Reactive Chlorine Species Research Articles

Evaluation of the bactericidal activity of plasma-activated NaCl solution (PAN) against foodborne pathogens: Inactivation mechanism and application to mackerel

The objective of this study was to assess the bactericidal effect of plasma-activated NaCl solution (PAN) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes and apply PAN as a brine salting solution for mackerel. To enhance the bactericidal effect of plasma-activated water (PAW), NaCl solutions (0, 3.5, 7, and 10 %) were treated with plasma for 20 and 40 min to generate PAN. Plasma-activated glycerol solution (PAG) was also included to evaluate the influence of water activity on plasma activation and its effect on microbial activity. Physicochemical analysis revealed that elevating the NaCl concentration of PAN led to a decrease in pH, an increase in oxidation–reduction potential, and higher levels of reactive species such as H2O2 and HOCl. PAN showed greater antibacterial activity compared to PAW and PAG, except for L. monocytogenes, where 40 min activation time and treatment time exceeding 20 min was required for significantly higher reduction to occur. PAN exhibited greater antibacterial activity at higher NaCl concentrations, which was attributed to increased ionic strength and reactive chlorine species. Additionally, we evaluated the microbial mechanisms of PAN by assessing cellular damage and alterations. The common observation across the three pathogens was that PAN resulted in increased cell membrane damage, reduced intermembrane enzyme activity, higher intracellular ROS levels, and changes in zeta potential values, while DNA damage was observed only in PAN-treated L. monocytogenes. Furthermore, when PAW and PAN were stored for up to four weeks, PAN showed higher efficacy compared to PAW. 10 % PAN was also effective against foodborne pathogens on mackerel, achieving log reductions of 3.62 for E. coli O157:H7, 4.62 for S. Typhimurium, and 3.18 for L. monocytogenes after a 20 min treatment without adversely affecting quality. Our results demonstrated the antibacterial activity and action mechanism of PAN, presenting its potential application in the seafood industry.

Space heterogeneity of nitrogen removal functional genes, nitrogen transformation pathways and mechanisms in MEBR treating mariculture wastewater

Membrane Electro-Bioreactor (MEBR), exhibited excellent nitrogen removal in mariculture wastewater treatment. However, the differences of microbial community and nitrogen transformation pathways on spatial scale caused by the mass transfer of reactive chlorine species (RCS) generated by electrooxidation were unclear. This study provided new insights into the space heterogeneity of ammonia transformation pathways and mechanisms. The results demonstrated an increase in the reduction of nitrate to nitrite through partial denitrification on the membrane. Coupling with dissimilatory nitrate reduction to ammonium and RCS, higher nitrate (0.3 mgN/L) and TN removal (2.6 mgN/L) was obtained in MEBR. Higher relative abundance of narGHI and nirB were obtained. Ammonia oxidation by biofilters was enhanced in MEBR, that ammonia removal contributed by biofilters was 32.0 %, 5.2 times higher than the membrane. Relative abundance of amoABC and Nitrosomonas in biofilters were the highest. The results will provide theoretical basis for reactor configuration and operation optimization.

Periodic polarity reversal triggered sequential electrochemical redox of blue TiO2 nanotube arrays for efficient removal of antibiotics and inorganic nitrogen from actual mariculture wastewater

Using blue TiO2 nanotube arrays (Ti/BTNAs) as both anode and cathode, a novel bipolar system that is suitable for operating in periodic polarity reversal (PR) and flow-through mode was established, aiming to effectively remove antibiotics and inorganic nitrogen from actual mariculture wastewater. Under optimal conditions of 4.0 mA/cm2 current density, 3.0 mL/min water flow rate and 2.0 cycles/h polarity reversal frequency, the PR-induced synergistic effect endows Ti/BTNAs abundant oxygen vacancies (20.53 %), high activity and extended service lifetime (5.25 years), realizing continuous production of surface-adsorbed active hydrogen (Hads∗) at cathode and reactive chlorine species (Cl, ClO, Cl2−, ClO−) over anode through Cl− reacting with reactive oxygen species (HO, SO4−, O2−) and ∼0.7 kWh/gTOC energy consumption. Bipolar system can also flexibly regulate denitrification mechanism through the PR-triggered sequential electrochemical redox of Ti/BTNAs in water flow direction, achieving minimum energy consumption of ∼43.4 ± 3.2 kWh/kg(NH4+-N) and 33.7 ± 2.3 % current efficiency. Berberine antibiotic degradation pathway was suggested by DFT calculations and LC-MS/MS analyses. This study provides a long-acting and low-consuming strategy for comprehensive treating mariculture wastewater.

Effect of chloride ions on the reactive species change pathway and removal performance in the MW/Co2+/PMS process

Efficient removal of refractory organic matter is the focus of research in the treatment of actual wastewater with Peroxymonosulfate-based advanced oxidation processes(PMS-AOPs). However, the high concentration of chloride ions (Cl−) ubiquitously present in wastewater changes the reactive species (RS) present in the process, and the mechanism of this change is still not clear. This study investigated the effects of Cl− on the transformation behavior of RS and treatment performance under microwave (MW) combined with cobalt ions (Co2+)-activated PMS. The results showed that increasing the Cl− concentration can enhance the removal performance of the MW/Co2+/PMS process for ammonia (NH4+-N), accompanied by the production of nitrate nitrogen (NO3-N), monochloramine (NH2Cl) and dichloramine (NHCl2), which was related to the transformation of existing reactive oxygen species (ROS, •OH and SO4•−) into reactive chlorine species (RCS), with hypochlorous acid (HClO) as an important intermediate. Cl− can generate HClO through free radical oxidation (•OH and SO4•−) dominance and non-free radical oxidation (PMS and Co3+) promotion, and the heat transferred by MW can further trigger conversion between HClO and PMS, thereby changing the distribution of RS. Increasing the dosage of PMS and the dosage of Co2+ promoted the transformation of Cl− to reactive species in the MW/Co2+/PMS process. At MW power=160W, [Co2+]=17.5μM,[PMS]=10mM, and [Cl−]=17mM, the maximum cumulative [HClO] reached 1910.97μM, and the decay rate was 0.1815 min-1.The macromolecular organic matter in aqueous environment competes with ammonia for the RCS. Through regulating [Cl−] in mature leachate, RCS has a certain oxidation effect to organic matter, but its limited oxidation ability leads to a decrease in the mineralization effect.

Novel Catalyst-free Activation of Chlorine by Visible Light for Micropollutant Abatement

This study systematically investigated the direct activation of chlorine by visible light emitting diode (Vis-LED). Vis-LED could effectively activate chlorine to degrade micropollutants with degradation efficiency and pseudo-first-order degradation rate constant range of 64.3%-100% and 0.0340-0.195min-1, respectively. Quenching experiments and modeling results suggested that reactive chlorine species (RCS, including ClO•, Cl2•-, and Cl•) and hydroxyl radical (•OH) were involved in the degradation of atenolol (ATL). The contribution ratio of ClO•, free available chlorine, Cl•, Cl2•-, and •OH to ATL degradation were 58.7%, 17.4%, 15.6%, 1.8%, and 5.9%, respectively, in Vis-LED448/chlorine process. Moreover, the innate quantum yields of HClO and ClO- decreased from 0.229 and 0.0206 to 0.0489 and 0.0109mol·Einstein-1, respectively, as the wavelength increased from 448 to 513nm, leading to a decrease in ATL degradation, which was consistent with the model results. Experimental and modeling results have confirmed that ATL degradation decreased when pH increased from 4.0 to 9.0. Cl- had little effect on the degradation of ATL, while HA and HCO3- affected ATL degradation by scavenging reactive species and/or shielding effect. The concentration of disinfection by-products decreased with the increase of wavelength and pH. In summary, Vis-LED/chlorine is an efficient water treatment process even without a catalyst.

Exploring electrochemical mechanisms for clindamycin degradation targeted at the efficient treatment of contaminated water

Numerous studies reveal pollutants like clindamycin (CLD) in the environment, posing environmental and health risks. Conventional water treatment methods are ineffective at removing these contaminants, typically found in low concentrations. An innovative treatment approach is introduced through pre-concentration via adsorption, which is highly efficient, energy-saving, and reusable. The innovation uses solvents like methanol or ethanol to desorb pollutants, creating concentrated CLD solutions for more effective electrochemical degradation than conventional methods. Thus, this study explores, for the first time, the behavior of CLD electro-oxidation in different media—water, methanol, and ethanol—using a Dimensionally Stable Anode (DSA®-Cl₂). The study reveals distinct degradation mechanisms and offers new insights into solvent-assisted electrochemical treatments. After 30 min of electrolysis, all the current densities evaluated promoted significant degradation, ranging from 90 to 92%. The energy consumption was 2.9 Wh m⁻³ per percentage point at current densities of 2 and 3.5 mA cm⁻2. This demonstrates that using higher current densities over shorter electrolysis times is feasible, achieving removal rates of approximately 90%.The performance of chloride-based electrolytes was superior to that of sulfate-based electrolytes due to the ability of DSA®-Cl2 electrodes to generate reactive chlorine species more efficiently. A higher concentration of supporting electrolytes initially improved CLD removal, but no significant changes were observed after 1 h. Neutral pH conditions optimized CLD degradation, achieving up to 91% removal. Higher pollutant concentrations were associated with lower kinetic constants and decreased removal percentages. Methanol and ethanol enhanced removal rates to 98.3% and 92.3%, respectively, by generating oxidizing species such as methoxy, hydroxymethyl, and ethoxy radicals. The degradation by-products differed across the three media, with each solvent exhibiting distinct oxidation mechanisms. These findings highlight the potential of using methanol or ethanol as an electrolytic medium with efficiency comparable to water.

Simultaneous desalination and removal of organic pollutants in a novel bifunctional FCDI system: The enhancement by in-situ reutilization of chloride ions

A new system of flow electrode capacitive deionization coupled with in situ electro-oxidation (FCDI-EO) was constructed to achieve the degradation of organic pollutants while maintaining high desalination efficiency. The effects of different parameters of potential difference, electrolyte, and pH on the chloride removal performance of FCDI-EO and the removal performance of rhodamine B (RhB) were investigated. Under the optimal conditions of potential difference = 4 V, [Cl−] = 500 mg/L, and pH = 4, the removal of chloride ions, RhB and total organic carbon (TOC) was as high as 88.74 %, 99.02 %, and 87.82 %, respectively. The quenching experiments showed that hydroxyl radicals (OH) and reactive chlorine species (RCS) were involved in the FCDI-EO system. The liquid chromatography-mass spectrometry (LC-MS) analysis suggested the possible mineralization pathway of RhB. Furthermore, toxicity assessment analysis showed that the toxicity of the degradation intermediates of RhB decreased during the treatment. Overall, this study provides a promising process that reducing the generation of high-concentration saline water in desalination and in situ utilizing the Cl− gathered from the feed water. The findings of this study could provide useful guidance for the development of flow electrode capacitive deionization technology and organic pollutant control.

Chloride-mediated transformation of sulfate radicals to hydroxyl radicals for enhanced contaminant degradation in high-temperature wastewater

Using the excess heat in high-temperature industrial wastewater to activate oxidant precursors for organic contaminant degradation would make advanced oxidation processes more sustainable in the context of carbon neutrality, while the salt-mediated radical transformation chemistry in such wastewater is poorly understood. We herein demonstrate that chloride (Cl–) in the range of 0.28–56.34 mM accelerates the degradation of organic contaminants in the heat activated persulfate process in both synthetic and authentic wastewater. We use laser flash photolysis technique together with kinetic modeling to directly determine the bimolecular rate constants of radical–radical (e.g., kSO4·--SO4·- = 2.50 × 109 M−1 s−1), Cl–-radical (e.g., kSO4·--Cl- = 3.75 × 108 M−1 s−1), and radical-contaminant (e.g., kHO·-BA = 1.02 × 1010 M−1 s−1) reactions at 55 °C, which are substantially higher than those at ambient water temperature (25 °C). We also combine experimental and modelling results to calculate the contribution of different radicals to contaminant removal at 55 °C. The results clearly demonstrate that Cl– in the range of 0.28–56.34 mM promotes the transformation of SO4– to HO at 55 °C, with reactive chlorine species involved as the intermediates. The findings improve fundamental understanding of Cl–-mediated radical chemistry in high-temperature water and provide an engineering strategy to recycle the heat in wastewater to enhance contaminant remediation.

Impact of metal ions on PMS/Cl− disinfection efficacy: Enhancing or impeding microbial inactivation?

Peroxymonosulfate (PMS) is an eco-friendly disinfectant gaining attention. This study examined the influence of metal ions (Co(II), Cu(II), Fe(II)) on PMS disinfection with chloride ions (Cl−) against waterborne microorganisms, encompassing both bacteria and fungal spores. The findings elucidated that metal ions augment the inactivation of bacteria in the PMS/Cl− system while concurrently impeding the inactivation of fungal spores. Specifically, the PMS/Co(II)/Cl− process increased E. coli inactivation rates by 2.25 and 2.75 times compared to PMS/Co(II) and PMS/Cl−, respectively. Conversely, PMS/Me(II)/Cl− generally exhibited a diminished inactivation capacity against the three fungal spores compared to PMS/Cl−, albeit surpassing the efficacy of PMS/Me(II). For instance, the inactivation levels of A. niger by PMS/Cl−, PMS/Cu(II)/Cl−, and PMS/Cu(II) are 4.47-log, 1.92-log, and 0.11-log, respectively. Notably, fungal spores demonstrated a substantially higher resistance to disinfectants compared to bacteria. Differences in microbial susceptibility were linked to cell wall structure, composition, antioxidant defenses, and reactive species generation, such as hydroxyl radicals (•OH), sulfate radicals (SO4•−), and reactive chlorine species (RCS). This study demonstrated the novel and unique phenomenon of metal ions' dual role in modulating the PMS/Cl− disinfection process, which has not been reported before and has important implications for the field of water treatment.

The solar/chlorine system degradation of iopamidol: Roles of reactive species, degradation pathway, and toxicity

The study systematically examined the changing trends of the percentage contributions of reactive species to the degradation of Iopamidol (IPM) under various conditions in the solar/chlorine system. Results showed that compared with chlorine alone and solar radiation system, the solar/chlorine system had an obvious effect on the degradation of IPM under control experiment conditions. HO• and reactive chlorine species played major roles in the apparent pseudo-first-order rate constants of IPM degradation (kobs). The increase of oxidant dosage significantly promoted the steady-state concentration of RCS and the contributions of those to kobs. The concentrations of various radicals at pH 9 were lower than those of at pH 5, while only the percentage contribution of ClO• on kobs was increased. The presence of Cl- could increase the steady-state concentration of Cl•, which further promoted the kobs. NOM would significantly reduce the steady-state concentration of ClO• and Cl•, which leads to a significant increase in the percentage contribution of Cl2−• to kobs, indicating the important role of Cl2−• on organic removal in real water. HCO3-/CO32- had almost no effect on kobs. The degradation products of IPM were identified through HPLC-MS, and the Fukui function was employed to ascertain the sites of degradation, which were formulated by deiodination reactions and the production of organic radicals. According to structure Activity Relationships Program v.2.2 and acute toxicological tests, byproducts did not exhibit significant toxicity. The findings illustrate that the solar/chlorine system is effective in decomposing IPM into non-toxic and non-hazardous small molecules, with potential applications.

Impacts of chloride ions on the electrochemical decomplexation and degradation of Cr(III)-EDTA: Reaction mechanisms of HO• and RCS

The removal of Cr(III)-organic complexes, encompassing both decomplexation and ligand degradation, presents significant challenges in industrial wastewater treatment. As one of the most common anions in wastewater, Cl− significantly improves the efficiency of electrochemically removing Cr(III)-organic complexes through generated reactive chlorine species (RCS). In the electrochemical chlorine (EC/Cl2) process, extensive experimentation revealed that ClO• plays a dominant role in the degradation of Cr(III)-EDTA, surpassing the effects of free chlorine, direct electrooxidation, HO•, and other RCS. Density functional theory calculations indicated that RCS, primarily Cl• and ClO•, preferentially oxidize the ligand in Cr(III)-EDTA via H-abstraction, whereas HO• trends to attack the Cr atom through electron transfer. The influential factors on the degradation efficiency of Cr(III)-EDTA, Cr(VI) yield, and total organic carbon removal in EC/Cl2 were also assessed, including Cl− concentration, current density, and pH. Real industrial wastewater was employed as a reaction matrix to evaluate the application of the EC/Cl2 process for treating Cr(III)-EDTA, accompanied by energy efficiency calculations. Additionally, a two-chamber reactor was established to simultaneously oxidize Cr(III)-EDTA at the anode and reduce Cr(VI) at the cathode. This study provided insight into developing RCS-dominated AOPs to effectively decomplex and decompose organic Cr(III)-complexes in Cl−-containing industrial wastewater.

Solar powered electrocatalytic treatment of actual human urine coupled with hydrogen production using Ti/MMO for the removal of nitrogen-based compounds

This study addresses a significant micropollutant removal from actual human urine (AHU) through photovoltaic driven electrochemical treatment. This study showed significant reductions of 69.09 % urea, 67.95 % creatinine, and 95.180 % uric acid under optimal continuous recirculation conditions. Analysis revealed that the degradation of pollutants were facilitated by in-situ generation of reactive oxygen and chlorine species during treatment. Further, the procedure employed with molecular hydrogen generation during anodic oxidation as useful byproduct. Thus, the findings suggest that novel mixed metal oxide exhibits high efficacy for AHU treatment, offering a prominent concept for onsite treatment of complex mixed wastewater in real-life applications.

Genetic determinants of increased sodium hypochlorite and ciprofloxacin susceptibility in Pseudomonas aeruginosa PA14 biofilms

Reactive chlorine species (RCS) like sodium hypochlorite (NaOCl) are potent oxidizing agents and widely used biocides in surface disinfection, water treatment, and biofilm elimination. Moreover, RCS are also produced by the human immune system to kill invading pathogens. However, bacteria have developed mechanisms to survive the damage caused by RCS. Using the comprehensive Pseudomonas aeruginosa PA14 transposon mutant library in a genetic screen, we identified a total of 28 P. aeruginosa PA14 mutants whose biofilms showed increased susceptibility to NaOCl in comparison to PA14 WT biofilms. Of these, ten PA14 mutants with a disrupted apaH, PA0793, acsA, PA1506, PA1547, PA3728, yajC, queA, PA3869, or PA14_32840 gene presented a 4-fold increase in NaOCl susceptibility compared to wild-type biofilms. While none of these mutants showed a defect in biofilm formation or attenuated susceptibility of biofilms toward the oxidant hydrogen peroxide (H2O2), all but PA14_32840 also exhibited a 2–4-fold increase in susceptibility toward the antibiotic ciprofloxacin. Further analyses revealed attenuated levels of intracellular ROS and catalase activity only for the apaH and PA1547 mutant, providing insights into the oxidative stress response in P. aeruginosa biofilms. The findings of this paper highlight the complexity of biofilm resistance and the intricate interplay between different mechanisms to survive oxidative stress. Understanding resistance strategies adopted by biofilms is crucial for developing more effective ways to fight resistant bacteria, ultimately contributing to better management of bacterial growth and resistance in clinical and environmental settings.

Multiple Roles of Anions on the Degradation Efficiency and Mineralization Pathway of Recalcitrant Acetaldehyde by Vacuum Ultraviolet (185 nm) Oxidation.

Vacuum-UV (185 nm, VUV) is widely applied to polish reverse osmosis permeate (ROP), such as the production of electronics-grade ultrapure water. In this study, the VUV oxidation of acetaldehyde, a common carbonyl in ROP, was found to be influenced by anions even at low concentrations. Interestingly, the influencing extent and mechanism varied depending on the anions. Bicarbonate minimally affected the VUV-photon absorption and •OH consumption, but at 5000 μg-C·L-1, it decreased the degradation of acetaldehyde by 58.7% possibly by scavenging organic radicals or other radical chain reactions. Nitrate strongly competed for VUV-photon absorption and •OH scavenging through the formation of nitrite, and at 500 μg-N·L-1, it decreased the removal rate of acetaldehyde degradation by 71.2% and the mineralization rate of dissolved organic carbon by 53.4%. Chloride competed for VUV-photon absorption and also generated reactive chlorine species, which did not affect acetaldehyde degradation but influenced the formation of organic byproducts. The radical chain reactions or activation of anions under VUV irradiation could compensate for the decrease in oxidation performance and need further investigation. In real ROPs, the VUV oxidation of acetaldehyde remained efficient, but mineralization was hindered due to nitrate and chloride anions. This study deepens the understanding of the photochemistry and feasibility of VUV in water with low concentrations of anions.

The degradation of 8 PPCPs in UV/chlorine: Effect of chloride and sulfate ions and the role of radicals

The degradation of 8 structurally diverse pharmaceuticals and personal care products (PPCPs) by UV/chlorine and the effects of Cl- and SO42- were systematically investigated. The 8 PPCPs are divided into 2 groups based on their stability under UV light. This is the first time the kinetic degradation of the 8 PPCPs in the presence of Cl- and SO42- have been intensively evaluated. The results showed that the high degradation rate of the PPCPs is attributed to their second-order rate constants towards radicals (i.e., •OH, and reactive chlorine species (RCS)). 0–500 mM Cl- enhanced kPPCPs, while this range of SO42- inhibited most of them (except in the case of ciprofloxacin (CIP)). An overview of the effect of 0–200 mM Cl-/SO42- on the degradation of micropollutants by different UV-based advanced oxidation processes (UV-AOPs) in previous studies indicates that Cl-/SO42- can cause varying effects on the degradation rate of pollutants (i.e., “increase”, “decrease”, or “no effect”), depending on the type of pollutant and the process. Cl-/SO42- did not change the trend of pH influence on the PPCPs degradation rates by UV/chlorine. Acidic and neutral conditions were favorable to the degradation of the PPCPs in all cases. Under three operating conditions (i.e., the control process, 500 mM Cl- and SO42-), [•OH]ss decreased considerably, but [Cl•]ss or ([Cl•]ss + [SO4•-]ss) slightly fluctuated when the pH was increased from 2.5 to 10.5. The synergistic effect of seawater and Cl-/SO42- significantly improved some kPPCPs, thereby, decreasing energy consumption.

Magnéli Ti4O7 nanowire induces locally enhanced electric field and electrocatalytic activity for rapid and efficient water disinfection via synergistic electroporation and electrochlorination

Waterborne pathogens in drinking water remains a prominent cause of morbidity and mortality in developing rural areas. Electrochlorination offers a promising alternative for decentralized water disinfection, but its efficiency was strongly restrained by the low Cl- concentrations in natural waters. Herein, we proposed a Magnéli Ti4O7 nanowire-assisted electrochemical approach via forming locally enhanced electric field over nanowire tips for reinforcing electroporation, and via forming locally enhanced charge density for promoting Cl- enrichment and subsequent electrochlorination on anodic nanowire tips. Ti4O7 nanowires induced the enhancement of electric field by 4 orders of magnitude and Cl- concentration by ∼3 orders of magnitude at nanowire tips, and promoted ∼3 times more active chlorine generation at a voltage of 3.0 V compared with non-nanostructured one. Synergistic electroporation and electrochlorination on nanowires enabled rapid and efficient disinfection with hydraulic retention time of ∼36 s and energy consumption below ∼5 J/L/log for inactivation of various gram-positive/negative bacteria and phage MS2. Analyses of cell morphology and membrane integrity revealed that electroporation-induced cell pores provided diffusion channels of reactive chlorine species for oxidative damage of cell structures, resulting in ∼1.5–4.5 times lower energy consumption than the thin-film, nanoparticle, and nanorod ones. The excellent operation stability for tap water with significant bacterial inactivation and active chlorine generation demonstrated its great application potential for decentralized water disinfection.

Unveiling the transformation of organics in leachate during the composite anode-supported electro-hybrid ozonation-coagulation (CA-E-HOC) treatment: Significance of synergy among chlorine, coagulant, and ozone

Ozone and its advanced oxidation processes are commonly used to remove organics from leachate because of their high efficiency in eliminating toxic and resistant compounds. However, the mechanisms underlying the transformations of these organic compounds remain unclear. This study aimed to investigate the organic transformation during the composite anode (CA)-supported electro-hybrid ozonation-coagulation (CA-E-HOC) treatment of leachate, where chlorine and coagulants were coproduced on CA for ozone activation, to gain a deeper understanding of the synergistic effects when chlorine, coagulant, and ozone are simultaneously present within a single unit for leachate treatment. Reactive oxygen species and reactive chlorine species in the CA-E-HOC facilitated the degradation of unsaturated and aromatic compounds into hydrophilic organic chains or mineralisation to CO2/H2O. The aromatic carbon (C=C) and long-chain aliphatic carbon (C-C) are converted to carbonyl carbon (O-C=O), which is easily degraded by microorganisms during the subsequent treatment process. The reactive chlorine species and ozone molecules transform CHOS into CHO through −S and −SH2, whereas •OH converts CHNOS into CHNO. The enriched CHO substances were further converted into saturated fatty acids and carbohydrates through oxidation reactions (+O), making them more susceptible to conversion into CO2 and H2O by interacting with •OH and Cl•/ClO•, ultimately resulting in organic mineralisation. This study contributes to a better understanding of molecular changes in organics during leachate treatment using the CA-E-HOC process, with potential implications for practical applications.

Chloride-mediated enhancement in Cu(II)-catalyzed Fenton-like reaction: The overlooked reactive chlorine species

The practical application of Cu(II)-catalyzed Fenton-like reaction (Cu(II)/H2O2) exhibits a low efficiency in the degradation of refractory compounds of wastewater. The impact of chloride ions (Cl−) on Fenton-like reactions have been investigated, but the influence mechanism is still unclear. Herein, the presence of Cl− (5 mM) significantly accelerated the degradation of benzoic acid (BA) under neutral conditions. The degradation of BA follows pseudo-first-order kinetics, with a degradation rate 7.3 times higher than the Cu(II)/H2O2 system. Multiple evidences strongly demonstrated that this reaction enables the production of reactive chlorine species (RCS) rather than HO• and high-valent copper (Cu(III)). The kinetic model revealed that Cl− could shift reactive species from the key intermediate (Cu(III)-chloro complexes) to RCS. Dichlorine radicals (Cl2•-) was discovered to play a crucial role in BA degradation, which was largely overlooked in previous reports. Although the reaction rate of Cl2•- with BA (k = 2.0 × 106 M−1 s−1) is lower than that of other species, its concentration is 10 orders of magnitude higher than that of Cu(III) and HO•. Furthermore, the exceptional efficacy of the Cu(II)/H2O2 system in BA degradation was observed in saline aquatic environments. This work sheds light on the previously unrecognized role of the metal-chloro complexes in production the RCS and water purification.

Degradation of emerging contaminants in synthetic hydrolyzed urine by UV/peracetic acid: Free radical chemistry, and toxicity analysis

The ecological impact of emerging contaminants (ECs) in aquatic environments has raised concerns, particularly with regards to urine as a significant source of such contaminants in wastewater. The current investigation used the UV/Peracetic Acid (UV/PAA) processes, an innovative advanced oxidation technology, to effectively separate two emerging pollutants from urine at its source, namely, ciprofloxacin (CIP) and bisphenol A(BPA). The research findings demonstrate that the presence of the majority of characteristic ions has minimal impact on the degradation of ECs. However, in synthetic hydrolyzed urine, only NH4+ inhibits the degradation of two types of ECs, with a more pronounced effect observed on CIP degradation compared to BPA.The impact of halogen ions, specifically Cl− and I−, on the degradation of CIP in synthetic hydrolyzed urine was a complex phenomenon. When these two halogen ions are present individually, the generation of reactive halogen species (RHS) within the system enhances the degradation of CIP. However, when both types of ions coexist, the formation of diatomic radical species partially inhibits degradation. In terms of BPA degradation, while the production of reactive chlorine species (RCS) to some extent hinders the reaction rate, the generation of reactive iodine species (RIS) promotes the overall process. CIP undergoes fragmentation of the piperazine and quinoline rings, decarboxylation, defluorination reactions, as well as substitution reactions, leading to the formation of products with simplified structures. The degradation of BPA occurs gradually through hydroxyl and halogen substitution as well as isopropyl cleavage. The preliminary toxicity analysis confirmed that the presence of halogen ions in urine resulted in the formation of halogenated products in two types of ECs, albeit with an overall reduction in toxicity. The UV/PAA processes was considered to be an effective and relatively safe approach for the separation of ECs in urine.

Electrochemically producing high-valent iron–oxo species for phenolics-laden high chloride wastewater pretreatment

Electrochemical advanced oxidation processes (EAOPs) have shown great promise for treating industrial wastewater contaminated with phenolic compounds. However, the presence of chloride in the wastewater leads to the production of undesirable chlorinated organic and inorganic byproducts, limiting the application of EAOPs. To address this challenge, we investigated the potential of incorporating Fe(II) and Fe(III) into the EAOPs with a boron-doped diamond (BDD) anode under near-neutral conditions. Our findings revealed that both Fe(II) and Fe(III) facilitated the generation of high-valent iron-oxo species (Fe(IV) and Fe(V)) in the anodic compartment, thereby reducing the oxidation contribution of reactive chlorine species. Remarkably, the addition of 1000 μM Fe(II) under high chloride conditions resulted in over a 2.8-fold increase in the oxidation rate of 50 μM phenolic contaminants at pH 6.5. Furthermore, 1000 μM Fe(II) contributed to a reduction of more than 66% in the formation of chlorinated byproducts, consequently enhancing the biodegradability of the treated water. Additionally, transitioning from batch mode to continuous flow mode further amplified the positive effects of Fe(II) on the EAOPs. Overall, this study presents a modified electrochemical approach that simultaneously enhanced the degradation of phenolic contaminants and improved the biodegradability of wastewater with high chloride concentrations.