Generation mechanism of singlet oxygen from the interaction of peroxymonosulfate and chloride in aqueous systems

Sulfite induced degradation of sulfamethoxazole by a silica stabilized ZIF-67(Co) catalyst via non-radical pathways: Formation and role of high-valent Co(IV) and singlet oxygen

The sulfite (S(IV))-based advanced oxidation process (AOP) has emerged as an appealing alternative to the traditional persulfate-based AOP for the elimination of organic contaminants from diverse water matrices. In this work, a silica reinforced ZIF-67(Co) catalyst (CZS) is fabricated, characterized and tested in the activation of S(IV) for the sulfamethoxazole (SMX) degradation. The prepared CZS demonstrates superior stability and catalytic ability for the degradation of SMX compared to ZIF-67(Co) across a broad pH range. Unlike the conventional radical-dominated oxidation systems, the CZS/S(IV) system for SMX degradation operates through a non-radical mechanism, featuring high-valent Co(IV) and singlet oxygen (1O2) as the predominated reactive species. The hydroxylated Co species exposed on the CZS surface is identified as the pivotal active site, realizing the S(IV) activation through a complexation-electron transfer process, resulting in the production of various reactive intermediates. Co(II) undergoes the conversion to Co(IV) by generated HSO5-, and 1O2 predominantly originates from the intermediate SO4•-. Profiting from the highly selective oxidation capacities of Co(IV) and 1O2, the established oxidative system demonstrates a remarkable interference resistance and exhibits an exceptional decontamination performance under real-world water conditions. In short, this work provides a sustainable S(IV)-based oxidation strategy for environmental remediation via non-radical mechanism.

Enhancing singlet oxygen production of dioxygen activation on the carbon-supported rare-earth oxide nanocluster and rare-earth single atom catalyst to remove antibiotics

Singlet oxygen (1O2) is extensively employed in the fields of chemical, biomedical and environmental. However, it is still a challenge to produce high- concentration 1O2 by dioxygen activation. Herein, a system of carbon-supported rare-earth oxide nanocluster and single atom catalysts (named as RE2O3/RE-C, RE=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y) with similar morphology, structure, and physicochemical characteristic are constructed to activate dissolved oxygen (DO) to enhance 1O2 production. The catalytic activity trends and mechanisms are revealed experimentally and are also proven by theoretical analyses and calculations. The 1O2 generation activity trend is Gd2O3/Gd-C>Er2O3/Er-C>Sm2O3/Sm-C>pristine carbon (C). More than 95.0% of common antibiotics (ciprofloxacin, ofloxacin, norfloxacin and carbamazepine) can be removed in 60 min by Gd2O3/Gd-C. Density functional theory calculations indicate that Gd2O3 nanoclusters and Gd single atoms exhibit the moderate adsorption energy of ·O2- to enhance 1O2 production. This study offers a universal strategy to enhance 1O2 production in dioxygen activation for future application and reveals the natural essence of basic mechanisms of 1O2 production via rare-earth oxide nanoclusters and rare-earth single atoms.

The synergistic effect of oxidant-peroxide coupling systems for water and wastewater treatments

With the growing complexity and severity of water pollution, it has become increasingly challenging to effectively remove contaminants or inactivate microorganisms just by traditional chemical oxidants such as O3, chlorine, Fe(VI) and Mn(VII). Up till now, numerous studies have indicated that these oxidants in combination with peroxides (i.e., hydrogen peroxide (H2O2), peroxymonosulfate (PMS), peracetic acid (PAA) and periodate (PI)) exhibited excellent synergistic oxidation. This paper provided a comprehensive review on the combination of aforementioned oxidant-peroxide applied in water and wastewater treatments. From one aspect, the paper thoroughly elucidated the synergy mechanism of each oxidant-peroxide combination in turn. Among these combinations, H2O2 or PMS generally performed as the activator of four traditional oxidants above to accelerate reactive species generation and therein various reaction mechanisms, including electron transfer, O atom abstraction and oxo ligand substitution, were involved. In addition, although neither PAA nor PI was able to directly activate Fe(VI) and Mn(VII), they could act as the stabilizer of intermediate reactive iron/manganese species to improve the latter utilization efficiency. From another aspect, this paper summarized the influence of water quality parameters, such as pH, inorganic ions and natural organic matter (NOM), on the oxidation performance of most combined systems. Finally, this paper highlighted knowledge gaps and identified areas that require further research.

Cobalt(II) mediated calcium sulfite activation for efficient oxidative decontamination in waters: Performance, kinetics and mechanism

To overcome the drawback that excess SO32- from soluble Na2SO3 captures the generated reactive intermediates in sulfite (S(IV))-based advanced oxidation processes (AOP), CaSO3 of the ability to slowly release SO32- is selected as an alternative S(IV) source to establish an enduring S(IV)-based AOP with Co(II). Herein, the Co(II)/CaSO3 process triggers a much better ofloxacin (OFL) degradation than the Co(II)/Na2SO3 process (degradation rate constant: 12.1 > 3.18 mM-1 min-1). The mechanism investigation corroborates that the Co(II) mediated CaSO3 activation follows a Fenton-like process (complexation followed by intramolecular electron transfer). Apart from the conventional sulfate radical (SO4•-), Co(IV) species and singlet oxygen (1O2) are also certifiably involved in Co(II)/CaSO3 process, and their role and formation mechanisms are elucidated comprehensively. Further, the proposed Co(II)/CaSO3 process exhibits an excellent tolerance to complex water matrices (e.g., background ions and humic acid), suggesting its practical application potential for various contaminants abatement in actual wastewater.

Singlet oxygen-mediated fluconazole degradation during the activation of chlorine dioxide with sulfite

Singlet oxygen (1O2)-mediated advanced oxidations have received considerable attention due to their strong capacity to resist the water matrix and high selectivity for organic pollutants. In this study, the activation of chlorine dioxide with sulfite (sulfite/ClO2 process) to effectively produce 1O2 was proposed to degrade fluconazole (FLC) and simultaneously control the formation of disinfection byproducts (DBPs). The results revealed that FLC could be rapidly degraded by 78.6 % within 10 s by the sulfite/ClO2 process. Radical quenching tests and electron paramagnetic resonance (EPR) measurements confirm that 1O2 produced by the cleavage of epoxides formed by the combination of triazole electron-rich groups in FLC with peroxymonosulfate (PMS) was the main active species in the sulfite/ClO2 process. The degradation of FLC was favored under alkaline conditions because of the fast electron transfer rate at higher pH values. The presence of chloride (Cl-), bicarbonate (HCO3-), and humic acid (HA) hindered the degradation of FLC mainly because they compete with PMS for the electron-rich groups produced by the reaction. The degradation intermediates of FLC were identified by UPLC‒MS/MS, and their transformation pathways were deduced by the condensed Fukui function (CFF) theory. Using sulfite/ClO2 as a pretreatment process to treat real potable water, aldehydes, ketones, carboxylic acids and other intermediates may be produced via the carboxylation and carbonylation reactions mediated by 1O2, therefore promoting the formation of DBPs during the following chlorination. This study provided a new perspective that while 1O2 is effectively produced in the sulfite/ClO2 process for contaminant degradation, the formation of DBPs during subsequent chlorination should be cautioned.

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

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

Site Engineering of Covalent Organic Frameworks for Regulating Peroxymonosulfate Activation to Generate Singlet Oxygen with 100 % Selectivity

Singlet oxygen (1 O2 ) is an excellent reactive oxygen species (ROSs) for the selective conversion of organic matter, especially in advanced oxidation processes (AOPs). However, due to the huge dilemma in synthesizing single-site type catalysts, the control and regulation of 1 O2 generation in AOPs is still challenging and the underlying mechanism remains largely obscure. Here, taking advantage of the well-defined and flexibly tunable sites of covalent organic frameworks (COFs), we report the first achievement in precisely regulating ROSs generation in peroxymonosulfate (PMS)-based AOPs by site engineering of COFs. Remarkably, COFs with bipyridine units (BPY-COFs) facilitate PMS activation via a nonradical pathway with 100 % 1 O2 , whereas biphenyl-based COFs (BPD-COFs) with almost identical structures activate PMS to produce radicals (⋅OH and SO4 .- ). The BPY-COFs/PMS system delivers boosted performance for selective degradation of target pollutants from water, which is ca. 9.4 times that of its BPD-COFs counterpart, surpassing most reported PMS-based AOPs systems. Mechanism analysis indicated that highly electronegative pyridine-N atoms on BPY-COFs provide extra sites to adsorb the terminal H atoms of PMS, resulting in simultaneous adsorption of O and H atoms of PMS on one pyridine ring, which facilitates the cleavage of its S-O bond to generate 1 O2 .

Activation of peroxymonosulfate by Fe-Mn-modified MWCNTs for selective decontamination: Formation of high-valent metal-oxo species and superoxide anion radicals

The extensive presence of organic micropollutants in complex water matrices requires the development of selective oxidation technologies. In this study, a novel selective oxidation process was developed via the conjunction of FeMn/CNTs with peroxymonosulfate and successfully used to remove micropollutants such as sulfamethoxazole (SMX) and bisphenol A from aqueous solutions. FeMn/CNTs were prepared using a facile co-precipitation method, characterized using a series of surface characterization techniques, and then tested for pollutant removal. The results showed that the FeMn/CNTs had much greater reactivity than CNTs, manganese oxide, and iron oxide. The pseudo-first-order rate constant with FeMn/CNTs was more than 2.9-5.7 times that of the other tested materials. The FeMn/CNTs had great reactivity in a wide range of pH values from 3.0 to 9.0, with the best reactivity found at pH values of 5.0 and 7.0. High-valent metal-oxo species such as Fe(IV)O and Mn(IV)O and superoxide anion radicals were determined to be the reactive species and were responsible for the oxidation of SMX. These reactive species were selective; therefore, the overall removal performance of SMX was not obviously influenced by high levels of water components including chloride ions, bicarbonates, and natural organic matters. The results from this study may promote the design and application of selective oxidation technologies for micropollutant abatement.

Predicting chlorine demand by peracetic acid in drinking water treatment

Peracetic acid (PAA) may be used in drinking water treatment for pre-oxidation and mussel control at the intake. PAA may exert a downstream chlorine demand, but full details of this reaction have not been reported. There are three possible mechanisms of this demand: (1) PAA may react directly with chlorine; (2) PAA exists in equilibrium with hydrogen peroxide, which is known to react with chlorine; and (3) as H2O2 reacts with chlorine, PAA will hydrolyze to form more H2O2 to re-establish PAA/H2O2 equilibrium, thereby serving as an indirect reservoir of chlorine demand. While the H2O2 reaction with chlorine is well known, the other mechanisms of possible PAA-induced chlorine demand have not previously been investigated. The observed molar stoichiometric ratio of PAA to free chlorine (n) for the presumed direct PAA + free chlorine reaction was determined to be approximately 2, and the corresponding observed reaction rate coefficients at pH 6, 7, 8, and 9 were 2.76, 3.14, 1.61, 10.1 M-n·s-1, respectively (at 25 °C). With these estimated values, a kinetic model was built to predict the chlorine demand by PAA. The results suggest that chlorine demand from PAA is likely to be negligible over the course of several days (e.g., < 20% chlorine loss) for most conditions except for high pH (e.g., >8) and high PAA:Cl2 molar ratios (e.g., >2:1).

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Do We Appropriately Detect and Understand Singlet Oxygen Possibly Generated in Advanced Oxidation Processes by Electron Paramagnetic Resonance Spectroscopy?

Electron paramagnetic resonance (EPR) spectroscopy using sterically hindered amine is extensively applied to detect singlet oxygen (¹O₂) possibly generated in advanced oxidation processes. However, EPR-detectable ¹O₂ signals were observed in not only the ¹O₂-dominated hydrogen peroxide (H₂O₂)/hypochlorite (NaClO) reaction but surprisingly also the ¹O₂-absent Fe(II)/H₂O₂, UV/H₂O₂, and ferrate [Fe(VI)] process with even stronger intensities. By taking advantage of the characteristic reaction between ¹O₂ and 9,10-diphenyl-anthracene and near-infrared phosphorescent emission of ¹O₂, ¹O₂ was excluded in the Fe(II)/H₂O₂, UV/H₂O₂, and Fe(VI) process. The false detection of ¹O₂ was ascribed to the direct oxidation of hindered amine to piperidyl radical by reactive species [e.g., ^•OH and Fe(VI)/Fe(V)/Fe(IV)] via hydrogen transfer, followed by molecular oxygen addition (forming a piperidylperoxyl radical) and back reaction with piperidyl radical to generate a nitroxide radical, as evidenced by the successful identification of a piperidyl radical intermediate at 100 K and theoretical calculations. Moreover, compared to the highly oxidative species (e.g., ^•OH and high-valence Fe), the much lower reactivity of ¹O₂ and the profound nonradiative relaxation of ¹O₂ in H₂O resulted it too selective and inefficient in organic contaminant destruction. This study demonstrated that EPR-based ¹O₂ detection could be remarkably misled by common oxidative species and thereby jeopardize the understandings on ¹O₂.