactive chlorine species (RCS) than in-situ chlorination. Besides, RCS were the dominant reactive species, contributing to 78.3%, 75.6%, and 71.6% of CBZ removal in MP, LP UV/chlorine, and in-situ chlorination, respectively. According to the Gibbs free energy barriers between CBZ and RCS/•OH calculated based on density functional theory (DFT), RCS had more reaction routes with CBZ and showed lower energy barrier in the main CBZ degradation pathways like epoxidation and formation of iminostilbene. When applied to secondary wastewater effluent, UV/chlorine and in-situ chlorination produced overall DBPs ranging from 104.77 to 135.41 µg/L. However, the production of chlorate during UV/chlorine processes was 15 times higher than that during in-situ chlorination.

Disinfection by-products control in wastewater effluents treated with ozone and biological activated carbon followed by UV/Chlor(am)ine processes

Sequential utilization of ozone (O3) and biological activated carbon (BAC) followed by UV/chlor(am)ine advanced oxidation process (AOP) has drawn attention in water reuse. However, the formation of disinfection by-products (DBPs) in this process is less evaluated. This study investigated the DBP formation and the relevant toxicity during the O3-BAC-UV/chlor(am)ine treatment of sand-filtered municipal secondary effluent. DBP formation in UV/chlorine and UV/dichloramine (NHCl2) processes were compared, where the impact of key operational parameters (e.g., UV wavelength, pH) on DBP formation were comprehensively evaluated. O3-BAC significantly reduced DBP formation potential (DBPFP) (58.2 %). Compared to UV/chlorine AOP, UV/NHCl2 AOP reduced DBP formation by 29.7 % in short-time treatment, while insignificantly impacting on DBPFP (p > 0.05). UV/NHCl2 AOP also led to lower calculated cytotoxicity (67.7 %) and genotoxicity (55.9 %) of DBPs compared to UV/chlorine AOP. Compared to 254 nm UV light, the utilization of 285 nm UV light decreased the formation of DBPs in wastewater treated with the UV/chlorine AOP and UV/NHCl2 AOP by 31.3 % and 19.2 %, respectively. However, the cytotoxicity and genotoxicity in UV/NHCl2 AOP using 285 nm UV light increased by 83.4 % and 58.5 %, respectively, compared to 254 nm. The concentration of DBPs formed in the UV/NHCl2 AOP at pH 8 was 54.3 % lower than that at pH 7, suggesting a better control of DBPs at alkaline condition. In the presence of bromide, UV/NHCl2 AOP tended to generate more brominated DBPs than UV/chlorine AOP. Overall, UV/NHCl2 AOP resulted in lower concentration and toxicity of DBPs compared to UV/chlorine AOP.

Evaluating the Impact of Cl2•- Generation on Antibiotic-Resistance Contamination Removal via UV/Peroxydisulfate

The removal of antibiotic-resistant bacteria (ARB) and antibiotic-resistance genes (ARGs) using sulfate anion radical (SO4•-)-based advanced oxidation processes has gained considerable attention recently. However, immense uncertainties persist in technology transfer. Particularly, the impact of dichlorine radical (Cl2•-) generation during SO4•--mediated disinfection on ARB/ARGs removal remains unclear, despite the Cl2•- concentration reaching levels notably higher than those of SO4•- in certain SO4•--based procedures applied to secondary effluents, hospital wastewaters, and marine waters. The experimental results of this study reveal a detrimental effect on the disinfection efficiency of tetracycline-resistant Escherichia coli (Tc-ARB) during SO4•--mediated treatment owing to Cl2•- generation. Through a comparative investigation of the distinct inactivation mechanisms of Tc-ARB in the Cl2•-- and SO4•--mediated disinfection processes, encompassing various perspectives, we confirm that Cl2•- is less effective in inducing cellular structural damage, perturbing cellular metabolic activity, disrupting antioxidant enzyme system, damaging genetic material, and inducing the viable but nonculturable state. Consequently, this diminishes the disinfection efficiency of SO4•--mediated treatment owing to Cl2•- generation. Importantly, the results indicate that Cl2•- generation increases the potential risk associated with the dark reactivation of Tc-ARB and the vertical gene transfer process of tetracycline-resistant genes following SO4•--mediated disinfection. This study underscores the undesired role of Cl2•- for ARB/ARGs removal during the SO4•--mediated disinfection process.

Effects of pH on the triplet state dissolved organic matter induced free available chlorine decay and radical formation

Triplet state dissolved organic matter (3DOM*) plays a significant role in inducing oxidant decay and radical generation in light-based advanced oxidation processes. However, the effects of pH still need investigation. This work quantitatively analyzed the pH-dependent free available chlorine (FAC) decay and radical formation (i.e., HO• and Cl•) induced by 3DOM* or triplet state photosensitizer (3PS*). Upon UV irradiation at 254 nm, the decay rate of FAC by 3DOM* or 3PS* was the highest at neutral pH, while those by dark reaction of DOM and the direct photolysis of FAC were the highest at acidic conditions. This is attributed to the variation of FAC species, 3DOM* or 3PS* formation, and the reaction rate constants of FAC with 3DOM* or 3PS* at pH 5.0-10.0. 3DOM* and 3PS* formed increasingly with pH varying from 5.0 to 10.0, while their reactivity with FAC decreased due to the speciation from HOCl to OCl-. Radical formation (i.e., HO• and Cl•) from FAC reaction with 3DOM* or 3PS* occurred at all the testing pH range (5.0-10.0). This work highlighted the pH-dependent role of 3DOM* in oxidant decay and radical formation in treating DOM containing waters through oxidant photolysis. ENVIRONMENTAL IMPLICATIONS: Triplet state dissolved organic matter (3DOM*) plays a significant role in inducing oxidant decay and radical generation in light-based AOPs. This study revealed the effects of pH in 3DOM* induced free available chlorine (FAC) decay and radical formation (i.e., HO• and Cl•). With DOM at 3 mgC L-1, FAC decayed fastest under neutral conditions and radical formation (i.e., HO• and Cl•) was enhanced at 5.0-10.0 due to 3DOM* reaction with FAC. These results highlighted the pH-dependent role of 3DOM* in oxidant transformation and radical formation in treating DOM containing waters by AOPs based on oxidant photolysis.

Advanced oxidation of recalcitrant chromophores in full-scale MBR effluent for non-potable reuse of leachate co-treated municipal wastewater

Wastewater non-potable reuse involves further processing of secondary effluent to a quality level acceptable for reuse and is a promising solution to combating water scarcity. Recalcitrant chromophores in landfill leachate challenge the water quality for non-potable reuse when leachate is co-treated with municipal wastewater. In this study, we first use multivariate statistical analysis to reveal that leachate is an important source (with a Pearson's coefficient of 0.82) of recalcitrant chromophores in the full-scale membrane bioreactor (MBR) effluent. We then evaluate the removal efficacies of chromophores by chlorination, breakpoint chlorination, and the chlorination-UV/chlorine advanced oxidation treatment. Conventional chlorination and breakpoint chlorination only partially remove chromophores, leaving a colour level exceeding the standards for non-potable reuse (>20 Hazen units). We demonstrate that pre-chlorination (with an initial chlorine dosing of 20 mg/L as Cl2) followed by UV radiation (with a UV fluence of 500 mJ/cm2) effectively degraded recalcitrant chromophores (>90%). By quantifying the electron donating capacity (EDC) and radical scavenging capacity (RSC) of the reclaimed water, we demonstrate that pre-chlorination reduces EDC and RSC by up to 64%, increases UV transmittance by 32%, and increases radical yields from UV photolysis of chlorine by 1.7-2.2 times. The findings advance fundamental understanding of the alteration of dissolved coloured substances by (photo)chlorination treatment and provide implications for applying advanced oxidation processes in treating wastewater effluents towards sustainable non-potable reuse.

Oxidant-mediated radical reactions of the azole fungicide TEB in aquatic media: Degradation mechanism and toxicity evolution

The degradation of tebuconazole (TEB) by UV/H2O2, UV/NaClO, and ozonation was investigated in this research. The experimental findings unveiled that under the specified conditions, the degradation percentages of TEB were raised to 99% within 40 s, 5 min, and 3 min for UV/H2O2, UV/NaClO and ozonation, respectively. The mineralization percentages within 1 h were 59%, 31% and 8% for the three AOPs. UV/H2O2 and UV/NaClO technologies mainly acted through OH·, while O3 treatment primarily relied on the free radicals such as 1O2 and O2·-. UV-based AOPs achieved almost complete dechlorination within 1 h, whereas O3 treatment had a less effective dechlorination, reaching only 27.61%. Notably, UV alone achieved a dechlorination percentage of 43.07%. By identifying the TPs, we found that the three AOPs shared three similar degradation pathways. The degradation mechanism of TEB mainly entailed the removal of the benzene ring, tert-butyl group and triazolyl group. Toxicity assessment revealed an initial increase followed by a gradual decrease in toxicity for UV/NaClO and O3 treatments, whereas UV/H2O2 treatment exhibited a sustained decrease. This was due to the presence of TP278 and TP303 by UV/NaClO and TP168 and TP153 by ozonation. After estimating the costs of the three AOPs, UV/H2O2 standed out as the best choice for achieving a 90% degradation percentage and exhibiting lower toxicity performance, while O3 treatment was favored for low TOC demands. These research findings provided valuable reference for understanding the degradation mechanism and developing a new technology of the removal of TEB.

Identifying the Transients and Transformation Products in Hydroxyl Radical-Methimazole Reactions Using DFT and UPLC-Q-TOF MS/MS Approaches

Oxidative reactions of the hydroxyl radical (·OH) with methimazole (MMI), an antithyroid drug, are crucial for understanding its fate in oxidizing environments. By synergistically integrating density functional theory and ultraperformance liquid chromatography-quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF MS/MS) techniques, we elucidated the transients and transformation products (TPs) arising from the ·OH-MMI reactions. We probed two hydrogen-atom abstraction (HA) reactions, three radical adduct formation reactions, and single electron transfer (SET) at the M06-2X/6-311++G(d,p)/SMD(water) level. All proposed reaction channels, except for HA from the methyl group and SET, were found to be barrier-free. SET is the dominant oxidation pathway, accounting for 44% of oxidations, as determined by branching ratio analysis. The selenium analogue, MSeI, exhibited minor reactivity differences compared to MMI, yet its overall patterns resembled those of ·OH-MMI reactions. TPs were generated experimentally by reacting MMI with ·OH produced by UV-photolysis of H2O2. Eight TPs were identified from an approximately 24% degradation of MMI using UPLC-Q-TOF MS/MS analysis, and an additional two TPs were identified from the approximately 52% degraded MMI sample. The exact identities of all of the TPs were established through their corresponding fragmentation patterns. This study elucidates the drug's susceptibility to free radical species under physiologically relevant conditions.

Electro-oxidation and UV irradiation coupled method for in-site removing pollutants from human body fluids in swimming pool

A comprehensive study was conducted to investigate how ultraviolet (UV) irradiation combined with electrochemistry (EC) can efficiently remove human body fluids (HBFs) related pollutants, such as urea/creatinine/hippuric acid, from swimming pool water (SPW). In comparison with the chlorination, UV, EC, and UV/chlorine treatments, the EC/UV treatment exhibited the highest removal rates for these typical pollutants (TPs) from HBFs in synthetic SPW. Specifically, increasing the operating current of the EC/UV process from 20 to 60 mA, as well as NaCl content from 0.5 to 3.0 g/L, improved urea and creatinine degradation while having no influence on hippuric acid. In contrast, EC/UV process was resilient to changes in water parameters (pH, HCO3-, and actual water matrix). Urea removal was primarily attributable to reactive chlorine species (RCS), whereas creatinine and hippuric acid removal were primarily related to hydroxyl radical, UV photolysis, and RCS. In addition, the EC/UV procedure can lessen the propensity for creatinine and hippuric acid to generate disinfection by-products. We can therefore draw the conclusion that the EC/UV process is a green and efficient in-situ technology for removing HBFs related TPs from SPW with the benefits of needless chlorine-based chemical additive, easy operation, continuous disinfection efficiency, and fewer byproducts production.

Combined applications of UV and chlorine on antibiotic resistance control: A critical review

Environmental health problems caused by antibiotic-resistant bacteria (ARB) and antibiotic-resistant genes (ARGs) have become a global concern. ARB and ARGs have been continuously detected in various water environments, which pose a new challenge for water quality safety assurance. Disinfection is a key water treatment process to eliminate pathogenic microorganisms in water, and combined chlorine and UV processes (the UV/Cl2 process, the UV-Cl2 process, and the Cl2-UV process) are considered potential disinfection methods to control antibiotic resistance. This review documented the efficacy and mechanism of combined UV and chlorine processes for the control of antibiotic resistance, as well as the effects of chlorine dose, solution pH, UV wavelength, and water matrix on the effectiveness of the processes. There are knowledge gaps in research on the combined chlorine and UV processes for antibiotic resistance control, in particular the UV-Cl2 process and the Cl2-UV process. In addition, changes in the structure of microbial communities and the distribution of ARGs, which are closely related to the spread of antibiotic resistance in the water, induced by combined processes were also addressed. Whether these changes could lead to the re-transmission of antibiotic resistance and harm human health may need to be further evaluated.

Novel UV-LED-driven photocatalysis-chlorine activation for carbamazepine degradation by sulfur-doped NH2-MIL 53 (Fe) composites: Electronic modulation effect and the dual role of chlorine

Chlorine activation-inefficient and the generation of disinfection by-products (DBPs) has indeed limited the application of UV/chlorine process. In this study, the typical metal-organic frameworks (MOFs) NH2-MIL53(Fe) were successfully modified with organic ligands containing sulfur functional groups and applied to construct a novel UV-LED-driven heterogeneous chlorine activation system. The generation of intermediate energy levels and the charge redistribution effect on Fe-S bond facilitated the excitation of electrons and realized the effective separation of photohole (hvb+) and photoelectron (ecb-). The involvement of S-NH2-MIL53(Fe) improved the efficiency of UV-LED/chlorine process by 6 times. The effective activation of HOCl/OCl- by hvb+ and ecb- significantly enhanced the yield of HO· and Cl·. More importantly, HOCl/OCl- played a dual role in UV-LED/chlorine/S-NH2-MIL53(Fe) process as a precursor for the generation of free radicals and a catalyst for the enhancement of HO· yield, which could achieve efficient removal of the target pollutants at lower chlorine doses. In addition, the presence of low-valent sulfur species and ecb- accelerated the cycle of Fe(II)/Fe(III) and in-situ generation of HO· and Cl·. The known generation of DBPs in UV-LED/chlorine/S-NH2-MIL53(Fe) process decreased by 37.9% compared to UV-LED/chlorine process. Developing novel UV-LED/chlorine/S-NH2-MIL53(Fe) processes provided a reliable strategy to efficiently purify actual micro-polluted water bodies.

Chemical-free vacuum ultraviolet irradiation as ultrafiltration membrane pretreatment technique: Performance, mechanisms and DBPs formation

Membrane fouling induced by natural organic matter (NOM) has seriously affected the further extensive application of ultrafiltration (UF). Herein, a simple, green and robust vacuum ultraviolet (VUV) technology was adopted as pretreatment before UF and ultraviolet (UV) technology was used for comparison. The results showed that control effect of VUV pretreatment on membrane fouling was better than that of UV pretreatment, as evidenced by the increase of normalized flux from 0.27 to 0.38 and 0.73 after 30 min UV or VUV pretreatment, respectively. This is related to the fact that VUV pretreatment exhibited stronger NOM degradation ability than UV pretreatment owing to the formation of HO•. The steady-state concentration of HO• was calculated as 3.04 × 10-13 M and the cumulative exposure of HO• reached 5.52 × 10-10 M s after 30 min of VUV irradiation. And the second-order rate constant between NOM and HO• was determined as 1.36 × 104 L mg-1 s-1. Furthermore, fluorescence EEM could be applied to predict membrane fouling induced by humic-enriched water. Standard blocking and cake filtration were major fouling mechanisms. Moreover, extension of UV pretreatment time increased the disinfection by-products (DBPs) formation, the DBPs concentration was enhanced from 322.36 to 1187.80 μg/L after 210 min pretreatment. However, VUV pretreatment for 150 min reduced DBPs content to 282.57 μg/L, and DBPs content continued to decrease with the extension of pretreatment time, revealing that VUV pretreatment achieved effective control of DBPs. The variation trend of cytotoxicity and health risk of DBPs was similar to that of DBPs concentration. In summary, VUV pretreatment exhibited excellent effect on membrane fouling alleviation, NOM degradation and DBPs control under a certain pretreatment time.