Insight into PPCP degradation by UV/NH2Cl and comparison with UV/NaClO: Kinetics, reaction mechanism, and DBP formation

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

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

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

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

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

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

Hypochlorite-mediated degradation and detoxification of sulfathiazole in aqueous solution and soil slurry

Although various activated sodium hypochlorite (NaClO) systems were proven to be promising strategies for recalcitrant organics treatment, the direct interaction between NaClO and pollutants without explicit activation is quite limited. In this work, a revolutionary approach to degrade sulfathiazole (STZ) in aqueous and soil slurry by single NaClO without any activator was proposed. The results demonstrated that 100% and 94.11% of STZ could be degraded by 0.025 mM and 5 mM NaClO in water and soil slurry, respectively. The elimination of STZ was shown to involve superoxide anion (O2•-), chlorine oxygen radical (ClO•), and hydroxyl radical (•OH), according to quenching experiments and the analysis of electron paramagnetic resonance. The addition of Cl-, HCO3-, SO42-, and humic acid (HA) marginally impeded the decomposition of STZ, while NO3-, Fe3+, and Mn2+ facilitated the process. The NaClO process exhibited significant removal effectiveness at a neutral initial pH. Moreover, the NaClO facilitated application in various soil samples and water matrices, and the procedure was also successful in effectively eliminating a range of sulfonamides. The suggested NaClO degradation mechanism of STZ was based on the observed intermediates, and the majority of the products exhibited lower ecotoxicity than STZ. Besides, the experiment results by using X-ray diffraction (XRD) and a fourier transform infrared spectrometer (FTIR) indicated the negligible effects on the composition and structure of soil by the treatment of NaClO. Simultaneously, the experimental results also illustrated that the bioavailability of heavy metals and the physiochemical characteristics of the soil before and after the remediation did not change to a significant extent. Following the remediation of NaClO, the phytotoxicity tests showed reduced toxicity to wheat and cucumber seeds. As a result, treating soil and water contaminated with STZ by using NaClO was a reasonably practical and eco-friendly method.

Efficient photochemical conversion of naproxen by butanedione: Role of energy transfer

Photochemical active species generated from photosensitizers, e.g., dissolved organic matter (DOM), play vital roles in the transformation of micropollutants in water. Here, butanedione (BD), a redox-active moiety in DOM and widely found in nature, was employed to photo-transform naproxen (NPX) with peracetic acid (PAA) and H2O2 as contrasts. The results obtained showed that the BD exhibited more applicable on NPX degradation. It works in the lake or river water under UV and solar irradiation, and its NPX degradation efficiency was 10-30 times faster than that of PAA and H2O2. The reason for the efficient transformation of pollutants is that the BD system was proved to be a non-free radical dominated mechanism. The quantum yield of BD (Ф254 nm) was calculated to be 0.064, which indicates that photophysical process is the dominant mode of BD conversion. By adding trapping agents, direct energy transfer from 3BD* to NPX (in anoxic environment) or dissolved oxygen (in aerobic environment) was proved to play a major role (> 91 %). Additionally, the BD process reduces the toxicity of NPX and promotes microbial growth after irradiation. Overall, this study significantly deepened the understanding of the transformation between BD and micropollutants, and provided a potential BD-based process for micropollutants removal under solar irradiation.

Refinement of kinetic model and understanding the role of dichloride radical (Cl2•-) in radical transformation in the UV/NH2Cl process

The ultraviolet/monochloramine (UV/NH2Cl) process is an emerging advanced oxidation process with promising prospects in water treatment. Previous studies developed kinetic models of UV/NH2Cl for simulating radical concentrations and pollutant degradation. However, the reaction rate constants of Cl2•- with bicarbonate and carbonate (kCl2•-, HCO3- and kCl2•-, CO32-) were overestimated in literature. Consequently, when dosing 1 mM chloride and 1 mM bicarbonate, the current models of UV/NH2Cl severely under-predicted the experimental concentrations of three important radicals (i.e., hydroxyl radical (HO•), chlorine radical (Cl•), and dichloride radical (Cl2•-)) with great deviations (> 90 %). To investigate this issue, the transformation reactions among these three radicals in UV/NH2Cl were systematically studied. For the first time, it was found that in addition to Cl•, Cl2•- was also an important parent radical of HO• in the presence of chloride, and chloride could effectively compensate the inhibitory effect of bicarbonate on HO• generation in the system. Moreover, reactions and rate constants in current models were scrutinized from corresponding literature, and the reaction rate constants of Cl2•- with bicarbonate and carbonate (kCl2•-, HCO3- and kCl2•-, CO32-) were reevaluated to be 1.47 × 105 and 3.78 × 106 M-1s-1, respectively, by laser flash photolysis. With the newly obtained rate constants, the refined model could accurately simulate concentrations of all three radicals under different chloride and bicarbonate dosages with satisfactory deviations (< 30 %). Meanwhile, the refined model performed much better in predicting pollutant degradation and radical contribution compared with the unrefined model (with the previously estimated kCl2•-, HCO3- and kCl2•-, CO32-). The results of this study enhanced the accuracy and applicability of the kinetic model of UV/NH2Cl, and deepened the understanding of radical transformation in the process.

Removal of tetracycline by ultraviolet/sodium percarbonate (UV/SPC)advanced oxidation process in water

Tetracycline (TC) was widely used and frequently detected in various water bodies, where the presence of TC posed a significant threat to the health of aquatic organisms. Furthermore, antibiotics were hardly degraded by biological treatment. Thus, in order to enhance the removal of TC, we proposed the use of a novel ultraviolet/sodium percarbonate (UV/SPC) advanced oxidation process and initiated an in-depth study. The study investigated the influence of oxidant dosage, initial pH, UV intensity, and TC concentration on the removal of TC. The results demonstrated that the UV/SPC system efficiently removed TC, with removal efficiency increasing as the SPC concentration increased. Within the pH range of 3-11, TC degradation exhibited minimal variation, indicating the UV/SPC system's strong adaptability to pH variations. The research on the impact of the water matrix on TC removal revealed that HCO3- had an inhibitory effect on TC degradation, while NO3- promoted TC degradation. Additionally, the presence of free radical species (·OH, ·CO3-, ·O2-) were detected and rate constants for the secondary reactions (k·OH,TC = 6.3 × 109 L mol-1·s-1, k·CO3-,TC = 3.4 × 108 L mol-1·s-1) were calculated, indicating that ·OH exhibited a stronger oxidative performance compared to ·CO3-. This study did not only present a novel strategy via UV/SPC to remove TC but also uncovered the unique role of ·CO3- for contaminant removal.

Bio-mitigation of organic pollutants using horseradish peroxidase as a promising biocatalytic platform for environmental sustainability

A wide array of environmental pollutants is often generated and released into the ecosystem from industrial and human activities. Antibiotics, phenolic compounds, hydroquinone, industrial dyes, and Endocrine-Disrupting Chemicals (EDCs) are prevalent pollutants in water matrices. To promote environmental sustainability and minimize the impact of these pollutants, it is essential to eliminate such contaminants. Although there are multiple methods for pollutants removal, many of them are inefficient and environmentally unfriendly. Horseradish peroxidase (HRP) has been widely explored for its ability to oxidize the aforementioned pollutants, both alone and in combination with other peroxidases, and in an immobilized way. Numerous positive attributes make HRP an excellent biocatalyst in the biodegradation of diverse environmentally hazardous pollutants. In the present review, we underlined the major advancements in the HRP for environmental research. Numerous immobilization and combinational studies have been reviewed and summarized to comprehend the degradability, fate, and biotransformation of pollutants. In addition, a possible deployment of emerging computational methodologies for improved catalysis has been highlighted, along with future outlook and concluding remarks.

Transformation and toxicity studies of UV filter diethylamino hydroxybenzoyl hexyl benzoate in the swimming pools

Diethylamino hydroxybenzoyl hexyl benzoate (DHHB), an ultraviolet (UV) filter, can be found in sunscreens and other personal care products and thus can be introduced into swimming pools through the swimmers. In outdoor pools, DHHB will inevitably interact with free chlorine and sunlight. Therefore, the mechanism of solar‑chlorine chemical transformation of DHHB, as well as the environmental risk, were investigated in this work. In chlorinated with solar (Cl + solar) process, free chlorine was the dominant contributor to 85% of the DHHB degradation, while hydroxyl radicals and reactive chlorine species contributed only 15% because of low free radical generation and fast DHHB and free chlorine reaction rates. Scavenging matrices, such as Cl^-, NH₄⁺, and dissolved organic matter (DOM), inhibited the degradation of DHHB in the Cl + solar process, while Br^-, HCO₃^-, NO₃^-, and urea promoted DHHB degradation. DHHB degradation was inhibited in tap water swimming pool samples, while it was enhanced in seawater pool samples by the Cl + solar process. Seven transformation by-products (TBPs) including mono-, dichlorinated, dealkylate, and monochloro-hydroxylated TBPs were identified. Three degradation pathways, chlorine substitution, chlorine and hydroxyl substitution, and dealkylation were proposed for DHHB transformation in the Cl + solar process. Both Quantitative structure-activity relationship and Aliivibrio fischeri toxicity tests demonstrated increased toxicity for the chlorinated TBPs. A risk assessment of the DHHB and its TBPs suggested that both DHHB and its chlorinated TBPs pose a significant health risk.

Hydroxyl radical dominated ibuprofen degradation by UV/percarbonate process: Response surface methodology optimization, toxicity, and cost evaluation

Ibuprofen (IBP) is a typical nonsteroidal anti-inflammatory drug with a wide range of applications, large dosages, and environmental durability. Therefore, ultraviolet-activated sodium percarbonate (UV/SPC) technology was developed for IBP degradation. The results showed that IBP could be efficiently removed using UV/SPC. The IBP degradation was enhanced with prolonged UV irradiation time, with the decreasing IBP concentration and the increasing SPC dosage. The UV/SPC degradation of IBP was highly adaptable to pH ranging from 4.05 to 8.03. The degradation rate of IBP reached 100% within 30 min. The optimal experimental conditions for IBP degradation were further optimized using response surface methodology. IBP degradation rate reached 97.3% under the optimal experimental conditions: 5 μM of IBP, 40 μM of SPC, 7.60 pH, and UV irradiation for 20 min. Humic acid, fulvic acid, inorganic anions, and natural water matrix inhibited the IBP degradation to varying degrees. Scavenging experiments of reactive oxygen species indicated that hydroxyl radical played a major role in the UV/SPC degradation of IBP, while carbonate radical played a minor role. Six IBP degradation intermediates were detected, and hydroxylation and decarboxylation were proposed as the primary degradation pathways. An acute toxicity test, based on the inhibition of luminescence in Vibrio fischeri, indicated that the toxicity of IBP during UV/SPC degradation decreased by 11%. An electrical energy per order value of 3.57 kWh m^-3 indicated that the UV/SPC process was cost-effective in IBP decomposition. These results provide new insights into the degradation performance and mechanisms of the UV/SPC process, which can potentially be used for practical water treatment in the future.

Toxicological aspect of water treated by chlorine-based advanced oxidation processes: A review

As well established in the literature, residual toxicity is an important parameter for evaluating the sanitary and environmental safety of water treatment processes, and this parameter becomes even more crucial when chlorine-based processes are applied for water treatment. Eliminating initial toxicity or preventing its increase after water treatment remains a huge challenge mainly due to the formation of highly toxic disinfection by-products (DBPs) that stem from the degradation of organic contaminants or the interaction of the chlorine-based oxidants with different matrix components. In this review, we present a comprehensive discussion regarding the toxicological aspects of water treated using chlorine-based advanced oxidation processes (AOPs) and the recent findings related to the factors influencing toxicity, and provide directions for future research in the area. The review begins by shedding light on the advances made in the application of free chlorine AOPs and the findings from studies conducted using electrochemical technologies based on free chlorine generation. We then delve into the insights and contributions brought to the fore regarding the application of NH₂Cl- and ClO₂-based treatment processes. Finally, we broaden our discussion by evaluating the toxicological assays and predictive models employed in the study of residual toxicity and provide an overview of the findings reported to date on this subject matter, while giving useful insights and directions for future research on the topic.

Visible light-driven NH2Cl activation by g-C3N4 photocatalysis producing reactive nitrogen species to degrade bisphenol A

The photolysis of monochloramine (NH₂Cl), a widely used disinfectant, under UVC irradiation produces different radicals for the micropollutant degradation. For the first time, this study demonstrates the degradation of bisphenol A (BPA) via the NH₂Cl activation by graphitic carbon nitride (g-C₃N₄) photocatalysis using visible light-LEDs at 420 nm, termed as the Vis₄₂₀/g-C₃N₄/NH₂Cl process. The process produces •NH₂, •NH₂OO, •NO and •NO₂ via the e_CB^-- and O₂^•--induced activation pathways and •NHCl and NHClOO• via the h_VB⁺-induced activation pathway. The produced reactive nitrogen species (RNS) enhanced 100% of the BPA degradation compared with the Vis₄₂₀/g-C₃N₄. Density functional theory calculations confirmed the proposed NH₂Cl activation pathways and further demonstrated that e_CB^-/O₂^•- and h_VB⁺ induced the cleavage of N-Cl and N-H bonds in NH₂Cl, respectively. The process converted 73.5% of the decomposed NH₂Cl to nitrogen-containing gas, compared with that of approximately 20% in the UVC/NH₂Cl process, leaving much less ammonia, nitrite and nitrate in water. Among different operating conditions and water matrices tested, of particular significance is natural organic matter of 5 mgDOC/L only reduced 13.1% of the BPA degradation compared against that of at least 46% reduction in the UVC/NH₂Cl process. Only 0.017-0.161 µg/L of disinfection byproducts were produced, two orders of magnitudes lower than that in the UVC/chlorine and UVC/NH₂Cl processes. The combined use of visible light-LEDs, g-C₃N₄ and NH₂Cl significantly improves the micropollutant degradation and reduces the energy consumption and byproduct formation of the NH₂Cl-based AOP.

Efficiently photocatalysis activation of peroxydisulfate by Fe-doped g-C3N5 for pharmaceuticals and personal care products degradation

Peroxydisulfate (PDS) based advanced oxidation processes (AOPs) are widely used for the degradation of pharmaceutical and personal care products (PPCP) in wastewater treatment. In this study, a Fe-doped g-C₃N₅ (Fe@g-C₃N₅) was synthesized as a photocatalyst for catalyzing the PDS-based AOPs to degrade tetracycline hydrochloride (TH) at pH 3 and Naproxen (NPX) at pH 7. The photocatalytic performance of Fe@g-C₃N₅ was 19% and 67% higher than g-C₃N₅ and g-C₃N₄ for degradation of TH at pH 3, respectively, while it was 21% and 35% at pH 7. The Fe:N ratio in Fe@g-C₃N₅, was calculated as 1:3.79, indicating that the doped Fe atom formed a FeN₄ structure with an adjacent two-layer graphite structure of g-C₃N₅, which improved the charge separation capacity of g-C₃N₅ and act as a new reaction center that can efficiently combine and catalyze the PDS to radicals. Although the intrinsic photo-degradation performance is weak, the photocatalytic performance of Fe@g-C₃N₅ has great room for the improvement and application in wastewater treatment.

Improvement of Fe(VI) oxidation by NaClO on degrading phenolic substances and reducing DBPs formation potential

Ferrate(VI) is a green oxidant and can effectively oxidize micropollutants. However, the instability of Fe(VI), i.e., self-decomposition, in the aqueous solution limited its application. Herein, it was found that the degradation of phenolic substances had been substantially improved through the combination of Fe(VI) with NaClO. At the condition of pH 8.0, 50 μM of Fe(VI) degraded 18.66 % of BPA (bisphenol A) at 0.5 min or 21.67 % of phenol at 2 min. By contrast, Fe(VI)/NaClO (50/10 μM) oxidized 38.21 % of BPA at 0.5 min or 38.08 % of phenol at 2 min with a synergistic effect. At the end of the reaction, the concentration of Fe(VI) in Fe(VI)/NaClO (50/10 μM) was 28.97 μM for BPA degradation, higher than the 25.62 μM of Fe(VI) group. By active species analysis, intermediate iron species [i.e., Fe(V) and Fe(IV)] played a vital role in the synergistic effect in Fe(VI)/NaClO system, which would react with the applied NaClO to regenerate Fe(VI). In natural water, the Fe(VI)/NaClO could also degrade phenolic substances of natural organic matter (NOM). Although the NaClO reagent was applied, disinfection by-products (DBPs) formation potential decreased by 22.75 % of the raw sample after Fe(VI)/NaClO treatment. Significantly, THMs, mainly caused by phenolic substances of NOM, even declined by 29.18 % of raw sample. Based on that, this study explored a novel ferrate(VI) oxidation system using the cheap NaClO reagent, which would present a new insight on ferrate(VI) application.

Orientated construction of visible-light-assisted peroxymonosulfate activation system for antibiotic removal: Significant enhancing effect of Cl. JOURNAL OF HAZARDOUS MATERIALS 2023;445:130476. [PMID: 36455327 DOI: 10.1016/j.jhazmat.2022.130476]

Antibiotic contaminants can migrate over long distances in the water, thus possibly causing severe detriment to the environment and even potential harm to human health. Heterogeneous activation of peroxymonosulfate (PMS) assisted by visible light is an emerging and promising technology for the purification of such wastewater. This study designed an ultra-efficient and stable PMS activator (FeCN) to restore the typical antibiotic-polluted water under harsh conditions. About 90.94% of sulfamethoxazole (SMX) was degraded in 35 min in the constructed FeCN+PMS/vis system, and the reaction rate constant was nearly 50-fold higher than direct photocatalysis. Electron spin resonance, quenching experiments, LC/MS technique, eco-toxicity assessment, and density functional theory validated that the SMX removal was dominated by the attack of h⁺, ^•O₂^- and ¹O₂ on the active atoms of SMX molecules with high Fukui index, presenting as a simultaneous degradation and detoxification process. Such a visible-light-assisted PMS activation system also had good resistance to the environmental water bodies and a broad spectrum in the degradation of various pollutants. In particular, Cl^- (50 mM) could significantly accelerate the removal of SMX with a 32.6-fold increase in catalytic activity, and the mineralization efficiency could reach 56.6% under identical conditions. Moreover, this Cl^- containing system excluded the degradation products of disinfection by-products, and such a system was also versatile for different contaminants. This work demonstrates the feasibility of the FeCN+PMS/vis system for the remediation of antibiotic-contaminated wastewater in the presence and absence of Cl^-, and also highlights their great potential in WWTPs.

Transformation of Graphitic Carbon Nitride by Reactive Chlorine Species: "Weak" Oxidants Are the Main Players

Graphitic carbon nitride (g-C₃N₄) nanomaterials hold great promise in diverse applications; however, their stability in engineering systems and transformation in nature are largely underexplored. We evaluated the stability, aging, and environmental impact of g-C₃N₄ nanosheets under the attack of free chlorine and reactive chlorine species (RCS), a widely used oxidant/disinfectant and a class of ubiquitous radical species, respectively. g-C₃N₄ nanosheets were slowly oxidized by free chlorine even at a high concentration of 200-1200 mg L^-1, but they decomposed rapidly when ClO· and/or Cl₂^•- were the key oxidants. Though Cl₂^•- and ClO· are considered weaker oxidants in previous studies due to their lower reduction potentials and slower reaction kinetics than ·OH and Cl·, our study highlighted that their electrophilic attack efficacy on g-C₃N₄ nanosheets was on par with ·OH and much higher than Cl·. A trace level of covalently bonded Cl (0.28-0.55 at%) was introduced to g-C₃N₄ nanosheets after free chlorine and RCS oxidation. Our study elucidates the environmental fate and transformation of g-C₃N₄ nanosheets, particularly under the oxidation of chlorine-containing species, and it also provides guidelines for designing reactive, robust, and safe nanomaterials for engineering applications.

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

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

Peroxymonosulfate activated by composite ceramic membrane for the removal of pharmaceuticals and personal care products (PPCPs) mixture: Insights of catalytic and noncatalytic oxidation

A composite manganese-based catalytic ceramic membrane (Mn-CCM) was developed by a solid-state sintering method, and its effectiveness toward activation of peroxymonosulfate (PMS) for the degradation of 11 pharmaceutical and personal care products (PPCPs) mixture was tested. The optimized Mn-CCMs/PMS system showed remarkable degradation efficiencies for PPCPs mixture with total removal >90% in ultrapure water, river water and natural organic matter (NOM) solution. The Mn-CCMs/PMS system showed the contribution of different phenomena in PPCPs removal in the order of catalytic oxidation (54.7%, Mn-CCMs/PMS) > noncatalytic oxidation (42.3%, PMS oxidation) > adsorption (3.0%, by Mn-CCMs). The singlet oxygen (¹O₂) was the dominant reactive oxygen specie for the degradation of PPCPs in all water matrices proved by the quenching experiments and electro-paramagnetic resonance (EPR) spectroscopy. The extraordinary stability of Mn-CCMs for the activation of PMS has been noted in terms of repeatability experiments for PPCPs degradation with fewer leaching of Mn (1.9 to 3.6 µg/L). Mineralization was achieved in the range of 28-65% for different water matrices. The toxicity of the PPCPs mixture was reduced by 85.9%. The Mn-CCMs/PMS system showed a reduction (25-100%) in precursors of different carbon- and nitrogen-based disinfection by-products. This study found the Mn-CCMs/PMS system as a feasible purification unit for removing trace concentrations of PPCPs (ng/L) in real drinking water matrices.

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

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

Degradation of carbamazepine by high-voltage direct current gas-liquid plasma with the addition of H2O2 and Fe2

Carbamazepine (CBZ) is a typical psychotropic pharmaceutical which is one of the most commonly detected persistent pharmaceuticals in the environment. The degradation of CBZ in the aqueous solution was studied by a direct current (DC) gas-liquid phase discharge plasma combined with different catalysts (H₂O₂ or Fe²⁺) in this study. The concentrations of reactive species (H₂O₂, O₃, and NO₃^-) and •OH radical yield in the liquid were measured during the discharge process. The various parameters that affect the degradation of CBZ, such as discharge powers, initial concentrations, initial pH values, and addition of catalysts, were investigated. The energy efficiency was 25.2 mg·kW^-1·h^-1 at 35.7 W, and the discharge power at 35.7 W was selected to achieve the optimal balance on the degradation effect and energy efficiency. Both acidic and alkaline solution conditions were conducive to promoting the degradation of CBZ. Both H₂O₂ and Fe²⁺ at low concentration (10-100 mg/L of Fe²⁺, 0.05-2.0 mmol/L of H₂O₂) were observed contributing to the improvement of the CBZ degradation rate, while the promotional effect of CBZ degradation was weakened even inhibition would occur at high concentrations (100-200 mg/L of Fe²⁺, 2.0-5.0 mmol/L of H₂O₂). The degradation rate of CBZ was up to 99.1%, and the total organic carbon (TOC) removal efficiency of CBZ was up to 67.1% in the plasma/Fe²⁺ (100 mg/L) system at 48 min, which suggested that high degradation rate and mineralization efficiency on CBZ could be achieved by employing Fe²⁺ as a catalyst. Based on the intermediate products identified by Ultra Performance Liquid Chromatography Tandem Mass Spectrometry (UPLC-MS), the possible degradation pathways were proposed. Finally, the growth inhibition assay with Escherichia coli (E. coli) showed that the toxicity of plasma/Fe²⁺-treated CBZ solution decreased and a relatively low solution toxicity could be achieved. Thus, the plasma/catalyst could be an effective technology for the degradation of pharmaceuticals in aqueous solutions.

Impact of water matrices on oxidation effects and mechanisms of pharmaceuticals by ultraviolet-based advanced oxidation technologies: A review

The binding between water components (dissolved organic matters, anions and cations) and pharmaceuticals influences the migration and transformation of pollutants. Herein, the impact of water matrices on drug degradation, as well as the electrical energy demands during UV, UV/catalysts, UV/O₃, UV/H₂O₂-based, UV/persulfate and UV/chlorine processes were systemically evaluated. The enhancement effects of water constituents are due to the powerful reactive species formation, the recombination reduction of electrons and holes of catalyst and the catalyst regeneration; the inhibition results from the light attenuation, quenching effects of the excited states of target pollutants and reactive species, the stable complexations generation and the catalyst deactivation. The transformation pathways of the same pollutant in various AOPs have high similarities. At the same time, each oxidant also can act as a special nucleophile or electrophile, depending on the functional groups of the target compound. The electrical energy per order (EEO) of drugs degradation may follow the order of EEO_UV > EEO_UV/catalyst > EEO_UV/H2O2 > EEO_UV/PS > EEO_UV/chlorine or EEO_UV/O3. Meanwhile, it is crucial to balance the cost-benefit assessment and toxic by-products formation, and the comparison of the contaminant degradation pathways and productions in the presence of different water matrices is still lacking.

Catalytic removal of attached tetrabromobisphenol A from microplastic surface by biochar activating oxidation and its impact on potential of disinfection by-products formation

There are numerous studies concerning the impacts of widespread microplastic pollution on the ecological environment, and it shows synergistic effect of microplastics and co-exposed pollutants in risk enhancement. However, the control methods for removing harmful pollutants from microplastic surface to reduce their ecological toxicity has rarely been explored. In this paper, magnetic graphitized biochar as a catalyst is shown to achieve 97% removal of tetrabromobisphenol A (TBBPA) from microplastics by biochar mediated electron transfer. The changes in the surface and structure of microplastics caused by various aging processes affected the pollutant attachment and subsequent removal efficiency. After chlorination, the highest disinfection by-product (DBP) generation potential was observed by the group of microplastics attached with TBBPA. The oxidation system of biochar activating peroxodisulfate (PDS) can not only reduce the kinds of DBPs, but also greatly reduce the total amount of detected DBPs by 76%, as well as reducing the overall toxicity. This paper highlights an overlooked contribution of pollutant attachment to the potential risks of DBP generated from natural microplastics during chlorination process, and provides the underlying insights to guide the design of a biochar-based catalyst from wastes to achieve the removal of TBBPA from microplastics and reduce the risks and hazards of co-contamination.

Quantification of the diverse inhibitory effects of dissolved organic matter on transformation of micropollutants in UV/persulfate treatment

Dissolved organic matter (DOM), ubiquitous in natural waters, is known to inhibit the degradation of micropollutants in the advanced oxidation processes such as the UV/peroxydisulfate process. However, the quantitative understanding of the inhibitory pathways is missing. In this study, guanosine, aniline and catechol belonging to amines, purines and phenols were first investigated due to their resistance to UV irradiation at 254 nm and similar reactivity with SO₄^•- and HO^•, respectively. The presence of 0.5 mg_C L^-1 Suwannee River NOM (SRNOM) inhibited their degradation rates by 72.9%, 54.5%, and 32.4%, respectively, despite their similar degradation rates in the absence of SRNOM. The results highlight the importance of reverse reduction of oxidation intermediates to the parent compound by antioxidant moieties in SRNOM besides the inner filtering and radical scavenging effects. The three inhibitory pathways were quantified for 34 common micropollutants. In the presence of 0.5 mg_C L^-1 SRNOM, inner filtering effect was found to contribute less than 2.8% of the inhibitory percentages (IP). Radical scavenging effects contribute between 10.7% and 38.9% and compounds having lower reactivity with SO₄^•- (< 4.0 × 10⁹ M^-1 s^-1) tended to be inhibited more strongly. The IP of reverse reduction effects of SRNOM varied significantly from none up to 70.8%. It was linearly related with a micropollutant's reduction potential. Purines and amines generally exhibited more pronounced reverse reduction inhibition than phenols. The results of this study provide guidance on improving the elimination efficiency of micropollutants.

Antibiotics degradation by UV/chlor(am)ine advanced oxidation processes: A comprehensive review

Antibiotics are emerging contaminants in aquatic environments which pose serious risks to the ecological environment and human health. Advanced oxidation processes (AOPs) based on ultraviolet (UV) light have good application prospects for antibiotic degradation. As new and developing UV-AOPs, UV/chlorine and derived UV/chloramine processes have attracted increasing attention due to the production of highly reactive radicals (e.g., hydroxyl radical, reactive chlorine species, and reactive nitrogen species) and also because they can provide long-lasting disinfection. In this review, the main reaction pathways of radicals formed during the UV/chlor (am)ine process are proposed. The degradation efficiency, influencing factors, generation of disinfection by-products (DBPs), and changes in toxicity that occur during antibiotic degradation by UV/chlor (am)ine are reviewed. Based on the statistics and analysis of published results, the effects caused by energy consumption, defined as electrical energy per order (EE/O), increase in the following order: UV/chlorine < UV/peroxydisulfate (PDS)< UV/H₂O₂ < UV/persulfate (PS) < 265 nm and 285 nm UV-LED/chlorine (EE/O). Some inherent problems that affect the UV/chlor (am)ine processes and prospects for future research are proposed. The use of UV/chlor (am)ine AOPs is a rich field of research and has promising future applications, and this review provides a theoretical basis for that.

Marine bacteria-mediated abiotic-biotic coupling degradation mechanism of ibuprofen

Knowledge on the behavior and fate of pharmaceuticals and personal care products (PPCPs) is poorly explored in marine aphotic environment. In this study, the degradation mechanism of a typical PPCPs-ibuprofen (IBP) by a ubiquitous marine Pseudoalteromonas sp. was investigated based on transcriptome and key enzymes analysis. More importantly, a novel enzymatic-nonenzymatic coupling degradation mechanism was uncovered for the first time, namely, the degradation of IBP was firstly initiated by extracellular reactive oxygen species (ROS), then the intermediate (e.g.4-ethylresorcinol) was further degraded by intracellular enzymes. It was showed that biogenic •OH, O₂^•-and H₂O₂ were responsible for extracellular nonenzymatic degradation, in which IBP was degraded to 4-ethylresorcinol through hydrogenation, isobutyl moiety cleavage, oxidation and decarboxylation. 4-Hydroxyphenylpyruvate dioxygenase, homogentisate 1,2-dioxygenase, long-chain acyl-CoA synthetase, acetyl-CoA acyltransferase and enoyl-CoA hydratase were identified to be involved in intracellular degradation, leading 4-ethylresorcinol cracked and eventually mineralized. Ultimately, this novel degradation mechanism was demonstrated to be amino acids-driven through KEGG enrichment analysis and experimental data. Overall, our work uncovered a yet undiscovered abiotic-biotic coupling degradation mechanism in PPCPs biotransformation, thereby updating the conventional concept that contaminants transformation is solely accomplished by enzymes or non-enzymes, which can also provide new insights into PPCPs environmental behavior and fate.

Insight into the degradation of ciprofloxacin by medium-pressure UV-activated monochloramine process

The degradation efficiency and mechanisms of ciprofloxacin (CIP), a typical antibiotic, by a medium-pressure ultraviolet/chloramine (MPUV/NH₂Cl) treatment were investigated. The results showed that CIP degradation by MPUV/NH₂Cl was significantly higher than that by NH₂Cl oxidation and MPUV photolysis, and that this degradation processes were consistent with pseudo-first-order kinetics. The initial CIP concentration (7.5-30.2 μM) and the presence of HCO₃^- (0.5-10 mM) significantly inhibited CIP degradation with k_obs,CIP 0.0090-0.0069 and 0.0078-0.0048 cm²/mJ. In contrast, NO₃^- (50-500 μM) and Br^- (0.5-10 mM) significantly promoted the degradation with k_obs,CIP 0.0078-0.0102 and 0.0078-0.0124 cm²/mJ. The effect of Cl^- (0.5-10 mM) and natural organic matter (1-5 mg/L) were negligible. The NH₂Cl dosage (30-60 μM) presented a dual effect, in which its increase within the optimal concentration range (30-40 μM) accelerated CIP degradation due to the formation of reactive radicals, whereas an excessive increase (40-60 μM) quenched the free radicals, ultimately quenching the free radicals and inhibiting the degradation. The optimum pH for CIP degradation under MPUV/NH₂Cl treatment was 7.0. The contribution of reactive halogen species (i.e., reactive chlorine species and reactive nitrogen species) to CIP degradation was substantially greater than that of hydroxyl radicals under acidic or neutral conditions. We identified the degradation products of CIP and proposed degradation pathways, which included defluorination and cracking of the piperazine ring, with the latter being dominant. Compared to haloacetic acid (HAA) and nitrogenous disinfection byproducts (N-DBPs), MPUV/NH₂Cl significantly reduced trihalomethane (THM) production and theoretical cytotoxicity by 80.1% and 78.4% respectively, compared to the background experiment in natural water at a UV dose of 300 mJ/cm².

Inactivation and subsequent reactivation of Aspergillus species by the combination of UV and monochloramine: Comparisons with UV/chlorine

Ultraviolet (UV)/monochloramine (NH₂Cl) as an advanced oxidation process was firstly applied for Aspergillus spores inactivation. This study aims to: i) clarify the inactivation and photoreactivation characteristics of UV/NH₂Cl process, ii) compared with UV/Cl₂ in inactivation efficiency, photoreactivation and energy consumption. The results illustrated that UV/NH₂Cl showed better inactivation efficiency than that of UV alone and UV/Cl₂, and could effectively control the photoreactivation. For instance, the inactivation rates for Aspergillus flavus, Aspergillus niger and Aspergillus fumigatus in the processes of UV/NH₂Cl (2.0 mg/L) was 0.034, 0.030 and 0.061 cm²/mJ, respectively, which were higher than that of UV alone (0.027, 0.026 and 0.024 cm²/mJ) and UV/Cl₂ (0.023, 0.026 and 0.031 cm²/mJ). However, there was no synergistic effect for Aspergillus flavus and Aspergillus fumigatus. As for Aspergillus niger, the best synergistic effect can reach 1.86-log₁₀. This may be due to their different resistance to disinfectants, which were related to the size, an outer layer of rodlets (hydrophobins) and pigments. After UV/NH₂Cl inactivation, the degree of cell membrane damage and intracellular reactive oxygen species were higher than that of UV alone. UV/NH₂Cl had the advantages of high inactivation efficiency and inhibition of photoreactivation, which provides a new entry point for the disinfection of waterborne fungi.

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

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

Multi-angle comparison of UV/chlorine, UV/monochloramine, and UV/chlorine dioxide processes for water treatment and reuse

Advanced oxidation processes (AOPs) have been increasingly studied and practiced for micropollutant abatement in drinking water treatment and potable water reuse. This study conducted the multi-angle comparison of the UV/chlorine, UV/monochloramine (UV/NH₂Cl), and UV/chlorine dioxide (UV/ClO₂) AOPs with respect to reactive species generation, micropollutant degradation, byproduct formation, and toxicity change. The concentrations of radicals (HO^·, Cl^·, and ClO^·) generated in the three AOPs followed the order of UV/chlorine > UV/NH₂Cl > UV/ClO₂ at an oxidant dose of 70 μM, an irradiation wavelength of 254 nm, and a pH of 7.5. The concentration of ozone generated in the UV/ClO₂ AOP was higher than that in the UV/chlorine AOP, while ozone was not generated in the UV/NH₂Cl AOP. The effects of pH (pH 6.0, 7.5, and 9.0) and UV wavelength (254 nm, 285 nm, and 300 nm) on the three AOPs were evaluated and compared. Using the radical and ozone concentrations determined in this study, the pseudo-first-order degradation rate constants of 24 micropollutants by the three AOPs were predicted and compared. When the three AOPs were used to treat the water containing the same concentration of natural organic matter, the formation of total organic chlorine (TOCl) and the organic byproduct-associated toxicity followed the same order of UV/chlorine > UV/NH₂Cl > UV/ClO₂. On the contrary, the inorganic byproduct-associated toxicity followed the order of UV/ClO₂ > UV/chlorine > UV/NH₂Cl, due to the high concentrations of chlorite and chlorate formed in the UV/ClO₂ AOP. Findings in this study offer fundamental information useful for the selection and operation of AOPs for micropollutant abatement in drinking water treatment and potable water reuse.

Simulation degradation of bromophenolic compounds in chlorine-based advanced oxidation processes: Mechanism, microscopic and apparent kinetics, and toxicity assessment

Chlorine-based advanced oxidation processes (AOPs) have been extensively studied to remove contaminants through generating HO^• and reactive chlorine species, including ClO^• and Cl^•. In this work, 2,4,6-tribromoanisole (246TBA) and 2,4,6-tribromophenol (246TBP) were selected as model to investigate the reaction mechanisms and micro-kinetics of brominated contaminants with HO^•, ClO^• and Cl^• in chlorine-based AOPs. Also, the apparent degradation kinetics of two compounds were simulated at pH 3.0-9.5 under UV/H₂O₂, UV/chlorine and UV/NH₂Cl. Calculated results showed that neutral 246TBA and 246TBP exhibited similar reactivity to HO^• and ClO^•, which was different from anionic 2,4,6-tribromophenolate (246TBPT): radical adduct formation (RAF) and H atom abstraction (HAA) were predominant mechanisms for the HO^• and ClO^• initiated reactions of 246TBA and 246TBP, while RAF and single electron transfer (SET) for 246TBPT; the reaction rate constants of 246TBA and 246TBP with HO^• and ClO^• were lower than 10⁷ M^-1 s^-1, and such rate constants dramatically increased to 10¹⁰ M^-1 s^-1 once 246TBP was deprotonated to 246TBPT. The apparent degradation kinetics of 246TBA at pH 3.0-9.5 was simulated in the order of UV/NH₂Cl > UV/chlorine > UV/H₂O₂, and UV/chlorine and UV/NH₂Cl were more effective for the removal of 246TBP and 246TBPT than UV/H₂O₂. UV and/or Cl^• dominated 246 TBA degradation under three AOPs. The main radicals mediating 246TBP and 246TBPT degradation are respectively HO^• under UV/H₂O₂, ClO^• under UV/chlorine, and HO^• and Cl^• under UV/NH₂Cl. The transformation products of 246TBA, 246TBP and 246TBPT, especially methoxylated and hydroxylated polybrominated diphenyl ethers (MeO-PBDEs and HO-PBDEs), were still toxic pollutants.

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

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

Ultraviolet-Light-emitting-diode activated monochloramine for the degradation of carbamazepine: Kinetics, mechanisms, by-product formation, and toxicity

Monochloramine (NH₂Cl) oxidant combined with a Ultraviolet (UV)-Light-emitting-diode (LED) light source forms a new advanced oxidation process (AOP), which can achieve high-efficiency degradation of carbamazepine (CBZ). The degradation of CBZ displayed pseudo-first-order reaction kinetics (R² > 0.98, k_CBZ = 0.0043 cm² mJ^-1 at pH 7). The degradation of CBZ was dependent on UV-LED wavelength, with maximum degradation efficiency observed at 265 nm since it was the lowest wavelength studied among UV-LEDs. Variation in pH across the range, which might be expected under normal environmental conditions (pH 6-8), and the presence of Cl^- had no significant effect on the degradation efficiency of CBZ, while the presence of HCO₃^- and natural organic matter (NOM) inhibited degradation. Electron paramagnetic resonance (EPR) experiments detected OH in the system. Probe compounds were used to distinguish the contribution of reactive chlorine species (RCS). It was proved that OH and Cl played major roles and OH was responsible for around 50% of the observed degradation of CBZ. Eight transformative products (TPs) in the degradation process of CBZ were identified, with a generally decreasing toxicity. The concentration of disinfection by-products (DBPs) formed during CBZ degradation was all within limits of WHO and China standard for drinking water. Although the concentration of nitrogen-containing DBPs (N-DBPs) was the lowest, N-DBPs were the main contributors to toxicity, and these would require more attention in practical applications. UV-LED/NH₂Cl AOP was identified as an effective way to degrade pharmaceutically active compounds.

UVA-LED-Assisted Activation of the Ferrate(VI) Process for Enhanced Micropollutant Degradation: Important Role of Ferrate(IV) and Ferrate(V)

This paper investigated ultraviolet A light-emitting diode (UVA-LED) irradiation to activate Fe(VI) for the degradation of micropollutants (e.g., sulfamethoxazole (SMX), enrofloxacin, and trimethoprim). UVA-LED/Fe(VI) could significantly promote the degradation of micropollutants, with rates that were 2.6-7.2-fold faster than for Fe(VI) alone. Comparatively, UVA-LED alone hardly degraded selected micropollutants. The degradation performance was further evaluated in SMX degradation via different wavelengths (365-405 nm), light intensity, and pH. Increased wavelengths led to linearly decreased SMX degradation rates because Fe(VI) has a lower molar absorption coefficient at higher wavelengths. Higher light intensity caused faster SMX degradation, owing to the enhanced level of reactive species by stronger photolysis of Fe(VI). Significantly, SMX degradation was gradually suppressed from pH 7.0 to 9.0 due to the changing speciation of Fe(VI). Scavenging and probing experiments for identifying oxidative species indicated that high-valent iron species (Fe(V)/Fe(IV)) were responsible for the enhanced degradation. A kinetic model involving target compound (TC) degradation by Fe(VI), Fe(V), and Fe(IV) was employed to fit the TC degradation kinetics by UVA-LED/Fe(VI). The fitted results revealed that Fe(IV) and Fe(V) primarily contributed to TC degradation in this system. In addition, transformation products of SMX degradation by Fe(VI) and UVA-LED/Fe(VI) were identified and the possible pathways included hydroxylation, self-coupling, bond cleavage, and oxidation reactions. Removal of SMX in real water also showed remarkable promotion by UVA-LED/Fe(VI). Overall, these findings could shed light on the understanding and application of UVA-LED/Fe(VI) for eliminating micropollutants in water treatments.

Influence of chemical speciation on enrofloxacin degradation by UV irradiation: Kinetics, mechanism and disinfection by-products formation

Fluoroquinolones (FQs) were frequently detected in aqueous environment. The UV irradiation have been reported as an efficient method for FQs degradation. This study investigated the influence of chemical speciation on enrofloxacin (ENR) photolysis process by UV irradiation. The results showed that chemical speciation of ENR significantly affected the photodegradation kinetics, and the highest degradation rate was observed in the zwitterion form. Presence of natural organic matter (NOM) and inorganic anions had different degrees of influences on ENR photodegradation for three chemical speciation of ENR. The contribution of ¹O₂ on ENR degradation in neutral and alkalinity condition was significantly higher than that in acidic condition. The cation and zwitterion of ENR was beneficial to the formation of trichloromethane (TCM) and haloacetonitrile (HAN) during the chlorination alone. Compared with the chlorination of ENR, the UV pretreatment respectively caused 4.06-fold and 3.14-fold decrease in TCM formation at acidic and neutral reaction condition during subsequent chlorination. Also the decrease in HAN formation at neutral and alkalinity condition was found after UV treatment followed by chlorination. The UV pretreatment caused higher yield of HAN in the subsequent chlorination at acidic condition than that at neutral and alkalinity condition. Through the UV pretreatment at neutral condition, the generated concentration of halonitromethane (HNM) reached the maximum value during the subsequent chlorination. Potential toxic risk analysis showed the toxicity decreased in zwitterion form of ENR, while toxicity increased in cationic and anionic form after UV irradiation pretreatment.

A critical review on ibuprofen removal from synthetic waters, natural waters, and real wastewaters by advanced oxidation processes

Ibuprofen (IBP) is one ubiquitous drug prescribed as anti-inflammatory, analgesic, and antipyretic. It has been detected in effluents of wastewater plant treatments, sewage sludge, hospital wastewaters, surface waters, and drinking water due to its continuous release to the environment, mainly from the excretion in the urine of animals and humans. IBP is a carcinogenic and non-steroidal endocrine disrupting drug with harmful effects over fungal, bacterial, algae, microorganisms, crustacean, and fish species, and can be potentially hazard for human health. Since conventional treatments remove inefficiently this drug, many advanced oxidation processes (AOPs) have been developed aiming their abatement from waters to avoid their harmful health problems. This paper presents an exhaustive and critical review on the application of AOPs to treat synthetic waters, natural waters, and real wastewaters polluted with IBP alone or mixed with other common drugs covering up to 2020. The characteristics and main results obtained for single, hybrid, and sequential treatments are described. Dielectric barrier or pulsed-corona discharges are detailed among the single processes. Hybrid processes such as photocatalysis (UV/H₂O₂, UV/chlorine, TiO₂/UV), hybrid ozonation (O₃/H₂O₂, electro-peroxone, catalytic ozonation), Fenton-based processes (photo-Fenton, electro-Fenton, photoelectro-Fenton), zero-valent iron, ultrasonic, peroxymonosulfate, and persulfate, are discussed. The effect of the kind of irradiation (UV, visible, solar) on photo-assisted processes is analyzed. Sequential processes with biological pre- or post-treatments using or not membranes for natural water and real wastewater remediation are described. Finally, 38 by-products detected during IBP removal by AOPs are reported, allowing envisaging three parallel pathways for its initial degradation.

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

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

A review on degradation of perfluorinated compounds based on ultraviolet advanced oxidation

Perfluorinated compounds (PFCs), as emerging persistent pollutants, can exist for a long time in the environment due to their high stability. PFCs have been detected in drinking water, wastewater, and the human body. Studies have shown that PFCs pose a threat to human health and the ecological environment, which is expected to be listed in new drinking water regulations. Traditional processes, including coagulation, biological filtration, chlorination, ozonolysis, and ultraviolet light have ineffective removal efficiency on PFCs; however, advanced oxidation processes (AOP) based on ultraviolet (UV) light have good application prospects for the removal of PFCs. This study provides an overview of the removal of PFCs by UV-based AOPs; systematically introduces the research status of various UV-based AOPs from the perspectives of degradation pathways, degradation efficiency, influencing factors, formation of by-products; and comprehensively compares these different UV-based AOPs. Finally, the limitations of existing research and future research needs are discussed. This review aims to provide an overview for a better understanding of the degradation status and prospects of UV-based AOPs for the degradation of PFCs.

Revisiting the Graphitized Nanodiamond-Mediated Activation of Peroxymonosulfate: Singlet Oxygenation versus Electron Transfer

Graphitized nanodiamonds (ND) exhibit outstanding capability in activating peroxymonosulfate (PMS) for the removal of aqueous organic micropollutants (OMPs). However, controversial observation and interpretation regarding the effect of graphitization degree on ND's activity and the role of singlet oxygen (¹O₂) in OMP degradation need to be clarified. Herein, we investigated graphitized ND-mediated PMS activation. Experiments show that the activity of ND increases first and then decreases with the monotonically increased graphitization degree. Further experimental and theoretical studies unveil that the intensified surface graphitization alters the degradation mechanism from singlet oxygenation to an electron-transfer pathway. Moreover, for the first time, we applied a self-constructed, time-resolved phosphorescence detection system to provide direct evidence for ¹O₂ production in the PMS-based system. This work not only elucidates the graphitization degree-dependent activation mechanism of PMS but also provides a reliable detection system for in situ analysis of ¹O₂ in future studies.

ABTS as an electron shuttle to accelerate the degradation of diclofenac with horseradish peroxidase-catalyzed hydrogen peroxide oxidation

Horseradish peroxidase (HRP)-catalyzed hydrogen peroxide (H₂O₂) oxidation could degrade a variety of organic pollutants, but the intrinsic drawback of slow degradation rate limited its widespread application. In this study, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was introduced into HRP/H₂O₂ system as an electron shuttle to enhance diclofenac degradation under neutral pH conditions. The green-colored ABTS radical (ABTS^•+), generated by the oxidation of ABTS with HRP-catalyzed H₂O₂ oxidation, was proved to be the main reactive species for the rapid degradation of diclofenac in HRP/H₂O₂/ABTS system. There was no destruction of ABTS/ABTS^•+ in HRP/H₂O₂/ABTS system, and ABTS was verified as an ideal electron shuttle. The reaction conditions including solution pH (4.5-10.5), HRP concentration (0-8 units mL^-1) and H₂O₂ concentration (0-500 μM) would impact the formation of ABTS^•+, and affect the degradation of diclofenac in HRP/H₂O₂/ABTS system. Moreover, compared with Fenton and hydroxylamine/Fenton systems, HRP/H₂O₂/ABTS system had better diclofenac degradation efficiency, higher H₂O₂ utilization efficiency and stronger anti-interference capacity in actual waters. Overall, the present study provided a meaningful and promising way to enhance the degradation of organic pollutants in water with HRP-catalyzed H₂O₂ oxidation.

Micropollutant abatement and byproduct formation during the co-exposure of chlorine dioxide (ClO2) and UVC radiation

Photolysis of ClO₂ by UVC radiation occurs in several drinking water treatment scenarios (e.g., pre-oxidation by ClO₂ with post-UVC disinfection or a multi-barrier disinfection system comprising ClO₂ and UVC disinfection in sequence). However, whether micropollutants are degraded and undesired byproducts are formed during the co-exposure of ClO₂ and UVC radiation remain unclear. This study demonstrated that four micropollutants (trimethoprim, iopromide, caffeine, and ciprofloxacin) were degraded by 14.4-100.0% during the co-exposure of ClO₂ and UVC radiation in the synthetic drinking water under the environmentally relevant conditions (UV dose of 207 mJ cm^-2, ClO₂ dose of 1.35 mg L^-1, and pH of 7.0). Trimethoprim and iopromide were predominantly degraded by ClO₂ oxidation and direct UVC photolysis, respectively. Caffeine and ciprofloxacin were predominantly degraded by the radicals (HO^• and Cl^•) and the in-situ formed free chlorine from ClO₂ photolysis, respectively. The yields of total organic chlorine (12.5 µg L^-1 from 1.0 mg C L^-1 of NOM) and chlorate (0.14 mg L^-1 From 1.35 mg L^-1 of ClO₂) during the co-exposure were low. However, the yield of chlorite was high (0.76 mg L^-1 from 1.35 mg L^-1 of ClO₂), which requires attention and control.

Role of driven approach on the piezoelectric ozonation processes: Comparing ultrasound with hydro-energy as driving forces

Driven approach is vital for evaluating degradation and energy efficiencies of piezocatalysis process. Thus, piezoelectric ozonation processes driven by hydraulic (HPE-O₃) and ultrasonic (UPE-O₃) forces were compared systematically, using BaTiO₃ as piezoelectric material for ibuprofen (IBP) degradation. The synergy indexes of HPE-O₃ and UPE-O₃ processes were 4.51 and 5.78, respectively. Besides, UPE-O₃ process (88.84%) achieved better mineralization efficiency than HPE-O₃ process (68.80%) in 90 min. Nevertheless, the energy consumptions of HPE-O₃ process was only 4.01‰ of UPE-O₃ process. The formation rate and concentration of ^•OH (the dominant active species in both processes) in UPE-O₃ process were 2-3 times higher than that in HPE-O₃ process. Notably, piezoelectric potential and current density driven by ultrasound were approximately 47500-fold and 40-fold than those by hydro-energy, respectively. These led to the difference of ^•OH paths between HPE-O₃ and UPE-O₃ processes. Further analyses indicated that ^•OH was mainly generated by single-electron transfer without H₂O₂ generation in HPE-O₃ process, whereas both single- and double-electron transfer (with H₂O₂ generation) contributed to the production of ^•OH in UPE-O₃ process. This study revealed the mechanism of piezoelectric ozonation process with different driven approaches and may provide valuable reference for selection of driven approaches in piezocatalytic study and application.

Evaluation of hydroxyl radical and reactive chlorine species generation from the superoxide/hypochlorous acid reaction as the basis for a novel advanced oxidation process

The reaction of hypochlorous acid (HOCl) with superoxide radical (O₂^•-) - a source of hydroxyl radical (HO^•) and various reactive chlorine species (RCS) - was investigated as the basis for a novel non-photochemical advanced oxidation process (AOP). Moderately stable (t_1/2 ~ minutes) aqueous O₂^•- stocks were prepared by several approaches at pH>12 and either (a) added directly to aqueous free available chlorine (FAC; i.e., HOCl/OCl^-) at circumneutral pH, or (b) premixed with alkaline FAC and then acidified to pH 7, to degrade various organic probe compounds via in situ generated HO^• and RCS. Radical production was optimal at [HO₂^•/O₂^•-]₀/[FAC]₀ ~ 2, with ~0.8 mol HO^• formed/mol FAC consumed, and HO^• and RCS exposures reaching ~5×10^-10 and ~10^-9 M×s, respectively. Similar trends were observed in natural waters and organic matter-amended phosphate buffer containing up to 5 mgC/L of dissolved organic carbon. Direct formation of oxyhalides, trihalomethanes (THMs), and haloacetic acids (HAAs), was minimal, though THM and HAA formation was moderately enhanced during post-chlorination of O₂^•-/FAC-treated solutions. This process could provide a beneficial addition to the range of available AOPs due to its high radical exposures, simplicity, rapid time-scales, potential for on-site O₂^•- generation, and widespread accessibility of FAC and other reagents.

Treatment of iodine-containing water by the UV/NH2Cl process: Dissolved organic matters transformation, iodinated trihalomethane formation and toxicity variation

UV/NH₂Cl process is becoming increasingly important for water treatment, while its impact on iodine-containing water remains unknown. In this study, the structure transformation of dissolved organic matters (DOMs), generation of iodinated trihalomethanes (I-THMs), and variation of acute toxicity were evaulated during the UV/NH₂Cl treatment of iodine-containing water. The combination of exciation emission matrix-parallel factor analysis and two-dimensional correlation spectroscopy integrated with synchronous fluorescence and infrared absorption spectroscopy showed that fulvic-like fraction of DOM was more susceptible to UV/NH₂Cl process and particularly iodo and polysaccharide groups gave the fastest resopnses. Consequently, UV fluence lower than 60 mJ/cm² promoted the production of I-THMs, while excessive UV exhausted NH₂Cl and reactive iodine species and subsequently reduced I-THM generation. Moreover, DOM concentration and source, NH₂Cl dosage, and I^- concentration had significant impacts on I-THM formation in the UV/NH₂Cl process. Additionally, a positive correlation was found between acute toxicity variation and I-THM formation when treating iodine-containing waters with UV/NH₂Cl. These results together provide a comprehensive understanding on UV/NH₂Cl treatment of iodine-containing water.

Tetracycline antibiotics as precursors of dichloroacetamide and other disinfection byproducts during chlorination and chloramination

Pollution of natural water and even source water with pharmaceuticals is problematic worldwide and raises concern about the possibility of disinfection byproduct (DBP) formation during subsequent water treatment. In this study, the formation of DBPs, especially dichloroacetamide (DCAcAm), was investigated during chlorination and chloramination of tetracyclines, which are a class of broad-spectrum antibiotics. DBPs including DCAcAm were formed during chlorination and chloramination of tetracycline (TC). Although the concentrations and theoretical cytotoxicity of the DBPs formed from TC were affected by the contact time, disinfectant dose, and pH, DCAcAm was the main contributor determining the yields and cytotoxicity of the measured DBPs. The DCAcAm yields from four tetracycline antibiotics ranged from 0.43% to 54.26% for chlorination. For chloramination, the DCAcAm yields reached 44.57%, and the nitrogen in DCAcAm mainly came from tetracycline antibiotics rather than chloramines. ClO₂ pre-oxidation and UV photolysis decreased DCAcAm formation during chlorination and chloramination of TC. The high yields observed in this study suggest that tetracycline antibiotics are possible precursors of DCAcAm.