A High-Radical-Yield Advanced Oxidation Process Coupling Far-UVC Radiation with Chlorinated Cyanurates for Micropollutant Degradation in Water

Application of machine learning algorithms for the prediction of metformin removal with hydroxyl radical-based photochemical oxidation and optimization of process parameters

This study investigated the effectiveness of hydroxyl radical-based photochemical oxidation processes on metformin (METF) removal, and the experimental data were modeled by machine learning (ML) algorithms. Hydrogen peroxide (HP), sodium percarbonate (PC), and peracetic acid (PAA) were used as hydroxyl radicals sources. Modeling was conducted using ML algorithms with the integration of additional experiments. Under optimum conditions (UV/PC: pH 5, PC 6 mM, UV/HP: pH 3, HP 6 mM, UV/PAA: pH 9, PAA 6 mM), the METF removal efficiency was 74.1 %, 40.7 %, and 47.9 % with UV/PC, UV/HP, and UV/PAA, respectively. The scavenging experiments revealed that hydroxyl and singlet oxygen radicals were dominant in UV/PC and hydroxyl radicals were predominant in UV/HP and UV/PAA. Nitrate negatively affected UV/HP, UV/PC, and UV/PAA, whereas chlorine had a positive impact. The EE/O were 0.682, 1.75, and 1.41 kWh/L for UV/PC, UV/HP, and UV/PAA, respectively. The experimental results were successfully modeled by ML models with high R2 values and low MAE and RMSE values. XGBoost models effectively represent data with generalization by avoiding overfitting. Using ML algorithms to model hydroxyl radical-based photochemical oxidation processes is considered an effective and practical method for future research.

Research on the mechanism of flumequine degradation by ultraviolet light activated trichloroisocyanuric acid

Flumequine (FLU), a synthetic second-generation fluoroquinolone (FQ), is increasingly detected at ecologically concerning concentrations in aquatic and terrestrial environments. This study explores the synergistic application of UV irradiation and chlorine disinfectant-trichloroisocyanuric acid (TCCA) for the remediation of FLU-contaminated water and wastewater, aiming to develop an innovative decontamination strategy. The experimental results revealed that the UV/TCCA system significantly outperformed separate UV photolysis and TCCA oxidation processes, achieved 38.61- and 48.39-fold higher degradation rates. FLU degradation efficiency correlated positively with TCCA concentration, and the system also exhibited strong performance in degrading other contaminants. pH critically influences degradation dynamics, with optimal FLU removal (95.4 %) at pH 5 versus minimal efficiency (32.5 %) at pH 11. Quenching experiments and electron paramagnetic resonance (EPR) analyses confirmed hydroxyl radicals (·OH) and chlorine monoxide radicals (·ClO) as dominant reactive species, with acidic conditions favoring ·OH generation and ·ClO production peaking at pH 9 (1.7-fold higher than at pH 5). Density Functional Theory (DFT) was employed to optimize FLU's molecular structure and predict reactive sites, while LC-MS/MS analysis identified degradation pathways including defluorination, decarbonylation and chlorine substitution. Ecological structure activity relationship modeling demonstrated reduced ecotoxicity in degradation products through fragmentation into smaller molecules. Ultimately, the aggregate results highlight the suitability of the UV/TCAA system to treat FLU in real water and wastewater.

Efficient permanganate activation under UV 222 nm irradiation for enhanced pollutant abatement

As an emerging advanced oxidation process (AOP), the ultraviolet (UV)-driven permanganate [Mn(VII)] activation process has drawn increasing attention in water treatment. Hydroxyl radical (HO•) and various reactive manganese species (RMnS) were simultaneously generated, resulting in enhanced pollutant abatement. However, Mn(VII) activation efficiency by the current UV lights was quite limited, with unsatisfactory apparent quantum yields of Mn(VII) (Φapp, Mn(VII) < 0.25 mol/Einstein). Recently, the krypton chloride (KrCl*) excimer lamp, which emits UV light mainly at 222 nm (UV222), has received growing interest due to its superior photon energy, safety, environmental friendliness, and lifespan compared with currently applied UV lamps. Herein, the KrCl* excimer lamp was applied to activate Mn(VII) for the first time. Φapp, Mn(VII) was determined to be as high as 0.65 mol/Einstein in UV222/Mn(VII). The fluence-based pseudo-first-order reaction rate constants of target pollutants (flumequine and 4-hydroxybenzoic acid) in UV222/Mn(VII) were several to thousands of times higher than those in other UV/Mn(VII) AOPs reported in literature, suggesting significantly higher efficiency of UV222 in Mn(VII) activation. Compared with HO•, RMnS played more significant roles in pollutant abatement. The total contributions of RMnS to pollutant abatement were approximately 60-70% under pH 4.0-9.0, while the contributions of HO• were around 10-25%. Moreover, hypomanganate [Mn(V)] was important RMnS responsible for the abatement of two pollutants, while the role of trivalent manganese [Mn(III)] was limited based on experimental and computational results. Pollutant abatement was inhibited with the increase of pH, while was promoted with the increase of light intensity and Mn(VII) concentration. Furthermore, water matrix components (chloride, bicarbonate, and humic acid) showed negligible or slight influences on pollutant abatement due to the selective oxidation features of RMnS. This study demonstrates the superior potential of the UV222/Mn(VII) AOP in water treatment.

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.

Melamine derivatives in indoor dust from China: Temporal trends and human exposure before and during COVID-19 pandemic

Melamine-based compounds (MELs) are emerging indoor contaminants with potential health risks, yet their temporal variations and exposure implications remain poorly characterized. In this study, we analyzed MELs in 66 paired indoor dust samples from residential households in Tianjin, China, comparing pre- and during-COVID-19 periods. Four traditional MELs, i.e., MEL, ammeline, ammelide, and cyanuric acid (CYA), were detected in all samples, with total MEL concentrations (∑MELs) ranging from 61.2 to 5.83 × 104 ng/g (median: 6.73 ×103 ng/g). During the pandemic, ∑MEL concentrations increased 1.73-fold (8.25 ×103 vs. 4.76 ×103 ng/g, p < 0.01), with CYA emerging as the predominant compound (median: 2.82 ×103 ng/g), likely due to its extensive use in disinfectants (up to 0.4 % and 20 % in liquid and tablet formulations, respectively). Human exposure assessment revealed that infants had the highest estimated daily intakes (EDIs, 40.1-69.6 ng/kg bw/day), about an order of magnitude higher than adults (3.31-5.74 ng/kg bw/day), primarily through dust ingestion. Non-carcinogenic risks (HQs<1) and lifetime cancer risks (maximum median from teenagers: 7.98 ×10-8) remained within negligible limits. Monte Carlo simulations identified indoor dust concentration and body weight as key risk determinants. These findings underscore the environmental consequences of pandemic-driven disinfection practices and the urgent need for regulatory oversight of MEL-containing materials.

Distinct pathways for superoxide radical generation induced by Mn and Cu-based catalysts in electro-Fenton like process

Superoxide radicals (·O2-) has been regarded as one of the reactive oxygen species (ROS) for the elimination of complex contaminants via electro-Fenton like (EF-like) technology. However, the generation path of ·O2- is diverse, and the influence of the physicochemical properties of metals on the mechanism of ·O2- conversion is significant in the EF-like treatment of wastewater. Herein, metals (M = Mn, Cu) loaded zeolitic imidazolate frameworks catalytic materials (M-NC) were prepared for sulfamethoxazole (SMX) removal to analyze the effect of metals on the pathways of ·O2- generation. The removal kinetic rate of SMX by Cu-NC was 1.32 times higher than that of Mn-NC. Quenching experiments demonstrated that ·O2- is the most important oxidizing species to achieve SMX removal. The RRDE measurements and quantitative experiment on the concentration of H2O2 experiments indicated that Mn-NC was more inclined to generate ROS through activation of H2O2 and Cu-NC through other ways. Therefore, the transformation pathways of ·O2- in different catalytic systems were thoroughly analyzed. Electron paramagnetic resonance test and reactive oxygen species quenching experiments indicated that the pathway for ·O2- production of Mn-NC was O2 → H2O2 → ·O2-, and that of Cu-NC was O2 → ·O2-. The strategy of using Mn and Cu-based catalysts to investigate the mechanism of the ·O2- generation pathway provided a way to efficiently utilize the conversion of ·O2-.

Accelerated Pollutant Degradation by UV/H2O2 at the Air-Water Interface of Microdroplets

Ultraviolet light-induced homolysis of hydrogen peroxide (UV/H2O2) can generate powerful hydroxyl radicals (•OH) for sustainable water purification. However, the efficiency of the conventional bulk-phase UV/H2O2 system is limited by the low yield and utilization of •OH, in turn necessitating high UV energy input and long purification period. In this study, we present an innovative UV/H2O2 microdroplet system for enhanced pollutant degradation. The degradation of pollutants in sprayed microdroplets was accelerated by 8.5-63.3-fold compared to those in bulk water, demonstrating universal effectiveness across a range of pollutant types and diverse aqueous matrices. This enhancement stems from elevated •OH production at the air-water interface due to the enhanced UV absorbance of H2O2. The production of •OH in the microdroplet system was 45-fold higher than that in bulk water, facilitating rapid •OH-mediated pollutant degradation. Moreover, pollutants accumulate at the air-water interface, where •OH is concentrated, leading to higher utilization of •OH for mediating pollutant degradation before quenching. Our findings provide a solution to overcome the bottlenecks in •OH production and utilization, offering insights for improving the efficiency of UV/H2O2 water treatment systems.

Hydrated Electrons Trigger the Breakdown of Recalcitrant Cyanuric Acid in Wastewater

Cyanuric acid (CA), a triazine-ring compound commonly used as a stabilizer for free chlorine to enhance disinfection, often persists in wastewater for the production of chlorinated cyanurates (Cl-CAs), posing challenges for treatment. This study demonstrates that conventional advanced oxidation processes (UV/H2O2 and UV/peroxydisulfate) are ineffective in degrading CA, while the UV/sulfite system successfully achieves its breakdown. Hydrated electrons (eaq-) were identified as the primary reactive species responsible for cleaving the stable triazine ring, with minimal contributions from SO3•- and H•. The pH value influences both the activity of eaq- and the degradability of CA by altering its structure; lower pH increases the electron-deficient regions in dihydrogen CA, enhancing its susceptibility to nucleophilic attack by eaq-. The high concentrations of Cl- can inhibit CA removal, likely due to the formation of reactive chlorine species that react with sulfite and suppress eaq- production. Effective CA degradation was also demonstrated in real wastewater, highlighting the UV/sulfite system as a sustainable solution for water treatment. These findings offer valuable insights into CA transformation and present effective approaches for eliminating emerging contaminants in the context of the extensive use of disinfectants.

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.

Insights into the inhibitory effects of trichloroisocyanuric acid disinfectant on the phototransformation of polypropylene microplastics

The chemical components in the natural aquatic environment have the potential to be involved in phototransformation of microplastics (MPs). Little information is available regarding the mediation effects of artificially introduced chemicals on MP phototransformation, especially those used in aquaculture water that are vulnerable to human interference. Herein, this study investigated the phototransformation process and mechanism of polypropylene microplastic (PP MPs) in presence of trichloroisocyanuric acid (TCCA) disinfectant with unique properties unlike the conventional inorganic chlorine disinfectants. The results showed that the presence of TCCA inhibited the surface photooxidation of PP MPs. Analysis of PP MP surface and reaction filtrate indicated that the inhibitory effects were likely derived from TCCA derivatives and the weakening in promoting effect of polypropylene microplastic-derived dissolved organic matter (PP-DOM) as photolytic byproducts, with the more important role of free chlorine in initial period and that of other chlorine species (i.e., the adsorbed chloride ions (Cl-), newly formed carbon-chlorine (CCl) bonds, chlorinated cyanurates, and chlorinated products) in middle and later period. The study highlights for the first time the important role of chlorine species derived from TCCA in phototransformation process of co-existed PP MPs and proposes a previously unrecognized phototransformation pathway, which will provide a new understanding and knowledge for the environmental behavior of MPs in aquaculture environment.

Electron transfer mediated activation of periodate by contaminants to generate 1O2 by charge-confined single-atom catalyst

The electron transfer process (ETP) is able to avoid the redox cycling of catalysts by capturing electrons from contaminants directly. However, the ETP usually leads to the formation of oligomers and the reduction of oxidants to anions. Herein, the charge-confined Fe single-atom catalyst (Fe/SCN) with Fe-N3S1 configuration was designed to achieve ETP-mediated contaminant activation of the oxidant by limiting the number of electrons gained by the oxidant to generate 1O2. The Fe/SCN-activate periodate (PI) system shows excellent contaminant degradation performance due to the combination of ETP and 1O2. Experiments and DFT calculations show that the Fe/SCN-PI* complex with strong oxidizing ability triggers the ETP, while the charge-confined effect allows the single-electronic activation of PI to generate 1O2. In the Fe/SCN + PI system, the 100% selectivity dechlorination of ETP and the ring-opening of 1O2 avoid the generation of oligomers and realize the transformation of large-molecule contaminants into small-molecule biodegradable products. Furthermore, the Fe/SCN + PI system shows excellent anti-interference ability and application potential. This work pioneers the generation of active species using ETP's electron to activate oxidants, which provides a perspective on the design of single-atom catalysts via the charge-confined effect.

Reactive Oxygen Species Generated in Situ During Carbamazepine Photodegradation at 222 nm Far-UVC: Unexpected Role of H2O Molecules

When 222 nm far-UVC is used to drive AOPs, photolysis emerges as a critical pathway for the degradation of numerous organic micropollutants (OMPs). However, the photodegradation mechanisms of the asymmetrically polarized OMPs at 222 nm remain unclear, potentially posing a knowledge barrier to the applications of far-UVC. This study selected carbamazepine (CBZ), a prevalent aquatic antiepileptic drug that degrades negligibly at 254 nm, to investigate its photodegradation mechanisms at 222 nm. Accelerated CBZ treatment by 222 nm far-UVC was mainly attributed to in situ ROS generation via self-sensitized photodegradation of CBZ. By quenching experiments and EPR tests, •OH radicals were identified as the major contributor to the CBZ photodegradation, whereas O2•- played a minor role. By deoxygenation and solvent exchange experiments, the H2O molecules were demonstrated to play a crucial role in deactivating the excited singlet state of CBZ (1CBZ*) at 222 nm: generating •OH radicals via electron transfer interactions with 1CBZ*. In addition, 1CBZ* could also undergo a photoionization process. The transformation products and pathways of CBZ at 222 nm were proposed, and the toxicities of CBZ's products were predicted. These findings provide valuable insights into OMPs' photolysis with 222 nm far-UVC, revealing more mechanistic details for far-UVC-driven systems.

Matching periodate peak absorbance by far UVC at 222 nm promotes the degradation of micropollutants and energy efficiency

Periodate (PI)-based advanced oxidation processes have gained increasing interest. This study for the first time elevates the light-activation capacity of PI by using far UVC at 222 nm (UV222/PI) without extra chemical inputs. The effectiveness and the underlying mechanisms of UV222/PI for the remediation of micropollutants were studied by selecting atenolol (ATL) as a representative. PI possessed a high molar absorption coefficient of 9480-6120 M-1 cm-1 at 222 nm in the pH range of 5.0-9.0, and it was rapidly decomposed by UV222 with first-order rate constants of 0.0055 to 0.002 s-1. ATL and the six other organic compounds were effectively degraded by the UV222/PI process under different conditions with the fluence-based rate constants generally two to hundred times higher than by UVA photolysis. Hydroxyl radical and ozone were confirmed as the major contributors to ATL degradation, while direct photolysis also played a role at higher pH or lower PI dosages. Degradation pathways of ATL were proposed including hydroxylation, demethylation, and oxidation. The high energy efficiency of the UV222/PI process was also confirmed. This study provides a cost-effective and convenient approach to enhance PI light-response activity for the treatment of micropollutants.

Control of Aromatic Disinfection Byproducts in Potable Reuse Water by the UV222/H2O2 vs UV254/H2O2 Advanced Oxidation Processes

Research has demonstrated the difficulty associated with degrading the conventional 1-2 carbon aliphatic halogenated byproducts of disinfectant reactions with organic matter [disinfection byproducts (DBPs)] within advanced oxidation process (AOP) units in potable reuse trains, but the efficacy of AOP units for treating the emerging classes of halogenated aromatic DBPs is unclear. We herein demonstrate more effective removal of 28 halogenated aromatic DBPs in the UV/H2O2 AOP at 222 nm (UV222) than in the conventional UV/H2O2 AOP at 254 nm. Direct photolysis of 28 halogenated aromatic DBPs was greatly enhanced at 222 nm with fluence-based photodecay rate constants of 4.31 × 10-4-1.53 × 10-2 cm2 mJ-1, which was mainly attributed to the higher molar absorption coefficients of halogenated aromatic DBPs at 222 nm than 254 nm. Generally, quantum yields of halogenated aromatic DBPs at both 222 and 254 nm followed the order of halophenols > halohydroxybenzaldehydes > halonitrophenols. All 28 halogenated aromatic DBPs exhibit high reactivity toward HO• with second-order rate constants ranging from 2.18 × 109 to 1.15 × 1010 M-1 s-1 determined by X-ray radiolysis. The UV fluence required to achieve 90% loss of halogenated aromatic DBPs in the UV222/H2O2 AOP was 75-95% lower than that in the UV254/H2O2 AOP, and 90% removal of most tested halogenated aromatic DBPs can be achieved in the UV222/H2O2 AOP within the UV fluence levels commonly applied in potable reuse (700-1000 mJ cm-2).

Enhanced degradation of emerging contaminants by Far-UVC photolysis of peracetic acid: Synergistic effect and mechanisms

Krypton chloride (KrCl*) excimer lamps (222 nm) are used as a promising irradiation source to drive ultraviolet-based advanced oxidation processes (UV-AOPs) in water treatment. In this study, the UV222/peracetic acid (PAA) process is implemented as a novel UV-AOPs for the degradation of emerging contaminants (ECs) in water. The results demonstrate that UV222/PAA process exhibits excellent degradation performance for carbamazepine (CBZ), with a removal rate of 90.8 % within 45 min. Notably, the degradation of CBZ in the UV222/PAA process (90.8 %) was significantly higher than that in the UV254/PAA process (15.1 %) at the same UV dose. The UV222/PAA process exhibits superior electrical energy per order (EE/O) performance while reducing resource consumption associated with the high-energy UV254/PAA process. Quenching experiments and electron paramagnetic resonance (EPR) detection confirm that HO• play a dominant role in the reaction. The contributions of direct photolysis, HO•, and other active species (RO• and 1O2) are estimated to be 5 %, 88 %, and 7 %, respectively. In addition, the effects of Cl-, HCO3-, and humic acid (HA) on the degradation of CBZ are evaluated. The presence of relatively low concentrations of Cl-, HCO3-, and HA can inhibit CBZ degradation. The UV222/PAA oxidation process could also effectively degrade several other ECs (i.e., iohexol, sulfamethoxazole, acetochlor, ibuprofen), indicating the potential application of this process in pollutant removal. These findings will propel the development of the UV222/PAA process and provide valuable insights for its application in water treatment.

Far-UVC 222 nm Treatment: Effects of Nitrate/Nitrite on Disinfection Byproduct Formation Potential

Irradiation at far ultraviolet C (far-UVC) 222 nm by krypton chloride (KrCl*) excilamps can enhance microbial disinfection and micropollutant photolysis/oxidation. However, nitrate/nitrite, which absorbs strongly at 222 nm, may affect the formation of disinfection byproducts (DBPs). Herein, we evaluated model organic matter and real water samples and observed a substantial increase in the formation potential for trichloronitromethane (chloropicrin) (TCNM-FP), a nitrogenous DBP, by nitrate or nitrite after irradiation at 222 nm. At a disinfection dose of 100 mJ·cm-2, TCNM-FP of humic acids and fulvic acids increased from ∼0.4 to 25 and 43 μg·L-1, respectively, by the presence of 10 mg-N·L-1 nitrate. For the effect of nitrate concentration, the TCNM-FP peak was observed at 5-10 mg-N·L-1. Stronger fluence caused a greater increase of TCNM-FP. Similarly, the increase of TCNM-FP was also observed for wastewater and drinking water samples containing nitrate. Pretreatment using ozonation and coagulation, flocculation, and filtration or the addition of H2O2 can effectively control TCNM-FP. The formation potential of other DBPs was minorly affected by irradiation at 222 nm regardless of whether nitrate/nitrite was present. Overall, far-UVC 222 nm treatment poses the risk of increasing TCNM-FP of waters containing nitrate or nitrite at environmentally relevant concentrations and the mitigation strategies merit further research.

Dissolved Organic Matter-Mediated Photosensitized Activation of Monochloramine for Micropollutant Abatement in Wastewater Effluent

Utilizing solar light and water matrix components in situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. We herein propose a new strategy for micropollutant abatement through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH2Cl). Exposing the chlorinated wastewater effluent with residual NH2Cl to solar irradiation (solar/DOM/NH2Cl process) degrades six structurally diverse micropollutants at rate constants 1.26-34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH2Cl process are 1.13-4.32 times the summation of those by the solar/DOM and solar/NH2Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO·) from the photosensitized activation of NH2Cl. Triplet state-excited DOM (3DOM*) dominates the activation of NH2Cl, leading to the generation of RNS, while HO· is produced from the interactions between RNS and other photochemically produced reactive intermediates (e.g., O2·- and DOM·+/·-). The findings advance the knowledge of DOM-mediated photosensitization and offer a sustainable method for micropollutant abatement in wastewater effluents containing residual NH2Cl.

Assessing the Chemical-Free Oxidation of Trace Organic Chemicals by VUV/UV as an Alternative to Conventional UV/H2O2

Low-pressure mercury lamps with high-purity quartz can emit both vacuum-UV (VUV, 185 nm) and UV (254 nm) and are commercially available and promising for eliminating recalcitrant organic pollutants. The feasibility of VUV/UV as a chemical-free oxidation process was verified and quantitatively assessed by the concept of H2O2 equivalence (EQH2O2), at which UV/H2O2 showed the same performance as VUV/UV for the degradation of trace organic contaminants (TOrCs). Although VUV showed superior H2O activation and oxidation performance, its performance highly varied as a function of light path length (Lp) in water, while that of UV/H2O2 proportionally decreased with decreasing H2O2 dose regardless of Lp. On increasing Lp from 1.0 to 3.0 cm, the EQH2O2 of VUV/UV decreased from 0.81 to 0.22 mM H2O2. Chloride and nitrate hardly influenced UV/H2O2, but they dramatically inhibited VUV/UV. The competitive absorbance of VUV by chloride and nitrate was verified as the main reason. The inhibitory effect was partially compensated by •OH formation from the propagation reactions of chloride or nitrate VUV photolysis, which was verified by kinetic modeling in Kintecus. In water with an Lp of 2.0 cm, the EQH2O2 of VUV/UV decreased from 0.43 to 0.17 mM (60.8% decrease) on increasing the chloride concentration from 0 to 15 mM and to 0.20 mM (53.5% decrease) at 4 mM nitrate. The results of this study provide a comprehensive understanding of VUV/UV oxidation in comparison to UV/H2O2, which underscores the suitability and efficiency of chemical-free oxidation with VUV/UV.

Far-UVC Photolysis of Peroxydisulfate for Micropollutant Degradation in Water

Increasing radical yields to reduce UV fluence requirement for achieving targeted removal of micropollutants in water would make UV-based advanced oxidation processes (AOPs) less energy demanding in the context of United Nations' Sustainable Development Goals and carbon neutrality. We herein demonstrate that, by switching the UV radiation source from conventional low-pressure UV at 254 nm (UV254) to emerging Far-UVC at 222 nm (UV222), the fluence-based concentration of HO• in the UV/peroxydisulfate (UV/PDS) AOP increases by 6.40, 2.89, and 6.00 times in deionized water, tap water, and surface water, respectively, with increases in the fluence-based concentration of SO4•- also by 5.06, 5.81, and 55.47 times, respectively. The enhancement to radical generation is confirmed using a kinetic model. The pseudo-first-order degradation rate constants of 16 micropollutants by the UV222/PDS AOP in surface water are predicted to be 1.94-13.71 times higher than those by the UV254/PDS AOP. Among the tested water matrix components, chloride and nitrate decrease SO4•- but increase HO• concentration in the UV222/PDS AOP. Compared to the UV254/PDS AOP, the UV222/PDS AOP decreases the formation potentials of carbonaceous disinfection byproducts (DBPs) but increases the formation potentials of nitrogenous DBPs.

What is the role of nitrate/nitrite in trace organic contaminants degradation and transformation during UV-based advanced oxidation processes?

The effectiveness of UV-based advanced oxidation processes (UV-AOPs) in degrading trace organic contaminants (TrOCs) can be significantly influenced by the ubiquitous presence of nitrate (NO3-) and nitrite (NO2-) in water and wastewater. Indeed, NO3-/NO2- can play multiple roles of NO3-/NO2- in UV-AOPs, leading to complexities and conflicting results observed in existing research. They can inhibit the degradation of TrOCs by scavenging reactive species and/or competitively absorbing UV light. Conversely, they can also enhance the elimination of TrOCs by generating additional •OH and reactive nitrogen species (RNS). Furthermore, the presence of NO3-/NO2- during UV-AOP treatment can affect the transformation pathways of TrOCs, potentially resulting in the nitration/nitrosation of TrOCs. The resulting nitro(so)-products are generally more toxic than the parent TrOCs and may become precursors of nitrogenous disinfection byproducts (N-DBPs) upon chlorination. Particularly, since the impact of NO3-/NO2- in UV-AOPs is largely due to the generation of RNS from NO3-/NO2- including NO•, NO2•, and peroxynitrite (ONOO-/ONOOH), this review covers the generation, properties, and detection methods of these RNS. From kinetic, mechanistic, and toxicologic perspectives, future research needs are proposed to advance the understanding of how NO3-/NO2- can be exploited to improve the performance of UV-AOPs treating TrOCs. This critical review provides a comprehensive framework outlining the multifaceted impact of NO3-/NO2- in UV-AOPs, contributing insights for basic research and practical applications of UV-AOPs containing NO3-/NO2-.

Chlorination of Emerging Contaminants for Application in Potable Wastewater Reuse: Disinfection Byproduct Formation, Estrogen Activity, and Cytotoxicity

With increasing water scarcity, many utilities are considering the potable reuse of wastewater as a source of drinking water. However, not all chemicals are removed in conventional wastewater treatment, and disinfection byproducts (DBPs) can form from these contaminants when disinfectants are applied during or after reuse treatment, especially if applied upstream of advanced treatment processes to control biofouling. We investigated the chlorination of seven priority emerging contaminants (17β-estradiol, estrone, 17α-ethinylestradiol, bisphenol A (BPA), diclofenac, p-nonylphenol, and triclosan) in ultrapure water, and we also investigated the impact of chlorination on real samples from different treatment stages of an advanced reuse plant to evaluate the role of chlorination on the associated cytotoxicity and estrogenicity. Many DBPs were tentatively identified via liquid chromatography (LC)- and gas chromatography (GC)-high resolution mass spectrometry, including 28 not previously reported. These encompassed chlorinated, brominated, and oxidized analogs of the parent compounds as well as smaller halogenated molecules. Chlorinated BPA was the least cytotoxic of the DBPs formed but was highly estrogenic, whereas chlorinated hormones were highly cytotoxic. Estrogenicity decreased by ∼4-6 orders of magnitude for 17β-estradiol and estrone following chlorination but increased 2 orders of magnitude for diclofenac. Estrogenicity of chlorinated BPA and p-nonylphenol were ∼50% of the natural/synthetic hormones. Potential seasonal differences in estrogen activity of unreacted vs reacted advanced wastewater treatment field samples were observed.

Rapid Inactivation of Fungal Spores in Drinking Water by Far-UVC Photolysis of Free Chlorine

Effective and affordable disinfection technology is one key to achieving Sustainable Development Goal 6. In this work, we develop a process by integrating Far-UVC irradiation at 222 nm with free chlorine (UV222/chlorine) for rapid inactivation of the chlorine-resistant and opportunistic Aspergillus niger spores in drinking water. The UV222/chlorine process achieves a 5.0-log inactivation of the A. niger spores at a chlorine dosage of 3.0 mg L-1 and a UV fluence of 30 mJ cm-2 in deionized water, tap water, and surface water. The inactivation rate constant of the spores by the UV222/chlorine process is 0.55 min-1, which is 4.6-fold, 5.5-fold, and 1.8-fold, respectively, higher than those of the UV222 alone, chlorination alone, and the conventional UV254/chlorine process under comparable conditions. The more efficient inactivation by the UV222/chlorine process is mainly attributed to the enhanced generation of reactive chlorine species (e.g., 6.7 × 10-15 M of Cl•) instead of hydroxyl radicals from UV222 photolysis of chlorine, which is verified through both experiments and a kinetic model. We further demonstrate that UV222 photolysis damages the membrane integrity and benefits the penetration of chlorine and radicals into cells for inactivation. The merits of the UV222/chlorine process over the UV254/chlorine process also include the more effective inhibition of the photoreactivation of the spores after disinfection and the lower formation of chlorinated disinfection byproducts and toxicity.

Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

The susceptibility of 19 representative per- and polyfluoroalkyl substances (PFAS) to direct photolysis and defluorination under far-UVC 222 nm irradiation was investigated. Enhanced photolysis occurred for perfluorocarboxylic acids (PFCAs), fluorotelomer unsaturated carboxylic acids (FTUCAs), and GenX, compared to that at conventional 254 nm irradiation on a similar fluence basis, while other PFAS showed minimal decay. For degradable PFAS, up to 81% of parent compound decay (photolysis rate constant (k222 nm) = 8.19-34.76 L·Einstein-1; quantum yield (Φ222 nm) = 0.031-0.158) and up to 31% of defluorination were achieved within 4 h, and the major transformation products were shorter-chain PFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride, and humic acids had mild impacts, while nitrate significantly affected PFAS photolysis/defluorination at 222 nm. Decarboxylation is a crucial step of photolytic decay. The slower degradation of short-chain PFCAs than long-chain ones is related to molar absorptivity and may also be influenced by chain-length dependent structural factors, such as differences in pKa, conformation, and perfluoroalkyl radical stability. Meanwhile, theoretical calculations indicated that the widely proposed HF elimination from the alcohol intermediate (CnF2n+1OH) of PFCA is an unlikely degradation pathway due to high activation barriers. These new findings are useful for further development of far-UVC technology for PFAS in water treatment.