Assessing the Use of Probes and Quenchers for Understanding the Reactive Species in Advanced Oxidation Processes

MnO/MnS nanoparticles encapsulated in Lycopodium spores derived nitrogen-doped porous carbon as a cost-effective peroxymonosulfate activator for pollutant decontamination: Insight into the mechanism of electron transfer-dominated non-radical pathway

The rational design and exploitation of cost-effective and robust catalysts for peroxymonosulfate (PMS) activation is of great significance. Herein, MnO/MnS nanoparticles encapsulated in Nitrogen-doped porous carbon skeleton (abbreviated as MnO/MnS@NPC) were first constructed through an easy two-step of impregnation along with subsequent pyrolysis technique and utilized to activate PMS for the elimination and mineralization of tetracycline (TC). Benefiting from the strong coupling of transition metal Mn with carbon-based material, the co-doping of heteroatom N and S, the enhanced electrical conductivity, and the hierarchical porous microarchitecture, the obtained MnO/MnS@NPC composite has been expected to present superior PMS activation capacity and pollutant elimination efficiency to its benchmark NPC and MnO@NPC, with 92.5 % degradation rate of TC within 60 min. Comprehensive investigations, involving quenching experiments, electron paramagnetic resonance (EPR) studies, in situ Raman identification, and electrochemical tests, jointly indicated that the non-radical pathways including electron-transfer, single oxygen (1O2) and the high-valent Mn-oxo species, especially the electron transfer process (ETP) from TC molecule to the metastable MnO/MnS@NPC-PMS* complex were dominantly responsible for PMS activation and further decomposition of TC, which greatly enhanced the selective removal of TC and the anti-interference capacity of the PMS system. Furthermore, the possible TC degradation routes were predicted by Density Functional Theory (DFT) calculation and the toxicity of degradation intermediates were also analyzed by toxicity assessment software. In addition, the heterogeneous catalyst displayed outstanding stability and reusability owing to the shield effect of NPC framework to MnO/MnS nanoparticles. Overall, this work proposed a prospective strategy for rationally designing and exploring heterogeneous PMS activator towards wastewater purification.

Directed transformation of glyphosate to orthophosphate in EAOP based on Ti/SnO2/PbO2 anode: First unraveling the role of high-valent Pb in various reactive oxygen species

The discharge of wastewater containing non-reactive phosphorus (NRPs), which does not exist in the form of P(V) (PO43-), poses significant environmental risks and phosphorus loss. In this study, electrochemical advanced oxidation process (EAOP) based twenty pairs of electrodes is used to simultaneously achieve the pollution-reducing and targeted conversion of NRPs to P(V). Significantly, EAOP driven by Ti/SnO2/PbO2-Ti plate electrode pair achieved more than five-fold of glyphosate (Gly) removal with only 16 %-56 % of energy consumption compared with that in other anode-based EAOP systems. Usually, Ti/SnO2/PbO2 as a typical non-active anode is considered to produce ·OH as the main active species. Surprisingly, the high-valent Pb was also identified as a key active species through PMSO probe experiment combined with 18O isotope labelling technique in this study. Therefore, the synergistic effect of ·OH and high-valent Pb enhanced the Gly degradation and directed generation of PO43- through the cleavage of C-P bond in the EAOP system based on Ti/SnO2/PbO2 anode. The work provides promising electrodes and insights for identification of reactive species and mechanism in EAOP system to remove NRPs contaminants and targeted conversion to P(V).

Insight into homogeneous activation of sodium hypochlorite by dithionite coupled with dissolved oxygen (DO@NaClO/DTN) for carbamazepine degradation

Emerging contaminants (ECs) including carbamazepine (CBZ) in aquatic systems pose non-target risks to wildlife. We introduce an innovative advanced oxidation process (AOP) utilizing sodium hypochlorite (NaClO), which achieved 45.3 % degradation and mineralization of CBZ within 60 mins. Natural saturated state dissolved oxygen (DO, ∼7.5 mg·L-1) played a crucial role in synergistically activating NaClO with dithionite (DTN) without extra energy consumption. In DO@NaClO/DTN system, scavenging tests and electron spin resonance (ESR) analysis confirmed that ·OH and Cl· were dominant for CBZ degradation. The critical DO was responsible for the direct simultaneous production of ·OH and Cl·, confirmed by the greater thermodynamic data ΔG from density functional theory (DFT) calculation. These reactive species participate in subsequent transformations of SO4·-, O2·-, and 1O2. Preferential hydroxylation of CBZ first occurred due to the attacking at the reactive sites of C(21) and C(22) atoms. LC-MS/MS detection and DFT theoretical calculations also verified the sequent mechanisms of Meinwald rearrangement, deamidation and hydroxylation, cyclized hydroxylated and dehydrated with the decreasing ΔG. Ubiquitous Cl- accelerated CBZ degradation remarkably, regardless of its concentration. The significant enhancement of Cl- for CBZ degradation in DO@NaClO/DTN system suggest its promising application for ECs degradation in high-chloride seawater including offshore wastewater and tailwater in mariculture.

Ultrahigh Peroxymonosulfate Utilization Over a Single-Atom Iron-N-C Catalyst for Efficient Fenton-Like Chemistry via Surface-Bound Reactive Complexes

Transition metal single-atom catalysts (SACs) find extensive application in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). Yet, the disparity in intrinsic activity is often attributed to thermodynamics, but few studies focused on the electronic structure between different metals. Herein, transition metal catalysts in the form of single-atom M-N4 moieties moored to graphitic carbon nitride (denoted MSA CN, M = Fe, Co, and Cu) are developed and used for activating PMS for the degradation of 4-chlorophenol. Remarkably, FeSA CN achieves a catalyst-dose-normalized kinetic rate constant of 34.2 L min-1 g-1, surpassing reported systems by 2-551 times ─ even at ultralow catalyst (0.06 mg L-1) and PMS (0.2 mm) concentration. The in situ formation of surface-bound PMS* complexes enabled the degradation of 4-chlorophenol to achieve unprecedented utilization efficiency (≈100%) through highly efficient non-radical pathways. Density functional theory calculations revealed that large spin polarization of Fe-N-C sites facilitated the d orbitals to overlap with the PMS on the metal active sites and promoted electron transport, thereby facilitating PMS adsorption and enhancing the oxidation capacity. This work establishes a mechanistic foundation for designing a single Fe-atom catalyst/PMS system in Fenton-like water treatment.

Enhanced peroxone reaction with amphoteric oxide modulation for efficient decontamination of challenging wastewaters: Comparative performance, economic evaluation, and pilot-scale implementation

The peroxone reaction, a promising alternative technology for water treatment, is traditionally hampered by its restricted pH operational range and suboptimal oxidant utilization. In this study, we introduced a novel amphoteric metal oxide (ZnO)-regulated peroxone system that transcended the pH limitations of conventional peroxone processes. Our innovative approach exploited the unique properties of ZnO to regulate the reaction pathway of the traditional O3/H2O2 (or peroxymonosulfate, PMS) processes, resulting in a 52.4 % (64.9 %) increase in the removal efficiency of electron-deficient pollutant atrazine under acidic conditions (pH=5.8). This was achieved through the facilitated generation of hydroxyl radicals (•OH) and sulfate radicals (SO4•-), alongside a marked increase in the utilization efficiency of O3, thus reducing the requisite amount of oxidant. The primary active sites within this system were identified as zinc-oxidant sites, with the critical interfacial interactions between ZnO and oxidants elucidated through comprehensive analytical techniques. These studies reveal that ZnO acted as an electron acceptor, with H2O2 (or PMS) serving as the electron donor, leading to the formation of a reactive intermediate. This intermediate subsequently engaged with O3, producing secondary radicals such as HO2• (SO5•-) and O3•-, which were instrumental in generating the final radical species, •OH and SO4•-. The efficacy of this ZnO-regulated peroxone process was validated through resistance to interference tests, treatment of pilot-scale coking wastewater (mineralization rate of over 70 %), and extensive biological toxicity evaluations, all of which validated the system's robust degradation capability, stability, and significant detoxification potential. A detailed comparison of reaction systems with conventional technologies using Electrical Energy per Order (EE/O) and Life Cycle Assessment (LCA) further highlighted the advantages. This investigation offers a groundbreaking solution for the treatment of complex wastewater, showcasing the substantial promise of ZnO-catalyzed peroxone for practical wastewater treatment applications.

Generation of DBPs from dissolved organic matter by solar photolysis of chlorine: Associated changes of cytotoxicity and reactive species

Since elevated amounts of chlorine disinfectant were discharged into surface water, more attention should be paid to the reactions between dissolved organic matter (DOM) and chlorine under sunlight. However, disinfection byproducts (DBPs) formed from DOM by solar photolysis of chlorine, and changes of cytotoxicity during this process remain unclear. In this study, it was found that solar photolysis of chlorine significantly promoted the formation of aliphatic chlorinated DBPs and aromatic chlorinated DBPs (including chlorobenzoquinone) by 44.7-109 % and 81.7-121 %, respectively compared with dark chlorination. Unknown total organic chlorine contained in low molecular weight fraction (<1 kD) significantly positively correlated to the cytotoxicity of water samples. Several factors (bicarbonate, dissolved oxygen, pH, nitrate, ammonia, bromide, and iodide) affecting the radical chemistry, and the formation of DBPs under solar photolysis of chlorine were also investigated. Reactive species including HO•, Cl•, O3, and reactive nitrogen species (RNS) were responsible for forming different DBPs. Especially O3 increased the formation of most categories of DBPs tested in this study, and RNS contributed to the formation of nitrogenous DBPs. This study provided more understanding of the adverse impact of overused chlorine, and reaction mechanisms between reactive species and DOM.

Overlooked risks of photoaging of nitrogenous microplastics with natural organic matter in water: Augmenting the formation of nitrogenous disinfection by-products

In aqueous environments, microplastics (MPs) undergo photoaging, releasing dissolved organic matter (DOM). Disinfection byproducts (DBPs) formation from natural organic matter (NOM) phototransformation has been reported. However, the impact of NOM on the photoaging of MPs (especially nitrogen-containing MPs) and subsequent nitrogenous DBPs (N-DBPs) formation remains unknown. Herein, this study investigated polyamide (PA) with NOM (fulvic acid [FA], humic acid [HA] and biochar-derived DOM [BDOM]) on N-DBPs formation. Results showed that the levels of the main DBPs, N-nitrosamine, formed in the FA+PA, BDOM+PA, and HA+PA systems were 3.0. 2.7 and 1.6 folds higher, respectively, compared to those in the corresponding NOM treatments. NDMA was found to be the dominant N-nitrosamine species, with the highest level of 202 ng/L, exceeding the WHO guideline of 100 ng/L. The main reactive intermediates (RIs) were 1O2 and reactive nitrogen species (RNS) during the first stage (0-3d), and •OH and RNS during the second stage (3-7d), which were confirmed by quenching experiment. For the first time, we found the formation of N-DBPs during photoaging of N-containing MPs, and proposed a two-stages, two-processes, and two-pathways theory of N-DBPs formation. This work emphasizes the importance to understand the interactions between the MPs and NOM in photoaging to better assess the risk of DBPs formation in aqueous environment.

Binding interaction between chlorine and powder activated carbon driving nonradical oxidation toward diclofenac abatement: Surface-bound complexes generating on diverse sites performing diverse duties

Photolysis of chlorine by UV irradiation is commonly used as an advanced oxidation process for the abatement of micropollutants, but suffers from the energy-extensive consumption and potential risk, e.g., formation of disinfection byproduct and use of fragile mercury-containing lamps. This study demonstrates powder activated carbon (PAC) catalysis-mediated chlorine activation to significantly promote the degradation of diclofenac (DCF), a representative emerging contaminant, via nonradical oxidation pathways, thus reconsidering the interaction between PAC and chlorine in depth which are widely applied in actual water treatment. The chlorine/PAC process produces reactive metastable surface-bound complexes, i.e., PAC-HOCl*, via the cleavage of O-Cl bond in chlorine and formation of C-Cl by interfacial binding interaction, to regulate the charge distribution and electron density configuration. Carbonyl groups and structural defects of PAC are determined as the active sites via functional group derivatization and defect engineering for PAC modification, and performed diverse duties in the chlorine activation, producing PAC-C=O-HOCl* and PAC-D-HOCl*, responsible for the oxidation ability improvement and electron transfer acceleration, respectively. Of particular significance is that the chlorine/PAC process performs high efficiencies in the degradation of diverse micropollutants and is scarcely affected by water matrices, exhibiting a high potential of practical application for the decontamination of emerging micropollutants without the requirement of external energy input.

Ultra-efficient degradation of isoquinoline from shale gas wastewater with the diethylamine-ferrate(VI) system: The key role of Fe(IV)/Fe(V) active species

Although isoquinoline (IQL) in shale gas wastewater contributes minimally to chemical oxygen demand, its potential high toxicity makes it an environmental risk factor that cannot be overlooked. This study introduces a synergistic diethylamine (Di)/ferrate (Fe(VI)) system for efficient degradation of IQL. Compared with Fe(VI) alone, the Di/Fe(VI) system demonstrated superior performance, achieving degradation efficiency of 80.5 %. The degradation rate constant of the Di/Fe(VI) system was almost 3-fold larger than that measured with Fe(VI) alone in the degradation of IQL. Mechanistic studies, including radical quenching, electron paramagnetic resonance, pre-mixed experiments, Raman spectroscopy, and probe compounds tests suggested that high-valent iron intermediates (Fe(IV/V)) were responsible for IQL degradation in the Di/Fe(VI) system. The presence of Di promoted the generation of Fe(IV)/Fe(V) by donating electrons. Based on the intermediates identified with GC-MS measurements and density functional theory calculations, three reaction pathways for IQL degradation were proposed. ECOSAR prediction and Escherichia coli toxicity tests showed that the toxicity of IQL was significantly reduced after treatment with Di/Fe(VI) system. Optimal IQL degradation occurred at higher Fe(VI)/Di concentrations and lower pH, with minimal interference from common ions or matrix components. The system also effectively degraded other organics (e.g., 2,4-di-tert-butylphenol, 6-methylquinoline, diclofenac, carbamazepine), demonstrating broad applicability for refractory pollutant treatment.

High-efficiency and sustainable peroxymonosulfate activation on Fe single-atom catalyst through incorporating complementary S species for enhanced water decontamination

Single-atom FeNC catalyst is a promising alternative for peroxymonosulfate (PMS) activation based water treatment, but the trade-off between PMS adsorption and products desorption on Fe single site limits its performance for achieving sustainable PMS activation. Herein, the thiophene-like S (CSC) and oxidized S (C-SOx) with different electron-donating/withdrawing properties were introduced into the second coordination shell of single-atom FeNC catalyst (Fe-NSBC) to optimize its Fe electronic structure for high-efficiency and sustainable PMS activation. The S species composition was also regulated to study its effect on catalytic performance and mechanism of Fe-NSBC. Results showed the Fe-NSBC displayed highest PMS catalytic efficiency and radical (SO4•- and •OH) yield at moderate CSC/C-SOx ratio, whose catalytic activity was 3.8 and 21.1 times higher than that of Fe-NBC without S doping and homogeneous Co2+ (metal-based benchmark), and greatly outperformed those of the state-of-the-art FeNC based catalysts. Moreover, the Fe-NSBC/PMS favored free radicals production for preferential removal of hydrophilic pollutants, and displayed unique anti-interference and long-term effectiveness in real water decontamination. The CSC and C-SOx played complementary role in promoting sustainable PMS activation: CSC raised d-band center and electron density of Fe for enhanced adsorption and reduction of PMS into radicals, while C-SOx lowered d-band center of Fe for favorable radical desorption and active site regeneration.

Chloride-enhanced degradation of micropollutants in natural water by the iron/biochar/peroxymonosulfate system: Role of iron(IV) and radicals

The increased occurrence and concentration of micropollutants in water supplies raise public health concerns. Advanced oxidation of micropollutants in real water sources remains challenging due to scavenging reactions involving background anions and natural organic matter. For the first time, this paper demonstrates that chloride (Cl-) accelerates the activation of peroxymonosulfate (PMS) by iron-biochar (Fe/BC) composites. Under the tested conditions, this novel system completely degraded bisphenol A (BPA), a representative micropollutant, within 1.0 min. Micropollutant degradation was investigated at different Cl- contents, PMS levels, Fe/BC doses, and solution pH. The primary reactive species involved with BPA degradation were iron(IV) (Fe(IV)), sulfate radical (SO4•-), hydroxyl radical (•OH), and reactive chlorine species (Cl•, ClO•, Cl2•-). The steady-state concentrations of these reactive species were evaluated to determine their relationships to the Cl- and PMS contents. Fe(IV) was confirmed as the dominant reactive species, with Fe(IV) concentrations increasing with Cl- content and salinity to enhance the overall BPA degradation. Importantly, BPA degradation by the Fe/BC/PMS/Cl- system was not greatly affected by background anions or natural organic matter (NOM) present in real water sources, and the system was successfully applied for five sequential cycles of BPA treatment.

Investigation of OH Species in a Helium Atmospheric Pressure Plasma Jet: from Gas Phase to Liquid Phase through the Plasma-Liquid Interface

This work develops a semiempirical 1D numerical model with average measured [•OH(g)] (denoted as [•OH(g)]M) as the boundary condition and measured [•OH(aq)] (denoted as [•OH(aq)]M) to calibrate the accumulated [•OH(aq)] modeled (denoted as [•OH(aq)]S) in the solution treated by a plasma jet. The [•OH(g)]M obtained in the plasma plume is integrated from the [•OH(g)] distribution detected in the radial direction at position 1 mm above the interface. The [•OH(aq)]M in the solutions is determined from the fluorescence measurements by exciting 2-hydroxyterephthalic acid at 310 nm and detecting the fluorescence emitted at 425 nm for cases with different plasma treatment times. The developed numerical model considers both the diffusion and convection for the domain covering 1 mm above the interface with the dominant generation and consumption mechanisms considered in the discharge plume to evaluate the incoming flux of •OH(g) through the interface, which is calibrated with [•OH(aq)]M in the solution treated. The simulated results show that the transport behavior (i.e., diffusion and convection) plays only a minor role in the contribution of [•OH(aq)]S, while the electron-impact dissociation reactions play significant roles in the generation of •OH(g) in the discharge plume, leading to the high local [•OH(g)] and incoming flux of •OH(g) to the interface. The self-association reactions of •OH(g) contribute to the remarkable consumption of •OH(g). The simulated [•OH(g)] distribution increases from the [•OH(g)]M determined at the upstream boundary to its maximum near the central region as the density reaches 9.5 × 1019 m-3 and decreases rapidly above the interface.

Cu-O-Fe boosts electronic transport for efficient peroxymonosulfate activation over a wide range of pH

The transition metal-oxygen-transition metal (TM1-O-TM2) structural bonds served as electron transport bridges facilitates the charge flow, which guided developing bimetallic activators for peroxymonosulfate (PMS). In this work, a bimetallic CuO/Fe2O3 heterojunction with Cu-O-Fe bond in a core-shell structure was designed. The configuration enhanced PMS activation over a wide range of pH, as demonstrated by the degradation of the model contaminant sulfamethoxazole (SMX), particularly under acid and alkaline condition (>95 % within 30 min). Mechanistic investigations were conducted under various pH conditions. Cu(III) and 1O2 were the dominant active species at acid and alkaline condition, respectively, confirmed by DFT calculations, characterizations and experiments. The degradation pathway of SMX was speculated by mass spectrometry combined with calculations. In addition, the substrate guidance mechanism which was verified by 6 kinds of emerging contaminants (ECs) further highlights the excellence of the surveyed system. The potential for practical application was suggested, attributed to the excellent performance in SMX removal from raw river water (98.7 %), tap water (100 %) and domestic sewage (86.7 %). This study provides new prospect into PMS activation by bimetallic heterojunctions attributing to electronic transport via TM1-O-TM2 bond, which is significant for treating ECs in water.

Confinement-Modulated Singlet-Oxygen Nanoreactors for Water Decontamination

Water decontamination with singlet oxygen (1O2) has shown advantages over the traditional radical-based treatment processes, which are frequently inhibited by the background inorganic/organic substances and produce toxic byproducts. However, earlier reported treatment systems mostly suffer from side reactions against efficient and cost-effective production of 1O2, together with insufficient utilization of 1O2 limited by the extremely short diffusion length. To overcome the drawbacks, we here designed high-performance nanoreactors by modulating the MnO2 phase to nanotube structures (with ∼5 nm diameter, termed "NT5"). With nanoconfinement, our developed NT5 directed efficient and almost 100% utilization of peroxymonosulfate (PMS) to produce 1O2 and achieved maximal kinetics on organic pollutant elimination. The mechanism study revealed that the geometric strain of NT5 together with spatial confinement modulated the adsorption properties of PMS molecules and led to their transformation to 1O2. To demonstrate the applicability of NT5, we developed a reactive filter with a particulate catalyst (NT5 grown on an alumina substrate) that can effectively and stably work in a broad range of contaminated scenarios (surface water, groundwater, municipal secondary effluent, and industrial wastewaters), due to the confined treatment together with the fouling-resistance nature. Our study may boost the deployment of nanomaterials with confined catalysis and their applications in practical water treatment scenarios.

Fe1-xS@BC prepared from natural pyrite and biomass as peroxydisulfate activator for sulfadiazine degradation

The efficient degradation of SAs is a significant challenge for the treatment of wastewater. To address this, the Fe1-xS@BC was prepared by calcining a mixture of pyrite and biomass, and used to activate peroxydisulfate (PDS) to degrade sulfadiazine (SDZ). The effect of carbon sources (wheat straw, rice husk, and corn cob) on catalytic activity of Fe1-xS@BC were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), total Fe dissolution and free radical quantification. The results indicate that Fe1-xS@WBC with carbon defects and oxygenated functional groups facilitate the dissolution of Fe and the generation of ·OH and ·SO4-. Additionally, the electron-rich the thiophene S facilitate the regeneration of Fe(II). In the Fe1-xS@WBC/PDS system, 90.3% of SDZ degradation could be achieved under optimal conditions: Fe1-xS@WBC = 0.5 g L-1, SDZ = 10 mg L-1, PDS = 2.0 mM, initial solution pH = 7.0. In addition, Fe1-xS@WBC/PDS system exhibits strong resistance to interference from Cl-, and NO3-, while elevated concentrations of HCO3-, HPO42-, and HA hinder SDZ degradation. The Fe1-xS@WBC/PDS system shows excellent selectivity and recoverability. Quenching experiments and electron spin resonance (ESR) reveal the involvement of ·OH, ·SO4-, and 1O2 in the degradation of SDZ within Fe1-xS@WBC/PDS system. Furthermore, four possible degradation pathways for SDZ were proposed based on density functional theory (DFT) and liquid chromatography-mass spectrometry (LC-MS) analysis, while assessing the toxicity of degradation intermediates. This study not only introduces a novel catalytic system for the efficient degradation of antibiotic-contaminated wastewater, but also provides a theoretical foundation for the development and application of iron sulfide-biomass composite catalysts.

Photogenerated electron transfer in Ni/NiO supported on g-C3N4 enables sustainable catalytic activation of peroxymonosulfate for emerging pollutant removal

Emerging pollutants such as enrofloxacin (ENR), a widely used fluoroquinolone antibiotic, pose significant threats to aquatic ecosystems due to their persistence, bioaccumulation, and toxicity. This study reports the development of a stable and efficient Ni-NiO/g-C3N4 heterojunction photocatalyst for ENR degradation under visible light and in the presence of peroxymonosulfate (PMS). The catalyst, synthesized via a templated self-assembly and hydrothermal method, achieved 98.7 % ENR removal within 45 min. Mechanistic studies revealed that the charge transfer along lower energy bands in ternary heterojunctions enhances charge separation and promotes the generation of reactive oxygen species (ROS), including sulfate radicals (SO4•-), hydroxyl radicals (•OH), and singlet oxygen (1O2). Density functional theory calculations confirmed strong PMS adsorption on the heterojunction of metallic Ni and exposed Ni in NiO, facilitating efficient ROS production and bond polarization for pollutant degradation. The catalyst exhibited remarkable structural stability, maintaining consistent performance over six reuse cycles, attributed to the robust g-C3N4 matrix and dynamic redox cycling of Ni/NiO. Toxicity assessments showed significant detoxification of ENR into less harmful byproducts, emphasizing the environmental safety of the process. This work demonstrates the potential of the Ni-NiO/g-C3N4/PMS system as a sustainable and scalable approach to address the challenges posed by emerging pollutants in aquatic environments. The research highlights the significance of integrating photocatalysis and PMS activation for advanced oxidation processes, offering an effective pathway to mitigate antibiotic pollution and its ecological impact and can contribute to the development of next-generation catalysts for environmental remediation.

Selective oxidation of aminotrismethylene phosphonate (ATMP) by UV-Cu (II)/H2O2 system: Performance and mechanism

Phosphonates are important water-soluble organic phosphorus compounds that are widely present in contaminated water bodies. Their oxidation and conversion to orthophosphate are often prerequisite steps for achieving deep removal of total phosphorus from water. In this study, aminotrismethylene phosphonate (ATMP) was used as a representative phosphonate, under alkaline pH conditions, we innovatively employed the UV-Cu (II)/H₂O₂ system to achieve selective oxidation of ATMP and efficient conversion to orthophosphate. Under conditions of UV irradiation, trace amount of Cu (II) (0.020 mM), and 2 mM H₂O₂, 97.43 % of ATMP (0.10 mM) was converted into orthophosphate within 30 min at pH= 8.5. Furthermore, the UV-Cu (II)/H₂O₂ system was unaffected by the presence of common cations, anions, humic acid and partial competitive ligands. The results indicated that hydroxyl radicals, singlet oxygen and trivalent copper Cu (III) were the primary active species participating in the oxidation process. The complexation of Cu (II) with ATMP, promoted the intramolecular electron transfer process, thereby improving the selectivity of the process and increasing the conversion rate to orthophosphate. Further validation involved adding H₂O₂ to concentrated reverse osmosis water with existing trace Cu (II), confirming its effectiveness in degrading ATMP and highlighting its potential for removing phosphonate scale inhibitors.

Interfacial electronic structure engineering on nano-confined cobalt nanoparticles to enhance Fenton-like reaction

Rational design of nano-confined cobalt-based heterogeneous catalysts for peroxymonosulfate (PMS) activation remains challenging in Fenton-like systems, particularly in regulating interfacial electronic structures and establishing explicit electron transfer-activity correlations. To address this, we engineered a hierarchically structured Co@C-CNT composite featuring cobalt nanoparticles encapsulated in N-doped carbon polyhedrons interconnected by carbon nanotubes, forming a unique one-dimensional beads-on-string architecture. The strategic integration of CNTs significantly enhanced interfacial electron transfer kinetics, endowing the Co@C-CNT/PMS system with exceptional catalytic performance for carbamazepine degradation (kobs = 0.2281 min⁻¹), demonstrating a twofold enhancement over the CNT-free Co@C counterpart (0.1053 min⁻¹). Mechanistic studies through DFT calculations unveiled that the CNT-induced electronic string effect synergistically modulates the d-band center of confined Co sites and facilitates electron donation to PMS, preferentially cleaving O-H bonds to generate metastable SO5•- intermediates. This electronic configuration promotes selective O12 generation through PMS self-decomposition while suppressing radical pathways. The optimized system exhibited high efficiency across electron-rich pollutants, maintained robust performance in real wastewater matrices, and demonstrated operational stability in continuous-flow reactors. This work elucidates the critical role of electronic structure engineering in manipulating PMS activation pathways and provides a paradigm for designing advanced oxidation catalysts through targeted electron transport optimization.

From Quencher to Promoter: Revisiting the Role of 2,4,6-Trimethylphenol (TMP) in Triplet-State Photochemistry of Dissolved Organic Matter

Triplet-state dissolved organic matter (3DOM*) plays a crucial role in environmental aquatic photochemistry, with 2,4,6-trimethylphenol (TMP) frequently used as a chemical probe or quencher due to its high reactivity with 3DOM*. However, the influence of TMP-derived oxidation intermediates on the target photochemical reactions has not been comprehensively examined. This study investigated TMP's effect on the photolysis of sulfamethoxazole (SMX), a common antibiotic found in natural waters, in the presence of different DOM sources or model photosensitizer. Contrary to expectation, TMP significantly accelerated SMX photolysis, with the extent of enhancement depending on TMP and DOM concentrations. Laser flash photolysis and kinetic modeling suggested the long-lived TMP-derived reactive species (TMP-RS), including phenoxyl radicals, semiquinone radicals, and quinones, as the key factors in this process. Unlike 3DOM*, TMP-RS may react with SMX with the formation of non-SMX•+ intermediates. This process prevents the reduction of SMX•+ and the subsequent regeneration of SMX. The kinetic model successfully predicts the dynamic contributions of various factors to SMX oxidation during the reaction, highlighting the critical role of TMP-RS. This study advances our understanding of TMP's involvement in triplet-state photochemistry and suggests a reconsideration of the role long-lived organic RSs play in the transformation of environmental micropollutants.

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

A comprehensive and in-depth analysis of reaction mechanisms is essential for advancing chemical water treatment technologies. However, due to the limitations of conventional experimental and analytical methods, the types of reactive species and their generation pathways are commonly debatable in many aqueous systems. As highly sensitive diagnostic tools, isotope techniques offer deeper insights with minimal interference from reaction conditions. Nevertheless, precise interpretations of isotope results remain a significant challenge. Herein, we first scrutinized the fundamentals of isotope chemistry and highlighted key changes induced by the isotope substitution. Next, we discussed the application of isotope techniques in kinetic isotope effects, presenting a roadmap for interpreting KIE in sophisticated systems. Furthermore, we summarized the applications of isotope techniques in elemental tracing to pinpoint reaction sites and identify dominant reactive species. Lastly, we propose future research directions, highlighting critical considerations for the rational design and interpretation of isotope experiments in environmental chemistry and related fields.

MnOOH/carbon-based reactive electrochemical membrane for aqueous organic pollutants decontamination

The electrochemical filtration process (ECFP), which integrates the benefits of membrane separation with electrochemical advanced oxidation, exhibits significant potential for water decontamination. A key aspect in realizing practical applications of ECFP lies in the development of cost-effective, high-performance reactive electrochemical membranes (REM). In this work, a novel carbon-based REM (MCM-30) was prepared by coating the low-cost coal-based carbon membrane (CM) with MnOOH nano-catalyst through a simple and environmentally friendly electrochemical deposition method. Results indicated that the nano-MnOOH catalyst significantly improved the hydrophilicity and electrochemical properties of the CM, thereby enhancing its permeability and removal efficiency towards bisphenol A (BPA). The effects of deposition time, applied voltages, flow rates, electrolyte concentrations, and water matrixes on BPA removal efficiency were systematically investigated. Under optimal conditions, 30 min deposition, 2.0 V applied voltage, 2 mL min-1 flow rate, 0.1 mol L-1 Na2SO4 electrolyte concentration, the BPA removal efficiency of the MCM-30 reached to over 95%, which is much higher than that of the CM. The improved water treatment performance of MCM-30 during the electrochemical filtration could be attributed to the enhancement in both direct and indirect oxidation owing to the nano MnOOH deposition. Furthermore, the MCM-30 is recyclable and can be applied across various water backgrounds and pollutant types.

Walnut shell-based biochar-assisted Fe sites anchored carbon-rich g-C3N4: Boosting photodegradation of 2-Mercaptobenzothiazole though synergistic enhancement of Fe sites and C substitution

To expand the utilization of discarded walnut shells and enhance the photocatalytic activity of graphitic carbon nitride(g-C3N4), the carbon-rich graphitic carbon nitride anchored with Fe sites (FeW-CN) was synthesized via a walnut shell-based biochar-assisted strategy. Unlike the direct thermal copolymerization of melamine for g-C3N4, the iron-loaded walnut shell-based biochar (FeW) was first synthesized, followed by thermal copolymerization of melamine with FeW to form FeW-CN. The C substitution on the triazine ring enhanced the light absorption and electron migration for FeW-CN. And the Fe sites interacting with N-(C)3 further improved the migration and the utilization rate of photogenerated carriers. During the degradation of 2-Mercaptobenzothiazole, FeW-CN showed excellent photocatalytic performance and stability compared with g-C3N4. Moreover, FeW-CN maintained excellent photocatalytic performance in river water. Combination of electron paramagnetic resonance with active species quenching experiments, the synergistic mechanism of singlet oxygen, holes, and superoxide radicals was confirmed in the FeW-CN system. Compared to g-C3N4, the Fe sites and C substitution enhanced the production of singlet oxygen.

The Role of Natural Organic Matter in the Degradation of Phenolic Pollutants by Sulfate Radical Oxidation: Radical Scavenging vs Reduction

Dissolved natural organic matter (NOM) significantly influences the performance of water treatment processes. It is generally recognized that NOM acts as a radical scavenger, thus inhibiting the degradation of organic pollutants in advanced oxidation processes (AOPs). This study examined the impacts of 8 different NOM isolates on the degradation of 4-chlorophenol (CP), a representative phenolic pollutant, in sulfate radical (SO4•-)-based AOPs. We developed an improved probe method to measure the steady-state concentration of SO4•- ([SO4•-]ss) in both the absence and presence of NOM. Results show that adding 1.00 mgC L-1 NOM resulted in only a 1.3-3.4% decrease in [SO4•-]ss. However, the apparent rate constants of CP degradation decreased by 76-88%. This discrepancy indicates that radical scavenging cannot be the primary mechanism for observed inhibition. We proposed NOM primarily acts as a reducing agent, reacting with the phenoxy radical intermediates generated from the single-electron oxidation of CP by SO4•-. Based on this hypothesis, we developed and validated a kinetic model using experimental data. The reductive capacity of NOM, as determined by the kinetic model, correlates positively with its electron-donating capacity. These findings enhance the understanding of NOM's role in SO4•--based AOPs and provide a foundation for developing strategies to mitigate its adverse effects.

Synergistic activation of peroxymonosulfate by highly dispersed iron-based sulfur-nitrogen co-doped porous carbon for bisphenol a removal: mechanistic insights and selective oxidation

Efficient and pervasive solutions are urgently needed to mitigate pollution from emerging contaminants in aquatic environments. Activation of peroxymonosulfate (PMS) is commonly employed to remove refractory organic pollutants from water. Herein, we synthesized sulfur-nitrogen co-doped porous carbon materials loaded with highly dispersed iron species (FeSNC) using template-assisted and ligand site construction methods. The uniform doping of N, S, and Fe in the carbon substrate, along with their synergistic effects, significantly enhanced catalytic activity by creating a high degree of defects in the catalyst (I D/I G = 1.47). This enhancement facilitated efficient removal of BPA, achieving an apparent rate constant of up to 2.83 min-1, which was 30 times higher than that of SNC and 6 times higher than that of FeNC. The FeSNC/PMS system demonstrated robust catalytic stability across the pH 3-9 range, and showed minimal sensitivity to environmental factors like the aqueous matrix, with low iron ion dissolution (<0.01 mg L-1) and certain reusability. Mechanistic investigations employing quenching experiments, EPR tests, probe experiments, and electrochemical tests elucidated that the system catalyzed the degradation of BPA via two non-radical pathways: high-valent iron oxidation and singlet oxygen pathways. Additionally, the system further exhibits selective degradation of electron-rich organics (e.g., 4-chlorophenol, sulfamethoxazole, ofloxacin, etc.).

Efficient degradation of sulfadiazine via facilitated electron transfer by iron-carbon catalyst with highly exposed active sites

The inaccessible active sites, excessive metal leaching and radical mediated degradation pathway greatly hinder the performances of Fe-C composite catalyst oxidation process in the advanced oxidation water treatment. Herein, a facile method was developed to in situ growth of MIL-53 (Fe) on the powder active carbon (PAC) surface by a mild condition, which finally yields PAC supported Fe3O4@C particles (PAC@MOFs-2T) after heat treatment. The detailed characterizations indicate that the fine Fe3O4 particles encapsulated with carbon layers were evenly anchored on the PAC as active sites, which made the catalytic centers highly accessible for the peroxydisulfate activation and sulfadiazine degradation. In addition, the carbon layers, coated on the active sites could prevent the metal leaching during the catalytic process resulting in the high stability in a wide pH range. More attractively, the density functional theory (DFT) simulations and emperimental evidences further proved that the oxidation was dominated by a electron transfer process (ETP), during which, the peroxydisulfate (PDS) was adsorbed on Fe3O4 to form PDS∗ with high oxidation potential to initiate the ETP. Meanwhile, it was also demonstrated that the optimized sample PAC@MOFs-2T enriched with electron donating groups could selectively degrade the sulfadiazine, which avoid the negative impacts from the co-existed foreign ions and organic matters during the oxidation process. In addition, the toxicity analysis of intermediate products revealed that the sulfadiazine can be degradated into low-toxic or non-toxic products, which further permits viability of this ETP mediated advanced oxidation processes.

Degradation of carbamazepine by the UVA-LED365/ClO2/NaClO process: Kinetics, mechanisms and DBPs yield

A mixed oxidant of chlorine dioxide (ClO2) and NaClO was often used in water treatment. A novel UVA-LED (365 nm)-activated mixed ClO2/NaClO process was proposed for the degradation of micropollutants in this study. Carbamazepine (CBZ) was selected as the target pollutant. Compared with the UVA365/ClO2 process, the UVA365/ClO2/NaClO process can improve the degradation of CBZ, with the rate constant increasing from 2.11×10-4 sec-1 to 2.74×10-4 sec-1. In addition, the consumption of oxidants in the UVA365/ClO2/NaClO process (73.67%) can also be lower than that of UVA365/NaClO (86.42%). When the NaClO ratio increased, both the degradation efficiency of CBZ and the consumption of oxidants can increase in the UVA365/ClO2/NaClO process. The solution pH can affect the contribution of NaClO in the total oxidant ratio. When the pH range of 6.0-8.0, the combination process can generate more active species to promote the degradation of CBZ. The change of active species with oxidant molar ratio was investigated in the UVA365/ClO2/NaClO process. When ClO2 acted as the main oxidant, HO• and Cl• were the main active species, while when NaClO was the main oxidant, ClO• played a role in the system. Both chloride ion (Cl-), bicarbonate ion (HCO3-), and nitrate ion (NO3-) can promote the reaction system. As the concentration of NaClO in the reaction solution increased, the generation of chlorates will decrease. The UVA365/ClO2/NaClO process can effectively control the formation of volatile disinfection by-products (DBPs), and with the increase of ClO2 dosage, the formation of DBPs can also decrease.

Non-metallic iodine single-atom catalysts with optimized electronic structures for efficient Fenton-like reactions

In this study, we introduce a highly effective non-metallic iodine single-atom catalyst (SAC), referred to as I-NC, which is strategically confined within a nitrogen-doped carbon (NC) scaffold. This configuration features a distinctive C-I coordination that optimizes the electronic structure of the nitrogen-adjacent carbon sites. As a result, this arrangement enhances electron transfer from peroxymonosulfate (PMS) to the active sites, particularly the electron-deficient carbon. This electron transfer is followed by a deprotonation process that generates the peroxymonosulfate radical (SO5•-). Subsequently, the SO5•- radical undergoes a disproportionation reaction, leading to the production of singlet oxygen (1O2). Furthermore, the energy barrier for the rate-limiting step of SO5•- generation in I-NC is significantly lower at 1.45 eV, compared to 1.65 eV in the NC scaffold. This reduction in energy barrier effectively overcomes kinetic obstacles, thereby facilitating an enhanced generation of 1O2. Consequently, the I-NC catalyst exhibits remarkable catalytic efficiency and unmatched reactivity for PMS activation. This leads to a significantly accelerated degradation of pollutants, evidenced by a relatively high observed kinetic rate constant (kobs ~ 0.436 min-1) compared to other metallic SACs. This study offers valuable insights into the rational design of effective non-metallic SACs, showcasing their promising potential for Fenton-like reactions in water treatment applications.

Phenolic acid-containing compounds enhanced Fe3+/peroxides processes for efficient removal of sulfamethoxazole in surface waters

Sulfamethoxazole (SMX) in surface waters has raised widespread concerns due to its potential environmental and biological hazards. In this study, the performance, mechanism, and environmental application of phenolic acid-containing compounds (PACCs) enhanced Fe3+/peroxides processes for SMX degradation were investigated. PACCs with two Ar-OH groups exhibited the lowest toxicity and the best enhancement performance (65%-66% of PDS, 47%-58% of PMS and 61%-63% of H2O2), which were attributed to the excellent chelating and reducing ability towards Fe3+. Free radicals played the predominant role in PDS (37% of SO4-·, 34% of ·OH), PMS (37% of SO4-·, 35% of ·OH) and H2O2 (61% of ·OH) oxidation processes. FeIVO2+ play a non-negligible role in PDS and PMS processes (ŋ[PMSO2] = 52%-80% and ŋ[PMSO2] = 59%-72%). PDS and PMS processes were suitable for a pH range of 3.0-9.0, while the H2O2 process was 3.0-10.0. PDS and PMS processes exhibited stronger resistance to the common anions in surface waters. PMS process exhibited higher adaptability to surface waters quality (92%-98%). This study provides a novel approach for enhancing the degradation of SMX in natural surface waters.

Copper-Copper Oxide Heterostructural Nanocrystals Anchored on g-C3N4 Nanosheets for Efficient Visible-Light-Driven Photo-Fenton-like Catalysis

The development of efficient and sustainable photocatalysts for wastewater treatment remains a critical challenge in environmental remediation. In this study, a ternary photocatalyst, Cu-Cu2O/g-C3N4, was synthesized by embedding copper-copper oxide heterostructural nanocrystals onto g-C3N4 nanosheets via a simple deposition method. Structural and optical characterization confirmed the successful formation of the heterostructure, which combines the narrow bandgap of Cu2O, the high stability of g-C3N4, and the surface plasmon resonance (SPR) effect of Cu nanoparticles. The photocatalytic performance was evaluated through the degradation of Rhodamine B (RhB) in a photo-Fenton-like reaction system under visible light irradiation. Among the catalysts tested, the 30 wt% Cu-Cu2O/g-C3N4 composite exhibited the highest catalytic efficiency, achieving a reaction rate constant approximately 3 times and 1.5 times higher than those of Cu-Cu2O and g-C3N4, respectively. Mechanistic studies suggest that the heterostructure facilitates efficient charge separation and promotes the reduction of Cu2+ to Cu+, thereby enhancing ∙OH radical generation. The catalyst also demonstrated excellent stability and reusability across a wide pH range. These findings provide a new strategy for designing highly efficient photocatalysts for organic pollutant degradation, contributing to the advancement of advanced oxidation processes for environmental applications.

The profound review of Fenton process: What's the next step?

Fenton and Fenton-like processes, which could produce highly reactive species to degrade organic contaminants, have been widely used in the field of wastewater treatment. Therein, the chemistry of Fenton process including the nature of active oxidants, the complicated reactions involved, and the behind reason for its strongly pH-dependent performance, is the basis for the application of Fenton and Fenton-like processes in wastewater treatment. Nevertheless, the conflicting views still exist about the mechanism of the Fenton process. For instance, reaching a unanimous consensus on the nature of active oxidants (hydroxyl radical or tetravalent iron) in this process remains challenging. This review comprehensively examined the mechanism of the Fenton process including the debate on the nature of active oxidants, reactions involved in the Fenton process, and the behind reason for the pH-dependent degradation of contaminants in the Fenton process. Then, we summarized several strategies that promote the Fe(II)/Fe(III) cycle, reduce the competitive consumption of active oxidants by side reactions, and replace the Fenton reagent, thus improving the performance of the Fenton process. Furthermore, advances for the future were proposed including the demand for the high-accuracy identification of active oxidants and taking advantages of the characteristic of target contaminants during the degradation of contaminants by the Fenton process.

Mechanically Induced Green Targeted Conversion of Ammonia Nitrogen to N2: Based on Cavitation Effects and ROS Oxidation

Addressing the mounting challenge of ammonia nitrogen pollution in aquatic ecosystems necessitates the selective oxidation of ammonia nitrogen to nitrogen gas, a pivotal aspect of eco-friendly nitrogen removal processes. Ultrasound cavitation, renowned for its capacity to generate reactive oxygen species (ROS), has garnered considerable attention in environmental remediation. This study reveals a highly synergistic mechanism in ultrasound coupled stirring (US-ST), establishing optimal coupling conditions through sound field monitoring and quantification of ROS. In comparison to ultrasound treatment alone (US), the sound pressure amplitude significantly increased from ±18 to ±30 kPa in US-ST, markedly reducing the cavitation nucleation threshold and augmenting the steady-state concentration of hydroxyl radicals (HO•) by 13-fold. Further, with appropriate charge transfer conditions enabled by the acoustoelectric characteristics of the passive film on stirring paddles, the concentrations of superoxide (•O2-) and singlet oxygen (1O2) elevated to 9.54 × 10-10 M and 8.43 × 10-13 M, respectively. Under the regulation of 500 rpm stirring vortex, a maximum sonochemical efficiency of 6.5 × 10-5 mg J-1 was achieved. In the context of domestic wastewater, ammonia nitrogen degradation was achieved through the oxidation and thermal dissociation effects of US-ST. The concentration decreased from 27.5 to 3.4 mg/L after 2 h, with an impressive N2 selectivity of 96.8%. This study elucidates the targeted conversion mechanism of ammonia nitrogen in US-ST, introducing an emerging water treatment technology propelled by mechanical energy.

Direct and indirect photodegradation mechanism of ractopamine in aquatic environment

As a kind of typical lean meat essences and veterinary drugs, ractopamine (RAC) has been frequently detected in agricultural sewage and livestock, posing potential risk to both aquatic ecosystems and human health. Despite its widespread occurrence, the environmental fate of RAC remains unclear. Here, the mechanisms underlying the direct and indirect photodegradation of RAC was investigated under UV light irradiation at wavelengths of 275 and 365 nm, respectively. The effect of pH, initial concentration, and co-existing ions were examined. For direct photodegradation, the quantum yield of RAC increased with increasing pH values. In solutions containing dissolved organic matter (DOM), indirect photodegradation of RAC intensified with increasing pH values, and the initial concentration of DOM accelerated the process. The presence of Cu2+ was found to inhibit both direct and indirect photodegradation of RAC. Electron spin resonance (ESR) spectrometry and quenching experiments revealed that direct photodegradation was primarily attributed to the decomposition of the triplet state of RAC. Both the triplet state of DOM (3DOM*) and singlet oxygen contributed to the indirect photodegradation of RAC. LC-MS/MS analysis indicated that oxidation of the phenol group and subsequent decarboxylation were the principal photodegradation processes. The energies of each state of RAC and the active sites of RAC molecules were computed using frontier molecular orbitals and Fukui indices based on density functional theory. Combining the analysis of photoproducts with energy calculation, pathways of the direct and indirect photodegradation of RAC were proposed. These findings unveiled the photochemical behaviors of RAC concerning the removal and attenuation in aquatic environment.

Assessing the discrepant role of anions in the transformation of reactive oxygen species in H2O2 and PDS system: A comparative kinetic analysis

Clarifying reactive oxygen species (ROS) variation in the presence of co-existing anions is significant for understanding the catalytic effect of magnetite (Fe3O4)-induced advanced oxidation processes (AOPs) in natural environment, yet this remains controversial. Herein, we compare the specific impacts of NO3-, SO42-, and Cl- on ROS (•OH, SO4•-, O2•-, and 1O2) exposure concentration in H2O2 and peroxydisulfate (PDS) systems catalyzed by Fe3O4, as well as how these variations affect the catalytic efficiency by developing kinetic model. In both two systems, NO3- demonstrates no discernible effect on ROS, whereas SO42- inhibits the exposure of all ROS and thus micropollutants degradation. Through theoretical calculation, it is proposed that SO42- primarily exerts its influence through affecting the electronic structure over catalyst surface. Regarding Cl-, it affects ROS exposure mainly by reacting with ROS. It shows inhibitory effect on 1O2 in both systems, but its suppressive impact on •OH is markedly more pronounced in H2O2 system compared to PDS system, which may be related to its rapid reactivity with SO4•-. Besides, the chlorine radicals (mainly ClO•) generated through the reaction of Cl- may exert a selective influence on micropollutants degradation. This study can help to re-understand the influence behavior of co-existing anions during AOPs.

Effect of peroxydisulfate activated by B-doped NiFe2Ox for degrading contaminants and mitigating nanofiltration membrane fouling in the landfill leachate treatment

Catalytic oxidation pretreatment is a significant focus in the field of membrane fouling control; however, traditional catalytic materials are plagued by limitations in catalytic sites and challenges in recovery. In this study, a novel catalyst, B-doped NiFe2Ox, was prepared with magnetic recovery capabilities and abundant oxygen vacancies to address landfill leachate treatment and mitigate membrane fouling. The results demonstrated the efficient activation of persulfate (PS) by the catalytic sites on B-NiFe2Ox, which significantly degraded the complex organic pollutants like conjugated double bonds and aromatic compounds in landfill leachate. A large amount of humic acid and soluble microbial products in the landfill leachate were efficiently degraded upon contact with sulfate and hydroxyl radicals produced by B-NiFe2Ox/PS, thereby resulting in achieving a chemical oxygen demand removal efficiency of up to 72 % and more than a twofold enhancement in filtration flux. Moreover, the characteristics of the fouled layer reveal that the B-NiFe2Ox/PS system facilitated the formation of a porous cake layer, maximizing the retention of functional groups on the NF270 membrane surface. Notably, a minor presence of B-NiFe2Ox is uniformly distributed within the cake layer, indicating the in-situ occurrence of weak catalytic oxidation reactions. This study provides an effective and innovative approach utilizing catalytic oxidation for membrane fouling control.

Mechanistic insights into the pH-driven radical transformation of the Fe(II)/nCP in groundwater remediation

Calcium peroxide nanoparticles (nCP) as a versatile and safe solid H2O2 source, have attracted significant research interst for their application potential in groundwater remediation. Compared to the traditional Fenton system, the nCP-based Fenton-like system has a wider pH-working window for contaminants degradation. This results from the dominant radical transformation under different pH. Unlike the traditional Fenton system which is only effective in acid conditions with hydroxyl radical (•OH) as the main active species, the release of H2O2 and O2 from nCP provides multiple contaminants degradation pathways. In acidic environments, •OH and Fe(IV) predominate as the active species, facilitated by substantial H2O2 production which activates the Fenton reaction. In neutral or alkaline conditions, the production of H2O2 was dramatically decreased. While the O2 released from nCP can be catalyzed by Fe(II) to form superoxide radical (•O2-), which subsequently generate singlet oxygen (1O2). The formation pathway of •O2- was tracked by O18 isotope labeling experiment. The impact of the water matrix on radical generation in the Fe(II)/nCP Fenton-like system was also studied. This research deepens the understanding of the radical formation mechanisms in nCP-based Fenton-like system, offering insights to support their application in remediating contaminated groundwater.

Bromate-induced oxidation of carbamazepine and toxicity assessment of transformation products in the freezing-sunlight process: Effects of trivalent chromium

Bromate (BrO3-)-induced pharmaceutical and personal care products (PPCPs) oxidation is enhanced in freezing systems. Reduced forms of metals are widely present, often coexisting with various contaminants. However, their effects on the interaction of PPCPs with BrO3- in ice in cold regions may have been overlooked. Herein we investigated the effects of representative reducing metal Cr(III) on the interaction between the representative PPCP carbamazepine (CBZ) and BrO3- in the freezing system. Our findings demonstrated that the degradation rate constants of CBZ by BrO3- and Cr(III) were 29.4%-60.3% lower than those by BrO3- in ice, revealing the inhibition of Cr(III) on CBZ degradation by BrO3- in ice. In BrO3-/freezing/sunlight system, BrO3- contributed 62.8% to CBZ degradation. In BrO3-/Cr(III)/freezing/sunlight system, Cr(III) promoted the generation of hydroxyl radical (·OH), leading to 51.0% contribution of ·OH to CBZ degradation. Oxidants were consumed by Cr(III) to form Cr(VI) rather than reacting with CBZ, thereby decreasing CBZ degradation by BrO3- in ice. Due to sunlight-induced Cr(VI) reduction in ice, only 0.3% of Cr(III) was converted to Cr(VI) in BrO3-/Cr(III)/freezing/sunlight system. BrO3--induced CBZ degradation rate in ice decreased in order of Fe(II), Cr(III), and Mn(II), which was due to the different reducing capabilities. An effective reduction in comprehensive toxicity of systems followed the freezing-sunlight process, even in the presence of Cr(III). This work sheds new light on the environmental behaviors and fate of PPCPs, brominated disinfection by-products, and reducing metals during seasonal freezing.

Unveiling electron transfer and radical transformation pathways in coupled electrocatalysis and persulfate oxidation reactions for complex pollutant removal

The degradation of multiple organic pollutants in wastewater via advanced oxidation processes might involve different radicals, of which the types and concentrations vary upon interacting with different pollutants. In this study, electrochemical activation of peroxymonosulfate (E/PMS) using advanced activated carbon cloth (ACC) as electrode was applied for simultaneous degradation of mixed pollutants, e.g., metronidazole (MNZ) and p-chloroaniline (PCA). 92.5 % of MNZ and 91.4 % of PCA can be degraded at the cathode and anode at a low current density and PMS concentration, respectively. The rate constants for the simultaneous removal of MNZ and PCA in the E/PMS/MNZ(PCA) system were 118 times and 6 times higher than those in the sole PMS system, and 2.5 times and 1.6 times higher than those in the E/Na2SO4/MNZ(PCA) system, respectively. Different electrochemical characteristics, EPR spectra and radical quenching tests verified that the degradation of MNZ and PCA in the optimal system proceeded primarily through non-radical-dominated oxidation, involving electron transfer and 1O2 effect. The system also exhibited low energy consumption (0.215 kWh/m-3·order-1), broad operational pH range, excellent removal efficiency for water matrix, and low by-products toxicity, indicating its strong potential for practical applications. The ACC, with its super stable, low cost, and electrochemical activity, make it as a promising materials applicable in the E/PMS system for degradation of multiple pollutants. The study further elucidated the mechanism of pollutant interaction with electrode materials in terms of radical and non-radical transformation, providing fundamental insight into the application of this system for treatment of complex wastewater.

Selective degradation of sulfonamide antibiotics by peracetic acid alone: Direct oxidation and radical mechanisms

In this study, a peracetic acid (PAA) alone process was systematically demonstrated to give a high efficiency in the selective degradation of sulfonamide antibiotics (SAs). The employment of scavengers and probe compounds in this process demonstrates the predominant role of PAA in direct oxidation, and the limited role of carbon-centered radicals (R-O•) in the degradation of representative SA, sulfamethazine (SMT). The process also exhibits high tolerance towards solution pH and competing anions in wastewater, indicating its applicability in enhancing the biodegradation of SAs in wastewater. Furthermore, the relationships between the observed rate constants (kobs) and the molecule descriptors for ten SA compounds are demonstrated through the assessment of structure-activity relationships, calculated from density functional theory (DFT). This study gives new insights into the selectivity, performance and mechanism of PAA direct-oxidation in SA degradation.

Photolysis of p-phenylenediamine rubber antioxidants in aqueous environment: Kinetics, pathways and their photo-induced toxicity

The widespread use of rubber antioxidants, especially p-phenylenediamines (PPDs), has raised increasing concerns about their risk assessment. However, there is a notable lack of research on their transformation products (TPs). Photolysis, influenced by active components, plays a significant role in the environmental fates of PPDs. This study investigated four emerging PPDs (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), N, N'-diphenyl-p-phenylenediamine (DPPD), N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), and N-cyclohexyl-N'-phenyl-p-phenylenediamine (CPPD)) through a combination of experiments (photolysis kinetics, quenching experiments, acute toxicity test to Vibrio Fischeri (V. fischeri) and identification of photolytic products) and theoretical calculations. The results revealed different pathways for indirect photolysis mediated by the hydroxyl radicals (•OH) and singlet oxygen (1O2) of DPPD and IPPD under simulated sunlight irradiation. The effects of dissolved organic matter (DOM) and fulvic acid (FA) on the rates of photolysis of PPDs highlighted the complex interactions among the molecular structure, light absorption properties, and environmental variables. Quenching for reactive oxygen species (ROS) reduced photo-induced toxicity, whereas the addition of DOM and FA increased it, suggesting the crucial role of ROS in the formation of more toxic photolytic products. The study of photolysis pathways and the evaluation of the health risks provide a comprehensive understanding of the environmental effects of these pollutants.

Comprehensive Insight into the Common Organic Radicals in Advanced Oxidation Processes for Water Decontamination

Radical-based advanced oxidation processes (AOPs) are among the most effective technologies employed to destroy organic pollutants. Compared to common inorganic radicals, such as •OH, O2•-, and SO4•-, organic radicals are widespread, and more selective, but are easily overlooked. Furthermore, a systematic understanding of the generation and contributions of organic radicals remains lacking. In this review, we systematically summarize the properties, possible generation pathways, detection methods, and contributions of organic radicals in AOPs. Notably, exploring organic radicals in AOPs is challenging due to (1) limited detection methods for generated organic radicals; (2) controversial organic radical-mediated reaction mechanisms; and (3) rapid transformation of organic radicals as reaction intermediates. In addition to their characteristics and reactivity, we examine potential scenarios of organic radical generation in AOPs, including during the peroxide activation process, in water matrices or with coexisting organic pollutants, and due to the addition of quenching agents. Subsequently, we summarize various methods for organic radical detection as reported previously, such as electron paramagnetic resonance spectroscopy (EPR), 31P nuclear magnetic resonance spectroscopy (31P NMR), liquid/gas chromatography-mass spectroscopy (GC/LC-MS), and fluorescence probes. Finally, we review the contributions of organic radicals to decontamination processes and provide recommendations for future research.

Persulfate activation by biochar and iron: Effect of chloride on formation of reactive species and transformation of N,N-diethyl-m-toluamide (DEET)

Fenton-like processes using persulfate for oxidative water treatment and contaminant removal can be enhanced by the addition of redox-active biochar, which accelerates the reduction of Fe(III) to Fe(II) and increases the yield of reactive species that react with organic contaminants. However, available data on the formation of non-radical or radical species in the biochar/Fe(III)/persulfate system are inconsistent, which limits the evaluation of treatment efficiency and applicability in different water matrices. Based on competition kinetics calculations, we employed different scavengers and probe compounds to systematically evaluate the effect of chloride in presence of organic matter on the formation of major reactive species in the biochar/Fe(III)/persulfate system for the transformation of the model compound N,N‑diethyl-m-toluamide (DEET) at pH 2.5. We show that the transformation of methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2) cannot serve as a reliable indicator for Fe(IV), as previously suggested, because sulfate radicals also induce PMSO2 formation. Although the formation of Fe(IV) cannot be completely excluded, sulfate radicals were identified as the major reactive species in the biochar/Fe(III)/persulfate system in pure water. In the presence of dissolved organic matter, low chloride concentrations (0.1 mM) shifted the major reactive species likely to hydroxyl radicals. Higher chloride concentrations (1 mM), as present in a mining-impacted acidic surface water, resulted in the formation of another reactive species, possibly Cl2•-, and efficient DEET degradation. To tailor the application of this oxidation process, the water matrix must be considered as a decisive factor for reactive species formation and contaminant removal.

Comparative study on CeO2 catalysts with different morphologies and exposed facets for catalytic ozonation: performance, key factor and mechanism insight

Morphology and facet effects of metal oxides in heterogeneous catalytic ozonation (HCO) are attracting increasing interests. In this paper, the different HCO performances for degradation and mineralization of phenol of seven ceria (CeO2) catalysts, including four with different morphologies (nanorod, nanocube, nanooctahedron and nanopolyhedron) and three with the same nanorod morphology but different exposed facets, are comparatively studied. CeO2 nanorods with (110) and (100) facets exposed show the best performance, much better than that of single ozonation, while CeO2 nanocubes and nanooctahedra show performances close to single ozonation. The underlying reason for their different HCO performances is revealed using various experimental and density functional theory (DFT) calculation results and the possible catalytic reaction mechanism is proposed. The oxygen vacancy (OV) is found to be pivotal for the HCO performance of the different CeO2 catalysts regardless of their morphology or exposed facet. A linear correlation is discerned between the rate of catalytic decomposition of dissolved ozone (O3) and the density of Frenkel-type OV. DFT calculations and in-situ spectroscopic studies ascertain that the existence of OV can boost O3 activation on both the hydroxyl (OH) and Ce sites of CeO2. Conversely, various facets without OV exhibit similar O3 adsorption energies. The OH group plays an important role in activating O3 to produce hydroxyl radical (∙OH) for improved mineralization. This work may offer valuable insights for designing Facet- and OV-regulated catalysts in HCO for the abatement of refractory organic pollutants.

The Dual Role of Natural Organic Matter in the Degradation of Organic Pollutants by Persulfate-Based Advanced Oxidation Processes: A Mini-Review

Persulfate-based advanced oxidation processes (PS-AOPs) are widely used to degrade significant amounts of organic pollutants (OPs) in water and soil matrices. The effectiveness of these processes is influenced by the presence of natural organic matter (NOM), which is ubiquitous in the environment. However, the mechanisms by which NOM affects the degradation of OPs in PS-AOPs remain poorly documented. This review systematically summarizes the dual effects of NOM in PS-AOPs, including inhibitory and promotional effects. It encompasses the entire process, detailing the interaction between PS and its activators, the fate of reactive oxygen species (ROS), and the transformation of OPs within PS-AOPs. Specifically, the inhibiting mechanisms include the prevention of PS activation, suppression of ROS fate, and conversion of intermediates to their parent compounds. In contrast, the promoting effects involve the enhancement of catalytic effectiveness, contributions to ROS generation, and improved interactions between NOM and OPs. Finally, further studies are required to elucidate the reaction mechanisms of NOM in PS-AOPs and explore the practical applications of PS-AOPs using actual NOM rather than model compounds.

Overlooked Complexation and Competition Effects of Phenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate System: Inhibited Generation of Primary and Secondary High-Valent Manganese Species

Organic contaminants with lower Hammett constants are typically more prone to being attacked by reactive oxygen species (ROS) in advanced oxidation processes (AOPs). However, the interactions of an organic contaminant with catalytic centers and participating ROS are complex and lack an in-depth understanding. In this work, we observed an abnormal phenomenon in AOPs that the degradation of electron-rich phenolics, such as 4-methoxyphenol, acetaminophen, and 4-presol, was unexpectedly slower than electron-deficient phenolics in a Mn(II)/nitrilotriacetic acid/peroxymonosulfate (Mn(II)/NTA/PMS) system. The established quantitative structure-activity relationship revealed a volcano-type dependence of the degradation rates on the Hammett constants of pollutants. Leveraging substantial analytical techniques and modeling analysis, we concluded that the electron-rich phenolics would inhibit the generation of both primary (Mn(III)NTA) and secondary (Mn(V)NTA) high-valent manganese species through complexation and competition effects. Specifically, the electron-rich phenolics would form a hydrogen bond with Mn(II)/NTA/PMS through outer-sphere interactions, thereby reducing the electrophilic reactivity of PMS to accept the electron transfer from Mn(II)NTA, and slowing down the generation of reactive Mn(III)NTA. Furthermore, the generated Mn(III)NTA is more inclined to react with electron-rich phenolics than PMS due to their higher reaction rate constants (8314 ± 440, 6372 ± 146, and 6919 ± 31 M-1 s-1 for 4-methoxyphenol, acetaminophen, and 4-presol, respectively, as compared with 671 M-1 s-1 for PMS). Consequently, the two-stage inhibition impeded the generation of Mn(V)NTA. In contrast, the complexation and competition effects are insignificant for electron-deficient phenolics, leading to declined reaction rates when the Hammett constants of pollutants increase. For practical applications, such complexation and competition effects would cause the degradation of electron-rich phenolics to be more susceptible to water matrixes, whereas the degradation of electron-deficient phenolics remains largely unaffected. Overall, this study elucidated the intricate interaction mechanisms between contaminants and reactive metal species at both the electronic and kinetic levels, further illuminating their implications for practical treatment.

Confronting the Mysteries of Oxidative Reactive Species in Advanced Oxidation Processes: An Elephant in the Room

Advanced oxidation processes (AOPs) are rapidly evolving but still lack well-established protocols for reliably identifying oxidative reactive species (ORSs). This Perspective presents both the radical and nonradical ORSs that have been identified or proposed, along with the extensive controversies surrounding oxidative mechanisms. Conventional identification tools, such as quenchers, probes, and spin trappers, might be inadequate for the analytical demands of systems in which multiple ORSs coexist, often yielding misleading results. Therefore, the challenges of identifying these complex, short-lived, and transient ORSs must be fully acknowledged. Refining analytical methods for ORSs is necessary, supported by rigorous experiments and innovative paradigms, particularly through kinetic analysis based on in situ spectroscopic techniques and multiple-probe strategies. To demystify these complex ORSs, future efforts should be made to develop advanced tools and strategies to enhance the mechanism understanding. In addition, integrating real-world conditions into experimental designs will establish a reliable framework in fundamental studies, providing more accurate insights and effectively guiding the design of AOPs.

Non-radical oxidation driven by iron-based materials without energy assistance in wastewater treatment

Chemical oxidation is extensively utilized to mitigate the impact of organic pollutants in wastewater. The non-radical oxidation driven by iron-based materials is noted for its environmental friendliness and resistance to wastewater matrix, and it is a promising approach for practical wastewater treatment. However, the complexity of heterogeneous systems and the diversity of evolutionary pathways make the mechanisms of non-radical oxidation driven by iron-based materials elusive. This work provides a systematic review of various non-radical oxidation systems driven by iron-based materials, including singlet oxygen (1O2), reactive iron species (RFeS), and interfacial electron transfer. The unique mechanisms by which iron-based materials activate different oxidants (ozone, hydrogen peroxide, persulfate, periodate, and peracetic acid) to produce non-radical oxidation are described. The roles of active sites and the unique structures of iron-based materials in facilitating non-radical oxidation are discussed. Commonly employed identification methods in wastewater treatment are compared, such as quenching, chemical probes, spectroscopy, mass spectrometry, and electrochemical testing. According to the process of iron-based materials driving non-radical oxidation to remove organic pollutants, the driving factors at different stages are summarized. Finally, challenges and countermeasures are proposed in terms of mechanism exploration, detection methods and practical applications of non-radical oxidation driven by iron-based materials. This work provides valuable insights for understanding and developing non-radical oxidation systems.

Photothermal nano-confinement reactor with bimetallic sites for enhanced peroxymonosulfate activation in antibiotic degradation

In recent years, photothermal-assisted Fenton-like degradation of organic pollutants has become a prominent green method in environmental pollution control. Nevertheless, the design of suitable catalysts remains a significant challenge for this approach. Herein, zeolite-imidazolate framework-derived CoMn bimetallic nanoparticles embedded in hollow carbon nanofibers (CoMnHCF) have been developed as a photothermal nano-confinement reactor with multiple active sites to enhance reaction performance and promote peroxymonosulfate (PMS) activation. Under light irradiation, the local temperature within the porous spaces of CoMnHCF was significantly higher than the liquid temperature. The confined space concentrated heat, minimized thermal loss, and effectively utilizes this feature to activate PMS for antibiotic degradation. The results demonstrated that this system efficiently degraded various antibiotics, including tetracycline hydrochloride, levofloxacin, sulfamethoxazole, norfloxacin and chlorotetracycline. Photothermal contribution analysis revealed that thermal effects predominate in this system. Further DFT simulations explored the coordination environment of metal elements and the properties of related pollutants, predicting potential structures and reaction sites. A series of water quality experiments and cyclic tests demonstrated the system's significant application potential. This study offered new insights into advancing the integrated use of photothermal conversion and nano-confinement reactor activation of PMS in sewage purification.

Tailored MXene-derived nano-heterostructure oxides for peroxymonosulfate activation in the treatment of municipal wastewaters

Nowadays, in the field of environmental protection, a huge effort is focused on efficient and sustainable processes to treat wastewaters. The current study emphasizes the photocatalytic performance of TiNbOx, a nano-heterostructure material derived from the oxidation of (Ti0.75Nb0.25)2CTx MXene. The TiNbOx nano-heterostructure exhibited remarkable performance in the degradation of caffeine (CAF) and sulfamethoxazole (SMX) under UVA irradiation in the presence of peroxymonosulfate (PMS). Under optimal conditions, 0.2 g L-1 of TiNbOx, 0.5 mM PMS and 50 μM concentration of pollutants and natural pH of deionized water, we observed a complete degradation of SMX and 91% degradation of CAF. Scavenging studies provided evidence for the involvement of ˙OH and SO4˙- in the degradation of the pollutants, which was also supported by indirect techniques of electron paramagnetic resonance (EPR) spectroscopy. The degradation pathway of the pollutants was analyzed by liquid chromatography-mass spectrometry (LC-MS) and several mechanisms were suggested including hydroxylation and isoxazole ring-opening reactions. In addition, X-ray photoelectron spectroscopy (XPS) supported the proposed degradation mechanism. The reusability test underscored the high stability and efficiency of TiNbOx. Moreover, the significance of this research was emphasized by conducting degradation studies in tap water (TW) and tertiary effluents of the wastewater (WW) treatment plant in Bratislava. Under optimal conditions, 49% and 30% CAF were degraded in TW and WW, respectively, after 12 hours of reaction. For SMX, 68% and 67% degradations were obtained in TW and WW, respectively.

Challenging established norms: The unanticipated role of alcohols in UV/PDS radical quenching

UV/peroxydisulfate (UV/PDS) process is known to be highly efficient for degrading micropollutants from water by generating sulfate (SO4•-) and hydroxyl radicals (HO•). Reliable analyses of short-lived SO4•- and HO• are therefore critical for understanding reaction mechanisms and optimizing operating conditions. Currently, alcohols are commonly used as quenchers to distinguish radicals based on the assumption that they exclusively react with target radicals without other influences. However, this study for the first time reveals a series of unexpected effects that challenge this conventional wisdom because: 1) adding alcohols altered the decomposition rates of PDS by replacing the reactions between SO4•- and HO• with PDS by the reactions between secondary reactive species and PDS; and 2) SO4•- preferably reacted with alcohols to generate nonnegligible level of hydrogen peroxide (H2O2) under oxygen-rich conditions, which subsequently led to indirect formation of HO•. Additionally, the formation of H2O2 was substantially impacted by the types of alcohols, dosages, dissolved oxygen, and solution pH. Using probe tests as tools, we found that the actual SO4•- levels after dosing alcohols were only slightly different from assumed/expected levels, whereas the actually HO• levels were 43.7, 3364.9, and 12.5 times higher than assumed/expected conditions for samples dosed with methanol, iso-propanol, and tert-butanol, respectively. These unanticipated effects thus suggest that cautions are needed when using alcohols to qualitative and quantitative determine HO• and SO4•- in UV/PDS process.

Efficient degradation of haloacetic acids by vacuum ultraviolet-activated peroxymonosulfate: Kinetics, mechanisms and theoretical calculations

Efficient degradation of haloacetic acids (HAAs) is crucial due to their potential risks. This study firstly proposed vacuum ultraviolet - activated peroxymonosulfate (VUV/PMS) to remove HAAs (i.e., monochloroacetic acid (MCAA), monobromoacetic acid (MBAA), dichloroacetic acid (DCAA), etc). VUV/PMS achieved 99.51 % MCAA and 63.29 % TOC removal within 10 min. Electron paramagnetic resonance (EPR), quenching and probe experiments demonstrated that •OH was responsible for MCAA degradation. MCAA degradation followed pathways of dehalogenation (major) and decarboxylation (minor). VUV/PMS showed application potential under various reaction parameters. Broad spectrum of VUV/PMS on various HAAs was further explored. Chlorinated HAAs (Cl-HAAs) were primarily degraded by oxidation reactions, while brominated HAAs (Br-HAAs) by direct VUV photolysis. The density functional theory-based calculations (DFT) revealed that reaction rates of HAAs correlated with the highest occupied molecular orbital (HOMO) and energy gap (ΔE), indicating that HAAs degradation depends on their chemical structures. The Fukui function (f0 values) and bond length showed vulnerability of the halogen atom in Cl-HAAs and C-Br bond in Br-HAAs. Overall, this study provides an in-depth perspective on the oxidation performance and mechanism of HAAs using VUV/PMS. It not only demonstrates a green and efficient method but also inspires new strategies for HAAs remediation.