Oxidation Mechanisms of the UV/Free Chlorine Process: Kinetic Modeling and Quantitative Structure Activity Relationships

Degradation behaviors of Nabumetone and its metabolite during UV/monochloramine process: Experimental and theoretical study

This study investigated degradation behaviors of a nonsteroidal anti-inflammatory drug Nabumetone (NMT) and its major metabolite 6-methoxy-2-naphthylacetic acid (MNA) in the coupling process of ultraviolet and monochloramine (UV/NH2Cl). The second-order rate constants of the contaminants reacting with reactive radicals (HO•, Cl•, Cl2•⁻, and CO3•⁻) were determined by laser flash photolysis experiments. HO• and Cl• contributed predominantly with 52.3% and 21.7% for NMT degradation and 60.8% and 22.3% for MNA degradation. The presence of chlorides retarded the degradation of NMT, while promoted the destruction of MNA, which was ascribed to the photosensitization effects of MNA under UV irradiation. Density functional theory (DFT) calculations revealed that radical adduct formation (RAF) was dominant pathway for both HO• and Cl• reacting with the contaminants, and hydrogen atom transfer (HAT) preferred to occur on side chains of NMT and MNA. NMT reacted with NO2• through single electron transfer (SET) with the second-order rate constant calculated to be 5.35 × 107 (mol/L)-1 sec-1, and the contribution of NO2• was predicted to be 13.0% of the total rate constant of NMT in pure water, which indicated that NO2• played a non-negligible role in the degradation of NMT. The acute toxicity and developmental toxicity of NMT were enhanced after UV/NH2Cl treatment, while those of MNA were alleviated. The transformation products of both NMT and MNA exhibited higher mutagenicity than their parent compounds. This study provides a deep understanding of the mechanism of radical degradation of NMT and MNA in the treatment of UV/NH2Cl.

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.

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.

Quantitative structure-activity relationships for the reaction kinetics of trace organic contaminants with one-electron oxidants

Understanding the reactivity between trace organic contaminants (TrOCs) and radicals involved in advanced oxidation processes (AOPs) is necessary for a good process design, but the experimentally determined rate constants (k values) are not sufficient for numerous artificial TrOCs. Thus, the development of quantitative structure-activity relationships (QSARs) for predicting k values may be an effective way to address this limitation. In this work, we developed QSARs for the reactions of TrOCs with AOP-related one-electron oxidants. Specifically, 15 QSARs using Hammett constants and 8 cross-correlations were developed based on the k values of over 400 reactions between TrOCs (most contain electron-rich moieties, such as phenol, aniline, and alkoxy benzene) and 5 one-electron oxidants (SO4˙-, Br˙, Br2˙-, Cl2˙-, and CO3˙-). Overall, the developed QSARs show a good predictive performance with 94% (237/251, for Hammett constant-based QSARs) and 80% (218/274, for cross-correlations) of the k values predicted within a factor of 3. All the Hammett constant-based QSARs show negative slope values and all cross-correlations show positive relationships, suggesting all 5 one-electron oxidants mainly share similar electrophilic mechanisms with the TrOCs highlighted in this work. Previous QSAR studies on the k values of one-electron oxidants were compared and integrated into their model analysis. Furthermore, k values predicted herein from the QSARs were used to evaluate the degradation of TrOCs during UV/persulfate and UV/chlorine treatment in multiple wastewater matrices, which were demonstrated to be useful. Finally, remarks on the use of the developed QSARs were presented.

Photochemical Transformation of Free Chlorine Induced by Triplet State Dissolved Organic Matter

Photolysis of free chlorine is an increasingly recognized approach for effectively inactivating microorganisms and eliminating trace organic contaminants. However, the impact of dissolved organic matter (DOM), which is ubiquitous in engineered water systems, on free chlorine photolysis is not yet well understood. In this study, triplet state DOM (³DOM*) was found to cause the decay of free chlorine for the first time. By using laser flash photolysis, the scavenging rate constants of triplet state model photosensitizers by free chlorine at pH 7.0 were determined to be in the range of (0.26-3.33) × 10⁹ M^-1 s^-1. ³DOM*, acting as a reductant, reacted with free chlorine at an estimated reaction rate constant of 1.22(±0.22) × 10⁹ M^-1 s^-1 at pH 7.0. This study revealed an overlooked pathway of free chlorine decay during UV irradiation in the presence of DOM. Besides the DOM's light screening ability and scavenging of radicals or free chlorine, ³DOM* played an important role in the decay of free chlorine. This reaction pathway accounted for a significant proportion of the decay of free chlorine, ranging from 23 to 45%, even when DOM concentrations were below 3 mg_C L^-1 and a free chlorine dose of 70 μM was present during UV irradiation at 254 nm. The generation of HO^• and Cl^• from the oxidation of ³DOM* by free chlorine was confirmed by electron paramagnetic resonance and quantified by chemical probes. By inputting the newly observed pathway in the kinetics model, the decay of free chlorine in UV₂₅₄-irradiated DOM solution can be well predicted.

UV-chlorine advanced oxidation for potable water reuse: A review of the current state of the art and research needs

This paper reports conclusions from a recent study completed for the Water Research Foundation and the State of California to offer guidance on UV-chlorine advanced oxidation for potable water reuse. The fundamentals of UV-chlorine advanced oxidation are discussed, and lessons learned from some of the early adopters of this technology are presented. Important highlights include the significant impact of ammonia and chloramines on UV-chlorine treatment, challenges associated with predicting UV-chlorine performance due to complex photochemistry, and an ongoing need to monitor potential byproducts and transformation products when employing any form of advanced oxidation for potable reuse.

Rate constants for the reactions of chloride monoxide radical (ClO•) and organic molecules of environmental interest

ClO^• plays a key role in the UV/chlorine process besides Cl^•, Cl2^• - , and ^•OH. In many experiments, ClO^• proved to be the main reactant that destroyed the organic pollutants in advanced oxidation process. About 200 rate constants of ClO^• reactions were collected from the literature, grouped together according to the chemical structure, and the molecular structure dependencies were evaluated. In most experiments, ClO^• was produced by the photolytic reaction of HClO/ClO-. For a few compounds, the rate constants were determined by the absolute method, pulse radiolysis. Most values were obtained in steady-state experiments by competitive technique or by complex kinetic calculations after measuring the pollutant degradation in the UV/chlorine process. About 30% of the listed rate constant values were derived in quantum chemical or in structure-reactivity (QSAR) calculations. The values show at least six orders of magnitude variations with the molecular structure. Molecules having electron-rich parts, e.g., phenol/phenolate, amine, or sulfite group have high rate constants in the range of 10⁸-10⁹ mol^-1 dm³ s^-1. ClO^• is inactive in reactions with saturated molecules, alcohols, or simple aromatic molecules.

Thiosulfate enhanced Cu(II)-catalyzed Fenton-like reaction at neutral condition: Critical role of sulfidation in copper cycle and Cu(III) production

Thiosulfate (S₂O₃^2-) has been proven to be an effective promoter of Fenton-like reactions by accelerating the metal ions cycle. However, up to now, little is known about the role of sulfur transformation and intermediate sulfur in the regulation of metal chemical cycle and reactive species production. Herein, free Cu(II) was selected as catalyst for the activation of H₂O₂. The introduction of S₂O₃^2- significantly enhanced the degradation of benzoic acid, and the degradation rate (k_obs) was 5.8 times that of Cu(II)/H₂O₂ system. The kinetic model revealed the transformation of sulfur species and demonstrated that sulfides (i.e., HS^-/S^2-, S₂O₃^2-) and S⁰ were the dominant electron donor for the reduction of Cu(II) into Cu(I). Consequently, the reduction and complexation roles of S₂O₃^2- significantly resolve the rate-limiting step and broaden the pH range of in Fenton-like reactions. Evidence for the critical role of high-valent copper (Cu(III)) and HO^• on BA degradation was obtained by scavengers experiments, electron paramagnetic resonance and fluorescent probes. Meanwhile, the Cu(II)/H₂O₂/S₂O₃^2- system also exhibited satisfactory anti-interference ability of the various matrix. Overall, this study offers mechanistic insight into sulfidation in Cu chemical cycle and Cu(III) generation, and highlights the potential of S₂O₃^2- for Fenton-like reactions to control pollutants.

Insight into the mechanisms of trichloronitromethane formation by vacuum ultraviolet: QSAR model and FTICR-MS analysis

Vacuum ultraviolet (VUV) photolysis is recognized as an environmental-friendly treatment process. Nitrate (NO₃^-) and natural organic matter (NOM) are widely present in water source. We investigated trichloronitromethane (TCNM) formation during chlorination after VUV photolysis, because TCNM is an unregulated highly toxic disinfection byproduct. In this study: (1) we found reactive nitrogen species that is generated under VUV photolysis of NO₃^- react with organic matter to form nitrogen-containing compounds and subsequently form TCNM during chlorination; (2) we found the mere presence of 0.1 mmol/L NO₃^- can result in the formation of up to 63.96 µg/L TCNM; (3) we found the changes in pH (6.0-8.0), chloride (1-4 mmol/L), and bicarbonate (1-4 mmol/L) cannot effectively diminish TCNM formation; and, (4) we established the quantitative structure-activity relationship (QSAR) model, which indicated a linear relationship between TCNM formation and the Hammett constant (σ) of model compounds; and, (5) we characterized TCNM precursors in water matrix after VUV photolysis and found 1161 much more nitrogen-containing compounds with higher aromaticity were generated. Overall, this study indicates more attention should be paid to reducing the formation risk of TCNM when applying VUV photolysis process at scale.

Analysis of degradation kinetic modeling and mechanism of chlorinated-halonitromethanes under UV/monochloramine treatment

Chlorinated-halonitromethanes (Cl-HNMs) including chloronitromethane (CNM), dichloronitromethane (DCNM), and trichloronitromethane (TCNM) are nitrogenous disinfection by-products, which have high cytotoxicity and genotoxicity to human. This study aimed to investigate the degradation kinetic modeling and mechanism of Cl-HNMs under monochloramine activated by ultraviolet of 254 nm (UV/NH₂Cl) treatment. The first-principle kinetic model of UV/NH₂Cl process was developed to simulate Cl-HNMs degradation. Of note, the second-order rate constants of Cl-HNMs reacting with HO• (∼10⁸ M^-1 s^-1), Cl• (k_{Cl•,CNM or DCNM} = ∼10¹⁰ M^-1 s^-1, k_Cl•,TCNM = ∼10² M^-1 s^-1), Cl₂•^- (k_{Cl•,CNM or DCNM} = ∼10⁹ M^-1 s^-1, k_Cl•,TCNM = ∼10¹ M^-1 s^-1), ClO• (∼10⁵-10⁶ M^-1 s^-1) and CO₃•^- (∼10⁶-10⁷ M^-1 s^-1) were obtained by the first-principle kinetic model. Overall, Cl-HNMs degradation under UV/NH₂Cl treatment was successfully predicted by the kinetic model under various conditions. It was found that UV (>60%) was dominant in Cl-HNMs degradation, followed by HO• (3.8%-24.5%), reactive chlorine species (RCS, 0.9%-28.8%) and CO₃•^- (0-26.1%). Among the contributions of RCS, Cl• and Cl₂•^- were main radicals in the degradation of CNM and DCNM, while ClO• was responsible for the abatement of TCNM. The minimum EE/O values under UV/NH₂Cl treatment were approximately 30% lower than those under UV treatment. Finally, the possible degradation pathways were proposed, including hemolytic/heterolytic cleavage of Cl-HNMs by UV irradiation, hydrogen abstraction/electron transfer of CNM and DCNM and adduct reaction of TCNM by free radicals. This study based on the kinetic model is beneficial to predict and control the concentrations of Cl-HNMs under UV/NH₂Cl treatment.

Radical chemistry, degradation mechanism and toxicity evolution of BPA in the UV/chlorine and UV/H2O2

UV-assisted advanced oxidation processes (AOPs) are widely used and studied in degradation of bisphenol A (BPA). However, detailed information on their radical chemistry and degradation mechanisms is still lacking. In this study, degradation of BPA was comparatively evaluated to investigate the radical mechanisms, products and the toxicity variation in UV/chlorine and UV/H₂O₂ processes. In comparison with UV/H₂O₂, UV/chlorine had a higher BPA degradation efficiency and higher pH-dependency due to chlorination and the synergy of •OH and RCS. The •OH and Cl• played a pivotal role as the primary radicals in BPA degradation by UV/chlorine process at all pH investigated (6-8). The relative contributions of the secondary radicals ClO• gradually decreased with a variation of pH from 6 to 8 in this process. Presence of HCO₃^─ and HA inhibited BPA degradation to different extents in UV/chlorine process, while the effect of Cl^─ could be neglected. According to the identified transformation products, chlorination (major), hydroxylation and breakage of the isopropylidene chain were BPA decomposition pathways in the UV/chlorine system. In the UV/H₂O₂ system, only hydroxylation (major) and breakage of the isopropylidene chain occurred. The toxicity analysis, based on the proposed degradation pathways, indicated that the generation of chlorinated products in the UV/chlorine system led to a higher toxicity of the resulting mixture than in the UV/H₂O₂ system. Although UV/chlorine has an excellent BPA degradation effect and it is cost-effective, the possible environmental risk should be carefully considered when UV/chlorine system is used to remove BPA in real waters.

Enhanced trichloronitromethane formation during chlorine-UV treatment of nitrite-containing water by organic amines

This study explored the risk of trichloronitromethane (TCNM) formation during chlorination of the nitrite-containing water after pre-chlorination and subsequent UV irradiation (i.e., the chlorine-UV process). The competitive reaction between amino acid (AA) and NO₂^- for chlorine produced organic chloramine and reduced the oxidation from NO₂^- to NO₃^-, resulting in a significant enhancement of TCNM in the presence of AA (>5.52 μg L^-1) as compared to the absence of AA (0.42 μg L^-1). The generation of HO^• during UV photolysis of organic chloramines was confirmed. Among the process parameters, pre-chlorination time (from 5 min to 30 min) had no significant effect on TCNM formation; the highest TCNM formation occurred at pH 7 (from pH 6 to pH 8); prolonged UV irradiation time (from 5 min to 30 min) and increased chlorine to AA ratio (Cl₂:AA) (from 1 to 3) decreased the TCNM formation. The hydroxylated, chlorinated and nitrosated products were detected. The quantum chemical calculation results indicated the attack of NO₂^• was more likely to occur at the meta and para positions of benzoic acid (BZA), because of the steric hindrance of the carboxylic group in BZA to the ortho position. Based on the results of the toxicity assessment, pre-chlorination with a higher chlorine dosage could be an effective method of controlling both TCNM formation and acute toxicity. Overall, the results of this study contributed to the understanding of the TCNM formation mechanism as well as optimizing the parameters of the chlorine-UV process to reduce the risk of TCNM formation.

Degradation of micropollutants in flow-through UV/chlorine reactors: Kinetics, mechanism, energy requirement and toxicity evaluation

The degradation of three micropollutants (i.e., atrazine (ATZ), sulfamethoxazole (SMX) and metoprolol (MET)) was comprehensively investigated in flow-through UV/chlorine reactors. Results showed that the micropollutants degradation fitted well with pseudo-first-order kinetics (R² > 0.92) with the order of rate constants following SMX > MET > ATZ. The developed steady-state approximation (SSA) model was roughly applicable in flow-through UV/chlorine reactors with the predictions deviated within 44%. UV photolysis here stood as the major degradation pathway for ATZ while the contribution of non-radical processes (UV photolysis and chlorination) to SMX degradation increased as the reactor internal diameter enlarged. The degradation rates were reduced to varying extents with complex water matrices (chloride, bicarbonate and dissolved organic matter (DOM)) where the inhibition from the DOM was most prominent (up to 73.6%). Although reactors with a larger internal diameter resulted in reduced degradation rate constants, the energy requirements were also lowered. The E_EO values of micropollutants degradation by UV/chlorine fell mostly within 1.0 kWh m^-3 order^-1 in deionized water and under different water matrices. The acute toxicity was observed to be higher after UV/chlorine treatment in tap water, but still stayed low in general. This study revealed the different kinetics and mechanisms of micropollutants degradation in flow-through reactors and demonstrated the potential of the UV/chlorine process in terms of low energy consumption and acute toxicity.

An efficient Fe2+ assisted UV/electrogenerated-chlorine process for carbamazepine degradation: The role of Fe(IV)

To improve the performance and solve the restrictions of UV/chlorine process (e.g., the narrow pH application range and high disinfection by-products (DBPs) formation), a Fe²⁺ assisted advanced oxidation process with electrochemically generated chlorine (UV/E-Cl/Fe²⁺) was proposed for carbamazepine (CBZ) degradation, which eliminated CBZ (5 mg/L) within 4 min under the optimal conditions. Compared with UV/electro-generated chlorine (UV/E-Cl) and anodic oxidation-chlorination/Fe²⁺ (AO-Cl/Fe²⁺) processes, the apparent first-order kinetics constant in UV/E-Cl/Fe²⁺ increased by 2.56 and 3.18 times respectively, and the energy consumption was lower (1.15 kWh/m³-log). Simultaneously, the pH application range could be expanded to 9, and DBPs formed in this process were 17.1% less than those in UV/E-Cl. Through quenching tests, electron paramagnetic resonance (EPR) experiments, measurement of ^•OH concentration, quantification of methyl phenyl sulfoxide (PMSO) and benzosulfone (PMSO₂) and processes comparison, possible CBZ degradation pathways and mechanism of UV/E-Cl/Fe²⁺ were proposed, in which Fe(IV) played the dominant role in the early stage, while the production of radicals (i.e., ^•OH and Cl^•) was enhanced with the increase of chlorine generation, accelerating the CBZ removal. Furthermore, this process demonstrated wide application prospect in treating various contaminants and real wastewaters. In conclusion, this study offers an effective and energy-efficient method for organic pollutants degradation.

Electrochemical Advanced Oxidation of Perfluorooctanoic Acid: Mechanisms and Process Optimization with Kinetic Modeling

Electrochemical advanced oxidation processes (EAOPs) are promising technologies for perfluorooctanoic acid (PFOA) degradation, but the mechanisms and preferred pathways for PFOA mineralization remain unknown. Herein, we proposed a plausible primary pathway for electrochemical PFOA mineralization using density functional theory (DFT) simulations and experiments. We neglected the unique effects of the anode surface and treated anodes as electron sinks only to acquire a general pathway. This was the essential first step toward fully revealing the primary pathway applicable to all anodes. Systematically exploring the roles of valence band holes (h⁺), hydroxyl radicals (HO^•), and H₂O, we found that h⁺, whose contribution was previously underestimated, dominated PFOA mineralization. Notably, the primary pathway did not generate short-chain perfluoroalkyl carboxylic acids (PFCAs), which were previously thought to be the main degradation intermediates, but generated other polyfluorinated alkyl substances (PFASs) that were rapidly degraded upon formation. Also, we developed a simplified kinetic model, which considered all of the main processes (mass transfer with electromigration included, surface adsorption/desorption, and oxidation on the anode surface), to simulate PFOA degradation in EAOPs. Our model can predict PFOA concentration profiles under various current densities, initial PFOA concentrations, and flow velocities.

Rate constants of chlorine atom reactions with organic molecules in aqueous solutions, an overview

Rate constants of chlorine atom (Cl^•) reactions (k_Cl•) determined using a large variation of experimental methods, including transient measurements, steady-state and computation techniques, were collected from the literature and were discussed together with the reaction mechanisms. The k_Cl• values are generally in the 10⁸-10⁹ mol^-1 dm³ s^-1 range when the basic reaction between the Cl^• and the target molecule is H-atom abstraction. When Cl^• addition to double bonds dominates the interaction, the k_Cl• values are in the 1 × 10⁹-2 × 10¹⁰ mol^-1 dm³ s^-1 range. In the k_Cl• = 1 × 10¹⁰-4 × 10¹⁰ mol^-1 dm³ s^-1 range, single-electron-transfer reactions may also contribute to the mechanism. The Cl^• reactions with organic molecules in many respects are similar to those of ^•OH, albeit Cl^• seems to be less selective as ^•OH. However, there is an important difference, as opposed to Cl^• in the case of ^•OH single-electron-transfer reactions have minor importance. The uncertainty of Cl^• rate constant determinations is much higher than those of ^•OH. Since Cl^• reactions play very important role in the emerging UV/chlorine water purification technology, some standardization of the rate constant measuring techniques and more k_Cl• measurements are recommended.

Wastewater treatment plants act as essential sources of microplastic formation in aquatic environments: A critical review

According to extensive in situ investigations, the microplastics (MPs) determined in current wastewater treatment plants (WWTPs) are mostly aged, with roughened surfaces and varied types of oxygen-containing functional groups (i.e., carbonyl and hydroxyl). However, the formation mechanism of aged MPs in WWTPs is still unclear. This paper systematically reviewed MP fragmentation and generation mechanisms in WWTPs at different treatment stages. The results highlight that MPs are prone to undergo physical abrasion, biofouling, and chemical oxidation-associated weathering in WWTPs at different treatment stages and can be further decomposed into smaller secondary MPs, including in nanoplastics (less than 1000 nm or 100 nm in size), suggesting that WWTPs can act as a formation source for MPs in aquatic environments. Sand associated mechanical crashes in the primary stage, microbes in active sewage sludge-related biodegradation in the secondary stage, and oxidant-relevant chemical oxidation processes (light photons, Cl₂, and O₃) in the tertiary stage are the dominant causes of MP formation in WWTPs. For MP formation mechanisms in WWTPs, external environmental forces (shear and stress forces, UV radiation, and biodegradation) can first induce plastic chain scission, destroy the plastic molecular arrangement, and create abundant pores and cracks on the MP surface. Then, the physicochemical properties (modulus of elasticity, tensile strength and elongation at break) of MPs shift consequently and finally breakdown into smaller secondary MPs or nanoscale plastics. Overall, this review provides new insights to better understand the formation mechanism, occurrence, fate, and adverse effects of aged microplastics/nanoplastics in current WWTPs.

Influence of nitrate/nitrite on the degradation and transformation of triclosan in the UV based disinfection

This study investigated the influence of nitrate/nitrite on the degradation and transformation pathway of triclosan (TCS) in UV, UV/peracetic acid (PAA) and UV/HClO processes. The results indicated that the function of nitrate/nitrite significantly depended on the UV source and wavelength, especially nitrate. Generally, the presence of nitrate decreased the direct photo-degradation of TCS in the UV based disinfection. In the LED-UV and LED-UV/HClO processes, the presence of nitrate improved the radical oxidation, and transformation pathway of TCS was varied accordingly. However, nitrate more played a role of photo-competitor in the UV/PAA process, and the reactive nitrogen species (RNS) was difficult to participant in the degradation of TCS due to low redox potential. Compared to nitrate, the presence of nitrite decreased the degradation of TCS in three different UV based disinfection processes. Under UV irradiation, nitrite primarily acted as an irradiation competitor and radical scavenger. Thus, the indirect photo-degradation of TCS was reduced. Noticeably, nitrate/nitrite were the improtant precersors of nitrogenous products in the UV base disinfection. Many new nitrogenous products were identified. But RNS preferentially reacted with the intermediates by -NO₂ addition compared to directly reacted with TCS.

The structure-activity relationship of aromatic compounds in advanced oxidation processes：a review

Advanced oxidation processes (AOPs) are widely used as efficient technologies to treat highly toxic and harmful substances in wastewater. Taking the most representative aromatic compounds (monosubstituted benzenes, substituted phenols and heterocyclic compounds) as examples, this paper firstly introduces their structures and the structural descriptors studied in AOPs before, and the influence of structural differences in AOPs with different reactive oxygen species (ROS) on the degradation rate was discussed in detail. The structure-activity relationship of pollutants has been previously analyzed through quantitative structure-activity relationship (QSAR) model, in which ROS is a very important influencing factor. When electrophilic oxidative species attacks pollutants, aromatic compounds with electron donating groups are more favorable for degradation than aromatic compounds with electron donating groups. While nucleophilic oxidative species comes to the opposite conclusion. The choice of advanced oxidation processes, the synergistic effect of various active oxygen species and the used catalysts will also change the degradation mechanism. This makes the structure-dependent activity relationship uncertain, and different conclusions are obtained under the influence of various experimental factors.

Molecular insights into formation of nitrogenous disinfection byproducts from algal organic matter in UV-LEDs/chlorine process based on FT-ICR analysis

Eutrophication is a globally concerned issue, which brings algal cells and algal organic matter (AOM) into drinking water treatment plants. AOM is an important branch of nitrogenous disinfection byproduct (N-DBP) precursors. The variation of AOM composition in UV-LEDs/chlorine process, and its relationship with N-DBP formation still remain much uncertainty. Herein, we used fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to investigate AOM transformation in UV-LEDs/chlorine process, with UV₂₈₅ and UV₃₆₅ as light source, and screen for typical precursors of N-DBPs. We found that more nitrogen-containing compounds were generated after UV-LEDs/chlorine process, leading to the larger formation of N-DBPs in postchlorination. Compounds such as lignin, proteins, and amino sugars tends to be oxidized by reactive species in UV-LEDs/chlorine process. Further, compounds with higher O/C and higher weighted average double bond equivalence (DBE_w) are easier to form N-DBPs, including dichloroacetonitrile and trichloronitromethane. Also, influence factors including pH, UV fluence, post-chlorination time and bromide concentration on N-DBP formation were evaluated. The results show that N-DBP formation generally followed the order of UV₂₈₅/chlorine-postchlorination, UV₃₆₅/chlorine-postchlorination, and direct chlorination. Our study provides comprehensive information on N-DBP formation from AOM in UV-LEDs/chlorine-postchlorination from molecular levels.

Micropollutant abatement by the UV/chloramine process in potable water reuse: A review

The need in using reclaimed water increased significantly to address the water shortage and its continuing quality deterioration in sustaining societal development. Degrading micropollutants in wastewater treatment plant effluents is one of the most important tasks in supplying safe drinking water, which is often achieved by full advanced treatment technologies (FATs), including reverse osmosis (RO) and the UV-based advanced oxidation process (AOP). As an emerging AOP, UV/chloramine process shows many noteworthy advantages in the scenario of potable water reuse, including membrane biological fouling control by chloramine, producing highly reactive radicals (e.g., Cl^•, HO^•, Cl₂^•-, and reactive nitrogen-containing species) to degrade the RO permeated pollutants, and acting as long-lasting disinfectant in the potable water distribution system. In addition, chloramine is often designedly produced by taking advantage of the ammonia in source. Thus, UV/chloramine processes gather much attention from researcher and published papers on UV/chloramine process have drastically increased since 2016, which were thoroughly reviewed in this paper. The fundamentals of chloramine photolysis, including the photolysis kinetics, the quantum yield, the generation and transformation of radicals and the final products, were scrutinized. Further, the impacts of reaction conditions such as pH, chloramine dosage and water matrix on the degradation of micropollutants by the UV/chloramine process are discussed. Moreover, the formation potential of disinfection by-products is debated. The opportunity of application of the UV/chloramine process in real-world practice is also presented, emphasizing the need for extensive efforts to remove currently prevalent knowledge roadblocks.

Kinetics and Mechanism of Degradation of Reactive Radical-Mediated Probe Compounds by the UV/Chlorine Process: Theoretical Calculation and Experimental Verification

The UV/chlorine process, by combining chlorination with UV irradiation, has been recently considered as a highly efficient advanced oxidation process (AOP) technology in water treatment. Nitrobenzene (NB), benzoic acid (BA), and p-chlorobenzoic acid (pCBA) are widely used as model probe compounds in the UV/chlorine system to calculate the second-order rate constants of the specific radical reaction with target contaminates by a competitive kinetics method. A comprehensive understanding of probe compounds' reaction mechanism with reactive radicals is critical for investigation of the UV/chlorine reaction system. Here, we evaluated the radical-mediated reaction kinetics and mechanism of NB, BA, and pCBA in the UV/chlorine process using theoretical calculations and experimental studies. The main reactive radicals ^•OH, ^•ClO, and ^•Cl in the UV/chlorine process for the initial reaction with NB, BA, and pCBA can be explained by H-abstraction and addition pathways. The ΔE ^0,≠ values for the ^•OH reaction with NB, BA, and pCBA were in the range of 5.0-8.0, 3.7-8.2, and 3.4-8.2 kcal mol^-1, respectively. The ΔE ^0,≠ values for ^•ClO and ^•Cl reactions with these three probe compounds were higher than those of ^•OH, indicating slower ^•ClO- and ^•Cl-initiated reactions than that of the ^•OH-initiated reaction. The theoretically calculated radical-mediated reaction kinetic rate constants (k _CP ^C) for NB, BA, and pCBA were 4.58 × 10^-3, 1.28 × 10^-2, and 1.6 × 10^-2 s^-1, respectively, which was consistent with the experimentally determined pseudo-first-order rate constant (k _CP ^RR) in the UV/chlorine process. Interestingly, theoretical calculations showed that ^•ClO and ^•Cl played an important role in subsequent reactions of NB-OH radicals, converting to hydroxylated and chlorinated products, which were further confirmed by experimental products' identification. The findings from this study indicated that quantum chemistry calculations provide an effective means to investigate the reaction kinetics and mechanism of chemicals in the UV/chlorine process.

UV-induced activation of organic chloramine: Radicals generation, transformation pathway and DBP formation

Organic chloramines of little disinfection efficacy commonly exist in disinfection process (chlor(am)ination) due to the wide presence of organic amines in water, of which N-chlorodimethylamine (CDMA) is a typical one. For the first time, UV photolysis for the activation of CDMA was investigated. UV photolysis caused the cleavage of N-Cl bond in CDMA to form Cl^• and subsequently HO^•, both of which are dominant contributors to the destruction of model contaminant bisphenol A (BPA). Typical spectra of HO^• were detected by electron paramagnetic resonance (EPR) experiments, while spectra of reactive nitrogen species (RNS) were not detected during UV photolysis of CDMA. The increase of pH (6.0-8.0), HCO₃^-/CO₃^2-, Cl^- and nature organic matter inhibited the degradation of BPA. We proposed pathways of CDMA and BPA degradation based on the identified transformation products. UV photolysis of CDMA and BPA reduced the formation of N-nitrosodimethylamine (NDMA) at pH 8.0, but increased the formation of trichloronitromethane (TCNM) at pH 7.0 and 8.0. The increasing toxicity and the formation of TCNM and NDMA gave us a hint that formation of organic chloramines should be concerned.

Misconceptions about the Chemistry of Aqueous Chlorine Atoms and HClOH•(aq), and a Revised Mechanism for the Photochemical Peroxydisulfate/Chloride Reaction

It is widely considered that aqueous chlorine atoms (Cl•) convert to the species HClOH• with a half life of about 3 µs and that this species plays an important role...

Elimination of β-N-methylamino-l-alanine (BMAA) during UV/chlorine process: Influence factors, transformation pathway and DBP formation

As a new cyanobacterial neurotoxin generated by cyanobacteria, BMAA was closely related to amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS/PDC). In this study, the degradation of BMAA by UV/chlorine process was investigated under the impacts of chlorine dosage, NOM dosage, pH and alkalinity. Results showed that only 10% of BMAA was removed by UV irradiation and 46.8% by chlorination in 5 min, however, 98.6% of BMAA was removed by UV/chlorine process in 5 min. The reaction rates were increased under alkaline conditions, but all achieved complete degradation in 5 min. Besides, HCO₃^- had slight inhibition, while NOM had significant inhibition on the degradation of BMAA. Furthermore, based on the detected degradation products of BMAA during UV/chlorine process, the possible degradation pathways were concluded. Overall, outcomes of this study exhibited that the use of the UV/chlorine process for BMAA degradation was appropriate in practical applications.

Degradation of micropolluants in flow-through VUV/UV/H2O2 reactors: Effects of H2O2 dosage and reactor internal diameter

The degradation of atrazine (ATZ), sulfamethoxazole (SMX) and metoprolol (MET) in flow-through VUV/UV/H₂O₂ reactors was investigated with a focus on the effects of H₂O₂ dosage and reactor internal diameter (ID). Results showed that the micropollutants were degraded efficiently in the flow-through VUV/UV/H₂O₂ reactors following the pseudo first-order kinetics (R² > 0.92). However, the steady-state assumption (SSA) kinetic model being vital in batch reactors was found invalid in flow-through reactors where fluid mixing was less sufficient. With the increase of H₂O₂ dosage, the ATZ removal efficiency remained almost constant while the SMX and MET removal was enhanced to different extents, which could be explained by the different reactivities of the pollutants towards HO^•. A larger reactor ID resulted in lower degradation rate constants for all the three pollutants on account of the lower average fluence rate, but the change in energy efficiency was much more complicated. In reality, the electrical energy per order (E_EO) of the investigated VUV/UV/H₂O₂ treatments ranged between 0.14-0.20, 0.07-0.14 and 0.09-0.26 kWh/m³/order for ATZ, SMX and MET, respectively, with the lowest E_EO for each pollutant obtained under varied H₂O₂ dosages and reactor IDs. This study has demonstrated the efficiency of VUV/UV/H₂O₂ process for micropollutant removal and the inadequacy of the SSA model in flow-through reactors, and elaborated the influential mechanisms of H₂O₂ dosage and reactor ID on the reactor performances.

The role of carbonate radicals on the kinetics, radical chemistry, and energy requirement of UV/chlorine and UV/H2O2 processes

Quantitative insight into the HCO₃^--dependent degradation kinetics is critical to improve understanding of the UV processes for the most-cost effective application. In this study, we developed a kinetic model to precisely predict the kinetics in UV/H₂O₂ and UV/chlorine processes. The second-order rate constants of HO, Cl, ClO, Cl₂^-, and CO₃^- with carbamazepine (CBZ) were fitted as 1.3 × 10⁹, 1.9 × 10⁹, 1.8 × 10⁶, 1.1 × 10⁵, and 4.5 × 10⁶ M^-1 s^-1, respectively. Based on the model, we investigated the significant impact of bicarbonate (HCO₃^-) and subsequently generated carbonate radical (CO₃^-) on CBZ degradation, radical chemistry, and energy requirement of UV/H₂O₂ and UV/chlorine processes. The presence of HCO₃^- inhibited CBZ degradation in UV/H₂O₂ and UV/chlorine processes to different degree. Contributions of HO, Cl, ClO, Cl₂^-, and CO₃^- to CBZ degradation in UV/H₂O₂ and UV/chlorine processes in the absence/presence of HCO₃^- were investigated. HO and CO₃^- make comparable contributions to CBZ degradation in UV/H₂O₂ process in the presence of HCO₃^- (2 mM), while ClO is always the main contributor at various HCO₃^- concentration of 0-2 mM. Furthermore, the presence of HCO₃^- in both processes increased the corresponding EE/O, when CBZ was degraded by an order of magnitude. Overall, HCO₃^- and CO₃^- influence the reactions and mechanism of UV/H₂O₂ and UV/chlorine processes, and have higher impact on UV/H₂O₂ process.

Modeling degradation kinetics of gemfibrozil and naproxen in the UV/chlorine system: Roles of reactive species and effects of water matrix

The UV/chlorine system has been regarded as an efficient oxidation technology for the removal of aqueous micropollutants. However, the roles of the possible radical species for this system on the elimination under environmentally relevant conditions/real waters were still largely unknown. Herein, the specific roles of radical species in the UV/chlorine oxidation degradation of gemfibrozil and naproxen as representative micropollutants were quantified by a steady-state kinetic prediction model considering the effects of water matrices. Overall, the model predicted results are consistent with the experimental data well. •OH and reactive chlorine species (RCS, such as Cl•, ClO•, and Cl₂•^-) contributions to gemfibrozil and naproxen degradation were water matrix specific. In pure water, both primary reactive species (i.e., •OH and Cl•) and secondary species ClO• dominated gemfibrozil and naproxen degradation, and their individual and the sum of the contributions to degradation rates reduced with pH increase of from 5 to 9. In the presence of Cl^-, we found that Cl₂•^- and in particular ClO• were responsible for the enhanced degradation with increasing Cl^- concentrations due to the considerable ClO• reactivity of gemfibrozil (1.93 × 10⁹ M^-1 s^-1) and naproxen (9.24 × 10⁹ M^-1 s^-1) and the rapid transformation of Cl₂•^- to ClO•. The presence of HCO₃^- notably facilitated the degradation in the UV/chlorine process because of the generation of CO₃•^-. CO₃•^- showed high reactivity with gemfibrozil and naproxen corresponding to respective second-order reaction rate constants of 2.45 × 10⁷ and 3.50 × 10⁷ M^-1 s^-1. Dissolved organic matter induced obvious scavenging for •OH, Cl•, and ClO• and greatly retarded the degradation. The constructed model considering the effects of above water matrix has successfully predicted the oxidation degradation kinetics in real waters, and both •OH and CO₃•^- are the predominant reactive species in the degradation. This study is helpful for comprehensive understanding the roles of possible radical species in micropollutant removal by UV/chlorine oxidation under real water matrix.

Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review

Natural waters, water droplets in the air at coastal regions and wastewaters usually contain chloride ions (Cl-) in relatively high concentrations in the milimolar range. In the reactions of highly oxidizing radicals (e.g., •OH, •NO3, or SO4•-) in the nature or during wastewater treatment in advanced oxidation processes the chloride ions easily transform to chlorine containing radicals, such as Cl•, Cl2•-, and ClO•. This transformation basically affects the degradation of organic molecules. In this review about 400 rate constants of the dichloride radical anion (Cl2•-) with about 300 organic molecules is discussed together with the reaction mechanisms. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation state), and molecules with conjugated electron systems are suggested to take place with electron transfer mechanism. The rate constant is high (107-109 M-1 s-1) when the reduction potential the one-electron oxidized species/molecule couple is well below that of the Cl2•-/2Cl- couple.

Chloride-Mediated Enhancement in Heat-Induced Activation of Peroxymonosulfate: New Reaction Pathways for Oxidizing Radical Production

This study is the first to demonstrate the capability of Cl^- to markedly accelerate organic oxidation using thermally activated peroxymonosulfate (PMS) under acidic conditions. The treatment efficiency gain allowed heat-activated PMS to surpass heat-activated peroxydisulfate (PDS). During thermal PMS activation at excess Cl^-, accelerated oxidation of 4-chlorophenol (susceptible to oxidation by hypochlorous acid (HOCl)) was observed along with significant degradation of benzoic acid and ClO₃^- occurrence, which involved oxidants with low substrate specificity. This indicated that heat facilitated HOCl formation via nucleophilic Cl^- addition to PMS and enabled free chlorine conversion into less selective oxidizing radicals. HOCl acted as a key intermediate in the major oxidant transition based on temperature-dependent variation in HOCl concentration profiles, kinetically retarded organic oxidation upon NH₄⁺ addition, and enabled rapid organic oxidation in heated PMS/HOCl mixtures. Chlorine atom that formed via the one-electron oxidation of Cl^- by the sulfate radical served as the primary oxidant and was involved in hydroxyl radical production. This was corroborated by the quenching effects of alcohols and bicarbonates, reactivity toward multiple organics, and electron paramagnetic resonance spectral features. PMS outperformed PDS in degrading benzoic acid during thermal activation operated in reverse osmosis concentrate, which was in conflict with the well-established superiority of heat-activated PDS.

Effect of UV/chlorine treatment on photophysical and photochemical properties of dissolved organic matter

Dissolved organic matter (DOM) is a ubiquitous component in effluents, DOM discharged with an effluent can affect the composition and properties of natural DOM in the receiving waters. As the photophysical and photochemical properties of effluent DOM can be changed by wastewater treatment processes, the effect of UV/chlorine treatment on the photophysical and photochemical properties of DOM was investigated using Suwannee River fulvic acid (SRFA) and Suwannee River natural organic matter (SRNOM) as representatives. Results showed that the absorbance of the two DOM was significantly decreased. The evolution trends of three representative photophysical parameters upon increase of chlorine dosages were observed. Also, a decrease in DOM aromaticity, molecular weight and electron-donating capacity was observed upon increasing chlorine dosage. Quantum yields of excited triplet state of DOM (³DOM*), singlet oxygen (¹O₂) and hydroxyl radicals (·OH) first decreases and then increased in the UV/chlorine systems upon increasing chlorine dosages due to the different reaction pathways of the two DOM. Moreover, ³DOM* can not only be regarded as a "controller" of other reactive intermediates, but also effectively promote the photodegradation of bezafibrate, which is classified as a persistent organic contaminant. This study gives deep insights into effects of UV/chlorine on the photophysical and photochemical properties of DOM, and is helpful for understanding the dynamic roles of DOM in the photodegradation of micropollutants.

Mechanistic insight into the degradation of ibuprofen in UV/H2O2 process via a combined experimental and DFT study

The study investigated the degradation kinetic and transformation mechanism of ibuprofen (IBP) in UV/H₂O₂ process from both experimental and theoretical aspects. Impacts of H₂O₂ dosage, solution pH, quenching agent, and concentration of nitrite (NO₂^-) on IBP degradation in UV/H₂O₂ process were evaluated. Both experimental results and theoretical calculations indicated that ^•OH played an important role in the degradation of IBP and its transformation products. The second-order rate constants of ^•OH and ^•NO₂ with IBP were calculated as 3.93 × 10⁹ M^-1 s^-1 and 5.59 × 10^-3 M^-1 s^-1, based on the transition state theory, which explained the phenomenon that addition of NO₂^- inhibited IBP degradation. Further, according to the results of ultra-high-resolution mass and density functional theory calculations, mechanisms of a detailed degradation pathway for IBP were clarified. Namely, the detailed mechanistic formation pathways for hydroxylated and keto-based products were proposed. Then, possible active sites of the keto-based products, as well as the corresponding subsequent products were predicted by Condensed Fukui Function. Our study can broaden the knowledge of the reactions of emerging contaminants with ^•OH, and provide theoretical foundation for the optimization of UV/H₂O₂ process.

Computerized Pathway Generator for the UV/Free Chlorine Process: Prediction of Byproducts and Reactions

The ultraviolet (UV)/free chlorine process is a very promising treatment technology to remove persistent organic contaminants (POCs, e.g., pharmaceutical and personal care products) from water. The radical chain reactions involved in the UV/free chlorine process are very complicated, and the reaction pathways for organic contaminants degradation are largely unknown. Therefore, we developed a computerized pathway generator that uses graph theory and experimentally determined reaction rules that were reported for the UV/free chlorine process. Our pathway generator predicts all possible intermediates, byproducts, and elementary reactions that are involved in the oxidation of organic contaminants. For example, the degradation of tricholoroethylene (TCE) produces 497 species (i.e., intermediates and byproducts) and 6608 elementary reactions. The predicted species from our pathway generator not only predict the major and stable byproducts that were observed in our experiments (e.g., CHCl₂COOH, CHCl(OCl)COOH, etc.) but also include many other minor and toxic byproducts that were produced but not measured because they have a short lifetime. Overall, our pathway generator significantly improves our understanding of the reaction pathways that are involved in organic contaminant degradation in the UV/free chlorine process.

Simultaneous removal of chlorite and contaminants of emerging concern under UV photolysis: Hydroxyl radicals vs. chlorate formation

It is well known that using chlorine dioxide (ClO₂) as a disinfectant inevitably produces a common disinfection byproducts chlorite (ClO₂^‒). In this study, we found that UV photolysis after ClO₂ disinfection can effectively eliminate both ClO₂^‒ and contaminants of emerging concern (CECs). However, the kinetic mechanisms of UV/ClO₂^‒ process destructing CECs, as well as transformation of ClO₂^‒ in UV/ClO₂^‒ system are not clear yet. Therefore, we systematically investigated the UV/ClO₂^‒ system to assist us appropriately design this process under optimal operational conditions. In this work, we first investigated the impact of water matrix conditions (i.e., pH, bicarbonate and natural organic matter (NOM)) and ClO₂^‒ dosage on the UV/ClO₂^‒ process. We found that bicarbonate and NOM have inhibition effects, while lower pH and higher ClO₂^‒ dosage have enhancement effects. Besides, hydroxyl radical (HO^•) and reactive chlorine species (RCS) are generated from UV/ClO₂^‒ system, and RCS are main contributors to CBZ degradation. Then we proposed a possible degradation pathway of CBZ based on the determined products from experiments. Additionally, we found that photolysis of ClO₂^‒ resulted in the generation of chloride (Cl^‒) and chlorate (ClO₃^‒). As the ClO₂^‒ dosage increases, the yield of ClO₃^‒ increased while that of Cl^‒ decreased. Finally, we elucidated the second order rate constant of the target organic compound with HO• has a strong correlation with the formation of ClO₃^‒.

Transformation of diazepam in water during UV/chlorine and simulated sunlight/chlorine advanced oxidation processes

Psychoactive drug diazepam is one of benzodiazepines widely used in human medicine. It has been found to be relatively resistant to chlorination and photolysis. Here we investigated the transformation mechanism of diazepam in aqueous solution through UV/chlorine and simulated sunlight/chlorine treatments. The results showed that the UV/chlorine and sunlight/chlorine processes significantly increased the degradation of diazepam in water. These observed degradations can be elucidated by in-situ generation of reactive species including hydroxyl radical (HO), reactive chlorine species (RCS) and ozone (O₃) during photolysis of chlorine. In the UV/chlorine treatment, the degradation efficiency of diazepam for HO, chlorine, UV and RCS reaction at 90 min was calculated to be 62.1%, 3.8%, 11.9% and 12.3%, respectively. In the simulated sunlight/chlorine treatment, the calculated degradation of 53.1%, 8.1% and 11.2% was attributed to HO, chlorine and RCS reaction, with negligible loss by O₃ reaction and sunlight irradiation. In the UV/chlorine and sunlight/chlorine treatments, a total of 70 transformation products was detected using a high-resolution TripleTOF mass system. Six transformation pathways have been tentatively proposed for the diazepam, which includes hydroxylation, chlorination, hydrolyzation, N-demethylation, loss of phenyl group, benzodiazepine ring rearrangement and contraction. Most of the obtained transformation products were less toxic to aquatic organisms including fish, daphnia and green algae than diazepam itself according to the toxicity prediction tool, and did not cause significant changes in toxicity to luminescent bacteria.

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

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

UV365 induced elimination of contaminants of emerging concern in the presence of residual nitrite: Roles of reactive nitrogen species

The presence of nitrite (NO₂^-) is inevitable with concentrations of several mg L^-1 in some typical water bodies. In this study, UV at wavelength of 365 nm was investigated to degrade contaminants of emerging concern (CECs) in the presence of NO₂^- at environmentally relevant concentrations (0.1-5.0 mg L^-1). Six selected CECs with different structures were efficiently removed because of the generation of reactive nitrogen species (RNS) and hydroxyl radical (HO^•) from photolysis of NO₂^-. Contributions of UV₃₆₅ photolysis, RNS, and HO^• to CEC degradation in UV₃₆₅/NO₂^- system were calculated, and RNS were found to be the predominant species that are responsible for CEC degradation. The second major contributor is HO^• for the degradation of selected CECs except for the case of sulfadiazine. Impacts of water matrix components (including dissolved oxygen, solution pH, and natural organic matter) on CEC degradation in UV₃₆₅/NO₂^- system were evaluated. Furthermore, evolution profiles of CECs and NO₂^- in UV₃₆₅/NO₂^- system were tracked when actual water samples were used as background, and a simultaneous removal of CECs and NO₂^- was observed. Transformation products of bisphenol A and carbamazepine were proposed according to the results of HPLC/MS and quantum chemistry calculations. Nitration induced by RNS and hydroxylation induced by HO^• are main reactions occurred during CEC degradation in UV₃₆₅/NO₂^- system. Overall, UV₃₆₅ is a potential technology to remove CECs and NO₂^- in aquatic environment when residual NO₂^- is present. Our present study also provides possibility for the application of sunlight to remediate water co-polluted by CECs and NO₂^-.

In Situ Electrochemical Generation of Reactive Chlorine Species for Efficient Ultrafiltration Membrane Self-Cleaning

Reactive membranes based on hydroxyl radical generation are hindered by the need for chemical dosing and complicated module and material design. Herein, we utilize an electrochemical approach featuring in situ generation of reactive (radical) chlorine species (RCS) through anodization of chloride ions for membrane self-cleaning. A hybridized carbon nanotube (CNT)-functionalized ceramic membrane (h-CNT/CM), possessing high hydrophilicity, permeability, and conductivity, was fabricated. Using carbamazepine (CBZ) as a probe, we confirmed the presence of RCS in the electrified h-CNT/CM. The rapid and complete degradation of CBZ in a single-pass ultrafiltration indicates a high localized RCS concentration within the three-dimensional porous CNT interwoven layer. We further demonstrate that the electrogeneration of RCS is a critical prestep for free chlorine (HClO and ClO^-) formation. The self-cleaning efficiency of the membrane after fouling with a model organic foulant (alginate) was assessed using an electrified cross-flow membrane filtration system. The fouled h-CNT/CM exhibits a near complete water flux recovery following a short (1 min) self-cleaning with an applied voltage of 3 or 4 V and feed solutions of 100 or 10 mM sodium chloride, respectively. Considering the superior performance of the RCS-mediated self-cleaning compared to conventional membrane chemical cleaning using sodium hypochlorite, our results exemplify an effective strategy for in situ electrogeneration of RCS to achieve a highly efficient membrane self-cleaning.

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process

Advanced oxidation processes tend to have very complex reaction mechanisms, and models containing over 150 steps have been developed to describe the chemistry. Without the aid of automation, it is extremely difficult to avoid the development of kinetic mechanisms that violate the principle of detailed balancing. Here, we apply DETBAL, a computer application, to systematically identify many violations of the principle of detailed balancing in a model proposed for the UV/chloramine process. We then show that these violations can also be found in dozens of other proposed models for advanced oxidation processes. Suggested repairs to these violations are provided. These repairs lead to no significant changes in the model predictions because the illegal loops include steps that are unnecessary under the conditions modeled. The model omits certain steps that do have significant effects on the model predictions.

Nitrogen conversion from ammonia to trichloronitromethane: Potential risk during UV/chlorine process

In this study, the potential formation of trichloronitromethane (TCNM) from model organic compounds in ammonia-containing water treated by UV/chlorine process was evaluated. Monochloramine generated from the reaction of chlorine and ammonia can be photolyzed to produce NO₂^- and reactive nitrogen species (RNS), which play important roles in the formation of TCNM during the subsequent chlorination. The results showed that increase of nitrogen to chlorine molar ratio (from 0 to 1.0) and pH (from 6.5 to 8.0) enhanced the formation of TCNM, mainly due to the increased yield of NO₂^- and RNS from the photolyzed monochloramine. The formation of TCNM was interestingly found to be linearly correlated with Hammett constants of the model precursors, which is theoretically related to the rate constants of RNS with model compounds. Enhanced formation of TCNM was also observed during the treatment of natural organic matter by UV/chlorine process in ammonia-containing water. The toxicity assessment showed that TCNM significantly increased the genotoxicity of formed DBPs. Furthermore, the electrophilic substitution reaction of ^•NO₂ was proved to more likely occur on the ortho and para position of phenol according to the calculation of Gaussian program, and a possible reaction pathway of phenol and ^•NO₂ was proposed based on the calculated results.

Oxidation of Microcystic-LR via the solar/chlorine process: Radical mechanism, pathways and toxicity assessment

Microcystin-LR (MC-LR) is the most widely distributed and harmful variant toxins released by cyanobacteria, which poses potential threaten to people and aquatic animals when entering natural water. In our research, solar/chlorine process was comprehensively investigated to degrade and detoxify MC-LR. Under the chlorine concentration of 1.0 mg L^-1, MC-LR (1.0 μM) was decreased by 96.7%, 26%, and 9% by solar/chlorine process, chlorination, and solar irradiation respectively. Quenching experiments confirmed that reactive chlorine species (RCS) and hydroxyl radical (HO) were the predominant reactive species in solar/chlorine process at neutral condition, and ozone was generated because of the participation of triplet-state oxygen (O(³P)). The respective contributions of each reactive species were calculated with the order as: RCS, HO, ozone, and solar irradiation. The presence of HCO₃^- and natural organic matter in water inhibited the degradation efficiency of MC-LR. Moreover, the transformation products of MC-LR generated during the solar/chlorine process were identified and a possible pathway was proposed. The hepatotoxicity of MC-LR and its transformation products was compared using protein phosphatase 2A. Our experimental results revealed that the concentration and hepatotoxicity of MC-LR both significantly decreased, and most products were not hepatoxic. Overall, the solar/chlorine process is a promising alternative technology to degrade MC-LR during eutrophication.