Application of Cobalt/Peracetic Acid to Degrade Sulfamethoxazole at Neutral Condition: Efficiency and Mechanisms

A review on reactive oxygen species-induced mechanism pathways of pharmaceutical waste degradation: Acetaminophen as a drug waste model

Innately designed to induce physiological changes, pharmaceuticals are foreknowingly hazardous to the ecosystem. Advanced oxidation processes (AOPs) are recognized as a set of contemporary and highly efficient methods being used as a contrivance for the removal of pharmaceutical residues. Since reactive oxygen species (ROS) are formed in these processes to interact and contribute directly toward the oxidation of target contaminant(s), a profound insight regarding the mechanisms of ROS leading to the degradation of pharmaceuticals is fundamentally significant. The conceptualization of some specific reaction mechanisms allows the design of an effective and safe degradation process that can empirically reduce the environmental impact of the micropollutants. This review mainly deliberates the mechanistic reaction pathways for ROS-mediated degradation of pharmaceuticals often leading to complete mineralization, with a focus on acetaminophen as a drug waste model.

Revisiting iodide species transformation in peracetic acid oxidation: unexpected role of radicals in micropollutants decontamination and iodate formation

Peracetic acid (PAA) is an alternative disinfectant for saline wastewaters, and hypohalous acids are typically regarded as the reactive species for oxidation and disinfection. However, new results herein strongly suggest that reactive radicals instead of HOI primarily contributed to decontamination during PAA treatment of iodine-containing wastewater. The presence of I- could greatly accelerate the micropollutants (e.g., sulfamethoxazole (SMX)) transformation by PAA. Chemical probes experiments and electron paramagnetic resonance analysis demonstrate acetylperoxyl radical rather than reactive iodine species primarily responsible for SMX degradation. The kinetic model was developed to further distinguish and quantify the contribution of radicals and iodine species, as well as to elucidate the transformation pathways of iodine species. Density functional theory calculations indicated that the nucleophilic attack of I- on the peroxide bond of PAA could form unstable O-I bond, with the transition state energy barrier for radical generation lower than that for HOI formation. The transformation of iodine species was regulated by acetylperoxyl radical to generate nontoxic IO3-, greatly alleviating the iodinated DBPs formation in saline wastewaters. This work provides mechanistic insights in radical-regulated iodine species transformation during PAA oxidation, paving the way for the development of viable and eco-friendly technology for iodide containing water treatment.

Task decomposition strategy based on machine learning for boosting performance and identifying mechanisms in heterogeneous activation of peracetic acid process

Heterogeneous activation of peracetic acid (PAA) process is a promising method for removing organic pollutants from water. Nevertheless, this process is constrained by several complex factors, such as the selection of catalysts, optimization of reaction conditions, and identification of mechanism. In this study, a task decomposition strategy was adopted by combining a catalyst and reaction condition optimization machine learning (CRCO-ML) model and a mechanism identification machine learning (MI-ML) model to address these issues. The Categorical Boosting (CatBoost) model was identified as the best-performing model for the dataset (1024 sets and 7122 data points) in this study, achieving an R2 of 0.92 and an RMSE of 1.28. Catalyst composition, PAA dosage, and catalyst dosage were identified as the three most important features through SHAP analysis in the CRCO-ML model. The HCO3- is considered the most influential water matrix affecting the k value. The errors between all reverse experiment results and the predictions of the CRCO-ML and MI-ML models were <10 % and 15 %, respectively. This interdisciplinary work provides novel insights into the design and application of the heterogeneous activation of PAA process, significantly contributing to the rapid development of this technology.

Advances of carbon-based materials for activating peracetic acid in advanced oxidation processes: A review

In recent years, the peracetic acid (PAA)-based advanced oxidation process (AOPs) has garnered significant attention in the field of water treatment due to rapid response time and environmentally-friendliness. The activation of PAA systems by diverse carbon-based materials plays a crucial role in addressing emerging environmental contaminants, including various types, structures, and modified forms of carbon materials. However, the structural characteristics and structure-activity relationship of carbon-based materials in the activation of PAA are intricate, while the degradation pathways and dominant active species exhibit diversity. Therefore, it is imperative to elucidate the developmental process of the carbon-based materials/PAA system through resource integration and logical categorization, thereby indicating potential avenues for future research. The present paper comprehensively reviews the structural characteristics and action mechanism of carbon-based materials in PAA system, while also analyzing the development, properties, and activation mechanism of heteroatom-doped carbon-based materials in this system. In conclusion, this study has effectively organized the resources pertaining to prominent research direction of comprehensive remediation of environmental water pollution, thereby elucidating the underlying logic and thought process. Consequently, it establishes robust theoretical foundation for future investigations and applications involving carbon-based materials/PAA system.

Rapid Peracetic acid activation by CoO under neutral condition: The contribution of multiple reactive species

This paper proposes a novel process of cobalt monoxide (CoO)-activated peracetic acid (PAA) for treating emerging micropollutant in water. PAA was activated under neutral conditions by combining a dominant heterogeneous phase on the catalyst surface and a homogeneous phase by dissolved Co2+. The system produced several reactive oxygen species, including hydroxyl radicals (HO∙HO•), singlet oxygen (1O2), organic radicals (RO•(CH3C(O)O•, CH3C(O)OO•) and high-valent cobalt (Co(IV)). Organic radicals and high-valent cobalt primarily drove the emerging micropollutants degradation, interacting via electron transfer. Further density functional theory calculations supported that the spontaneous adsorption of PAA onto the catalyst could break peroxy bonds that generate radicals. Furthermore, the CoO surface structure underwent minimal changes during the reaction, making it highly reusable. Thus, the novel CoO/PAA system could be an effective advanced oxidation process for water treatment.

Revisiting the synergistic oxidation of peracetic acid and permanganate(Ⅶ) towards micropollutants: The enhanced electron transfer mechanism of reactive manganese species

Synergistic actions of peroxides and high-valent metals have garnered increasing attentions in wastewater treatment. However, how peroxides interact with the reactive metal species to enhance the reactivity remains unclear. Herein, we report the synergistic oxidation of peracetic acid (PAA) and permanganate(Ⅶ) towards micropollutants, and revisit the underlying mechanism. The PAA-Mn(VII) system showed remarkable efficiency with a 28-fold enhancement on sulfamethoxazole (SMX) degradation compared to Mn(Ⅶ) alone. Extensive quenching experiments and electron spin resonance (ESR) analysis revealed the generation of unexpected Mn(V) and Mn(VI) beyond Mn(III) in the PAA-Mn(VII) system. The utilization efficiency of Mn intermediates was quantified using 2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), and the results indicated that PAA could enhance the electron transfer efficiency of reactive manganese (Mn) species, thus accelerating the micropollutant degradation. Density functional theory (DFT) calculations showed that Mn intermediates could coordinate to the O1 of PAA with a low energy gap, enhancing the oxidation capacity and stability of Mn intermediates. A kinetic model based on first principles was established to simulate the time-dependent concentration profiles of the PAA-Mn complexes and quantify the contributions of the PAA-Mn(III) complex (50.8 to 59.3 %) and the PAA-Mn(Ⅴ/Ⅵ) complex (40.7 to 49.2 %). The PAA-Mn(VII) system was resistant to the interference from complex matrix components (e.g., chloride and humic acid), leading to the high efficiency in real wastewater. This work provides new insights into the interaction of PAA with reactive manganese species for accelerated oxidation of micropollutants, facilitating its application in wastewater treatment.

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.

Overlooked Role of Coexistent Hydrogen Peroxide in Activated Peracetic Acid by Cu(II) for Enhanced Oxidation of Organic Contaminants

Cu(II)-catalyzed peracetic acid (PAA) processes have shown significant potential to remove contaminants in water treatment. Nevertheless, the role of coexistent H2O2 in the transformation from Cu(II) to Cu(I) remained contentious. Herein, with the Cu(II)/PAA process as an example, the respective roles of PAA and H2O2 on the Cu(II)/Cu(I) cycling were comprehensively investigated over the pH range of 7.0-10.5. Contrary to previous studies, it was surprisingly found that the coexistent deprotonated H2O2 (HO2-), instead of PAA, was crucial for accelerating the transformation from Cu(II) to Cu(I) (kHO2-/Cu(II) = (0.17-1) × 106 M-1 s-1, kPAA/Cu(II) < 2.33 ± 0.3 M-1 s-1). Subsequently, the formed Cu(I) preferentially reacted with PAA (kPAA/Cu(I) = (5.84 ± 0.17) × 102 M-1 s-1), rather than H2O2 (kH2O2/Cu(I) = (5.00 ± 0.2) × 101 M-1 s-1), generating reactive species to oxidize organic contaminants. With naproxen as the target pollutant, the proposed synergistic role of H2O2 and PAA was found to be highly dependent on the solution pH with weakly alkaline conditions being more conducive to naproxen degradation. Overall, this study systematically investigated the overlooked but crucial role of coexistent H2O2 in the Cu(II)/PAA process, which might provide valuable insights for better understanding the underlying mechanism in Cu-catalyzed PAA processes.

Activation of peracetic acid by electrodes using biogenic electrons: A novel energy- and catalyst-free process to eliminate pharmaceuticals

Peracetic acid (PAA) has received increasing attention as an alternative oxidant for wastewater treatment. However, existing processes for PAA activation to generate reactive species typically require external energy input (e.g., electrically and UV-mediated activation) or catalysts (e.g., Co2+), inevitably increasing treatment costs or introducing potential new contaminants that necessitate additional removal. In this work, we developed a catalyst-free, self-sustaining bioelectrochemical approach within a two-chamber bioelectrochemical system (BES), where a cathode electrode in-situ activates PAA using renewable biogenic electrons generated by anodic exoelectrogens (e.g., Geobacter) degrading biodegradable organic matter (e.g., acetic acid) in wastewater at the anode. This innovative BES-PAA technique achieved 98 % and 81 % removal of 2 µM sulfamethoxazole (SMX) in two hours at pH 2 (cation exchange membrane) and pH 6 (bipolar membrane) using 100 μM PAA without external voltage. Mechanistic studies, including radical quenching, molecular probe validation, electron spin resonance (ESR) experiments, and density functional theory (DFT) calculations, revealed that SMX degradation was driven by reactive species generated via biogenic electron-mediated OO cleavage of PAA, with CH3C(O)OO• contributing 68.1 %, •OH of 18.4 %, and CH3C(O)O• of 9.4 %, where initial formation of •OH and CH3C(O)O• rapidly reacts with PAA to produce CH3C(O)OO•. The presence of common water constituents such as anions (e.g., Cl-, NO3-, and H2PO4-) and humic acid (HA) significantly hinders SMX removal via the BES-PAA technique, whereas CO32- and HCO3- ions have a comparatively minor impact. Additionally, the study investigated the removal of various pharmaceuticals present in secondary treated municipal wastewater, attributing differences in removal efficiency to the selective action of CH3C(O)OO•. This research demonstrates a novel PAA activation method that is ecologically benign, inexpensive, and capable of overcoming catalyst deactivation and secondary pollution issues.

Enhanced degradation of emerging contaminants by Far-UVC photolysis of peracetic acid: Synergistic effect and mechanisms

Krypton chloride (KrCl*) excimer lamps (222 nm) are used as a promising irradiation source to drive ultraviolet-based advanced oxidation processes (UV-AOPs) in water treatment. In this study, the UV222/peracetic acid (PAA) process is implemented as a novel UV-AOPs for the degradation of emerging contaminants (ECs) in water. The results demonstrate that UV222/PAA process exhibits excellent degradation performance for carbamazepine (CBZ), with a removal rate of 90.8 % within 45 min. Notably, the degradation of CBZ in the UV222/PAA process (90.8 %) was significantly higher than that in the UV254/PAA process (15.1 %) at the same UV dose. The UV222/PAA process exhibits superior electrical energy per order (EE/O) performance while reducing resource consumption associated with the high-energy UV254/PAA process. Quenching experiments and electron paramagnetic resonance (EPR) detection confirm that HO• play a dominant role in the reaction. The contributions of direct photolysis, HO•, and other active species (RO• and 1O2) are estimated to be 5 %, 88 %, and 7 %, respectively. In addition, the effects of Cl-, HCO3-, and humic acid (HA) on the degradation of CBZ are evaluated. The presence of relatively low concentrations of Cl-, HCO3-, and HA can inhibit CBZ degradation. The UV222/PAA oxidation process could also effectively degrade several other ECs (i.e., iohexol, sulfamethoxazole, acetochlor, ibuprofen), indicating the potential application of this process in pollutant removal. These findings will propel the development of the UV222/PAA process and provide valuable insights for its application in water treatment.

Organic radicals driving polycyclic aromatic hydrocarbon polymerization with peracetic acid activation in soil

The use of peracetic acid (PAA) in advanced oxidation processes has gained significant attention recently, but the knowledge of activating PAA to degrade polycyclic aromatic hydrocarbons (PAHs) is limited due to the variety and selectivity of reactive substances in PAA oxidation system. This paper presented the first systemically study on the degradation of PAHs by PAA activation in soil. It was found that heat-activated peracetic acid (heat/PAA) was capable of degrading phenanthrene (PHE) efficiently with degradation efficiency > 90 % within 30 min. Experimental results demonstrated that a series of reactive oxygen species (ROS) including organic radicals (RO•), hydroxyl radicals (HO•) and singlet oxygen (1O2) were generated, while acetylperoxyl (CH3C(O)OO•) and acetyloxyl (CH3C(O)O•) radicals were primarily responsible for PHE degradation in soil. Further analysis shows that polymerization products such as diphenic acid, 2'-formyl-2-biphenylcarboxylic acid and other macromolecules were dominant products of PHE degradation, suggesting polymerization driving PHE degradation instead of the conventional mineralization process. Toxicity analysis shows that most of the polymerization products had less toxicity than that of PHE. These results indicate that PAA activation was a highly effective remediation method for PAHs contaminated soil, which also provided a novel mechanism for pollutant degradation with the PAA activation process for environmental remediation.

Efficient disinfection of combined sewer overflows by ultraviolet/peracetic acid through intracellular oxidation with preserving cell integrity

Combined sewer overflows (CSOs) introduce microbial contaminants into the receiving water bodies, thereby posing risks to public health. This study systematically investigated the disinfection performance and mechanisms of the combined process of ultraviolet and peracetic acid (UV/PAA) in CSOs with selecting Escherichia coli (E. coli) as a target microbial contaminant. The UV/PAA process exhibited superior performance in inactivating E. coli in simulated CSOs compared with UV, PAA, and UV/H2O2 processes. Increasing the PAA dosage greatly enhanced the disinfection efficiency, while turbidity and organic matter hindered the inactivation performance. Singlet oxygen (1O2), hydroxyl (•OH) and organic radicals (RO•) contributed to the inactivation of E. coli, with •OH and RO• playing the prominent role. Variations of intracellular reactive oxygen species, malondialdehyde, enzymes activities, DNA contents and biochemical compositions of E. coli cells suggested that UV/PAA primarily caused oxidative damage to intracellular molecules rather than the damage to the lipids of the cell membrane, therefore effectively limited the regrowth of E. coli. Additionally, the UV/PAA process displayed an outstanding performance in disinfecting actual raw CSOs, achieving a 2.90-log inactivation of total bacteria after reaction for 4 min. These results highlighted the practical applicability and effectiveness of the UV/PAA process in the disinfection of CSOs.

Pre-exposure to peracetic acid followed by UV treatment for deactivating vancomycin-resistant Enterococcus faecalis through intracellular attack

Antimicrobial resistance (AMR) poses a global health threat to aquatic environments and its propagation is a hot topic. Therefore, deactivating antibiotic-resistant bacteria (ARB) and removing antibiotic resistance genes (ARGs) from water is crucial for controlling AMR transmission. Peracetic acid (PAA), which is known for its potent oxidizing properties and limited by-product formation, is emerging as a favorable disinfectant for water treatment. In this study, we aimed to assess the efficacy of pre-exposure to PAA followed by UV treatment (PAA-UV/PAA) compared with the simultaneous application of UV and PAA (UV/PAA). The focus was on deactivating vancomycin-resistant Enterococcus faecalis (VREfs), a typical ARB in water. Pre-exposure to PAA significantly enhanced the efficacy of subsequent UV/PAA treatment. At a UV fluence of 7.2 mJ cm-2, the PAA-UV/PAA method achieved a 6.21 log reduction in VREfs, surpassing the 1.29 log reduction observed with UV/PAA. Moreover, compared to UV/PAA, PAA-UV/PAA showed increased efficacy with longer pre-exposure times and higher PAA concentrations, maintaining superior performance across a broad pH range and in the presence of humic acid. Flow cytometry analysis indicated minimal cellular membrane damage using both methods. However, the assessments of superoxide dismutase (SOD) activity and adenosine triphosphate content revealed that PAA-UV/PAA induced greater oxidative stress under similar UV irradiation conditions, leading to slower bacterial regrowth. Specifically, SOD activity in PAA-UV/PAA surged to 3.06 times its baseline, exceeding the 1.73-fold increase under UV/PAA conditions. Additionally, pre-exposure to PAA amplified ARGs degradation and reduced resistance gene leakage, effectively mitigating the spread of AMR. Pre-exposure to 200 μM PAA for 10 and 20 min enhanced vanB gene removal efficiency by 0.14 log and 1.29 log, respectively. Our study provides a feasible approach for optimizing UV/PAA disinfection for efficient removal of ARB and ARGs.

New insights into the pH-dependent removal of sulfamethoxazole in peracetic acid activation systems: From mechanistic exploration to practical application potentials

Peracetic acid (PAA) as emerging oxidant in advanced oxidation processes (AOPs) has attracted widespread attention in purifying water pollution. In this research, the removal of target contaminant (sulfamethoxazole, SMX) was investigated through PAA activation by a facile catalyst (Co@C), and the active sites of catalyst were identified as sp3-C, Oads, and Co0 by correlation analysis. Especially, different pH adjustment strategies were designed, including System A (adjusting pH after adding PAA) and System B (adjusting pH before adding PAA), to investigate the impact of oxidant acidity and alkalinity on solution microenvironment as well as effect and mechanism of pollutant removal. The results showed that HO· and CH3C(O)OO· dominated in System A, while Co(IV)O2+ was also observed in System B. Both systems showed optimal SMX degradation (98 %). However, System A exhibited excellent water quality tolerance (efficiency > 78 %), superior sustained catalyst activation (efficiency > 80 % in 40 h), less ion leaching (41 μg L-1), and lower products toxicity. Moreover, the pH of solution after reaction in System B was intensely acidic, requiring costly pH adjustments for discharge. This study unveils the strategy of adjusting pH after adding PAA is preferable for water purification, enriching the emerging research of PAA-based AOPs for the remediation of environments.

Protocatechuic acid enhanced the selective degradation of sulfonamide antibiotics in Fe(III)/peracetic acid process under actually neutral pH conditions

The practical application of the Fe-catalyzed peracetic acid (PAA) processes is seriously restricted due to the need for narrow pH working range and poor anti-interference capacity. This study demonstrates that protocatechuic acid (PCA), a natural and eco-environmental phenolic acid, significantly enhanced the removal of sulfonamide antibiotics in Fe(III)/PAA process under actually neutral pH conditions (6.0-8.0) by complexing Fe(III). With sulfamethoxazole (SMX) as the model contaminant, the pseudo-first-order rate constant of SMX elimination in PCA/Fe(III)/PAA process was 63.5 times higher than that in Fe(III)/PAA process at pH 7.0, surpassing most of the previously reported strategies-enhanced Fe-catalyzed PAA processes (i.e., picolinic acid and hydroxylamine etc.). Excluding the primary contribution of reactive species commonly found in Fe-catalyzed PAA processes (e.g., •OH, R-O•, Fe(IV)/Fe(V) and 1O2) to SMX removal, the Fe(III)-peroxy complex intermediate (CH3C(O)OO-Fe(III)-PCA) was proposed as the primary reactive species in PCA/Fe(III)/PAA process. DFT theoretical calculations indicate that CH3C(O)OO-Fe(III)-PCA exhibited stronger oxidation potential than CH3C(O)OO-Fe(III), thereby enhancing SMX removal. Four potential removal pathways of SMX were proposed and the toxicity of reaction solution decreased with the removal of SMX. Furthermore, PCA/Fe(III)/PAA process exhibited strong anti-interference capacity to common natural anions (HCO3-, Cl-and NO3-) and humic acid. More importantly, the PCA/Fe(III)/PAA process demonstrated high efficiency for SMX elimination in actual samples, even at a trace Fe(III) dosage (i.e., 5 μM). Overall, this study provided a highly-efficient and eco-environmental strategy to remove sulfonamide antibiotics in Fe(III)/PAA process under actually neutral pH conditions and to strengthen its anti-interference capacity, underscoring its potential application in water treatment.

Unveiling the environmental significance of acetylperoxyl radical: Reactivity quantification and kinetic modeling

Acetylperoxyl radical (CH3C(O)OO•) is among highly reactive organic radicals which are known to play crucial roles in atmospheric chemistry, aqueous chemistry and, most recently, peracetic acid (PAA)-based advanced oxidation processes. However, fundamental knowledge for its reactivity is scarce and severely hampers the understanding of relevant environmental processes. Herein, three independent experimental approaches were exploited for revelation and quantification of the reaction rates of acetylperoxyl radical. First, we developed and verified laser flash photolysis of biacetyl, ultraviolet (UV) photolysis of biacetyl, and pulse radiolysis of acetaldehyde, each as a clean source of CH3C(O)OO•. Then, using competition kinetics and selection of suitable probe and competitor compounds, the rate constants between CH3C(O)OO• and compounds of diverse structures were determined. The three experimental approaches complemented in reaction time scale and ease of operation, and provided cross-validation of the rate constants. Moreover, the formation of CH3C(O)OO• was verified by spin-trapped electron paramagnetic resonance, and potential influence of other reactive species in the systems was assessed. Overall, CH3C(O)OO• displays distinctively high reactivity and selectivity, reacting especially favorably with naphthyl and diene compounds (k ∼ 107-108 M-1 s-1) but sluggishly with N- and S-containing groups. Significantly, we demonstrated that incorporating acetylperoxyl radical-oxidation reactions significantly improved the accuracy in modeling the degradation of environmental micropollutants by UV/PAA treatment. This study is among the most comprehensive investigation for peroxyl radical reactivity to date, and establishes a robust methodology for investigating organic radical chemistry. The determined rate constants strengthen kinetic databases and improve modeling accuracy for natural and engineered systems.

Peracetic Acid Activation by Modified Hematite for Water Purification: Performance, Degradation Pathways, and Mechanism

Natural mineral-based advanced oxidation processes (AOPs) are now receiving increasing attention for the efficient degradation of pollutants. In this work, we used a common reducing agent (NaBH4) to treat natural Hematite to obtain modified Hematite (Hematite-(R)) and applied it to activate peracetic acid (PAA) for efficient degradation of cefazolin (CFZ). Compared with Hematite, the Hematite-(R)/PAA system increased the degradation rate of CFZ by 21.7% within 80 min under neutral conditions. Scavenging experiments and electron paramagnetic resonance (EPR) technology were introduced to identify the principal roles of 1O2, CH3C(O)OO•, and •OH for CFZ removal over the Hematite-(R)/PAA process. The outstanding capability of Hematite-(R) could be mainly due to the higher percentage of Fe(II) (52%) on the surface of catalysts. Furthermore, the possible degradation pathways of CFZ were explored. Moreover, the Hematite-(R)/PAA process showed a superior CFZ removal efficiency with a wide initial pH scope of 1.0-9.0. The degradation efficiency of CFZ showed a negligible effect in the presence of Cl-, SO42-, and NO3-, while significant inhibition was recorded after the addition of H2PO4- and CO32-. The inhibition of humic acid (HA) on CFZ degradation via the Hematite-(R)/PAA process showed an obvious concentration dependence. This work could provide strong support for the use of natural Hematite in water purification.

Insights into the pH-dependent mechanism of peracetic acid activation by biochar-supported zero-valent iron/cobalt bimetallic nanoparticles: The shift of reactive sites and the dual role of hydrogen peroxide

The peracetic acid (PAA)-based water purification process is often controlled by the solution pH. Herein, we explored the usage of biochar (BC) supported zero-valent iron/cobalt nanoparticles (Fe/Co@BC) for triggering PAA oxidation of sulfamethazine (SMT), and discovered the PAA activation mechanisms at different pHs. Fe/Co@BC exhibited extraordinary PAA activation efficiency over the pH range of 3.0-8.2, effectively broadening the working pH of the zero-valent iron nanoparticles (NZVI)-PAA process. Specifically, the SMT removal efficiency increased by 8.3 times in Fe/Co@BC-PAA system compared to the NZVI-PAA system at pH 8.2. Besides, the leaching and recycling experiments indicated the improved stability and reusability of the materials. For the mechanism study, the main reactive species was •OH under acidic conditions and R-O•/Fe(IV) under neutral/alkaline conditions. More interestingly, the reactive sites on Fe/Co@BC shifted from Fe species to Co species as pH increased, and the role of H2O2 in this reaction system also shifted from a radical precursor to a radical scavenger with increasing pH. This study highlights the distinct mechanism of PAA activation by bimetallic composites under different pH conditions and provides a new efficient approach for PAA activation to degrade organic contaminants.

Enhanced Degradation of Micropollutants in a Peracetic Acid/Mn(II) System with EDDS: An Investigation of the Role of Mn Species

Extensive research has been conducted on the utilization of a metal-based catalyst to activate peracetic acid (PAA) for the degradation of micropollutants (MPs) in water. Mn(II) is a commonly employed catalyst for homogeneous advanced oxidation processes (AOPs), but its catalytic performance with PAA is poor. This study showed that the environmentally friendly chelator ethylenediamine-N,N'-disuccinic acid (EDDS) could greatly facilitate the activation of Mn(II) in PAA for complete atrazine (ATZ) degradation. In this process, the EDDS enhanced the catalytic activity of manganese (Mn) and prevented disproportionation of transient Mn species, thus facilitating the decay of PAA and mineralization of ATZ. By employing electron spin resonance detection, quenching and probe tests, and 18O isotope-tracing experiments, the significance of high-valent Mn-oxo species (Mn(V)) in the Mn(II)-EDDS/PAA system was revealed. In particular, the involvement of the Mn(III) species was essential for the formation of Mn(V). Mn(III) species, along with singlet oxygen (1O2) and acetyl(per)oxyl radicals (CH3C(O)O•/CH3C(O)OO•), also contributed partially to ATZ degradation. Mass spectrometry and density functional theory methods were used to study the transformation pathway and mechanism of ATZ. The toxicity assessment of the oxidative products indicated that the toxicity of ATZ decreased after the degradation reaction. Moreover, the system exhibited excellent interference resistance toward various anions and humid acid (HA), and it could selectively degrade multiple MPs.

Unlocking high-efficiency decontamination by building a novel heterogeneous catalytic reduction system of thiourea dioxide/biochar

Herein, we reported a new contaminant purification paradigm, which enabled highly efficient reductive denitration and dechlorination using a green, stable reducing agent thiourea dioxide (TDO) coupled with biochar (BC) over a wide pH range under anoxic conditions. Specifically, BC acted as both activators and electron shuttles for TDO decomposition to achieve complete anoxic degradation of p-nitrophenol (PNP), p-nitroaniline, 4-chlorophenol and 2,4-dichlorophenol within 2 h. During this process, multiple strongly reducing species (i.e., SO22-, SO2•- and e-/H•) were generated in BC/TDO systems, accounting for 13.3%, 9.7% and 75.5% of PNP removal, respectively. While electron transfer between TDO and H+ or contaminants mediated by BC led to H• generation and contaminant reduction. These processes depended on the electron-accepting capacity and electron-conducting domains of biochar. Significantly, the BC/TDO systems were highly efficient at a pH of 2.0-8.0, especially under acidic conditions, which performed robustly in common natural water constituents.

Prehydrated Electrons Activated by Continuous Electron Transfer Stemmed from Peracetic Acid Homolysis Mediated by Diamond Surface Defects for Enhanced PFOA Destruction

Research on the use of peracetic acid (PAA) activated by nonmetal solid catalysts for the removal of dissolved refractory organic compounds has gained attention recently due to its improved efficiency and suitability for advanced water treatment (AWT). Among these catalysts, nanocarbon (NC) stands out as an exceptional example. In the NC-based peroxide AWT studies, the focus on the mechanism involving multimedia coordination on the NC surface (reactive species (RS) path, electron reduction non-RS pathway, and singlet oxygen non-RS path) has been confined to the one-step electron reaction, leaving the mechanisms of multichannel or continuous electron transfer paths unexplored. Moreover, there are very few studies that have identified the nonfree radical pathway initiated by electron transfer within PAA AWT. In this study, the complete decomposition (kobs = 0.1995) and significant defluorination of perfluorooctanoic acid (PFOA, deF% = 72%) through PAA/NC has been confirmed. Through the use of multiple electrochemical monitors and the exploration of current diffusion effects, the process of electron reception and conduction stimulated by PAA activation was examined, leading to the discovery of the dynamic process from the PAA molecule → NC solid surface → target object. The vital role of prehydrated electrons (epre-) before the entry of resolvable electrons into the aqueous phase was also detailed. To the best of our knowledge, this is the first instance of identifying the nonradical mechanism of continuous electron transfer in PAA-based AWT, which deviates from the previously identified mechanisms of singlet oxygen, single-electron, or double-electron single-path transfer. The pathway, along with the strong reducibility of epre- initiated by this pathway, has been proven to be essential in reducing the need for catalysts and chemicals in AWT.

Photo-assisted natural chalcopyrite activated peracetic acid for efficient micropollutant degradation

The effective activation of natural chalcopyrite (CuFeS2) on peracetic acid (PAA) to remove organic micropollutants was studied under visible light irradiation. Results showed than an effective sulfamethoxazole (SMX) degradation (95.0 %) was achieved under visible light irradiation for 30 min at pH 7.0. Quenching experiments, electron spin resonance analysis, and LC/MS spectrum demonstrated that HO• and CH3C(O)OO• were the main reactive species for SMX degradation, accounting for 43.3 % and 56.7 % of the contributions, respectively. Combined with X-ray photoelectron spectroscopy analysis, the photoelectrons generated on CuFeS2 activated by visible light enhanced the Fe3+/Fe2+ and Cu2+/Cu+ cycles on the surface, thereby activating PAA to generate HO•/CH3C(O)OO•. The removal rate of SMX decreased with the increase in wavelengths, due to the formation of low energy photons at longer wavelengths. Besides, the optimal pH for degradation of SMX by CuFeS2/PAA/Vis-LED process was neutral, which was attributed to the increasing easily activated anionic form of PAA during the increase in pH and the depletion of Fe species at alkaline conditions. Cl-, HCO3-, and HA slightly inhibited SMX degradation because of reactive species being quenched and/or shielding effect. Furthermore, the degradation efficiency of different pollutants by CuFeS2/PAA/Vis-LED was also measured, and the removal efficiency was different owing to the selectivity of CH3C(O)OO•. Finally, the process exhibited good applicability in real waters. Overall, this study provides new insight into visible light-catalyzed activation of PAA and suggests on further exploration of the intrinsic activation mechanism of PAA.

Tailoring the selective generation of oxidative organic radicals for toxic-by-product-free water decontamination

Peracetic acid (PAA) is emerging as a versatile agent for generating long-lived and selectively oxidative organic radicals (R-O•). Currently, the conventional transition metal-based activation strategies still suffer from metal ion leaching, undesirable by-products formation, and uncontrolled reactive species production. To address these challenges, we present a method employing BiOI with a unique electron structure as a PAA activator, thereby predominantly generating CH3C(O)O• radicals. The specificity of CH3C(O)O• generation ensured the superior performance of the BiOI/PAA system across a wide pH range (2.0 to 11.0), even in the presence of complex interfering substances such as humic acids, chloride ions, bicarbonate ions, and real-world water matrices. Unlike conventional catalytic oxidative methods, the BiOI/PAA system degrades sulfonamides without producing any toxic by-products. Our findings demonstrate the advantages of CH3C(O)O• in water decontamination and pave the way for the development of eco-friendly water decontaminations based on organic peroxides.

Regulating charge transfer for enhanced PAA activation over sulfur-doped magnetic CoFe2O4: A novel strategy for simultaneous micropollutants degradation and bacteria inactivation

Micropollutants and bacteria are prevalent pollutants in wastewater, posing significant risks to ecosystems and human health. As peracetic acid (PAA) is being increasingly used as a disinfectant, activation of PAA by low-cost and high-performance activators is a promising strategy for wastewater treatment. In this study, the sulfur-doped magnetic CoFe2O4 (SCFO) is successfully developed for efficient PAA activation to simultaneously decontaminate and disinfect wastewater. PAA/SCFO-0.3 exhibits exceptional performance, degrading 100 % of 8 μM sulfamethoxazole (SMX) with a first-pseudo reaction rate of 1.275 min-1, and achieving 5.3-log inactivation of Escherichia coli (E. coli) within 3 min at a PAA dosage of 0.2 mM and catalyst dosage of 0.025 g/L (initial pH 6.5). Scavenging experiments and electron paramagnetic resonance (EPR) analysis identify CH3C(O)O• and CH3C(O)OO• as the dominant reactive species for SMX degradation. The sulfur species in SCFO-0.3 facilitate Co2+ regeneration and regulate charge transfer, promoting PAA activation for SMX degradation. Moreover, the PAA/SCFO-0.3 system demonstrates operational feasibility over a broad range of water matrices and has excellent stability and reusability (maintaining 93 % removal of SMX after 5 cycles), demonstrating its potential for industrial applications. This study provides insights into enhancing PAA activation through sulfur doping in transition metal catalysts and highlights the practical applicability of the PAA/SCFO-0.3 system as an advanced alternative to conventional disinfection for simultaneous decontamination and disinfection in wastewater.

Efficient activation of peracetic acid by cobalt modified nitrogen-doped carbon nanotubes for drugs degradation: Performance and mechanism insight

Peracetic acid (PAA) has garnered significant attention as a novel disinfectant owing to its remarkable oxidative capacity and minimal potential to generate byproducts. In this study, we prepared a novel catalyst, denoted as cobalt modified nitrogen-doped carbon nanotubes (Co@N-CNTs), and evaluated it for PAA activation. Modification with cobalt nanoparticles (∼4.8 nm) changed the morphology and structure of the carbon nanotubes, and greatly improved their ability to activate PAA. Co@N-CNTs/PAA catalytic system shows outstanding catalytic degradation ability of antiviral drugs. Under neutral conditions, with a dosage of 0.05 g/L Co@N-CNT-9.8 and 0.25 mM PAA, the removal efficiency of acyclovir (ACV) reached 98.3% within a mere 10 min. The primary reactive species responsible for effective pollutant degradation were identified as acetylperoxyl radicals (CH3C(O)OO•) and acetyloxyl radicals (CH3C(O)O•). In addition, density functional theory (DFT) proved that Co nanoparticles, as the main catalytic sites, were more likely to adsorb PAA and transfer more electrons than N-doped graphene. This study explored the feasibility of PAA degradation of antiviral drugs in sewage, and provided new insights for the application of heterogeneous catalytic PAA in environmental remediation.

Doped Cu0 and sulfidation induced transition from R-O• to •OH in peracetic acid activation by sulfidated nano zero-valent iron-copper

Peracetic acid (PAA) has emerged as a new effective oxidant for various contaminants degradation through advanced oxidation process (AOP). In this study, sulfidated nano zero-valent iron-copper (S-nZVIC) with low Cu doping and sulfidation was synthesized for PAA activation, resulting in more efficient degradation of sulfamethoxazole (SMX, 20 μM) and other contaminants using a low dose of catalyst (0.05 g/L) and oxidant (100 μM). The characterization results suggested that S-nZVIC presented a more uniform size and distribution with fewer metal oxides, as the agglomeration and oxidation were inhibited. More significantly, doped Cu0 and sulfidation significantly enhanced the generation and contribution of •OH but decreased that of R-O• in S-nZVIC/PAA/SMX system compared with that of nZVIC and S-nZVI, accounting for the relatively high degradation efficiency of 97.7% in S-nZVIC/PAA/SMX system compared with 85.7% and 78.9% in nZVIC/PAA/SMX and S-nZVI/PAA/SMX system, respectively. The mechanisms underlying these changes were that (i) doped Cu° could promote the regeneration of Fe(Ⅱ) for strengthened PAA activation through mediating Fe(Ⅱ)/Fe(Ⅲ) cycle by Cu(Ⅰ)/Cu(Ⅱ) cycle; (ii) S species might consume part of R-O•, resulting in a decreased contribution of R-O• in SMX degradation; (iii) sulfidation increased the electrical conductivity, thus facilitating the electron transfer from S-nZVIC to PAA. Consequently, the dominant reactive oxygen species transited from R-O• to •OH to degrade SMX more efficiently. The degradation pathways, intermediate products and toxicity were further analyzed through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS) and T.E.S.T software analysis, which proved the environmental friendliness of this process. In addition, S-nZVIC exhibited high stability, recyclability and degradation efficiency over a wide pH range (3.0∼9.0). This work provides a new insight into the rational design and modification of nano zero-valent metals for efficient wastewater treatment through adjusting the dominant reactive oxygen species (ROS) into the more active free radicals.

Fe-N co-doped biochar derived from biomass waste triggers peracetic acid activation for efficient water decontamination

In this study, the porous carbon material (FeN-BC) with ultra-high catalytic activity was obtained from waste biomass through Fe-N co-doping. The prominent degradation rate (> 96.8%) of naproxen (NAP) was achieved over a wide pH range (pH 3.0-9.0) in FeN-BC/PAA system. Unlike previously reported iron-based peracetic acid (PAA) systems with •OH or RO• as the dominated reactive species, the degradation of contaminants was attributed to singlet oxygen (1O2) produced by organic radicals (RO•) decomposition, which was proved to be thermodynamically feasible and favorable by theoretical calculations. Combining the theoretical calculations, characteristic and experimental analysis, the synergistic effects of Fe and N were proposed and summarized as follows: i) promoted the formation of extensive defects and Fe0 species that facilitated electron transfer between FeN-BC and PAA and continuous Fe(II) generation; ii) modified the specific surface area (SSA) and the isoelectric point of FeN-BC in favor of PAA adsorption on the catalyst surface. This study provides a strategy for waste biomass reuse to construct a heterogeneous catalyst/PAA system for efficient water purification and reveals the synergistic effects of typical metal-heteroatom for PAA activation.

Mechanically treated Mn2O3 triggers peracetic acid activation for superior non-radical oxidation of micropollutants: Identification of reactive complexes

This study used a simple mechanical ball milling strategy to significantly improve the ability of Mn2O3 to activate peracetic acid (PAA) for sustainable and efficient degradation of organic micropollutant (like bisphenol A, BPA). BPA was successfully removed and detoxified via PAA activation by the bm-Mn2O3 within 30 min under neutral environment, with the BPA degradation kinetic rate improved by 3.4 times. Satisfactory BPA removal efficiency can still be achieved over a wide pH range, in actual water and after reuse of bm-Mn2O3 for four cycles. The change in hydrophilicity of Mn2O3 after ball milling evidently elevated the affinity of Mn2O3 for binding to PAA, while the reduction in particle size exposed more active sites contributing partially to catalytic oxidation. Further analysis revealed that BPA oxidation in the ball mill-treated Mn2O3 (bm-Mn2O3)/PAA process mainly depends on the bm-Mn2O3-PAA complex (i.e., Mn(III)-OO(O)CCH3) mediated non-radical pathway rather than R-O• and Mn(IV). Especially, the existence of the Mn(III)-PAA complex was definitely verified by in situ Raman spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Simultaneously, density functional theory calculations determined that PAA adsorbs readily on manganese sites thereby favoring the formation of Mn(III)-OO(O)CCH3 complexes. This study advances an in-depth understanding of the underlying mechanisms involved in the manganese oxide-catalyzed activation of PAA for superior non-radical oxidation of micropollutants.

N,S,O triply-doped carbon with nanotubes-interwoven nanosheets encapsulated Co nanoparticles for robust antibiotic destruction via activating peroxymonosulfate

The development of facile and effective approaches to regulate the stability and reusability of metallic Co catalytic materials towards peroxymonosulfate (PMS) activation for remediating antibiotic pollutants remains challenging. In this study, we develop a one-step pyrolysis strategy to fabricate three-dimensional porous architecture assembled with N,S,O-codoped carbon nanotube-interwoven hierarchically porous carbon nanosheets encapsulated Co nanoparticles (Co@NSOC), which serve as chainmail catalysts for stable and reusable degradation of tetracycline hydrochloride (TCH) through PMS activation. The optimal Co@NSOC-700-activated PMS system presents an excellent removal efficiency of 94.1 % for TCH within 10 min and a high cycling efficiency of 92.9 % after eight cycles. The encapsulated structure, abundant catalytic sites, superior hydrophilicity and strong magnetism contribute to the high performance. Further investigation demonstrates that both radical and nonradical pathways contribute to the TCH destruction, and 1O2 is verified as the dominant reactive substance. The possible degradation pathways and the toxicity of intermediates for TCH are evaluated. This work offers an innovative structure design and surface modulation strategy to fabricate robust catalysts towards environmental remediation.

Activating peroxymonosulfate with Fe-doped biochar for efficient removal of tetracycline: Dual action of reactive oxygen species and electron transfer

If pharmaceutical wastewater is not managed effectively, the presence of residual antibiotics will result in significant environmental contamination. In addition, inadequate utilization of agricultural waste represents a squandering of resources. The objective of this research was to assess the efficacy of iron-doped biochar (Fe-BC) derived from peanut shells in degrading high concentrations of Tetracycline (TC) wastewater through activated peroxymonosulfate. Fe-BC demonstrated significant efficacy, achieving a removal efficiency of 87.5% for TC within 60 min without the need to adjust the initial pH (20 mg/L TC, 2 mM PMS, 0.5 g/L catalyst). The degradation mechanism of TC in this system involved a dual action, namely Reactive Oxygen Species (ROS) and electron transfer. The primary active sites were the Fe species, which facilitated the generation of SO4•-, •OH, O2•-, and 1O2. The presence of Fe species and the C=C structure in the Fe-BC catalyst support the electron transfer. Degradation pathways were elucidated through the identification of intermediate products and calculation of the Fukui index. The Toxicity Estimator Software Tool (T.E.S.T.) suggested that the intermediates exhibited lower levels of toxicity. Furthermore, the system exhibited exceptional capabilities in real water and circulation experiments, offering significant economic advantages. This investigation provides an efficient strategy for resource recycling and the treatment of high-concentration antibiotic wastewater.

Trace Co(II) triggers peracetic acid activation in phosphate buffer: New insights into the oxidative species responsible for ciprofloxacin removal

Peracetic acid (PAA) emerges as a promising disinfectant and oxidant applied worldwide, and its application has been broadened for advanced oxidation processes (AOPs) in wastewater treatment. Current studies on transition metal-activated AOPs utilized relatively high concentrations of catalysts, leading to potential secondary pollution concerns. This study boosts the understanding of reaction mechanism in PAA activation system under a low-level concentration. Herein, trace levels of Co(II) (1 μM) and practical dosages of PAA (50-250 μM) were employed, achieving noticeable ciprofloxacin (CIP) degradation efficiencies (75.8-99.0%) within 20 min. Two orders of magnitude of the CIP's antibacterial activity significantly decreased after Co(II)/PAA AOP treatment, which suggested the effective ecological risk control capability of the reaction system. The degradation performed well in various water matrices and the primary reactive species is proposed to be CoHPO4-OO(O)CCH3 complexes with scavenging tests and electron paramagnetic resonance tests. The degradation pathway of fluoroquinolones including piperazine ring-opening (dealkylation and oxidation), defluorination, and decarboxylation, were systematically elucidated. This study boosts a comprehensive and novel understanding of PAA-based AOP for CIP degradation.

Peracetic acid activated by nickel cobaltite as effective oxidizing agent for BPA and its analogues degradation

The presented research concerns the use of nickel cobaltite nanoparticles (NiCo2O4 NPs) for the heterogeneous activation of peracetic acid and application of NiCo2O4-PAA system for degradation 10 organic micropollutants from the group of bisphenols. The bisphenols removal (initial concentration 1 μM) process was optimized by selecting the appropriate process conditions. The optimal amount of catalyst (115 mg/L), peracetic acid (PAA) concentration (7 mM) and pH (7) were determined using response surface analysis in the Design of Experiment. Then, NiCo2O4 NPs were used to check the possibility of reuse in subsequent oxidation cycles. The work also attempts to explain the mechanism of oxidation of the studied micropollutants. The participation of the sorption process on the catalyst was excluded and based on the experiments with radical scavengers it can be concluded that the oxidation proceeds in a radical pathway, mainly with participation of O2•- radicals. Experiments conducted in real water matrices exhibit low impact on degradation efficiency. Toxicity tests with green alga Acutodesmus obliquus and aquatic plant Lemna minor showed that post-reaction mixture influenced growth and the content of photosynthetic pigments in concentration dependent manner.

Hierarchical Porous Bimetallic FeMn Metal-Organic Framework Gel for Efficient Activation of Peracetic Acid in Antibiotic Degradation

Effective techniques for eliminating antibiotics from water environments are in high demand. The peracetic acid (PAA)-based advanced oxidation process has recently drawn increasing attention for its effective antibiotic degrading capability. However, current applications of PAA-based techniques are limited and tend to have unsatisfactory performance. An additional catalyst for PAA activation could provide a promising solution to improve the performance of PAA. Bulky metal-organic framework gels (MOGs) stand out as ideal catalysts for PAA activation owing to their multiple advantages, including large surface areas, high porosity, and hierarchical pore systems. Herein, a bimetallic hierarchical porous structure, i.e., FeMn13BTC, was synthesized through a facile one-pot synthesis method and employed for PAA activation in ofloxacin (OFX) degradation. The optimized FeMn MOG/PAA system exhibited efficient catalytic performance, characterized by 81.85% OFX degradation achieved within 1 h owing to the specific hierarchical structure and synergistic effect between Fe and Mn ions, which greatly exceeded the performance of the only PAA-catalyzed system. Furthermore, the FeMn MOG/PAA system maintained >80% OFX degradation in natural water. Quenching experiments, electron spin resonance spectra, and model molecular degradation revealed that the primary reactive oxygen species responsible for the catalytic effect was R-O•, especially CH3C(=O)OO•, with minor contributions of •OH and 1O2. Overall, introduction of the MOG catalyst strategy for PAA activation achieved high antibiotic degradation performance, establishing a paradigm for the design of heterogeneous hierarchical systems to broaden the scope of catalyzed water treatment applications.

Infancy of peracetic acid activation by iron, a new Fenton-based process: A review

The exacerbated global water scarcity and stricter water directives are leading to an increment in the recycled water use, requiring the development of new cost-effective advanced water treatments to provide safe water to the population. In this sense, peracetic acid (PAA, CH3C(O)OOH) is an environmentally friendly disinfectant with the potential to challenge the dominance of chlorine in large wastewater treatment plants in the near future. PAA can be used as an alternative oxidant to H2O2 to carry out the Fenton reaction, and it has recently been proven as more effective than H2O2 towards emerging pollutants degradation at circumneutral pH values and in the presence of anions. PAA activation by homogeneous and heterogeneous iron-based materials generates - besides HO• and FeO2+ - more selective CH3C(O)O• and CH3C(O)OO• radicals, slightly scavenged by typical HO• quenchers (e.g., bicarbonates), which extends PAA use to complex water matrices. This is reflected in an exponential progress of iron-PAA publications during the last few years. Although some reviews of PAA general properties and uses in water treatment were recently published, there is no account on the research and environmental applications of PAA activation by Fe-based materials, in spite of its gratifying progress. In view of these statements, here we provide a holistic review of the types of iron-based PAA activation systems and analyse the diverse iron compounds employed to date (e.g., ferrous and ferric salts, ferrate(VI), spinel ferrites), the use of external ferric reducing/chelating agents (e.g., picolinic acid, l-cysteine, boron) and of UV-visible irradiation systems, analysing the mechanisms involved in each case. Comparison of PAA activation by iron vs. other transition metals (particularly cobalt) is also discussed. This work aims at providing a thorough understanding of the Fe/PAA-based processes, facilitating useful insights into its advantages and limitations, overlooked issues, and prospects, leading to its popularisation and know-how increment.

Converting peracetic acid activation by Fe3O4 from nonradical to radical pathway via the incorporation of L-cysteine

Recently, peracetic acid (PAA) based Fenton (-like) processes have received much attention in water treatment. However, these processes are limited by the sluggish Fe(III)/Fe(II) redox circulation efficiency. In this study, L-cysteine (L-Cys), an environmentally friendly electron donor, was applied to enhance the Fe3O4/PAA process for the sulfamethoxazole (SMX) abatement. Surprisingly, the L-Cys incorporation was found not only to enhance the SMX degradation rate constant by 3.2 times but also to switch the Fe(IV) dominated nonradical pathway into the •OH dominated radical pathway. Experiment and theoretical calculation result elucidated -NH2, -SH, and -COOH of L-Cys can increase Fe solubilization by binding to the Fe sites of Fe3O4, while -SH of L-Cys can promote the reduction of bounded/dissolved Fe(III). Similar SMX conversion pathways driven by the Fe3O4/PAA process with or without L-Cys were revealed. Excessive L-Cys or PAA, high pH and the coexisting HCO3-/H2PO4- exhibit inhibitory effects on SMX degradation, while Cl- and humic acid barely affect the SMX removal. This work advances the knowledge of the enhanced mechanism insights of L-Cys toward heterogeneous Fenton (-like) processes and provides experimental data for the efficient treatment of sulfonamide antibiotics in the water treatment.

Organic peroxyl radicals from biacetyl accelerated the visible-light degradation of steroid estrogens in aqueous solution

Indirect photodegradation is an important pathway for the reduction of steroid estrogens in sunlit surface waters. Nevertheless, the kinetics and mechanisms governing the interaction between coexisting carbonyl compounds and estrogens under visible light (Vis) remain unexplored. This study systematically investigates the Vis-induced photodegradation of 17β-estradiol (E2) in the presence of five specific carbonyl compounds-biacetyl (BD), acetone, glyoxal, pyruvic acid, and benzoquinone. The results demonstrate that, among these compounds, only BD significantly enhanced the photodegradation of E2 under Vis irradiation (λ > 400 nm). The pseudo-first order photodegradation rate constants (k1) of E2 in the Vis/BD system were 0.025 min-1 and 0.076 min-1 in ultrapure water and river water, respectively. The enhancing effect of BD was found to be pH-dependent, increasing the pH from 3.0 to 11.0 resulted in a 76% reduction in the k1 value of E2 in the Vis/BD system. Furthermore, the presence of humic acid, NO3-, or HCO3- led to an increase of more than 35% in the k1 value of E2, while NO2- exerted a pronounced inhibitory effect, resulting in a 92% decrease. Peroxyacetyl and peroxymethyl radicals, derived from BD in a yield ratio of 9, played a crucial role in the degradation of E2. These peroxyl radicals primarily targeted electron-rich hydroxyl sites of E2, initiating hydroxylation and ring-opening reactions that culminated in the formation of acidic byproducts. Notably, toxicity evaluation indicates that these hydroxylated and acidic products exhibited lower toxicity than the parent compound E2. This study highlights the important role of peroxyl radicals in estrogen degradation within aquatic environment, and also helps to design efficient visible light-responsive photo-activators for the treatment of estrogen-contaminated waters.

Catalytic activation of peracetic acid for pelargonic acid vanillylamide degradation by Co3O4 nanoparticles in-situ anchored carbon-coated MXene nanosheets: Performance and mechanism insight

Pelargonic acid vanillylamide (PAVA), a capsaicin-type dacryagogue agent utilized for counter-terrorism and riot control, possesses a low stimulus threshold. This characteristic can lead to environmental contamination following its application and may easily result in secondary stimulation to personnel. Cobalt-doped Ti3C2-MXene nanosheets (Co3O4/Ti3C2@C) were synthesized for the purpose of activating peracetic acid (PAA) and degrading PAVA. A carbon layer was coated on the surface of Ti3C2-MXene nanosheets to address the challenge of poor oxygen resistance in MXenes, thus preventing a significant decline in surface reactivity. The BET surface area of Co3O4/Ti3C2@C was expanded to 149.6 m2/g, significantly exceeding that of Ti3C2 (13.0 m2/g) and Co3O4 (56.4 m2/g). With 0.5 mg/mL of Co3O4/Ti3C2@C and 0.35 mM of PAA, 100 mg/L of PAVA was completely degraded within 60 min. The augmented BET surface area and the presence of more active sites confer remarkable PAA activation and catalytic degradation properties toward PAVA. Parameters such as initial pH, PAVA concentration, catalyst dosage, and PAA concentration on PAVA degradation were systematically assessed. Furthermore, the reusability and stability of the nanocomposite were substantiated through recycling tests. Radical quenching experiments and electron paramagnetic resonance analysis demonstrated the acetylperoxy radical (CH3CO3) as the primary species responsible for PAVA degradation. This research serves as an illustration of the utilization of MXene and transition metal activated PAA in wastewater treatment.

Peracetic acid as an alternative disinfectant for micropollutants degradation and disinfection byproducts control in outdoor swimming pools

Peracetic acid (PAA) has garnered significant interest as a novel alternative to chlorine-based disinfectants for water treatment due to its broad-spectrum antimicrobial activity and its ability of reactive species generation when exposed to UV light. However, limited studies have investigated micropollutant degradation in the presence of PAA under solar irradiation. This is the first study to comprehensively investigate the photodegradation of caffeine (CAF) and 4-methylbenzylidene camphor (4-MBC) and the removal of disinfection byproducts (DBPs) in the presence of PAA under simulated solar light. The study revealed that the photodegradation of CAF and 4-MBC was significantly enhanced in the presence of PAA, following pseudo-first-order kinetics (R2 > 0.98) with reaction rates (kobs) of 0.220 and 0.111 h-1, respectively. In addition, substantial reduction of 21 DBPs, including trihalomethanes, haloacetic acids and haloacetonitriles, and no DBPs formation were observed in the presence of PAA and simulated solar irradiation. The proportion of coexisting H2O2 in the PAA solution considerably influenced target compounds degradation. CAF and 4-MBC were degraded faster under acidic conditions than under alkaline conditions. Hydroxyl radicals (·OH) dominated the degradation of CAF at different pH values, while direct photolysis and other reactive species played a major role in the degradation of 4-MBC.

Unraveling Different Reaction Characteristics of Alkoxy Radicals in a Co(II)-Activated Peracetic Acid System Based on Dynamic Analysis of Electron Distribution

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) have shown broad application prospects in organic wastewater treatment. Alkoxy radicals including CH3COO• and CH3COOO• are primary reactive species in PAA-AOP systems; however, their reaction mechanism on attacking organic pollutants still remains controversial. In this study, a Co(II)/PAA homogeneous AOP system at neutral pH was constructed to generate these two alkoxy radicals, and their different reaction mechanisms with a typical emerging contaminant (sulfacetamide) were explored. Dynamic electron distribution analysis was applied to deeply reveal the radical-meditated reaction mechanism based on molecular orbital analysis. Results indicate that hydrogen atom abstraction is the most favorable route for both CH3COO• and CH3COOO• attacking sulfacetamide. However, both radicals cannot react with sulfacetamide via the radical adduct formation route. Interestingly, the single-electron transfer reaction is only favorable for CH3COO• due to its lower ESUMO. In comparison, CH3COOO• can react with sulfacetamide via a similar radical self-sacrificing bimolecular nucleophilic substitution (SN2) route owing to its high ESOMO and easy escape of unpaired electrons from n orbitals of O atoms in the peroxy bond. These findings can significantly improve the knowledge of reactivity of CH3COO• and CH3COOO• on attacking organic pollutants at the molecular orbital level.

Unexpected side reactions dominate the oxidative transformation of aromatic amines in the Co(II)/peracetic acid system

Aromatic amines (AAs), ubiquitous in industrial applications, pose significant environmental hazards due to their resistance to conventional wastewater treatments. Peracetic acid (PAA)-based advanced oxidation processes (AOPs) have been proposed as effective strategies for addressing persistent AA contaminants. While the organic radicals generated in these systems are believed to be selective and highly oxidative, acetate residue complicates the evaluation of AA removal efficiency. In this work, we explored transformation pathways of AAs in a representative Co(II)-catalyzed PAA system, revealing five side reactions (i.e. nitrosation, nitration, coupling, dimerization, and acetylation) that yield 17 predominantly stable and toxic by-products. The dominant reactive species was demonstrated as Co-OOC(O)CH3, which hardly facilitated ring-opening reactions. Our findings highlight the potential risks associated with PAA-based AOPs for AA degradation and provide insights into selecting suitable catalytic systems aimed at efficient and by-product-free degradation of pollutants containing aromatic -NH2.

Bimetallic metal-organic framework as a high-performance peracetic acid activator for sulfamethoxazole degradation

A novel 3D bimetallic metal-organic framework (MOF(Fe-Co)) was successfully prepared and its performance on sulfamethoxazole (SMX) removal in advanced oxidation process (AOP) based on peracetic acid (PAA) was evaluated. MOF(Fe-Co) exhibited an efficient catalytic performance on PAA activation for SMX degradation under neutral condition. Increasing PAA concentration could enhance SMX removal, while the variation of MOF(Fe-Co) dosage from 0.05 to 0.2 g/L had an inappreciable effect on SMX removal. According to the results of inductively coupled plasma mass spectrometry analyses and X-ray photoelectron spectroscopy, catalytic reactions mainly occurred on the surface of MOF(Fe-Co). Organic radicals (i.e., CH3C(O)OO• and CH3C(O)O•) were demonstrated to be the predominant reactive radicals for SMX degradation by MOF(Fe-Co)/PAA through radical quenching experiments. The presence of Cl- could enhance the degradation of SMX by MOF(Fe-Co)/PAA, while HCO3- and natural organic matter inhibited SMX degradation severely. Five identified degradation products were detected in this system and four possible SMX transformation pathways were proposed, including amino oxidation, S-N bond cleavage, coupling reaction and hydroxylation.

Embedding electronic perpetual motion into single-atom catalysts for persistent Fenton-like reactions

In our quest to leverage the capabilities of the emerging single-atom catalysts (SACs) for wastewater purification, we confronted fundamental challenges related to electron scarcity and instability. Through meticulous theoretical calculations, we identified optimal placements for nitrogen vacancies (Nv) and iron (Fe) single-atom sites, uncovering a dual-site approach that significantly amplified visible-light absorption and charge transfer dynamics. Informed by these computational insights, we cleverly integrated Nv into the catalyst design to boost electron density around iron atoms, yielding a potent and flexible photoactivator for benign peracetic acid. This exceptional catalyst exhibited remarkable stability and effectively degraded various organic contaminants over 20 cycles with self-cleaning properties. Specifically, the Nv sites captured electrons, enabling their swift transfer to adjacent Fe sites under visible light irradiation. This mechanism accelerated the reduction of the formed "peracetic acid-catalyst" intermediate. Theoretical calculations were used to elucidate the synergistic interplay of dual mechanisms, illuminating increased adsorption and activation of reactive molecules. Furthermore, electron reduction pathways on the conduction band were elaborately explored, unveiling the production of reactive species that enhanced photocatalytic processes. A six-flux model and associated parameters were also applied to precisely optimize the photocatalytic process, providing invaluable insights for future photocatalyst design. Overall, this study offers a molecule-level insight into the rational design of robust SACs in a photo-Fenton-like system, with promising implications for wastewater treatment and other high-value applications.

Accelerated removal of naproxen in the iron-based peracetic acid activation system by chloride ions: Enhancement of reactive oxidative species via the formation of iron-chloride complexes

Iron-based PAA activation process is a promising advanced oxidation process for water decontamination which depends on Fe(II) as the main reactive site for PAA activation, resulting in various reactive oxidative species (ROSs) generation. For practical application, the impact of water matrix chloride ion (Cl-) on ROSs production and contaminants removal should be carefully considered. In this study, it's found that the introduction of Cl- (0.1-10 mM) could significantly enhance the reaction rate of the rapid stage (kobs1) up to 2.15 times at the initial pH of 4.25 in the Fe(II)/PAA system. Further studies demonstrated that the improved removal capacity of NAP resulted from Cl- induced R-O• generation as indicated by the exposure dose of R-O• increasing from 7.74 × 10-11 M•s to 1.44 × 10-10 M•s, rather than chlorine-containing radicals' generation. DFT calculation results suggested that the formed Fe(II)-Cl- complexes could easily activate PAA to generate more ROSs for NAP removal. Moreover, Fe(II)/PAA treatment can alleviate the biological toxicity of pollutants via both the Escherichia coli test and toxicity assessment. The obtained new knowledge manifested that Cl- can boost ROSs generation and conversion in iron-based PAA systems, providing guidance for the efficient decontamination of chlorine-containing sewage with PAA-based AOPs.

Insight into cobalt substitution in LaFeO3-based catalyst for enhanced activation of peracetic acid: Reactive species and catalytic mechanism

In this study, a hollow sphere-like Co-modified LaFeO3 perovskite catalyst (LFC73O) was developed for peracetic acid (PAA) activation to degrade sulfamethoxazole (SMX). Results indicated that the constructed heterogeneous system achieved a 99.7% abatement of SMX within 30 min, exhibiting preferable degradation performance. Chemical quenching experiments, probe experiments, and EPR techniques were adopted to elucidate the involved mechanism. It was revealed that the superior synergistic effect of electron transfer and oxygen defects in the LFC73O/PAA system enhanced the oxidation ability of PAA. The Co atoms doped into LaFeO3 as the main active site with the original Fe atoms as an auxiliary site exhibited high activity to mediate PAA activation via the Co(III)/Co(II) cycle, generating carbon-centered radicals (RO·) including CH3C(O)O· and CH3C(O)OO·. The oxygen vacancies induced by cobalt substitution also served as reaction sites, facilitating the dissociation of PAA and production of ROS. Furthermore, the degradation pathways were postulated by DFT calculation and intermediates identification, demonstrating that the electron-rich sites of SMX molecules such as amino group, aromatic ring, and S-N bond, were more susceptible to oxidation by reactive species. This study offers a novel perspective on developing catalysts with the coexistence of multiple active units for PAA activation in environmental remediation.

Activation of Peracetic Acid by CoFe2O4 for Efficient Degradation of Ofloxacin: Reactive Species and Mechanism

Peroxyacetic acid (PAA)-based advanced oxidation processes (AOPs) have attracted much attention in wastewater treatment by reason of high selectivity, long half-life reactive oxygen species (ROS), and wider applicability. In this study, cobalt ferrite (CoFe2O4) was applied to activate PAA for the removal of ofloxacin (OFX). The degradation of OFX could reach 83.0% via the CoFe2O4/PAA system under neutral conditions. The low concentration of co-existing anions and organic matter displayed negligible influence on OFX removal. The contributions of hydroxyl radicals (·OH), organic radicals (R-O·), and other reactive species to OFX degradation in CoFe2O4/PAA were systematically evaluated. Organic radicals (especially CH3C(O)OO·) and singlet oxygen (1O2) were verified to be the main reactive species leading to OFX destruction. The Co(II)/Co(III) redox cycle occurring on the surface of CoFe2O4 played a significant role in PAA activation. The catalytic performance of CoFe2O4 remained above 80% after five cycles. Furthermore, the ecotoxicity of OFX was reduced after treatment with the CoFe2O4/PAA system. This study will facilitate further research and development of the CoFe2O4/PAA system as a new strategy for wastewater treatment.

Unveiling the mechanisms of peracetic acid activation by iron-rich sludge biochar for sulfamethoxazole degradation with wide adaptability

Advanced oxidation processes (AOPs) based on peracetic acid (PAA) has been extensively concerned for the degradation of organic pollutants. In this study, metallic iron-modified sludge biochar (Fe-SBC) was employed to activate PAA for the removal of sulfamethoxazole (SMX). The characterization results indicated that FeO and Fe2O3 were successfully loaded on the surface of the sludge biochar (SBC). Fe-SBC/PAA system achieved 92% SMX removal after 30 min. The pseudo-first-order kinetic reaction constant of the Fe-SBC/PAA system was 7.34 × 10-2 min-1, which was 2.4 times higher than the SBC/PAA system. The degradation of SMX was enhanced with increasing the Fe-SBC dosage and PAA concentration. Apart from Cl-, NO3- and SO42- had a negligible influence on the degradation of SMX. Quenching experiments and electron paramagnetic resonance (EPR) techniques identified the existence of reactive species, of which CH3C(O)OO•, 1O2, and O2•- were dominant reactive species in Fe-SBC/PAA system. The effect of different water matrices on the removal of SMX was investigated. The removal of SMX in tap water and lake water were 79% and 69%, respectively. Four possible pathways for the decay of SMX were presented according to the identification of oxidation products. In addition, following the ecological structure-activity relationship model (ECOSAR) procedure and the germination experiments with lettuce seeds to predict the toxicity of the intermediates. The acute and chronic ecotoxicity of SMX solution was dramatically diminished by processing with Fe-SBC/PAA system. In general, this study offered a prospective strategy for the degradation of organic pollutants.

Picolinic Acid-Mediated Catalysis of Mn(II) for Peracetic Acid Oxidation Processes: Formation of High-Valent Mn Species

Metal-based advanced oxidation processes (AOPs) with peracetic acid (PAA) have been extensively studied to degrade micropollutants (MPs) in wastewater. Mn(II) is a commonly used homogeneous metal catalyst for oxidant activation, but it performs poorly with PAA. This study identifies that the biodegradable chelating ligand picolinic acid (PICA) can significantly mediate Mn(II) activation of PAA for accelerated MP degradation. Results show that, while Mn(II) alone has minimal reactivity toward PAA, the presence of PICA accelerates PAA loss by Mn(II). The PAA-Mn(II)-PICA system removes various MPs (methylene blue, bisphenol A, naproxen, sulfamethoxazole, carbamazepine, and trimethoprim) rapidly at neutral pH, achieving >60% removal within 10 min in clean and wastewater matrices. Coexistent H2O2 and acetic acid in PAA play a negligible role in rapid MP degradation. In-depth evaluation with scavengers and probe compounds (tert-butyl alcohol, methanol, methyl phenyl sulfoxide, and methyl phenyl sulfone) suggested that high-valent Mn species (Mn(V)) is a likely main reactive species leading to rapid MP degradation, whereas soluble Mn(III)-PICA and radicals (CH3C(O)O• and CH3C(O)OO•) are minor reactive species. This study broadens the mechanistic understanding of metal-based AOPs using PAA in combination with chelating agents and indicates the PAA-Mn(II)-PICA system as a novel AOP for wastewater treatment.

Reinvestigation on the Mechanism for Algae Inactivation by the Ultraviolet/Peracetic Acid Process: Role of Reactive Species and Performance in Natural Water

This study provided an in-depth understanding of enhanced algae inactivation by combining ultraviolet and peracetic acid (UV/PAA) and selecting Microcystis aeruginosa as the target algae species. The electron paramagnetic resonance (EPR) tests and scavenging experiments provided direct evidence on the formed reactive species (RSs) and indicated the dominant role of RSs including singlet oxygen (1O2) and hydroxyl (HO•) and organic (RO•) radicals in algae inactivation. Based on the algae inactivation kinetic model and the determined steady-state concentration of RSs, the contribution of RSs was quantitatively assessed with the second-order rate constants for the inactivation of algae by HO•, RO•, and 1O2 of 2.67 × 109, 3.44 × 1010, and 1.72 × 109 M-1 s-1, respectively. Afterward, the coexisting bi/carbonate, acting as a shuttle, that promotes the transformation from HO• to RO• was evidenced to account for the better performance of the UV/PAA system in algae inactivation under the natural water background. Subsequently, along with the evaluation of the UV/PAA preoxidation to modify coagulation-sedimentation, the possible application of the UV/PAA process for algae removal was advanced.

pH-dependent bisphenol A transformation and iodine disinfection byproduct generation by peracetic acid: Kinetic and mechanistic explorations

Peracetic acid (PAA) is regarded as an environmentally friendly oxidant because of its low formation of toxic byproducts. However, this study revealed the potential risk of generating disinfection byproducts (DBPs) when treating iodine-containing wastewater with PAA. The transformation efficiency of bisphenol A (BPA), a commonly detected phenolic contaminant and a surrogate for phenolic moieties in dissolved organic matter, by PAA increased rapidly in the presence of I-, which was primarily attributed to the formation of active iodine (HOI/I2) in the system. Kinetic model simulations demonstrated that the second-order rate constant between PAA and HOI was 54.0 M-1 s-1 at pH 7.0, which was lower than the generation rate of HOI via the reaction between PAA and I-. Therefore, HOI can combine with BPA to produce iodine disinfection byproducts (I-DBPs). The transformation of BPA and the generation of I-DBPs in the I-/PAA system were highly pH-dependent. Specifically, acidic conditions were more favorable for BPA degradation because of the higher reaction rates of BPA and HOI. More iodinated aromatic products were detected after 5 min of the reaction under acidic and neutral conditions, resulting in higher toxicity towards E. coli. After 12 h of the reaction, more adsorbable organic iodine (AOI) was generated at alkaline conditions because HOI was not able to efficiency transform to IO3-. The presence of H2O2 in the PAA solution played a role in the reaction with HOI, particularly under alkaline conditions. This study significantly advances the understanding of the role of I- in BPA oxidation by PAA and provides a warning to further evaluate the potential environmental risk during the treatment of iodine-bearing wastewater with PAA.

Comprehensive assessment of toxicity and environmental risk associated with sulfamethoxazole biodegradation in sulfur-mediated biological wastewater treatment

Incomplete mineralization of sulfamethoxazole (SMX) in wastewater treatment systems poses a threat to ecological health. The toxicity and environmental risk associated with SMX biodegradation in the sulfur-mediated biological process were examined for the first time through a long-term (180 days) bioreactor study and a series of bioassays. The results indicated that the sulfur-mediated biological system was highly resistant and tolerant to SMX toxicity, as evidenced by the enrichment of sulfate-reducing bacteria (SRB), the improved microbial metabolic activity, and the excellent performance on pollutants removal under long-term SMX exposure. SMX can be effectively biodegraded by the cleavage and rearrangement of the isoxazole ring, hydrogenation and hydroxylation reactions in sulfur-mediated biological wastewater system. These biodegradation pathways effectively reduced the acute toxicity, antibacterial activity, and ecotoxicities of SMX and its biotransformation products (TPs) in the effluent of the sulfur-mediated biological system. The TPs produced via hydrogenation (TP1), hydroxylation, and isoxazole ring cleavage (TP3, TP4, TP5, TP8, and TP9) exhibited lower toxicity than SMX. Under SMX stress, although the abundance of sulfonamide resistance genes increased, the total abundance of ARGs decreased due to the extrusion of some intracellular SMX by the efflux pump genes and the inactivation of some SMX through the biodegradation process. Efflux pump and inactivation, as the main resistance mechanisms of antibiotics in the sulfur-mediated biological system, play a crucial role in microbial self-defense. The findings of this study demonstrate the great potential of the sulfur-mediated biological system in SMX removal, detoxication, and ARGs environmental risk reduction.