Photosensitized Transformation of Peroxymonosulfate in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

Efficient degradation of benzalkonium chloride (BAC) by zero-valent iron activated persulfate: Kinetics, reaction mechanisms, theoretical calculations and toxicity evolution

The present study systematically investigated the elimination of benzalkonium chloride (BAC) in the zero valent iron activated persulfate (Fe0/PS) system. The influence of operational parameters, including PS concentration, Fe0 dosage and pH, were investigated through a series of kinetic experiments. When the Fe0 dosage was 5.0 mM, the initial ratio of [PS]: [BAC] was 10:1, the degradation efficiency could achieve 91.7% at pH 7.0 within 60 min. Common inorganic anions and humic acid did not significantly affect BAC degradation, implying that Fe0/PS system had a potential application prospect in the actual wastewater remediation. Based on the electron paramagnetic resonance test and quenching experiments, the BAC degradation was found to be contributed by •OH, SO4•- and Fe(IV). A total of 23 intermediates were identified by the liquid chromatography-mass spectrometry, and the degradation pathways were proposed accordingly, including dealkylation and demethylation, hydroxylation, sulfate substitution and benzyl C-N cleavage reactions. Density functional theory based calculations were conducted to realize the rationality of the proposed reaction mechanisms. The toxicity of transformation products was predicted by ECOSAR program. This work demonstrated the possibility of BAC removal in hospital and municipal wastewater by Fe0/PS treatment, and also provides a safe choice for deep treatment of quaternary ammonium salt wastewater.

Low-peroxide-consumption fenton-like systems: The future of advanced oxidation processes

Conventional heterogeneous Fenton-like systems employing different peroxides have been developed for water/wastewater remediation. However, a large population of peroxides consumed during various Fenton-like systems with low utilization efficiency and associated secondary contamination have become the bottlenecks for their actual applications. Recent strategies for lowering the peroxide consumptions to develop economic Fenton-like systems are primarily devoted to the effective radical generation and subsequent high-efficiency radical utilization through catalysts/systems engineering, leveraging emerging nonradical oxidation pathways with higher selectivity and longer life of the reactive intermediate, as well as reactor designs for promoting the mass transfer and peroxides decomposition to improve the yield of radicals/nonradicals. However, a comparative review summarizing the mechanisms and pathways of these strategies has not yet been published. In this review, we endeavor to showcase the designated systems achieving the reduction of peroxides while ensuring high catalytic activity from the perspective of the above strategic mechanisms. An in-depth understanding of these aspects will help elucidate the key mechanisms for achieving economic peroxide consumption. Finally, the existing problems of these strategies are put forward, and new ideas and research directions for lowering peroxide consumption are proposed to promote the application of various Fenton-like systems in actual wastewater purification.

Revisiting triplet state dissolved organic matter (3DOM⁎): Advances in probes, photoreactivity, and environmental implications

Triplet-state dissolved organic matter (3DOM⁎) plays a critical role in the photodegradation of organic pollutants in aquatic environments. This review offers a comprehensive overview of 3DOM⁎, focusing on monitoring methods using various probes, formation mechanisms, and photoreactivity. Traditional probes, such as 2,4,6-trimethylphenol (TMP) and sorbic acid, are widely used, while novel probes promise improved accuracy and sensitivity. The E2:E3 ratio emerges as a promising indicator for 3DOM⁎ due to its simplicity and correlation with photoreactivity, though further validation is needed to confirm its broader applicability. This review highlights the higher photoreactivity of DOM with low molecular weight, low aromaticity, and autochthonous sources, although DOM with contrasting features can also show significant photoreactivity. The presence of inorganic ions and nanomaterials significantly influences 3DOM⁎'s degradation capacity, demonstrating complex interactions with surrounding species. Additionally, the review underscores the importance of various environmental factors, including light source and DOM concentration, in affecting the photodegradation rates of contaminants. Recent literature suggests that future research should focus on developing new probes to capture different aspects of 3DOM⁎, exploring the synergistic effects of plastic leachate, and investigating the role of co-existing ions and nanomaterials on 3DOM⁎ activity. Employing machine learning (ML) techniques to predict 3DOM⁎-related parameters from easily measurable DOM descriptors presents an exciting research avenue. Enhanced understanding of 3DOM⁎ can lead to more effective strategies in wastewater treatment and environmental remediation.

Antibiotics removal and antimicrobial resistance control by ozone/peroxymonosulfate-biological activated carbon: A novel treatment process

Biological activated carbon (BAC) is one of the important treatment processes in wastewater and advanced water treatment. However, the BAC process has been reported to have antimicrobial resistance (AMR) risks. In this study, a new BAC-related treatment process was developed to reduce AMR caused by BAC treatment: ozone/peroxymonosulfate-BAC (O3/PMS-BAC). The O3/PMS-BAC showed better treatment performance on the targeted five antibiotics and dissolved organic matter removal than O3-BAC and BAC treatments. The O3/PMS-BAC process had better control over the AMR than the O3-BAC and BAC processes. Specifically, the amount of targeted antibiotic-resistant bacteria in the effluent and biofilm of O3/PMS-BAC was only 0.01-0.03 and 0.11-0.26 times that of the BAC process, respectively. Additionally, the O3/PMS-BAC process removed 1.76 %-62.83 % and 38.14 %-99.27 % more of the targeted ARGs in the effluent and biofilm than the BAC process. The total relative abundance of the targeted 12 ARGs in the O3/PMS-BAC effluent was decreased by 86 % compared to the effluent after BAC treatment. In addition, Proteobacteria and Bacteroidetes were probably the main hosts for transmitting ARGs in this study, and their relative abundance decreased by 9.6 % and 6.0 % in the effluent of the O3/PMS-BAC treatment compared to that in BAC treatment. The relationship analysis revealed that controlling antibiotic discharge was crucial for managing AMR, as antibiotics were closely related to both ARGs and bacteria associated with their emergence. The results showed that the newly developed treatment process could reduce AMR caused by BAC treatment while ensuring effluent quality. Therefore, O3/PMS-BAC is a promising alternative to BAC treatment for future applications.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.

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

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

Development of a Five-Chemical-Probe Method to Determine Multiple Radicals Simultaneously in Hydroxyl and Sulfate Radical-Mediated Advanced Oxidation Processes

Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•-)-mediated oxidation, are attractive technologies used in water and wastewater treatments. To evaluate the treatment efficiencies of AOPs, monitoring the primary radicals (HO• and SO4•-) as well as the secondary radicals generated from the reaction of HO•/SO4•- with water matrices is necessary. Therefore, we developed a novel chemical probe method to examine five key radicals simultaneously, including HO•, SO4•-, Cl•, Cl2•-, and CO3•-. Five probes, including nitrobenzene, para-chlorobenzoic acid, benzoic acid, 2,4,6-trimethylbenzoic acid, and 2,4,6-trimethylphenol, were selected in this study. Their bimolecular reaction rate constants with diverse radicals were first calibrated under the same conditions to minimize systematic errors. Three typical AOPs (UV/H2O2, UV/S2O82-, and UV/HSO5-) were tested to obtain the radical steady-state concentrations. The effects of dissolved organic matter, Br-, and the probe concentration were inspected. Our results suggest that the five-probe method can accurately measure radicals in the HO•- and SO4•--mediated AOPs when the concentration of Br- and DOM are less than 4.0 μM and 15 mgC L-1, respectively. Overall, the five-probe method is a practical and easily accessible method to determine multiple radicals simultaneously.

Effects of pH on the triplet state dissolved organic matter induced free available chlorine decay and radical formation

Triplet state dissolved organic matter (3DOM*) plays a significant role in inducing oxidant decay and radical generation in light-based advanced oxidation processes. However, the effects of pH still need investigation. This work quantitatively analyzed the pH-dependent free available chlorine (FAC) decay and radical formation (i.e., HO• and Cl•) induced by 3DOM* or triplet state photosensitizer (3PS*). Upon UV irradiation at 254 nm, the decay rate of FAC by 3DOM* or 3PS* was the highest at neutral pH, while those by dark reaction of DOM and the direct photolysis of FAC were the highest at acidic conditions. This is attributed to the variation of FAC species, 3DOM* or 3PS* formation, and the reaction rate constants of FAC with 3DOM* or 3PS* at pH 5.0-10.0. 3DOM* and 3PS* formed increasingly with pH varying from 5.0 to 10.0, while their reactivity with FAC decreased due to the speciation from HOCl to OCl-. Radical formation (i.e., HO• and Cl•) from FAC reaction with 3DOM* or 3PS* occurred at all the testing pH range (5.0-10.0). This work highlighted the pH-dependent role of 3DOM* in oxidant decay and radical formation in treating DOM containing waters through oxidant photolysis. ENVIRONMENTAL IMPLICATIONS: Triplet state dissolved organic matter (3DOM*) plays a significant role in inducing oxidant decay and radical generation in light-based AOPs. This study revealed the effects of pH in 3DOM* induced free available chlorine (FAC) decay and radical formation (i.e., HO• and Cl•). With DOM at 3 mgC L-1, FAC decayed fastest under neutral conditions and radical formation (i.e., HO• and Cl•) was enhanced at 5.0-10.0 due to 3DOM* reaction with FAC. These results highlighted the pH-dependent role of 3DOM* in oxidant transformation and radical formation in treating DOM containing waters by AOPs based on oxidant photolysis.

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

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

Ozone Micronano-bubble-Enhanced Selective Degradation of Oxytetracycline from Production Wastewater: The Overlooked Singlet Oxygen Oxidation

The efficient and selective removal of refractory antibiotics from high-strength antibiotic production wastewater is crucial but remains a substantial challenge. In this study, a novel ozone micronano-bubble (MNB)-enhanced treatment system was constructed for antibiotic production wastewater treatment. Compared with conventional ozone, ozone MNBs exhibit excellent treatment efficiency for oxytetracycline (OTC) degradation and toxicity decrease. Notably, this study identifies the overlooked singlet oxygen (1O2) for the first time as a crucial active species in the ozone MNB system through probe and electron paramagnetic resonance methods. Subsequently, the oxidation mechanisms of OTC by ozone MNBs are systematically investigated. Owing to the high reactivity of OTC toward 1O2, ozone MNBs enhance the selective and anti-interference performance of OTC degradation in raw OTC production wastewater with complex matrixes. This study provides insights into the mechanism of ozone MNB-enhanced pollutant degradation and a new perspective for the efficient treatment of high-concentration industrial wastewater using ozone MNBs. In addition, this study presents a promising technology with scientific guidance for the treatment of antibiotic production wastewater.

Degradation of methotrexate by unactivated and solar-activated peroxymonosulfate in water: Moiety-specific reaction kinetics and transformation product-associated risks

Anticancer drugs have raised worldwide concern owing to their ubiquitous occurrence and ecological risks, necessitating the development of efficient removal strategies in water and wastewater treatment. Although peroxymonosulfate (PMS) is known to be a promising chemical in water decontamination, limited information is available regarding the removal efficiency of anticancer drugs by PMS and solar/PMS systems. This study first reports the moiety-specific reaction kinetics and mechanisms of methotrexate (MTX), an anticancer drug with widespread attention, by PMS (unactivated) and solar-activated PMS in water. It was found that MTX abatement by the direct PMS oxidation followed second-order kinetics, and the pH-dependent rate constants increased from 0.4 M-1 s-1 (pH 5.0) to 1.3 M-1 s-1 (pH 8.0), with a slight decrease to 1.1 M-1 s-1 at pH 9.0. The presence of chloride and bromide exerted no obvious influence on the removal of MTX by PMS. Furthermore, the chemical reactivity of MTX and its seven substructures with different reactive species was evaluated, and the degradation contributions of the reactive species involved were quantitatively analyzed in the solar/PMS system. The product analysis suggested similar reaction pathways of MTX by PMS and solar/PMS systems. The persistence, bioaccumulation, and toxicity of the transformation products were investigated, indicating treatment-driven risks. Notably, MTX can be removed efficiently from both municipal and hospital wastewater effluents by the solar/PMS system, suggesting its great potential in wastewater treatment applications. Overall, this study systematically evaluated the elimination of MTX by the unactivated PMS and solar/PMS treatment processes in water. The obtained findings may have implications for the mechanistic understanding and development of PMS-based processes for the degradation of such micropollutants in wastewater.

Quantitative probing of reactive oxygen species and their selective degradation on contaminants in peroxymonosulfate-based process enhanced by picolinic acid

The processes of Fe(III) activated peroxymonosulfate (PMS) in degrading contaminants have been extensively studied. Herein, a biodegradable chelating agent, picolinic acid (PICA), was introduced to the PMS/Fe(III) process to improve the reaction efficiency. The emphases of this study were placed on the quantification of steady-state concentrations of reactive oxygen species (ROS). Experiments presented that five types of ROS, including Fe(IV), SO4•-, HO•, 1O2 and O2•- coexisted in this system. Four typical probe compounds were used to quantify the steady-state concentration of ROS under different variables. The steady-state concentration of Fe(IV) ([Fe(IV)]ss) was 3-5 orders of magnitude higher than that of other ROS, followed by 1O2 and SO4•-, whereas HO• had the lowest concentration. The reaction between PMS and PICA was first explored in our study and results showed that 1O2 and O2•- would form in this reaction. Owing to the hybrid oxidation by multiple ROS, this system showed high oxidation capacity, and could effectively degrade a variety of pollutants. The contributions of ROS to the alleviation of pollutants varied depending on their concentrations and specific reactivity of substrates. Generally, organic contaminants with phenol structures were prone to react with Fe(IV). Overall, this study compared the steady-state concentrations of different ROS and revealed the intrinsic ROS formation mechanisms.

Mechanistic insight into humic acid-enhanced sonophotocatalytic removal of 17β-estradiol: Formation and contribution of reactive intermediates

In this study, humic acid (HA) enhanced 17β-estradiol (17β-E2) degradation by Er3+-CdS/MoS2 (ECMS) was investigated under ultrasonic and light conditions. The degradation reaction rate of 17β-E2 was increased from (14.414 ± 0.315) × 10-3 min-1 to (122.677 ± 1.729) × 10-3 min-1 within 90 min sonophotocatalytic (SPC) reaction with the addition of HA. The results of quenching coupled with chemical probe experiments indicated that more reactive intermediates (RIs) including reactive oxygen species (ROSs) and triplet-excited states were generated in the HA-enhanced sonophotocatalytic system. The triplet-excited states of humic acid (3HA*), hydroxyl radical (•OH), and superoxide radical (•O2-) were the dominant RIs for 17β-E2 elimination. In addition, the energy- and electron-transfer process via coexisting HA also account for 12.86% and 29.24% contributions, respectively. The quantum yields of RIs in the SPC-ECMS-HA system followed the order of 3HA* > H2O2 > 1O2 > •O2-> •OH. Moreover, the spectral and fluorescence characteristics of HA were further analyzed during the sonophotocatalytic reaction process. The study expanded new insights into the comprehension of the effects of omnipresent coexisting HA and RIs formation for the removal of 17β-E2 during the sonophotocatalytic process.

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

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

Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (³1NN*) and singlet (¹1NN*) excited states were estimated to be 1.5 × 10⁶ and 2.5 × 10⁸ s^-1, respectively. Our results provided quantitative evidence for the environmental relevance of ³1NN* in waters. Possible reactions of ³1NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, ³1NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (^•OH) and sulfate (SO₄^•-) radicals can be generated via the ³1NN*-induced oxidation of inorganic ions (OH^- and SO₄^2-, respectively). We further investigated the reaction kinetics of ³1NN* and OH^- forming ^•OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of ³1NN* with OH^- and 1NN with ^•OH were determined to be 4.22 × 10⁷ and 3.95 ± 0.01 × 10⁹ M^-1 s^-1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.

Surface engineering of Al2O3 nanotubes by ureasolysis method for activating persulfate degradation of antibiotics

Though ecofriendly, pure Al₂O₃ has never been used for activation of peroxodisulfate (PDS) to degrade pollutants. We report the fabrication of Al₂O₃ nanotubes by ureasolysis method for efficient activating PDS degradation of antibiotics. The fast ureasolysis in aqueous AlCl₃ solution produces NH₄Al(OH)₂CO₃ nanotubes, which are calcined to porous Al₂O₃ nanotubes, and the release of ammonia and carbon dioxide engineers the surface features of large surface area, numerous acidic-basic sites and suitable Zeta potentials. The synergy of these features facilitates the adsorption of the typical antibiotics ciprofloxacin and PDS activation, which is proved by experiment results and density functional theory simulation. The proposed Al₂O₃ nanotubes can catalyze 92-96% degradation of 10 ppm ciprofloxacin within 40 min, with chemical oxygen demand removal of 65-66% in aqueous, and 40-47% in whole including aqueous and catalysts. Ciprofloxacin at high concentration, other fluoroquinolones and tetracycline can also be effectively degraded. These data demonstrate the Al₂O₃ nanotubes prepared by the nature-inspired ureasolysis method has unique features and great potentials for antibiotics degradation.

Photosensitive Dye as an Ideal Peroxymonosulfate Activator for Efficient Self-Degradation: A Novel Idea of Using Waste to Treat Waste

Commonly used peroxydisulfate (PS) or peroxymonosulfate (PMS) activation methods have been limited in their practical application due to certain drawbacks, such as high cost, high energy consumption and secondary pollution. In this study, a catalyst-free alizarin green (AG) self-activating PMS catalytic system was constructed based on photosensitization properties of dye, which ultimately achieved efficient degradation of the dye activator, also the target pollutant. Here, 52.5% of the 100 mL mixture of 10 mg/L AG decomposed within 60 min with 1 mM PMS under visible-light irradiation, thereby showing a strong pH adaptation. Mechanism of AG self-activating PMS was revealed that the photo-excited AG can effectively transfer photo-induced electrons to PMS for its activation, which generates reactive oxidizing species dominated by singlet oxygen (¹O₂), and supplemented by hydroxyl radical (•OH), superoxide radical (O₂^•-) and sulfate radical (SO₄^•-) to realize the efficient self-degradation of the dye pollutants. Moreover, such self-catalytic system operated well under natural sunlight irradiation, indicating the great application potential in the actual wastewater treatment. Herein, photosensitive dye acted as an ideal PMS activator realizing its efficient self-degradation, which provides a novel idea of "using waste to treat waste" for developing wastewater treatment process in a high-efficiency and low-consumption way.

Aquatic photolysis of high-risk chemicals of emerging concern from secondary effluent mediated by sunlight irradiation for ecological safety and the enhanced methods

Natural sunlight can reduce the chemicals of emerging concern (CECs) and biological effects from the discharged domestic wastewater. But the aquatic photolysis and biotoxic variations of specific CECs detected in secondary effluent (SE) were not clear. In this study, 29 CECs were detected in the SE, and 13 medium- and high-risk CECs were identified as target chemicals based on their ecological risk assessment. To comprehensively explore the photolysis properties of the identified target chemicals, the direct and self-sensitized photodegradation of the target chemicals, even the indirect photodegradation in the mixture, were investigated and compared with these photodegradation in the SE. Of the 13 target chemicals, only five chemicals (including dichlorvos (DDVP), mefenamic acid (MEF), diphenhydramine hydrochloride (DPH), chlorpyrifos (CPF), and imidacloprid (IMI)) underwent direct and self-sensitized photodegradation processes. The removal of DDVP, MEF, and DPH was attributed to self-sensitized photodegradation, which was mainly mediated by •OH; CPF and IMI primarily relied on direct photodegradation. Synergistic and/or antagonistic actions that occurred in the mixture improved/decreased the rate constants of five photodegradable target chemicals. Meanwhile, the biotoxicities (acute toxicity and genotoxicity) of the target chemicals (including individual chemicals and the mixture) were significantly reduced, which can explain the reduction of biotoxicities from SE. For the two refractory high-risk chemicals, atrazine (ATZ) and carbendazim (MBC), algae-derived intracellular dissolved organic matter (IOM) on ATZ, and IOM and extracellular dissolved organic matter (EOM) on MBC had slightly promotion for their photodegradation; while peroxysulfate, and peroxymonosulfate served as sensitizers were activated by natural sunlight and effectively improved their photodegradation rate, and then reduced their biotoxicities. These findings will promote the development of CECs treatment technologies based on sunlight irradiation.

Unveiling the roles of dissolved organic matters derived from different biochar in biochar/persulfate system: Mechanism and toxicity

Biochar has been frequently used as a persulfate (PS) activator due to its attractive properties, but dissolved organic matter (DOM) derived from the non‑carbonized part of biochar has received less attention, not to mention its specific role and impact in biochar/PS systems. In this study, wheat straw, municipal sludge, and swine bone were selected as the representative feed stocks of biochar. Subsequently, these three types of biochar were adopted to explore the roles of DOM in biochar/PS systems. Although the composition and amount of DOM derived from different biochar were discrepant, they exhibited similar effect in biochar/PS systems. To be specific, the pore-clogging effect of DOM on biochar suppressed the adsorption capacity and catalytic performance of the three biochar. Furthermore, the removal of DOM decreased the environmental risk of these biochar/PS systems and enhanced the stability of the involved biochar. With respect to the variation in degradation mechanism, the removal of DOM increased the proportion of electron transfer pathway in unison, but the diminution in the roles of O₂^•¯ and ¹O₂ was more remarkable in bone-derived-biochar/PS systems. Additionally, the toxicity test illustrated that the leakage and accumulation of DOM were toxic to Chlorella sp., and the DOM from sludge-derived-biochar presented the highest toxicity. Overall, this study analyzes the roles of DOM derived from different biochar in biochar/PS systems and evaluates their environmental risk, which contributes to a comprehensive understanding of the fate of DOM derived from biochar.

Remediation of environmentally persistent organic pollutants (POPs) by persulfates oxidation system (PS): A review

Over the past few years, persistent organic pollutants (POPs) exhibiting high ecotoxicity have been widely detected in the environment. Persulfate-oxidation hybrid system is one of the most widely used novel advanced oxidation techniques and is based on the persulfate generation of SO₄^-∙ and ∙OH from persulfate to degrade POPs. The overarching aim of this work is to provide a critical review of the variety of methods of peroxide activation (e.g., light activated persulfate, heat-activated persulfate, ultrasound-activated persulfate, electrochemically-activated persulfate, base-activated persulfate, transition metal activated persulfate, as well as Carbon based material activated persulfate). Specifically, through this article we make an attempt to provide the important characteristics and uses of main activated PS methods, as well as the prevailing mechanisms of activated PS to degrade organic pollutants in water. Finally, the advantages and disadvantages of each activation method are analyzed. This work clearly illustrates the benefits of different persulfate activation technologies, and explores persulfate activation in terms of Sustainable Development Goals, technical feasibility, toxicity assessment, and economics to facilitate the large-scale application of persulfate technologies. It also discusses how to choose the most suitable activation method to degrade different types of POPs, filling the research gap in this area and providing better guidance for future research and engineering applications of persulfates.

Catalyst-Free Photochemical Activation of Peroxymonosulfate in Xanthene-Rich Systems for Fenton-Like Synergistic Decontamination: Efficacy of Proton Transfer Process

Catalyst-free visible light assisted Fenton-like catalysis offers opportunities to achieve the sustainable water decontamination, but the synergistic decontamination mechanisms are still unclear, especially the effect of proton transfer process (PTP). The conversion of peroxymonosulfate (PMS) in photosensitive dye-enriched system was detailed. The photo-electron transfer between excited dye and PMS triggered the efficient activation of PMS and enhanced the production of reactive species. Photochemistry behavior analysis and DFT calculations revealed that PTP was the crucial factor to determine the decontamination performance, leading to the transformation of dye molecules. The excitation process inducing activation of whole system was composed of low energy excitations, and the electrons and holes were almost contributed by LUMO and HOMO. This work provided new ideas for the design of catalyst-free sustainable system for efficient decontamination.

Mitigation of ultrafiltration membrane fouling by a simulated sunlight-peroxymonosulfate system with the assistance of irradiated NOM

Oxidation pretreatments prior to ultrafiltration are hindered by the need for energy input and sludge disposal. Herein, a simulated sunlight-induced natural organic matter (NOM) for peroxymonosulfate (PMS) activation was used as pretreatment to alleviate ultrafiltration membrane fouling caused by NOM itself in the Songhua River water. When light intensity was over 100 mW/cm², the pretreatment removed NOM effectively, characterized with UV₂₅₄, dissolved organic carbon (DOC) and maximum fluorescent intensity (F_max), and improved filtration flux. At 200 mW/cm² light intensity and 0.5 mM PMS, 57.5% of UV₂₅₄ and 18.5% of DOC were removed, and humic-like fluorescent component was degraded by 84%-94% while ∼60% for protein-like substance. Membrane flux was increased by 94%, and reversible and irreversible fouling resistances were reduced by 62.4% and 51.9%, respectively. Both total fouling index (TFI) and hydraulic irreversible fouling index (HIFI) were moderately correlated with the DOC, whereas they prominently correlated with the UV₂₅₄ and the F_maxs of all fluorescence components, which could be served as key indicators to predict and control membrane fouling. Mathematical modeling showed that the pretreatment alleviated the fouling in the membrane pores and cake layer. The simulated sunlight-induced NOM (³NOM* and e_aq^¯) could activate PMS to form active species, which enabled to oxidize high molecular weight (MW) substances and mineralize low MW compounds in NOM as well as hinder their linking with inorganic cations, thereby reducing organic and inorganic membrane fouling simultaneously. This study may provide a new strategy for decentralized potable water treatment, especially in a single household or community.

Reducing properties of triplet state organic matter (3DOM*) probed via the transformation from chlorine dioxide to chlorite

The triplet states of dissolved organic matter (³DOM*) have been well known to oxidize various organic contaminants, but evidence of their reducing properties are largely scarce. In this work, chlorine dioxide (ClO₂) as a single-electron oxidant was used as a probe to evaluate the reduction property of ³DOM*. The reduction of ClO₂ to chlorite was observed in the solutions of model photosensitizers (i.e., 4-carboxybenzophenone, benzophenone, acetophenone, 3-methoxyacetophenone, naphthalene, and xanthone) during UV irradiation with the presence of ClO₂, though they are resistant to ClO₂ oxidation in the dark. The reducing property of the triplet states of photosensitizers was verified and their second-order reaction rate constants with ClO₂ were determined to be in the range of 1.45(± 0.03)× 10⁹ - 2.18(± 0.06) × 10⁹ M^-1 s^-1 at pH 7.0. The quenching tests excluded the role of other reactive species (e.g., HO^•, O(³P), Cl^•, ClO^• and HOCl/OCl^-, O₂^•- and e_aq^-) in ClO₂ reduction to chlorite when using model photosensitizers and DOM isolates. Chlorite formation was 48.1-90.4% and 4812.8-7721.8% higher during UV irradiation with the presence of ClO₂ and DOM than those without UV irradiation or without DOM present, respectively. The enhancement was attributed to the enhanced electron donating capacity (chlorite precursors) of DOM upon UV irradiation and also to ³DOM* acting as an electron donor reducing ClO₂ to chlorite. This study highlighted the important role of ³DOM* as a reductant.

Photosensitized Transformation of Hydrogen Peroxide in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

Hydrogen peroxide plays an important role in photochemical processes in aquatic environments. However, whether it can be transformed by photoexcited chromophoric dissolved organic matter (CDOM) remains unclear. Therefore, this study examined the photosensitized degradation of H₂O₂ in CDOM-enriched solutions under simulated solar irradiation. Our results suggest that the presence of CDOM enhances the photodegradation rate of H₂O₂ via the photosensitization process and ^·OH is generated stoichiometrically with H₂O₂ attenuation. Experimental results with model photosensitizers indicate that one-electron reducing species of CDOM (CDOM^·-), not triplet CDOM, is the primary reactive species that reduces H₂O₂ to yield ^·OH. By monitoring the variation of CDOM^·-, the reaction rate constant of CDOM^·- with H₂O₂ was estimated to be 1.5-fold greater than that with O₂. Furthermore, a wastewater effluent was exposed to simulated solar irradiation with the addition of H₂O₂, and the results demonstrated that the photodegradation of trace organic contaminants (TrOCs) was significantly enhanced by the increased ^·OH level. Overall, the current study provided new insights into the photochemical formation of ^·OH via the one-electron reduction of H₂O₂ by CDOM^·-. The solar irradiation of wastewater with H₂O₂ enhancement could be a useful and economically beneficial advanced oxidation process for TrOC abatement.

Roles of humic substances redox activity on environmental remediation

Humic substances (HS) as representative natural organic matters and the most common organic compounds existing in the environment, has been applied to the treatment and remediation of environmental pollution. This review systematically introduces and summarizes the redox activity of HS for the remediation of environmental pollutants. For inorganic pollutants (such as silver, chromium, mercury, and arsenic), the redox reaction of HS can reduce their toxicity and mobilization, thereby reducing the harm of these pollutants to the environment. The concentration and chemical composition of HS, environmental pH, ionic strength, and competing components affect the degree and rate of redox reactions between inorganic pollutants and HS significantly. With regards to organic pollutants, HS has photocatalytic activity and produces a large number of reactive oxygen species (ROS) under the light which reacts with organic pollutants to accelerate the degradation of organic pollutants. Under the affection of HS, the redox of Fe(III) and Fe(II) can enhance the efficiency of Fenton-like reaction to degrade organic pollutants. Finally, the research direction of HS redox remediation of environmental pollution is prospected.

Preferential Destruction of Micropollutants in Water through a Self-Purification Process with Dissolved Organic Carbon Polar Complexation

Removing micropollutants in real water is a scientific challenge due to primary dissolved organic carbon (DOC) and high energy consumption of current technologies. Herein, we develop a self-purification process for the preferential destruction of various micropollutants in municipal wastewater, raw drinking water, and ultrapure water with humic acid (HA) driven by the surface microelectronic field of Fe⁰-Fe_yC_z/Fe_x-GZIF-8-rGO without any additional input. It was verified that a strongly polar complex consisting of an electron-rich HA/DOC area and an electron-poor micropollutant area was formed between HA/DOC and micropollutants, promoting more electrons of micropollutants in the adsorbed complex to delocalizing to electron-rich Fe species area and be trapped by O₂, which resulted in their surface cleavage and hydrolyzation preferentially. The higher micropollutant degradation efficiency observed in real wastewaters was due to the greater complex polarity of DOC. Moreover, the electron transfer process ensured the stability of the surface microelectronic field and continuous water purification. Our findings provide a new insight into low-energy combined-micropollution water treatment.