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The ubiquity of refractory organic matter in aquatic environments necessitates innovative removal strategies. Sulfate radical-based advanced oxidation has emerged as an attractive solution, offering high selectivity, enduring efficacy, and anti-interference ability. Among many technologies, sulfite activation, leveraging its cost-effectiveness and lower toxicity compared to conventional persulfates, stands out. Yet, the activation process often relies on transition metals, suffering from low atom utilization. Here we introduce a series of single-atom catalysts (SACs) employing transition metals on g-C3N4 substrates, effectively activating sulfite for acetaminophen degradation. We highlight the superior performance of Fe/CN, which demonstrates a degradation rate constant significantly surpassing those of Ni/CN and Cu/CN. Our investigation into the electronic and spin polarization characteristics of these catalysts reveals their critical role in catalytic efficiency, with oxysulfur radical-mediated reactions predominating. Notably, under visible light, the catalytic activity is enhanced, attributed to an increased generation of oxysulfur radicals and a strengthened electron donation-back donation dynamic. The proximity of Fe/CN's d-band center to the Fermi level, alongside its high spin polarization, is shown to improve sulfite adsorption and reduce the HOMO-LUMO gap, thereby accelerating photo-assisted sulfite activation. This work advances the understanding of SACs in environmental applications and lays the groundwork for future water treatment technologies.

Sulfate radical (SO4•−)–based advanced oxidation processes (AOPs) from sulfite activation have recently received attention for abatement of microorganic pollutants in the aquatic environments. Trace-level Co(II) has been demonstrated to be effective for promoting sulfite activation (simplified as the Co(II)/sulfite system) and the corresponding radical formation, yet this process is challenged by the limited valence inter-transformation of Co(II)/Co(III). In order to enhance this valence inter-transformation, a novel Co(II)/HPO42−/sulfite system is developed in this work, because HPO42−, as a typical radical scavenging agent, has the advantage of complexing with Co(II) without quenching effect. In this work, complexation of Co(II) with HPO42− can regulate the electronic structure of Co(II), accelerate electron transfer, and promote valence inter-transformation of Co(II)/Co(III) during the sulfite activation process. The Co(II)/HPO42−/sulfite system exhibits superior iohexol abatement performance under circumneutral conditions. For pH 8.0 and Co(II) dose of 1 μM, the iohexol abatement efficiency is as high as 98%, which is considerably higher than that of the Co(II)/sulfite system (50%). SO4•− is identified as the predominant reactive radical contributing to iohexol abatement. The presence of HPO42− broadens the pH adaptability of the Co(II)/sulfite system for iohexol abatement. In addition, the coexisting Cl− exerts an inhibitory effect on iohexol abatement while the other cations and anions show negligible effect. The Co(II)/HPO42−/sulfite system displays good reusability and adaptability towards various organic pollutants. This study highlights the important role of complexation of Co(II) with HPO42− in sulfite activation and provides a feasible idea for abatement of the microorganic pollutants. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sulfite-based catalyst/photoenergy-free process facilitated thiazine dye destruction. • Electron donating capability of sulfite promoted reaction kinetics via bimolecular collisions. • Sulfite mediated oxysulfur radicals and reductive spicies possed excellent activity and stability. • The coupling photocatalysis enhanced pollutants mineralization. Sulfite-based advanced reduction processes have been extensively studied as a purposeful option of wastewater treatment, while their direct chain reaction mechanisms involved reductive destruction of textile dye are still unclear. Here, we reported that sulfite source can selectively destructed cationic dyes of thiazines without photoenergy and catalysts activation in neutral pH. The series of electrochemical experiments (e.g., galvanic oxidation, chronoamperometry measurements and cyclic voltammograms) showed that sulfite attacks phenazine dye of methylene blue via electrons transfer to form the primary electron-donor radical cation species of sulfite radical, followed by the auto-catalytic decontamination concurrently with hydrated electrons and oxysulfur species from radical chain reactions under aerobic conditions. Based on radical quenching and electron paramagnetic resonance, the underlying mechanism of reaction kinetics and activation pathway can be expanded to other common thiazine dyes (Azure II, Azure A and thionine) and even capable for removing co-existing azo dye of methyl orange and their raw production material of aniline in textile effluents. The selective bimolecular collisions system between thiazine and sulfite triggered the spontaneous radical formation without clear deactivation and water matrix affect, which is furthered to provide new ideas for hybrid treatment construction with respect to sustainable management of textile dye wastewater at industrial applications prospect. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel oxidative degradation process, FeO x @C/sulfite/UVA, was developed. • Synergy of photo-generated holes and Fe active sites promotes sulfite activation. • Radicals and non-radicals are majorly derived from the crucial SO 5 −. • Both Fe(IV) and 1O 2 are the dominant non-radical species to OFL degradation. • Procedure to realize the remediation of antibiotic-spiked water is proposed. Advanced oxidation processes (AOPs) based on sulfite (S(IV)) activation are gaining recognition as a promising Fenton-like strategy, primarily due to their ability to generate potent radicals (e.g., SO 4 − and HO). However, the existence and role of a non-radical-driven oxidation pathway in the S(IV)-mediated degradation of contaminants remain under debate. Herein, a magnetic iron-carbon composite (FeO x @C) was fabricated using the MOF-templated method and utilized as a stable photocatalyst to activate S(IV) under UVA light, achieving >93.6 % degradation of 20 μM ofloxacin (OFL) with a high degradation rate constant of 0.073 min−1 within 45 min. The catalyst's porous carbon structure facilitated the transport of substrates to active sites and shortened the diffusion distance between reactive species and contaminants. Adsorbed S(IV) donated one electron to photo-generated holes and exposed Fe(III) sites with the formation of SO 5 −, which then followed two distinct pathways to produce radicals (SO 4 −) and non-radicals (Fe(IV) and 1O 2) responsible for OFL degradation. By leveraging the synergy of both radicals and non-radicals, this innovative oxidative system showcased robust anti-interference capacities and exceptional performance in degrading contaminants from complex water matrices. This study provides new insights into the significant role of non-radical species in S(IV)-mediated decontamination processes. [ABSTRACT FROM AUTHOR]

Iron-based sulfite (S(IV)) activation has emerged as a novel strategy to generate sulfate radicals (SO 4 •-) for contaminants degradation. However, numerous studies focused on dissolved iron-induced homogeneous activation processes while the potential of structural Fe(III) remains unclear. In this study, five iron (hydr)oxide soil minerals (FeO x) including ferrihydrite, schwertmannite, lepidocrocite, goethite and hematite, were successfully employed as sources of structural Fe(III) for S(IV) activation. Results showed that the catalytical ability of structural Fe(III) primarily depended on the crystallinity of FeO x instead of their specific surface area and particle size, with ferrihydrite and schwertmannite being the most active. Furthermore, in-situ ATR-FTIR spectroscopy and 2D-COS analysis revealed that HSO 3 - was initially adsorbed on FeO 6 octahedrons of FeO x via monodentate inner-sphere complexation, ultimately oxidized into SO 4 2- which was then re-adsorbed via outer-sphere complexation. During this process, strong oxidizing SO 4 •- and •OH were formed for pollutants degradation, confirmed by radical quenching experiments and electron spin resonance. Moreover, FeO x /S(IV) system exhibited superior applicability with respect to recycling test, real waters and twenty-six pollutants degradation. Eventually, plausible degradation pathways of three typical pollutants were proposed. This study highlights the feasibility of structural Fe(III)-containing soil minerals for S(IV) activation in wastewater treatment. [Display omitted] • Structural Fe(III) of five iron (hydr)oxides can activate S(IV) to generate ROS. • Ferrihydrite and schwertmannite presented the most active performance. • Catalytical ability of structural Fe(III) depended on the crystallinity of FeO x. • In-situ ATR-FTIR spectroscopy and 2D-COS analysis unveiled possible mechanisms. • The FeO x /S(IV) system exhibited superior applicability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co 3 FeW-LDHs with interconnected layered ultra-thin nanosheets were prepared. • The pollutant was efficiently removed by activation of sulfite by Co 3 FeW-LDHs. • Synergistic interaction between Co, Fe and W promotes CaSO 3 activation. • 1O 2 and SO 4 •− were predominately responsible for MTZ degradation. • MTZ sites with high Fukui index exhibited high reactivity. Tungsten-doped iron-cobalt layered bimetallic hydroxides (CoFeW-LDHs) were used for activating sulfite to degrade metronidazole (MTZ) in this study. The characterization results confirmed the presence of vertically aligned hierarchical ultrathin nanosheets forming a porous network, which enhances the material's ability to activate CaSO 3. The study investigated the optimal Co/Fe/W atomic ratio in the catalysts and examined the impact of various parameters, such as catalyst dosage, CaSO 3 concentration, initial pH, and MTZ concentration. Under the best experimental conditions identified, the degradation rate of MTZ reached 95% within 60 min. Mechanism analysis revealed that W doping promoted oxidation–reduction reactions between Co(Ⅱ)/Co(Ⅲ) and Fe(Ⅲ)/Fe(Ⅱ) on the catalyst surface, facilitating CaSO 3 activation and the continuous generation of reactive oxygen species (ROS) like 1O 2 and SO 4 •−. Based on experimental, density functional theory (DFT), and quantitative structure–activity relationship analyses, the study proposed a degradation pathway and evaluated the ecotoxicity of MTZ and its intermediates. MTZ was found to be converted via three pathways into low or non-toxic intermediates, ultimately mineralizing into small inorganic molecules. Even after five cycles, Co 3 FeW-LDHs maintained a removal efficiency of over 90% for MTZ. The material's ease of synthesis, high activity, and stability suggest significant potential for practical application in environmental cleanup efforts. [ABSTRACT FROM AUTHOR]

• A novel Fe/Mn@CF composite electrode is fabricated for the activation of sulfite. • Fe/Mn@CF-sulfite system exhibits satisfied degradation activity and stability. • Both free radical and non-free radical pathways boost pollutant degradation. • The presence of redox cycles of Fe and Mn promotes the activation of sulfite. • The synergistic interactions of electric field and metals lead to efficient sulfite activation. Sulfite has been proven to promote advanced oxidation processes (AOPs) to achieve low-cost and efficient degradation of contaminants, serving as an alternative to persulfate. In this study, a novel composite Fe/Mn@carbon felt (CF) electrode was successfully constructed to remove carbamazepine (CBZ). The capacity of the Fe/Mn@CF anode to activate sulfite was evaluated under various electrode preparation conditions and operating parameters. Results clearly indicated that the complete removal of CBZ was achieved in Fe/Mn@CF anode activated sulfite system under the specific conditions including Na 2 SO 4 electrolyte concentration of 50 mM, sulfite dosage of 1 mM, initial pH of 7, and current density of 1 mA/cm2. The capacity of Fe/Mn@CF anode for sulfite activation was further presented by its excellent reusability and stability in terms of CBZ removal and metal ion release. Both radical and non-radical pathways were significantly contributed to the CBZ removal, including OH, SO 4 · - , O 2 · - , 1O 2 , and other reasons. Density functional theory (DFT) calculations demonstrated that Fe-Mn bimetals effectively promoted sulfite activation, with the electric field further boosting the process of Fe-Mn valence cycle and oxidation. Overall, the novel Fe/Mn@CF activated sulfite system exhibited outstanding performance and had great potential for sustainable use in removing organics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oxygen vacancy (Vo)-enriched γ-Fe 2 O 3 is prepared via high temperature calcination. • Vo-enriched γ-Fe 2 O 3 can activate sulfite efficiently for iohexol degradation. • SO 4 •−and HO• are mainly involved in the degradation of iohexol. • Vo improves the electron transfer and the >Fe(II)/>Fe(III) valence transformation. Sulfite activation with iron-based heterogeneous activators is a promising technology for the removal of organic contaminants from water. However, the slow valence transformation of >Fe(II)/>Fe(III) compromises sulfite activation efficiency. Regarding that the oxygen vacancy (Vo) can modulate the charge distribution and thus enhance >Fe(II)/>Fe(III) valence transformation via rapid electron transfer, Vo-enriched γ-Fe 2 O 3 is constructed for sulfite activation (simplified as the γ-Fe 2 O 3 -Vo/sulfite system). The γ-Fe 2 O 3 -Vo/sulfite system exhibits excellent abatement performance for iohexol (IOX) with the efficiency as high as 90.2 % within 1 h of reaction at pH 8.5, in which sulfate radicals (SO 4 •−) and hydroxyl radicals (HO•) are the major species. Electrochemical and DFT calculations reveal that Vo-enriched γ-Fe 2 O 3 possesses lower electrical impedance and the concomitant higher electron transfer efficiency. According to a series of characterizations, the enhance >Fe(II)/>Fe(III) valence transformation is confirmed to contribute to sulfite activation via the regulation of the electronic structure by the Vo. In addition, the degradation pathways of IOX are proposed by analyzing the degradation products of IOX by UPLC-QTOF-MS. Finally, the effects of different influencing factors on the degradation of IOX and the cyclic performance suggest the system shows strong resistance to interference and reusability, exhibiting superior application potential. [ABSTRACT FROM AUTHOR]

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 (1O 2) 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 HSO 5 −, and 1O 2 predominantly originates from the intermediate SO 4 •−. Profiting from the highly selective oxidation capacities of Co(IV) and 1O 2 , 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. [Display omitted] ● Co-based MOF (CZS) is prepared via hydrothermal self-assembled coordination. ● CZS is of superior stability and activity in the sulfite mediated SMX degradation. ● CZS driven sulfite activation for SMX degradation follows a non-radical mechanism. ● Both Co(IV) and 1O 2 are the dominant reactive species to SMX degradation. ● CZS/sulfite system is of great interference resistance to complex water matrices. [ABSTRACT FROM AUTHOR]

In this study, a amorphous Fe 1-x Co x S were successfully prepared by ultrasonic-assisted method and exhibited high performance in activating sulfite for RhB degradation in acidic and neutral conditions. For the heterogeneous Fe 1-x Co x S/sulfite system, on the one hand, doping Co into the crystal textures of FeS would lead to the increase of electron density, the slowing down of corrosion and the reduction of impedance, which is beneficial to accelerate the electron transfer efficiency. In addition, a non-radical activation pathway has been proposed. On the other hand, the electron-rich S(-Ⅱ) is key to promote the self-cycling of Fe3+/Fe2+, and S(-Ⅱ) reduces the accumulated ≡Fe(Ⅲ) to ≡Fe(Ⅱ). As a result, the synergistic effect of these processes is conducive to RhB removal. [Display omitted] • A novel amorphous Fe 1-x Co x S were prepared by ultrasonic-assisted method. • The Fe 1-x Co x S was first reported as heterogeneous catalyst for sulfite activation. • The doping Co into FeS accelerated electron transfer of Fe 1-x Co x S/sulfite system. • The electron-rich S(-Ⅱ) is key to promote the self-cycling of Fe3+/Fe2+. • The mechanism of sulfite activation by Fe 1-x Co x S was proposed. Sulfite activation has been considered as a promising advanced oxidation technique for treating organic pollutants in water. In this work, a series of cobalt-doped FeS (Fe 1-x Co x S) samples were synthesized via ultrasonic-assisted precipitation route and applied them in sulfite activation and Rhodamine B (RhB) removal in water at the same time, and their physical properties were characterized by XRD, SEM, TEM, XPS and electrochemical analysis. Benefiting from its high electron density, low corrosion voltage and impedance that beneficial to accelerate the electron transfer rate in the Fe 1-x Co x S/sulfite system and the reducing sulfur species rich in electrons which beneficial to promote the self-cycling of Fe3+/Fe2+, the activity of Fe 1-x Co x S for sulfite activation on the RhB removal was better than that of FeS in both acidic and neutral conditions. The redox cycle of metal ions on the surface of Fe 1-x Co x S efficiently triggered sulfite activation. The hydroxyl radical (·OH), resulting from the conversion of oxysulfur radicals, plays a crucial role in the degradation of RhB. Additionally, a non-radical activation pathway of sulfite has been proposed. The present study provided a new idea and effective strategy for sulfite activation to environmental removal of organic matter in water. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel carbon/sulfite system showed efficient removal of pharmaceuticals. • Quinone groups and conductivity were the key factors in sulfite activation. • The toxicity of pharmaceuticals could be significantly reduced. • Carbon/sulfite system exhibited effectiveness in actual wastewater treatment. • A continuous flow membrane reactor showed excellent reactivity and durability. Sulfite [S(IV)] activation processes have aroused increasing attention in the field of wastewater treatment. In this study, a novel humic acid-derived carbon (HAC-10) material was demonstrated as highly active and environmentally friendly S(IV) activator for the efficient elimination of recalcitrant pharmaceutical contaminants. The results suggested that quinone groups and conductivity served as the active site/structure for S(IV) activation, while the rapid adsorption process of contaminant significantly reduced the migration distance of generated SO 4 –, thus facilitating the degradation processes. The carbon–S(IV)* complexes were confirmed to be the primary step to produce SO 4 –. The mineralization process of tetracycline hydrochloride (TC, a model pharmaceutical) could effectively reduce its biological toxicity. In addition, S(IV)/HAC-10 system exhibited excellent adaptability to different pharmaceuticals, natural water matrix and was capable of effectively eliminating contaminants in actual wastewater. Furthermore, a continuous flow reactor consisting of HAC-10 catalyst membrane showed excellent reactivity and durability. Overall, this study provides novel insights into the activation mechanism of S(IV) by carbon materials, and advances the practical application of S(IV) activation processes in the decontamination and detoxification of pharmaceutical wastewater. [ABSTRACT FROM AUTHOR]

[Display omitted] • The piezoelectric charges are applied to activate sulfite to generate reactive specie for enhancing the metronidazole degradation, and the dominant reactive specie is sulfate radical. • The mechanism for piezoelectric sulfite activation is proposed. That is, the piezoelectric holes react with sulfite to produce sulfite radical and trigger a chain reaction. • The chain reaction of sulfite elicited by piezoelectric charges can be applied for practical water remediation. • This system can degrade metronidazole and reduce its environmental toxicity. Taking use of mechanical-to-chemical energy is a promising way to develop sustainable catalysis, but it is hindered by the low reactive oxygen species (ROS) yield induced directly by intrinsic carriers. In this paper, by coupling piezocatalysis with sulfite (S(IV)) activation via a g-C 3 N 4 /S(IV)/US ternary system, piezoelectric holes react with S(IV), triggers a chain reaction and brings about a considerable ROS generation for enhancing the degradation of organic pollutants. As a result, profiting from the crucial electron-gaining ability of holes, the k obs of metronidazole removal after adding g-C 3 N 4 increases by 2.02 times from 0.0161 min−1 to 0.0325 min−1. This enhancement occurs because the piezoelectric holes can both promote the transfer of electrons from S(IV) to g-C 3 N 4 and the generation of sulfite radical (SO 3 −). By this way, the SO 3 − rapidly reacts with the dissolved oxygen to generate sulfate radical and hydroxyl radical through a chain reaction for water remediation. This study provides a novel strategy to maximize the utilization of piezoelectric holes. [ABSTRACT FROM AUTHOR]

Copper-manganese composite metal oxides (CuMnO y) were prepared by hydrolysis-driven oxidation-reduction method and used to activate sulfite to degrade chlortetracycline hydrochloride (CTC) for the first time. The Jahn-Teller ions Mn3+ and Cu2+ exist in CuMnO y , which form a solid electric charge transport redox system and ensure the continuous generation of reactive oxygen species (ROS). Through the systematic study of the experimental parameters such as sulfite concentration, catalyst metal molar ratio, catalyst amounts and initial pH, the optimal degradation rate of CTC could reach 91.74% within 10 min and 94.46% after 30 min. The major reactive radicals were determined by radical quenching experiments and electron paramagnetic resonance (EPR) trapping techniques, and it was confirmed that SO 4 •- and •O 2 - played a nonnegligible role in the process of degrading CTC. Density functional theory (DFT) calculations show that higher Fukui indices (f - and f 0 ) of CTC sites are more vulnerable to free radical attack. CuMnO y has low CTC degradation intermediate toxicity, high catalytic performance, good anti-interference ability, reusability and stability, and possesses decent application potential in the actual water treatment field. [Display omitted] • Jahn-Teller-driven oxygen vacancies Cu-Mn composite oxide was prepared. • Much high CTC degradation efficiency was achieved using CuMnO y. • Jahn-Teller ions Mn3+ and Cu2+ in CuMnO y formed a solid-state charge transfer redox system. • The redox cycles of Mn, Cu and synergistic interactions led to efficient S(IV) activation. • CTC sites with high Fukui index exhibited high reactivity. [ABSTRACT FROM AUTHOR]

The ubiquity of refractory organic matter in aquatic environments necessitates innovative removal strategies. Sulfate radical-based advanced oxidation has emerged as an attractive solution, offering high selectivity, enduring efficacy, and anti-interference ability. Among many technologies, sulfite activation, leveraging its cost-effectiveness and lower toxicity compared to conventional persulfates, stands out. Yet, the activation process often relies on transition metals, suffering from low atom utilization. Here we introduce a series of single-atom catalysts (SACs) employing transition metals on g-C 3 N 4 substrates, effectively activating sulfite for acetaminophen degradation. We highlight the superior performance of Fe/CN, which demonstrates a degradation rate constant significantly surpassing those of Ni/CN and Cu/CN. Our investigation into the electronic and spin polarization characteristics of these catalysts reveals their critical role in catalytic efficiency, with oxysulfur radical-mediated reactions predominating. Notably, under visible light, the catalytic activity is enhanced, attributed to an increased generation of oxysulfur radicals and a strengthened electron donation-back donation dynamic. The proximity of Fe/CN's d-band center to the Fermi level, alongside its high spin polarization, is shown to improve sulfite adsorption and reduce the HOMO-LUMO gap, thereby accelerating photo-assisted sulfite activation. This work advances the understanding of SACs in environmental applications and lays the groundwork for future water treatment technologies., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Authors.)