Transformation of humic acid and halogenated byproduct formation in UV-chlorine processes

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

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

Photochemical demethylation of methylmercury (MeHg) in aquatic systems: A review of MeHg species, mechanisms, and influencing factors

Photodemethylation is the major pathway of methylmercury (MeHg) demethylation in surface water before uptake by the food chain, whose mechanisms and influence factors are still not completely understood. Here, we review the current knowledge on photodemethylation of MeHg and divide MeHg photolysis into three pathways: (1) direct photodemethylation, (2) free radical attack, and (3) intramolecular electron or energy transfer. In aquatic environments, dissolved organic matter is involved into all above pathways, and due to its complex compositions, properties and concentrations, DOM poses multiple functions during the PD of MeHg. DOM-MeHg complex (mainly by sulfur-containing molecules) might weaken the C-Hg bond and enhance PD through both direct and indirect pathways. In special, synergistic effects of both strong binding sites and chromophoric moieties in DOM might lead to intramolecular electron or energy transfer. Moreover, DOM might play a role of radical scavenger; while triplet state DOM, which is generated by chromophoric DOM under light, might become a source of free radicals. Apart from DOMs, transition metals, halides, NO3-, NO2-, and carbonates also act as radical initialaters or scavengers, and significantly pose effects on radical demethylation, which is generally mediated by hydroxyl radicals and singlet oxygen. Environmental factors such as pH, light wavelength, light intensity, dissolved oxygen, salinity, and suspended particles also affect the PD of MeHg. This study assessed previously published works on three major mechanisms, with the goal of providing general estimates for photodemethylation under various environment factors according to know effects, and highlighting the current uncertainties for future research directions.

Enhanced recovery of phosphorus from hypophosphite-laden wastewater via field-induced electro-Fenton coupled with anodic oxidation

Electrochemical recovered ferric phosphate (FePO4) precipitates from hypophosphite-laden wastewater were shown to be an efficient method for phosphorus (P) recovery. However, the influence of chloride ions (Cl-) coexisting commonly in wastewater is not known for this treatment. Herein, a field-induced electro-Fenton coupled with anodic oxidation electrochemical system consisting of a Ti-RuO2 anode, an Fe inductive electrode and an activated carbon fiber (ACF) cathode, namely Ti-RuO2/Fe/ACF(NaCl) system, was established to recover phosphorus (P) as FePO4 from hypophosphite-laden wastewater in the presence of Cl-. This system enabled a hypophosphite (H2PO2-, 1.0 mM) removal ratio of ~100% and all P was recovered within 30 min at 5.0 V under the initial solution pH of 3.0. The Faradaic efficiency and energy consumption of P recovery achieved the maximum value (~94%) and the lowest value (~16 kW h kg-1 P), respectively. Reactive oxygen species including 1O2, FeIVO2+, •O2- and •OH contribute to convert H2PO2- to PO43-, which immediately formed FePO4 with the generated Fe3+ at the optimized conditions. Therein, the contribution of non-radical 1O2 was very considerable. This system exhibited good stability. The efficiency and cost for treatment of actual hypophosphite-laden wastewater were addressed to check its applicability for P recovery.

Synergistic mechanisms between chlorine-mediated electrochemical advanced oxidation and ultraviolet light for ammonia removal

The combination of chlorine-mediated electrochemical advanced oxidation (Cl-EAO) and ultraviolet (UV) radiation (UV-E/Cl) can efficiently remove ammonia from wastewater. However, the synergistic mechanisms between Cl-EAO and UV need to be explored in more detail. Thus, in this study, the ammonia oxidation performance of Cl-EAO and UV-E/Cl systems were compared, while the synergistic mechanisms were identified by the performance of UV/chlorine oxidation (UV-ClO) system and the results of electron paramagnetic resonance (EPR) analysis, free radical inhibition assays, and determination of steady-state concentration of free radicals. It was found that, compared with the Cl-EAO system, UV increased the ammonia removal rate by 42.85% and reduced the active chlorine concentration (56.64%) and nitrate yield (53.61%). In the Cl-EAO, and UV-E/Cl systems, Cl• were detected, and the free radical inhibition assays and determination of steady-state concentration of free radicals suggested that UV increased the concentration of Cl• by 51.47%, resulting in Cl• becoming the major contributor to ammonia oxidation in the UV-E/Cl system. Besides, UV also increase the concentrations of HO• and Cl2•-, which further promoted the organic matter removal in the real domestic wastewater. This study also discussed the ammonia oxidation performance of the UV-E/Cl system in real domestic wastewater, even with the presence of significant levels of organic and inorganic anions in the wastewater, UV increased the ammonia oxidation by 21.95%. The results of this study thus clarify the mechanisms and potential applications of UV-E/Cl technology.

Bromide induced the formation of brominated halonitromethanes from aspartic acid in the UV/chlorine disinfection process

Brominated halonitromethanes (Br-HNMs) are generated in water disinfection processes and present high toxicity to human health. This work used aspartic acid (ASP) as the precursor to reveal that bromide (Br-) induced the production of Br-HNMs in the UV/chlorine disinfection process. Consequently, six Br-HNMs were identified, and their yields presented an increasing and then declining evolution over the reaction time from 0 to 15 min. Also, the total Br-HNMs yield reached the maximum of 251.1 μg L-1 at 5 min and then declined to 107.1 μg L-1. The total Br-HNMs yield increased from 2.40 to 251.14 μg L-1 with the increase of Cl2:Br- ratios from 0.25 to 3.0 by increasing free chlorine dosage with a fixed Br- concentration, and it increased from 207.59 to 251.14 μg L-1 and then decreased to 93.44 μg L-1 with the increase of Cl2:Br- ratio from 1.0 to 3.6 by increasing Br- concentration with a fixed free chlorine dosage. Besides, the total Br-HNMs yield reached the highest value (251.14 μg L-1) at pH 7.0 and the lowest value (74.20 μg L-1) at pH 8.0. Subsequently, the possible reaction mechanism of Br-HNMs generated from ASP was deduced, and the changes in toxicity of Br-HNMs also followed an increasing and then declining trend, closely relating to Br-HNMs yields and Br- utilization. This work explored and illustrated the yields, influence factors, reaction mechanisms, and toxicity of Br-HNMs formed from Br- containing ASP water during UV/chlorine disinfection, which might help to control Br-HNMs formation.

Novel calcium hypochlorite/ferrous iron as an ultrafiltration membrane pretreatment process for purifying algae-laden water

Algal fouling has become one of the most critical factors hindering the large-scale development of membrane processes in algae-laden water treatment. Herein, novel calcium hypochlorite (Ca(ClO)2)/ferrous iron (Fe(II)) process was proposed as an ultrafiltration (UF) membrane pretreatment technology, and its effects on membrane fouling and water properties were systematically studied. Results showed that the terminal specific fluxes were significantly elevated to 0.925 and 0.933, with the maximum removal ratios of reversible resistance reaching 99.65% and 96.99% for algae-laden water and extracellular organic matter (EOM), respectively. The formation of cake filtration was dramatically delayed, accompanied by a significant reduction of the adhesion free energy, and the contaminants attached to the membrane surface were effectively decomposed. With respect to water quality, the removal ratios of OD685 and turbidity achieved 81.25-95.31% and 90.16-97.72%, individually. The maximum removal rates of DOC, UV254 and fluorescent organics in influent water reached 46.14%, 55.17% and 75.77%, respectively. Furthermore, the generated reactive species (e.g., •OH, Cl•, Cl2•- and ClO•) could efficiently degrade EOM, which appreciably reduced the electrostatic repulsion between the algal foulants while ensuring the integrity of algal cells. At the Ca(ClO)2/Fe(II) dosage of 0.04/0.24 mM, the zeta potential changed from -32.9 mV to -10.8 mV, and a large range of aggregates was formed. The macromolecules in the algal solution were significantly removed, and the proportion of micromolecular organics was increased to some extent. Coagulation of in-situ formed Fe(III) dominated the membrane fouling mitigation, and the reactive species also contributed to the improvement of filtration performance. Overall, Ca(ClO)2/Fe(II) pretreatment has an exceptional prospect for efficient degradation of algal pollutants and enhancement of UF capability.

Enhanced •Cl generation by introducing electrophilic Cu(II) in Co3O4 anode for efficient total nitrogen removal with hydrogen recovery in urine treatment

Urine is a nitrogen-containing waste, but can be used as an attractive alternative substrate for H2 recovery. However, conventional urea oxidation reaction is subject to complex six-electron transfer kinetics and requires alkaline conditions. Herein, an efficient method of enhancing •Cl generation by introducing electrophilic Cu(II) into Co3O4 nanowires anode was proposed, which realized the highly efficient TN removal and H2 production in urine treatment under neutral conditions. The key mechanism is that the electrophilic effect of Cu(II) attracts electrons from the oxygen atom, which causes the oxygen atom to further attract electrons from Co(II), reducing the charge density of Co(II). Electrophilic Cu(II) accelerates the difficult conversion step of Co(II) to Co(III), which enhances the generation of •Cl. The generated •Cl efficiently converts urea to N2, while the electron transport promotes H2 production on the CuO@CF nanowires cathode. Results showed that the steady-state concentration of •Cl was increased to about 1.5 times by the Cu(II) introduction. TN removal and H2 production reached 94.7% and 642.1 μmol after 50 min, which was 1.6 times and 1.5 times that of Co3O4 system, respectively. It was also 2.3 times and 2.1 times of RuO2, and 3.3 times and 2.5 times of Pt, respectively. Moreover, TN removal was 11.0 times higher than that of without •Cl mediation, and H2 production was 4.3 times higher. More importantly, excellent TN removal and H2 production were also observed in the actual urine treatment. This work provides a practical possibility for efficient total nitrogen removal and hydrogen recovery in urine wastewater treatment.

Effects of alternative disinfection methods on the characteristics of effluent organic matter and the formation of disinfection byproducts

The characteristics of effluent organic matter (EfOM) and the type of disinfection methods are closely related to the formation of disinfection byproducts (DBPs) in reclaimed water. In this study, five disinfection methods, i.e., chlorination, ultraviolet (UV) followed by chlorination (UV + Cl), UV/chlorine (UV/Cl), chloramination, and chlorine dioxide (ClO2), were applied to investigate the changes in the properties of EfOM, the formation of DBPs, and the relationship between EfOM properties and DBP formation during the disinfection of four secondary biological effluents. The results showed that EfOM with medium molecular weight (MW) (0.5-6 kDa) was the dominant fraction for all WWTPs. From a fluorescence perspective, the EfOM of the AAO process was rich in humic matter, while the EfOM of the oxidation ditch (OD) process was rich in protein matter. Disinfectants tended to transfer EfOM with high molecular weight (MW) (>6 kDa) to those with low MW (<0.5 kDa). Chlorination, UV + Cl and UV/Cl were more reactive to humic matter, while chloramination and chlorine dioxide were more reactive to protein matter. The formation of known DBPs was mainly dependent on humic matter, while protein matter was more likely to generate unknown DBPs. N-DBPs only accounted for 5.7%-17.7% of the total DBPs, but contributed more than 70% of the calculated toxicity, among which bromochloroacetonitrile (BCAN), dichloroacetonitrile (DCAN), and monobromoacetamide (MBAcAm) were the most important contributors to the calculated cytotoxicity. Monobromoacetic acid (MBAA) and MBAcAm were the primary drivers of the calculated genotoxicity. Overall, UV + Cl was the suggested optimal disinfection method for WWTPs.

New insights into the influence of pre-oxidation on membrane fouling during nanofiltration of brackish water considering inorganic-organic complexation and oxidant reduction byproducts

Even though pre-oxidation is usually considered as a promising method to alleviate membrane fouling, information on performance and inner mechanisms of pre-oxidation-influenced membrane fouling during nanofiltration of brackish water is still limited. This study is the first work in which oxidant reduction byproducts and interaction between different pollutants were particularly considered to address these problems. Herein, nanofiltration experiments with different pre-oxidized synthesis brackish water containing inorganic salts and organic pollutants were conducted. Membrane flux results showed that both NaClO and K2FeO4 aggravated membrane fouling, but 0.45 mg/mg TOC KMnO4 mitigated it when simulation results of NICA-Donnan model showed that the complexation between calcium ions and humic acid (HA) was weakened. However, membrane fouling was enhanced by higher dosage of KMnO4. Fourier transform infrared spectrometer using attenuated total reflection (ATR-FTIR) and X-ray diffraction (XRD) spectrum showed that the aggravated membrane fouling was mainly caused by the generation of amorphous manganese oxide, which was oxidant reduction byproduct and had strong capacity for adsorption of HA. Particle size distribution and zeta potential variation indicated that the accumulation of HA could enhance the crystallization process and then the electrostatic attraction between membrane and bulk crystallization was induced. According to SEM images and fitting results of Hermia's models, the already-formed bulk crystallization by 1.90 mg/mg TOC KMnO4 could deposit on membranes more easily, followed by the formation of a denser fouling layer. Overall, the present study provided new insights into the design of reliable pre-oxidation strategies for alleviating membrane fouling during nanofiltration of brackish water.

A Bipolar Membrane-Integrated Electrochlorination Process for Highly Efficient Ammonium Removal in Mature Landfill Leachate: The Importance of ClO• Generation

Electrochemical oxidation has been demonstrated to be a useful method for removing biorefractory organic pollutants in mature landfill leachate but suffers from low efficiency in eliminating ammonium because of its resistance to being oxidized by HO• or free chlorine (FC) at decreased pH. Here, we propose a new bipolar membrane-electrochlorination (BPM-EC) process to address this issue. We found that the BPM-EC system was significantly superior to both the undivided and divided reactors with monopolar membranes in terms of elevated rate of ammonium removal, attenuated generation of byproducts (e.g., nitrate and chloramines), increased Faradaic efficiency, and decreased energy consumption. Mechanistic studies revealed that the integration of BPM was helpful in creating alkaline environments in the vicinity of the anode, which facilitated production of surface-bound HO• and FC and eventually promoted in situ generation of ClO•, a crucial reactive species mainly responsible for accelerating ammonium oxidation and selective transformation to nitrogen. The efficacy of BPM-EC in treating landfill leachates with different ammonium concentrations was verified under batch and continuous-flow conditions. A kinetic model that incorporates the key parameters was developed, which can successfully predict the optimal number of BPM-EC reactors (e.g., 2 and 5 for leachates containing 589.4 ± 5.5 and 1258.1 ± 9.6 mg L-1 NH4+-N, respectively) necessary for complete removal of ammonium. These findings reveal that the BPM-EC process shows promise in treating ammonium-containing wastewater, with advantages that include effectiveness, adaptability, and flexibility.

Formation, toxicity, and mechanisms of halonitromethanes from poly(diallyl dimethyl ammonium chloride) during UV/monochloramine disinfection in the absence and presence of bromide ion

Bromide ion (Br^-) is known as a prevalent component in water environments, which exhibits significant impacts on halonitromethanes (HNMs) formation. This study was performed to explore and compare the formation, toxicity, and mechanisms of HNMs from poly(diallyl dimethyl ammonium chloride) (PDDACl) in the absence and presence of Br^- in the UV/monochloramine (UV/NH₂Cl) disinfection process. The results showed that chlorinated HNMs were found in the absence of Br^-, while brominated (chlorinated) HNMs and brominated HNMs were found in the presence of Br^-. Furthermore, the peaks of total HNMs were promoted by 2.0 and 2.4 times, respectively when 1.0 and 2.0 mg L^-1 Br^- were added. Also, the peaks of total HNMs were enhanced with the increase of the NH₂Cl dosage, which were reduced with the increase of pH. It should be noted that Br^- induced higher toxicity of HNMs, and the cytotoxicity and genotoxicity of HNMs with the addition of 2.0 mg L^-1 Br^- were 78.0 and 3.7 times those without the addition of Br^-, respectively. Meanwhile, both the reaction mechanisms of HNMs produced from PDDACl were speculated in the absence and presence of Br^-. Finally, different HNMs species and yields were discovered in these two real water samples compared to those in simulated waters. These findings of this work will be conducive to understanding the significance of Br^- affecting HNMs formation and toxicity in the disinfection process.

Insight into micropollutant abatement during ultraviolet light-emitting diode combined electrochemical process: Reaction mechanism, contributions of reactive species and degradation routes

Electrochemical process coupling with ultraviolet light-emitting diode for micropollutant abatement was evaluated in the treatment of wastewater containing Cl^-. Four representative micropollutants, atrazine, primidone, ibuprofen and carbamazepine, were selected as target compounds. The impacts of operating conditions and water matrix on micropollutant degradation were investigated. Fluorescence excitation-emission matrix spectroscopy spectra and high performance size exclusion chromatography were employed to characterize the transformation of effluent organic matter in treatment. The degradation efficiencies of atrazine, primidone, ibuprofen and carbamazepine are 83.6 %, 80.6 %, 68.7 % and 99.8 % after 15 min treatment, respectively. The increment of current, Cl^- concentration and ultraviolet irradiance promote the micropollutant degradation. However, the presence of bicarbonate and humic acid inhibit micropollutant degradation. The mechanism of micropollutant abatement was elaborated based on reactive species contributions, density functional theory calculation and degradation routes. Free radicals (HO^•, Cl^•, ClO^• and Cl₂^•-) could be generated by chlorine photolysis and subsequent propagation reactions. The concentrations of HO^• and Cl^• are 1.14 × 10^-13 M and 2.0 × 10^-14 M in optimal condition, respectively, and the total contributions of HO^• and Cl^• for the degradation of atrazine, primidone, ibuprofen and carbamazepine are 24 %, 48 %, 70 % and 43 %, respectively. The degradation routes of four micropollutants are elucidated based on intermediate identification, Fukui function and frontier orbital theory. Micropollutants can be effectively degraded in actual wastewater effluent, and the small molecule compound proportion increases during effluent organic matter evolution. Compared with photolysis and electrolysis, the coupling of the two processes has potential for energy saving in micropollutant degradation, which shed light on the prospects of ultraviolet light-emitting diode coupling with electrochemical process for effluent treatment.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

Degradation of natural organic matter and disinfection byproducts formation by solar photolysis of free available chlorine

Environment disinfection effectively curbs transmission of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). However, elevated concentration of free available chlorine (FAC) in disinfectants can be discharged into surface water, generating toxic disinfection byproducts (DBPs). The impact of solar photolysis of FAC on natural organic matter (NOM) to form DBPs has not been well studied. In this work, solar photolysis of FAC was found to result in higher formation of DBPs, DBPs formation potential (DBPsFP), total organic chlorine (TOCl) and lower specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), compared to dark chlorination. In solar photolysis of FAC, formation of total DBPs was promoted by pH=8, but hindered by the addition of HCO₃^-, radical scavenger or deoxygenation, while addition of NO₃^-and NH₄⁺both enhanced the formation of nitrogenous DBPs. Differences in the formation of DBPs in solar photolysis of FAC under various conditions were influenced by reactive species. The formation of trichloromethane (TCM) and haloacetic acids (HAAs) in solar photolysis of FAC positively correlated with the steady-state concentrations of ClO^• and O₃. The steady-state concentrations of ^•NO and ^•NH₂ positively correlated with the formation of halonitromethanes (HNMs). HAAs and haloacetonitriles (HANs) mainly contributed to calculated cytotoxicity of DBPs. This study demonstrates that solar photolysis of FAC may significantly impact the formation of DBPs in surface water due to extensive use of disinfectants containing FAC during SARS-CoV-2 pandemic.

Flow electrochemical inactivation of waterborne bacterial endospores

Waterborne pathogens have the risk of spreading waterborne diseases and even pandemics. Some Gram-positive bacteria can form endospores, the hardiest known life form that can withstand heat, radiation, and chemicals. Electrochemical inactivation may offer a promising solution, but is hindered by low inactivation efficiencies resulting from limitation of electrode/endospores interaction in terms of electrochemical reaction selectivity and mass transfer. Herein, these issues were addressed through modifying selectivity of active species formation using electroactive ceramic membrane with high oxygen evolution potential, improving mass transfer property by flow-through operation. In this way, inactivation (6.0-log) of Bacillus atrophaeus endospores was achieved. Theoretical and experimental results demonstrated synergistic inactivation to occur through fragmentation of coat via interfacial electron transfer and electro-produced transient radicals (•OH primarily, •Cl and Cl₂^•- secondarily), thereby increasing cell permeability to facilitate penetration of electro-produced persistent active chlorine for subsequent rupture of intracellular structures. Numbering-up electrode module strategy was proposed to scale up the system, achieving average 5.3-log inactivation of pathogenic Bacillus anthracis endospores for 30 days. This study demonstrates a proof-of-concept manner for effective inactivation of waterborne bacterial endospores, which may provide an appealing strategy for wide-range applications like water disinfection, bio-safety control and defense against biological warfare.

Heterogeneous photocatalytic activation of electrogenerated chlorine for the production of reactive oxygen and chlorine species: A new approach for Bisphenol A degradation in saline wastewater

UV-E-chlorination/hematite nanoparticles (UV/E-Cl/HNs) as a heterogeneous photocatalytic activation of electrogenerated chlorine was assessed for the degradation of bisphenol A (BPA) as a new approach based on the generation of reactive chlorine and oxygen species. The prepared sample was characterized using multiple techniques, such as XRD, FTIR, FESEM, EDS, and BET-BJH. An excellent decontamination efficiency of 99.4% was achieved within 40 min of electrolysis under optimum conditions (pH of 5, HNs dosage 100 mg/L, current density of 20 mA/cm², and NaCl concentration of 50 mM). The HOCl content was reduced more swiftly in the presence of ultraviolet (UV) irradiation and hematite, resulting in the production of oxidative radicals (i.e., ^•OH, Cl^•, and Cl₂^•-). The scavenging experiments also verified the vital role of these radicals in oxidative treatment. The UV/E-Cl/HNs process is readily supplied with hydroxyl radicals through several mechanisms. Bicarbonate ions showed a noticeable inhibitory impact, whereas nitrate and sulfate anions only slightly affected BPA degradation. The HNs were a recoverable and stable catalyst for six cycles. Furthermore, the ECOSAR program predicted that the UV/E-Cl/HNs can be labeled as an environmental-friendly process. Eventually, reasonable degradation pathways were proposed based on the identified by-products through experimental and theoretical approaches.

Evaluating UV254 absorbance reductions in landfill leachate for municipal sewage co-treatment through timed UV/electrooxidation

Landfill leachate contains dissolved organic matter (DOM) exhibiting high ultraviolet absorbance at 254 nm (UVA₂₅₄). The UVA₂₅₄ limits leachate co-treatment with municipal sewage by hindering the downstream UV disinfection efficiency at wastewater treatment plants. Here, we alleviated the UVA₂₅₄ by timing the radiation in a UV/electrooxidation (UV/EO) process to accelerate reactive species formation. At 200 A·m^-2, the UV radiation was delayed by 10 min to accumulate 21 mg·L^-1 as Cl₂, which enhanced the initial radical formation rate by 5.25 times compared with a simultaneous UV/EO. The timed operation increased the steady-state concentrations of ClO^• by 700 times to 4.11 × 10^-14 M and reduced the leachate UVA₂₅₄ by 78.2% after 60 min. We identified that aromatic formulas with low oxygen content were susceptible to UV/EO from Fourier-transform ion cyclotron resonance mass spectrometry analysis. The toxicity of the treated leachate and generated byproducts was assessed through specific oxygen uptake rates (SOUR) and developmental assays with Platynereis dumerilii. After quenching the residual chlorine, leachate co-treatment at 3.5% v/v presented minimal toxicological risk. Our findings provide operational insights for applying UV/EO in high UVA₂₅₄ matrices such as landfill leachate.

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.

Degradation of odorous 2,4,6-trichloroanisole in chlorinated water by UV-LED/chlorination: kinetics and influence factors

2,4,6-Trichloroanisole (2,4,6-TCA) has aroused a special concern for their odor problem and potential threats. In this study, the degradation of 2,4,6-TCA by UV/chlorination with different UV sources was compared, including low-pressure mercury lamp (LPUV, 254 nm) and ultraviolet light-emitting diode (UV-LED, 275 and 285 nm). The maximum removal of 2,4,6-TCA can be achieved by 275-nm UV-LED/chlorination in neutral and alkaline conditions which was 80.0%. The reaction, kinetics, and water matrix parameters on 2,4,6-TCA degradation were also evaluated. During UV-LED (275 nm)/chlorination, 2,4,6-TCA degradation was mainly caused by direct UV photolysis and indirect hydroxyl radical (HO·) oxidation, while reactive chlorine radicals (RCSs) had a negligible contribution. The second-order rate constant between HO· and 2,4,6-TCA was determined as 3.1 × 10⁹ M^-1 s^-1. Increasing initial chlorine dosage and decreasing 2,4,6-TCA concentration or pH value significantly promoted 2,4,6-TCA degradation during UV/chlorination process. The presence of natural organic matter (NOM) and bicarbonate (HCO₃^-) can inhibit 2,4,6-TCA degradation, while chloride ion (Cl^-) had a negligible effect. The kinetic model for 2,4,6-TCA degradation was established and validated, and the degradation pathways were proposed based on the identified intermediates. Furthermore, UV-LED (275 nm)/chlorination also exhibited a promising effect on 2,4,6-TCA removal in real water, which can be used to control 2,4,6-TCA pollution and odor problems.

Influence of water matrix components on the UV/chlorine process and its reactions mechanism

The UV/chlorine system has become an attractive alternative Advanced Oxidation Process (AOP) for the removal of recalcitrant pollutants in the last decade due to the simultaneous formation of chlorine and hydroxyl radicals. However, there is no consensus regarding the results and trends obtained in previous micropollutant removal studies by AOPs, highlighting the complexity of the UV/chlorine process and the need for further research. This study investigates the degradation of acetaminophen (ACTP) by UV/chlorine and the effects of the water matrix in the reaction kinetics. In particular, the effects of natural organic matter (NOM), alkalinity and mineral salts on the kinetics and reactive species were elucidated. The complexity of the system was revealed by the analysis of the radical generation and transformation in different water matrices, applying the kinetic modelling approach to complement the scavenger tests. The higher kinetic rates of ACTP at alkaline pH provided new insights into the chlorine reactions under UV radiation, where secondary and tertiary reactive oxygen species including ozone were proven to play the major role in degradation. On the contrary, at acidic pH, reaction kinetic modelling demonstrated that ClO^• radical occurs at high concentrations in the order of 10^-10 M, being therefore the main oxidant, followed by other chlorine radicals. It is noteworthy that at alkaline pH the presence of typical inorganic ions such as carbonate had little impact on ACTP degradation, contrary to the observed reduction of degradation rates at acidic pH. The expected detrimental effect of the NOM in AOPs was also evidenced, although the use of chlorine as radical source reduces the relevance of the inner filter effect in comparison to UV/H₂O₂.

Preparation of Hydrogarnet/Poly(Lactic Acid) Composite Adsorbents for Humic Substance Removal

Humic substances are constituents of organic matter that require removal from water environments because of their adverse ecological and sanitation effects. A mixture of hydrogarnet and poly(lactic acid) dissolved in chloroform was electrospun to prepare a composite as a adsorbent for humic substance removal. Here, humic acid was used as the model substance for evaluating the adsorbent's water remediation efficiency. Despite the hydrogarnet particles being embedded in its poly(lactic acid) fibers, the composites demonstrated a higher humic acid removal ability than the pure poly(lactic acid) sample prepared using an electrospinning process. Pores were introduced to the fiber surfaces of the composite by controlling the relative humidity during electrospinning, thus enhancing their humic acid removal ability (4.6 ± 2.4 mg/g), compared to the composite consisting of the fibers without pores (1.2 ± 0.9 mg/g).

Facile synthesis of waste-based CdS-loaded hierarchically porous geopolymer for adsorption-photocatalysis of organic contamination and its environmental risks

In order to obtain an adsorbent-photocatalyst with low-cost, strong stability and great reusability/recyclability, a waste-based and CdS-loaded hierarchically porous geopolymer (HPG) was prepared by facile synthesis. The adsorption-photocatalysis ability, reusability, and stability of HPG under different conditions were determined. Results indicated that HPG showed better adsorption-photocatalysis performance for organic dyes under alkaline environment, and it remained a high adsorption-photocatalysis efficiency after used for five times. Furthermore, HPG was stable in different environment conditions (strong acidic, acid raining, neutral, high salinity, and high alkali environment). The mass loss of HPG were around 3.22-6.68% (7 days extraction), and the immobilization rates of Cd²⁺ in neutral, high salinity, and high alkali environments were higher than 99.99%. Under visible light irradiation, HPG effectively photo-degraded the organic substances in overlying water of polluted sediments. After 330 min irradiation, the concentrations of COD and TOC were decreased from 47.52 mg/L and 20.9 mg/L to 16.58 mg/L and 11.19 mg/L, respectively. The humic-like and fulvic-like substances were transformed to protein-like substances under photo-degradation effect. This study confirmed that HPG possesses advantages in cost, chemical stability, and reusability, and it has a great potential to be used as in-situ remediation environmental functional material for organic contaminants in lake.

Removal of ofloxacin from wastewater by chloride electrolyte electro-oxidation: Analysis of the role of active chlorine and operating costs

Electro-oxidation (EO) has received increasing attention as an efficient and green method for removing pollutants from wastewater. Chloride anions (Cl^-), which commonly exist in wastewater, can act as an electrolyte for the EO process. However, the role of reactive chlorine species (RCS) generated near electrodes is often underestimated. In this study, we generated hydroxyl radicals (OH) and RCS in a boron-doped diamond (BDD) electrode system and investigated its degradation mechanism for ofloxacin (OFX) removal. The findings suggested that OFX degradation was dominated by OH existing near the anode in solution, with RCS playing a supporting role. Based on the produced intermediates, we proposed an OFX decomposition pathway. The biological toxicities of the intermediates were evaluated through the ECOSAR and T.E.S.T. procedure. Nearly half of the intermediates are less toxic than the parent compound. After optimizing the operating parameters by the response surface methodology, 20 mg/L OFX was almost completely degraded after 10 min of reaction in 1.45 g/L NaCl with a current density (j) of 18 mA/cm², and the total organic carbon was decreased by 30.55 %. The energy consumption and current efficiency were 0.648 kW·h/g_TOC and 8.65 %, respectively. Comparing the operating costs of the proposed and other EO methods, our method emerged as a viable new treatment scheme for similar polluted wastewaters. This study aims to comprehensively understand the potential application value of BDD electrodes in the treatment of Cl^- containing organic wastewater.

Pinpointing the axial ligand effect on platinum single-atom-catalyst towards efficient alkaline hydrogen evolution reaction

Developing active single-atom-catalyst (SAC) for alkaline hydrogen evolution reaction (HER) is a promising solution to lower the green hydrogen cost. However, the correlations are not clear between the chemical environments around the active-sites and their desired catalytic activity. Here we study a group of SACs prepared by anchoring platinum atoms on NiFe-layered-double-hydroxide. While maintaining the homogeneity of the Pt-SACs, various axial ligands (−F, −Cl, −Br, −I, −OH) are employed via a facile irradiation-impregnation procedure, enabling us to discover definite chemical-environments/performance correlations. Owing to its high first-electron-affinity, chloride chelated Pt-SAC exhibits optimized bindings with hydrogen and hydroxide, which favor the sluggish water dissociation and further promote the alkaline HER. Specifically, it shows high mass-activity of 30.6 A mgPt⁻¹ and turnover frequency of 30.3  H₂ s⁻¹ at 100 mV overpotential, which are significantly higher than those of the state-of-the-art Pt-SACs and commercial Pt/C catalyst. Moreover, high energy efficiency of 80% is obtained for the alkaline water electrolyser assembled using the above catalyst under practical-relevant conditions.

Establishing robust structure/performance correlations is critical for the development of single-atom-catalysts with improved activity. Here, the axial ligand on Pt single-atom-catalyst is precisely adjusted and studied, showing that the ligand’s first electron affinity is crucial for the catalysis.

Degradation of organics and formation of DBPs in the combined LED-UV and chlorine processes: Effects of water matrix and fluorescence analysis

Combined processes of light-emitting diodes ultraviolet (LED UV) and chlorination (Cl₂) are alternative disinfection technologies in drinking water, while the formation of disinfection by-products (DBPs) needs to be evaluated. This study investigated the impacts of critical water matrix factors on the DBP formation in the combined processes. Moreover, the correlation between the degraded natural organic matter (NOM) and the formed DBP was studied. Simultaneous UV/Cl₂ outperformed single Cl₂ and sequential combined processes in degrading humic acids (HA) and resulted in the highest DBP yield. Iodide at 5-20 μg/L and bromide at 0.05-0.2 mg/L slightly affected the degradation of organics, while increased the formation of brominated DBPs up to 36.6 μg/L. pH 6 was regarded as the optimum pH, achieving high efficiency of HA degradation and a lower level of total DBP formation than pH 7 and 8 by 11 % and 24 %, respectively. Compared to HA samples (46.8-103.9 μg/L per mg/L DOC), NOM in canal water were less aromatic and yielded fewer DBPs (19.6 and 21.2 μg/L per mg/L DOC). However, the extremely high bromide in site 1 samples (18.6 mg/L) shifted the chlorinated DBPs to their brominated analogues, posting around 1 order of magnitude higher levels of toxicities than HA samples. The reduction of absorbance at 254 nm (UV₂₅₄) correlated with all DBP categories in HA samples, while the correlation coefficients were compromised when included in the canal samples. For the first time, this study found that parallel factor analysis (PARAFAC) would neglect the fluorescence change caused by iodide/bromide in UV/Cl₂, while the changes could be captured by self-organising map (SOM) trained with full fluorescence spectra. Fluorescence Ex/Em pairs were proposed to predict DBP formation, suggesting a potential method to develop an online monitoring system for DBPs.

Persulfate-enhanced continuous flow three-dimensional electrode dynamic reactor for treatment of landfill leachate

Compared with sequencing batch reactor, continuous flow dynamic reactors are more conducive to promotion and application. In this study, the ability of a three-dimensional (3D) electrode dynamic reactor to remove pollutants in the landfill leachate was investigated, in which landfill leachate entered through continuous flow. Either increased of current density or the decreased of flow rate was conducive to the removal of pollutants. The optimal process parameters for current density and flow rate were 16 mA cm^-2 and 0.75 L h^-1, respectively. When the current density was constant at 16 mA cm^-2 and the flow rate was kept at 0.75 L h^-1, 60.02% of total organic carbon (TOC), 96.50% of chroma, 64.98% of chemical oxygen demand (COD) and 99.46% of ammonia nitrogen (NH₃-N) were removed. The characteristic peaks of refractory organic pollutants were reduced by 97.95%. After the reaction, the biological oxygen demand (BOD)/COD was increased from 0.24 to 0.32. As one of the emerging trace organics in landfill leachate, 85.90% of ibuprofen (IBU) was removed. The results showed that the 3D electrode dynamic reactor constructed in this study could reduce the TOC, refractory trace organic pollutant, NH₃-N and chroma in the landfill leachate. The 3D electrode dynamic reactor constructed in this research has application potential in the field of landfill leachate treatment.

Effects of bromide ion on the formation and toxicity alteration of halonitromethanes from nitrate containing humic acid water during UV/chlor(am)ine disinfection

UV/chlor(am)ine are efficient for achieving multiple-barrier disinfection and maintaining residuals, while bromide (Br^-) has notable impacts on the formation and toxicity of halonitromethanes (HNMs) during UV/chlor(am)ine disinfection. This study investigated the effects of Br^- on HNMs formation and toxicity alteration during UV/chlor(am)ine disinfection of nitrate containing humic acid (HA) water. Results reveal that the maximum concentration of HNMs during UV/chlorine disinfection was 12.03 μg L^-1 with 0.2 mg L^-1 Br^-, which was 22.5% higher than that without Br^-, and the predominant species of HNMs were converted from trichloronitromethane (TCNM) to dibromonitromethane (DBNM) and tribromonitromethane (TBNM). However, the maximum concentration of HNMs during UV/chloramine disinfection was 3.69 μg L^-1 with 0.2 mg L^-1 Br^-, which was increased by 26.0% than that without Br^-, and the predominant species of HNMs were converted from dichloronitromethane (DCNM) to bromochloronitromethane (BCNM) and DBNM. Notably, the HNMs species and yields during UV/chloramine disinfection were less than those during UV/chlorine disinfection, primarily due to the higher concentrations of HO· and reactive chlorine/bromine species in UV/chlorine. Also, in the ranges of the Br^-:Cl₂ molar ratio from 0 to 0.32 and pH from 6.0 to 8.0, the Br^-:Cl₂ molar ratio of 0.16 and acidic pH contributed to the HNMs formation during UV/chlorine disinfection, and a high Br^-:Cl₂ molar ratio and neutral pH contributed to the HNMs formation during UV/chloramine disinfection. Note that the incorporation of Br^- significantly improved the calculated cytotoxicity (CTI) and genotoxicity (GTI) of HNMs formed, and the calculated CTI and GTI of HNMs formed during UV/chloramine disinfection were 28.19 and 48.90% of those during UV/chlorine disinfection. Based on the diversity of nitrogen sources, the possible formation pathways of HNMs from nitrate containing HA water were proposed during UV/chlor(am)ine disinfection in the presence of Br^-. Results of this study indicated that UV/chloramine can reduce the formation and toxicity of HNMs efficiently.

Inactivation and photoreactivation of blaNDM-1-carrying super-resistant bacteria by UV, chlorination and UV/chlorination

The excessive dissemination of New Delhi metallo-β-lactamase-1 (NDM-1), which mediates resistance to a majority of clinical β-lactam antibiotics, has created a major public health problem worldwide. Herein, a bla_NDM-1-carrying (plasmid encoded) super-resistant bacterium, Acinetobacter sp. CS-2, was selected to reveal its mechanisms of inactivation and photoreactivation during UV, chlorination and UV/chlorination disinfection. The inactivated CS-2 underwent a certain photoreactivation after UV and chlorination. The logistic model precisely fitted the data obtained in the photoreactivation experiments by UV treatment, with the estimated kinetic parameters S_m (0.530%-12.071%) and k₂ (0.0009-0.0471). The photoreactivation of Acinetobacter sp. CS-2 was observed when treated by chlorination at a dosage of 0.5 mg/L with a survival ratio of 34.04%. UV/chlorination not only resulted in the high-efficiency reduction of CS-2 but also effectively controlled its photoreactivation with a survival ratio of 0%- 0.87%. UV/chlorination showed great advantages in causing the irreversible destruction of bacterial surface structures by making the cell membranes wrinkled and incomplete compared with UV disinfection. The singlet oxygen (¹O₂) generated during UV/chlorination treatment played a vital role in bla_NDM-1 removal. This study proposed new insights into the mechanism of inactivation and the characteristics of photoreactivation for the super-resistant bacteria by UV, chlorination and UV/chlorination.

ClO2 pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process

The formation of undesirable chloro-organic byproducts is of great concern in the UV/chlorine process. In this study, chlorine dioxide (ClO₂) pre-oxidation was applied to control the formation of chloro-organic byproducts and the toxicity in UV/chlorine-treated water. The molecular-level changes in dissolved organic matter (DOM) were tracked by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and ClO₂ pre-oxidation was found to preferentially react with DOM moieties with high aromaticity level and with a carbon number of > 18, producing compounds with a higher degree of oxidation and lower aromaticity. The ClO₂-treated DOM was found to be less susceptible to attack by radicals and free chlorine in the UV/chlorine process compared to the raw DOM. ClO₂ pre-oxidation resulted in a significant decrease in the number of unknown chloro-organic byproducts (i.e., -17%) and the total intensity of organic chlorine detected by FT-ICR-MS (i.e., -31%). The molecular characteristics, such as O/C, aromaticity index, and the average number of chlorine atoms, of these unknown chloro-organic byproducts generated in the scenarios with and without ClO₂ pre-oxidation were also different. Additionally, ClO₂ pre-oxidation reduced the genotoxicity (SOS/umu test) and cytotoxicity (Hep G2 cytotoxicity assay) of UV/chlorine-treated water by 26% and 20%, respectively. The findings in this study highlight the merits of ClO₂ pre-oxidation for controlling chloro-organic byproducts and reducing the toxicity of water treated by the UV/chlorine process in actual practice.

Effects of MPUV/chlorine oxidation and coexisting bromide, ammonia, and nitrate on DBP formation potential of five typical amino acids

Disinfection byproduct (DBP) formation is a potential concern with regard to MPUV/Cl₂ application in water treatment. In this study, five typical amino acids (AAs) were selected to investigate their DBP alteration during short-term medium pressure (MP) UV/chlorine oxidation following post-chlorination relative to parallel dark controls. The five selected AAs include two potent DBP precursors (aspartic acid and tryptophan), one modest precursor (asparagine) and two poor precursors (phenylalanine and proline). MPUV/chlorine increased the total DBP formation and DBP-associated cytotoxicity of the two poor precursors phenylalanine (Phe) and proline (Pro) as well as their chlorine demands. Conversely, DBP formation and DBP-associated cytotoxicity of the three modest-to-potent DBP precursors showed the opposite changing trends due to MPUV/Cl₂ oxidation. The two aromatic AAs (tryptophan and phenylalanine) were more readily to be affected by MPUV/Cl₂ oxidation especially at acidic pH condition. Conversely, DBP formation and DBP-associated cytotoxicity of the three modest-to-potent precursors showed the opposite changing trends due to MPUV/Cl₂ oxidation. Among the measured DBPs, the absolute formation potential changes of haloacetic acids and haloacetonitriles were the most prominent. Presence of bromide increased the trihalomethane formation potential of five AAs. Ammonia-spiked samples resulted in notably higher chlorine demands but slightly reduced DBPFP. Photonitration caused increased haloacetonitrile and trichloronitromethane formation but lower overall DBP formation potential and DBP-associated cytotoxicity. Results indicated that increased DBP formation of unreactive aromatic AAs may be problematic with respect to MPUV/Cl₂ application, while the presence of inorganic ions may not contribute to further increase in calculated cytotoxicity of measured DBPs.

Enhanced formation of iodinated trihalomethanes in a mixed chlorine/chloramine system and attenuation by UV-activated process

Iodinated trihalomethanes (I-THMs) have drawn increasing concerns due to their higher toxicity than those of their chlorinated and brominated analogues. In this study, I-THM formation was firstly evaluated for three treatment scenarios - (i) chlorine alone, (ii) chloramine alone, and (iii) mixed chlorine/chloramine - in the presence and absence of UV irradiation for the iodide-containing humic acid solution or natural water. The results indicated that I-THM formation decreased in the order of mixed chlorination/chloramination > chloramination > > chlorination, which fitted the trend of toxicity evaluation results using Chinese hamster ovary cells. Conversely, total organic halide concentration decreased in the order of chlorination > > chloramination ≈ mixed chlorination/chloramination. Besides, I-THM formation can be efficiently controlled in a UV-activated mixed chlorine/chloramine system. Influencing factors including pH values and Br^-/I^- molar ratios were also systematically investigated in a mixed chlorine/chloramine system. Enhanced I-THM formation was observed with increasing pH values (6.0-8.0) and Br^-/I^- molar ratios (1: 1-10: 1). The results obtained in this study can provide new insights into the increasing risk of I-THM formation in a mixed chlorine/chloramine system and the effective control of I-THMs in the iodide-containing water using UV irradiation.

Experimental and theoretical study on the degradation of Benzophenone-1 by Ferrate(VI): New insights into the oxidation mechanism

The oxidation of Benzophenone-1 (BP-1) by ferrate (Fe(VI)) was systemically investigated in this study. Neutral pH and high oxidant dose were favorable for the reaction, and the second order rate constant was 1.03 × 10³ M^-1·s^-1 at pH = 7.0 and [Fe(VI)]₀:[BP-1]₀ = 10:1. The removal efficiency of BP-1 was enhanced by cations (K⁺, Ca²⁺, Mg²⁺, Cu²⁺, and Fe³⁺), while inhibited by high concentrations of anions (Cl^- and HCO₃^-) and low concentrations of humic acid. Moreover, intermediates were identified by LC-MS, and five dominating reaction pathways were predicted, involving single hydroxylation, dioxygen transfer, bond breaking, polymerization and carboxylation. Theoretical calculations showed the dioxygen transfer could occur by Fe(VI) attacking the CC double-bond in benzene ring of BP-1 to form a five-membered ring intermediate, which was hydrolyzed twice followed by H-abstraction to generate the dihydroxy-added product directly from the parent compound. Dissolved CO₂ or HCO₃^- might be fixed to produce carboxylated products, and Cl^- led to the formation of two chlorinated products. In addition, the toxicity assessments showed the reaction reduced the environmental risk of BP-1. This work illustrates Fe(VI) could remove BP-1 in water environments efficiently, and the newly proposed dioxygen transfer mechanism herein may contribute to the development of Fe(VI) chemistry.

Formation and transformation of halonitromethanes from dimethylamine in the presence of bromide during the UV/chlorine disinfection

Halonitromethanes (HNMs) is a typical class of nitrogenous disinfection byproducts with high toxicity. The effect of Br^- on the formation and transformation of HNMs from dimethylamine (DMA) during the ultraviolet (UV)/chlorine disinfection has been investigated in current study. Results reveal that only chloronitromethane, dichloronitromethane and trichloronitromethane (TCNM) could be found during the UV/chlorine disinfection. Whereas in the presence of Br^-, nine species of HNMs could be observed simultaneously. When Br^- concentration increased from 0 to 15.0 mg L^-1, the predominant species of HNMs were gradually changed from TCNM to dibromonitromethane and tribromonitromethane, which contributed to 23.37% and 31.07% of total HNMs concentration at 15 mg L^-1 Br^-, respectively. The presence of Br^- not only shifted the chlorinated-HNMs (Cl-HNMs) towards brominated-HNMs (Br-HNMs) but also affected the dominant species and total concentration of HNMs. When Br^- concentration was 4.0 mg L^-1, the formation of HNMs decreased with the increase of pH from 6.0 to 8.0 and increased with the increase of free chlorine and DMA. When free chlorine concentration rose from 0.25 to 1.1 mmol L^-1, Br-HNMs were shifted to Br(Cl)-HNMs and then to Cl-HNMs. According to the findings, possible formation and transformation pathways of HNMs from DMA were proposed in the presence of Br^- during the UV/chlorine disinfection. Finally, it was proved that the effect of Br^- on the trend of HNMs in real water was similar to that in deionized water, but higher HNMs concentrations and delayed peak time were observed in real water. This study can provide the scientific evidence and fundamental data for the applications of UV/chlorine disinfection in the treatment of water containing Br^-.

Highly efficient removal of total nitrogen and dissolved organic compound in waste reverse osmosis concentrate mediated by chlorine radical on 3D Co3O4 nanowires anode

Reverse osmosis concentrate (ROC) from wastewater reclamation has posed significant disposal challenges due to its highly concentrated NH₃-N, chloride ion and bio-refractory organics, and developing technologies for their removal are essential. Herein, we developed an efficient electrochemical system to remove total nitrogen and dissolved organic compound (DOC) simultaneously mediated by chlorine radical (Cl•), which is generated by activation of chloride ion existing in ROC on an inexpensive, three-dimensional Co₃O₄ nanowires. Results showed that the total nitrogen and total organic carbon removal were 98.2% and 56.9% in 60 min for synthetic ROC with 56 mg/L of NH₃-N and 20 mg/L of DOC. The utilization of Co₃O₄ nanowires enhanced NH₃-N degradation by 2.58 times compared with Co₃O₄ nanoplates, which were 1.69 and 17.5 times these of RuO₂ and Pt. We found that structural Co³⁺/Co²⁺ acts as cyclic catalysis to produce Cl• via single-electron transfer, which convert NH₃-N to N₂ and lead to faster DOC degradation. This architecture provides abundant catalytic sites and sufficient accessibility of reactants. Small amount of nitrate generated by oxidation of NH₃-N was further reduced to N₂ on Pd-Cu/NF cathode. These findings provide new insights for utilization of waste Cl^- and development of novel electrochemical system for ROC disposal.

High efficiency electrochemical disinfection of Pseudomons putida using electrode of orange peel biochar with endogenous metals

The electrochemical disinfection efficiency of Pseudomons putida was studied using ruthenium iridium coated titanium (RICT) electrode as anode and carbonized orange peel biochar (OPB) or graphite as the cathode. The results indicated that RICT/OPB system induced 6.5 and 7.0 log of P. putia inactivation after 60 s at 2 V and 45 s at 10 V, respectively. RICT/OPB system showed better efficiency than RICT/graphite system. The energy consumption of OPB cathode (17.5 Wh m^-3 per log) was significantly lower than that of graphite cathode (23.09 Wh m^-3 per log). Both anode and cathode played great roles on the disinfection. The anode absorbed electric energy to generate electrical hole, which can oxidize chloride ions to chlorine free radicals. The continuous porous structure of OPB can provide more adsorption sites and reduce electrolyte transport resistance, resulting in more Cl· production. Moreover, P. putia was much easier adsorbed to the anode surface in the RICT/OPB system because of the stronger electrostatic repulsion between cells and OPB cathode. As a result, P. putia was more easily inactivated by the Cl· produced on the anode. Besides chlorine active species, superoxide radical (O₂·﹣) produced on surface of cathode may also result in P. putia inactivation. The endogenous CuO in OPB can induce persistent free radicals (PFRs) production during pyrosis process. O₂·﹣ can be produced by O₂ activation through the function of Cu₂O/CuO and PFRs existed in OPB cathode. The more superoxide radical production led to the better disinfection effect than the graphite cathode. As a consequence, OPB electrode showed high efficiency electrochemical disinfection of P. putida.

Comparative investigation of acetaminophen degradation in aqueous solution by UV/Chlorine and UV/H2O2 processes: Kinetics and toxicity assessment, process feasibility and products identification

The degradation of acetaminophen (ACM) was comparatively studied by UV/chlorine and UV/H₂O₂ systems. An apparent reduction in the removal rate was observed above the optimum pH levels of 7.0 and 3.0 in UV/chlorine and UV/H₂O₂ processes, respectively. The relative contribution of each oxidizing agent in ACM removal using the two advanced oxidation processes (AOPs) was evaluated. Even though hydroxyl radicals, with the contribution percentage of 90.1%, were determined as the primary oxidizing species in ACM removal using the UV/H₂O₂ process, reactive chlorine species (RCS), with 43.8% of contribution percentage, were also found to play a pivotal role in ACM removal using the UV/chlorine process. For instance, dichlorine radical (Cl₂^•-) showed an acceptable contribution percentage of 32.2% in the degradation of ACM by the UV/chlorine process. The rate of ACM degradation significantly rose to 99.9% and 75.6%, as higher amounts of oxidants were used in the UV/chlorine and UV/H₂O₂ processes, respectively, within 25 min. The introduction of HCO₃^- ions and humic acid remarkably decreased the rate of ACM degradation in both techniques used in this study. The presence of NO₃^- and Cl^- ions did not considerably affect the removal rate in the UV/chlorine process. The acute toxicity analysis revealed that a more pronounced reduction in the ACM solution toxicity could be achieved by the UV/H₂O₂ process compared to the UV/chlorine process, which should be ascribed to the formation of chlorinated products in the UV/chlorine treatment. Eventually, plausible oxidation pathways were proposed for each process.

Application of UV/chlorine processes for the DR83:1 degradation from wastewater: Effect of coexisting anions

In the present work, effect of coexisting anions on the degradation performance of UV/chlorine (UV/Cl) processes as an advanced oxidation treatment for the dye containing wastewater was investigated. The results showed that by increasing pH of the solution from 3 to 11, degradation efficiency of UV/Cl process was reduced from 96.2 ± 1.4% to 62.9 ± 3.1%. In the case of 100 mg/L of DR83:1, the removal efficiency was improved from 30.2 ± 1.5% to 93.3 ± 4.7% when the chlorine dose was increased from 100 to 1000 μM. The HCO₃- and Br^- ions were the main inhibitor and promotor anions responsible for the degradation of DR83:1. The relative contribution of Cl^• was higher than that for HO^• and UV, which was about three-times higher than that for HO^•. The lowest and highest amounts of trihalomethanes were generated at acidic and alkaline conditions as well as low disinfection by products at low pH due to the generation of more HO^• radicals at acidic pH in comparison to higher pH such that less intermediates were remained to react with chlorine.

Facile ammonium oxidation to nitrogen gas in acid wastewater by in situ photogenerated chlorine radicals

The treatment of low-concentration ammonium (e.g., <50 mg L^-1) in highly acidic wastewaters through traditional biological nitrification, physical separation, or chemical stripping remains a huge challenge. Herein, we report that photocatalytic ammonium oxidation using bismuth oxychloride (BiOCl) can successfully occur in Cl^--laden solutions within a pH range of 1.0-6.0. All reactions follow pseudo-zero-order kinetics (with rate constants of 0.27-0.32 mg L^-1 min^-1 at pH 2.0-6.0 and 0.14 mg L^-1 min^-1 at pH 1.0), indicating the saturation of reactive species by the reactants. The interlayer is self-oxidized by the valence band holes (VB h⁺), resulting in the formation of Cl^• and subsequently HClO, which is excited upon UV irradiation to provoke consecutive photoreactions for chlorine radical generation. Compared to the free chlorine, HO^•, Cl^•, and Cl₂^•-, the ClO^• produced using the UV/BiOCl system plays a predominant role in oxidizing ammonium under acidic conditions. BiOCl exhibits good stability because of the compensation of Cl^- from solution and maintains high activity under different conditions (i.e., different cations and co-existing anions, temperatures, and initial substrate concentrations). The successful removal of ammonium from real wastewater using the UV/BiOCl system suggests that this is a promising method for treating diluted ammonium under highly acidic conditions.

Degradation of carbamazepine by UVA/WO3/hypochlorite process: Kinetic modelling, water matrix effects, and density functional theory calculations

The rapid recombination of electron/hole pairs is a major setback in the application of WO₃-based photocatalysis in water treatment. In this study, hypochlorite (ClO^-) was used as an electron acceptor to enhance the photocatalytic degradation of carbamazepine (CBZ) using UVA-excited WO₃. The results showed that CBZ degradation in the UVA/WO₃/ClO^- system followed a pseudo-first order reaction kinetic model. The addition of 0.1 mM ClO^- to the UVA/WO₃ system at pH values of 8.2 and 6.2 increased the rate constant (k_obs) of the degradation process 5.3- and 11.5-fold, respectively. Further, increasing the WO₃ dosage or decreasing the initial CBZ concentration resulted in an increase in k_obs. However, at high concentrations, ClO^- inhibited CBZ degradation. Based on the kinetic model, it could be suggested that ClO played a dominant role in the degradation process. Furthermore, the water matrix effects were as follows: the optimal pH was 6.2; humic acid, chloride, bicarbonate, and ammonium exhibited inhibitory effects on CBZ degradation; and sulfate ion significantly enhanced the degradation. Density functional theory (DFT) calculations indicated a strong affinity between ClO^- and the WO₃ surface. Specifically, the electrical energy per order that was associated with the use of ClO^- varied in the range of 0.100-1.617 kWh/m³. In summary, this study shows that ClO^- is an excellent electron acceptor for excited WO₃, while clarifying the CBZ degradation-enhancing effect of ClO^- as well as the kinetic model and DFT calculations. These findings can be employed in the degradation of recalcitrant contaminants in a cost-effective manner, while being significant for the development of more effective catalysts of UV-assisted advanced oxidation processes.

Enhanced Radical Generation in an Ultraviolet/Chlorine System through the Addition of TiO2

Ultraviolet (UV)/chlorine draws increasing attention for the abatement of recalcitrant organic pollutants. Herein, it was found that TiO₂ would significantly promote the degradation of dimethyl phthalate (DMP) in the UV/chlorine system (from 19 to 84%). Hydroxyl radicals (HO^•) and chlorine radicals (Cl^•) were the dominant reactive species for DMP degradation in the UV/chlorine/TiO₂ system. Chlorine decayed much faster in UV/chlorine/TiO₂ compared with UV/chlorine, which is possibly because photogenerated electrons (e_cb^-) and superoxide radicals (O₂^•-) have high reactivity with chlorine. As a result, the recombination of photogenerated holes (h_vb⁺) and e_cb^- was inhibited and the accumulation of HO^• and Cl^• was facilitated. A kinetic model was established to simulate the reaction process, and it was found that the concentrations of HO^• and Cl^• were several times to dozens of times higher in UV/chlorine/TiO₂ than that in UV/chlorine. The contributions of HO^• and Cl^• to DMP degradation were 70.3 and 29.7% by model simulation, respectively, and were close to the probe experiment result. In the UV/chlorine/TiO₂ system, the degradation of DMP did not follow pseudo-first-order kinetics but the degradation of benzoate fitted well with pseudo-first-order kinetics. This phenomenon was elucidated by the structure of the pollutant and TiO₂ and further tested by calculating the adsorption energy (E_ads)/binding energy (E_b) with density functional theory. Due to faster decay of chlorine, lower amounts of disinfection byproducts formed in UV/chlorine/TiO₂ compared with UV/chlorine. Adding TiO₂ into the UV/chlorine system can promote the degradation of recalcitrant organic pollutants in an aqueous environment.

Formation of disinfection by-products in a UV-activated mixed chlorine/chloramine system

In recent years, ultraviolet (UV) irradiation coupled with chlor(am)ination process is ubiquitous in secondary water supply systems in many cities of China. However, the disinfection by-products (DBPs) formation in a UV-activated mixed chlorine/chloramine system (MCCS) still remains unclear. In this study, the DBPs formation in a UV-activated MCCS was systematically investigated, considering influencing factors including the mass ratios of free chlorine to NH₂Cl, UV irradiation, pH values, NOM types, Br^- concentration and toxicity of the DBPs. Results indicated that DBPs formation decreased remarkably as mass ratio of free chlorine to NH₂Cl changed from 5:0 to 0:5. The DBPs formation in humic acid (HA)-containing water was the highest, followed by those in fulvic acid (FA) and algal organic matter (AOM). Besides, better control of the DBP-related calculated toxicity can be achieved in acidic conditions regardless of the UV irradiation. Furthermore, in the presence of Br^-, a significant reduction of DBPs formation could be achieved in a UV-activated MCCS. The findings also demonstrated that DBPs formation in real water can be effectively reduced at high UV fluence in a MCCS.

Rational Surface and Interfacial Engineering of IrO2 /TiO2 Nanosheet Arrays toward High-Performance Chlorine Evolution Electrocatalysis and Practical Environmental Remediation

The chlorine evolution reaction (CER) is a critical and commercially valuable electrochemical reaction in industrial-scale utilization, including the Chlor-alkali industry, seawater electrolysis, and saline wastewater treatment. Aiming at boosting CER electrocatalysis, hybrid IrO₂ /TiO₂ nanosheet arrays (NSAs) with rational surface and interfacial tuning strategies are proposed in this study. The IrO₂ /TiO₂ NSAs exhibit superb CER electrocatalytic activity with a low overpotential (44 mV) at 10 mA cm^-2 , low Tafel slope of 40 mV dec^-1 , high CER selectivity (95.8%), and long-term durability, outperforming most of the existing counterparts. The boosting mechanism is proposed that the aerophobic/hydrophilic surface engineering and interfacial electronic structure tuning of IrO₂ /TiO₂ are beneficial for the Cl^- mass-transfer, Cl₂ release, and Volmer-Heyvrosky kinetics during the CER. Practical saline wastewater treatment by using the IrO₂ /TiO₂ NSAs electrode is further conducted, demonstrating it has a higher p-nitrophenol degradation ratio (95.10% in 60 min) than that of other electrodes. The rational surface and interfacial engineering of IrO₂ /TiO₂ NSAs can open up a new way to design highly efficient electrocatalysts for industrial application and environmental remediation.

Evidence of free radical generation from the interaction of polyethylene glycol with PVC medical tubing

The combination of polyethylene glycol (PEG) and polyvinyl chloride (PVC) medical tubing was previously demonstrated to degrade an active pharmaceutical ingredient (API), a phenomenon proposed to occur by free radical mechanisms. This study tests the hypothesis that dehydrochlorinated PVC at the tubing surface increases the oxidative potential of PEG autooxidation via radical propagation. The functional group composition at the surfaces of intact, autoclaved, or force-degraded medical grade PVC tubings was assessed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The content of double bonds in PVC was correlated with the extent of API degradation in the PEG-PVC system, with the repeated autoclaving cycle treatments yielding the most reactive tubing. After PEG exposure, new functional groups on the surface of PVC were observed, indicating the participation of PVC in the oxidation reactions. The PEG-PVC system was further probed by the fluorinated spin-trap reagent FDMPO, where trapped adducts were analyzed by ¹⁹F NMR, revealing the presence of three radical species. Trapped adducts were then analyzed by two-dimensional liquid chromatography tandem mass spectrometry (2D-LC-MS/MS), which revealed the presence of free chlorine atoms and/or hypochlorous acid and a PEG alkoxy radical. Chemical mechanisms describing the interaction between dehydrochlorinated PVC and PEG are proposed to explain the presence of free radicals and the functional group changes in the PVC surface.