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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]

Sulfite (S(IV)) is a promising substitute for sulfate radical-based advanced oxidation processes. Here, a composite of in-situ anchoring NiCo 2 O 4 nanosheets on biochar (BC) was firstly employed as a heterogeneous activator for sulfite (NiCo 2 O 4 @BC-sulfite) to degrade atrazine (ATZ) in the neutral environment. The synergistic coupling of BC and NiCo 2 O 4 endows the resulting composite excellent catalytic activity. 82% of the degradation ratio of ATZ (1 mg/L) could be achieved within 10 min at initial concentrations of 0.6 g/L NiCo 2 O 4 @BC, 3.0 mmol/L sulfite in neutral environment. When further supplementing sulfite into the system at 20 min (considering the depletion of sulfite), outstanding degradation efficiency (∼ 100%) were achieved in the next 10 min without any other energy input by the NiCo 2 O 4 @BC-sulfite system. The features of the prepared catalysts and the effects of some key parameters on ATZ degradation were systematically examined. A strong inner-sphere complexation (Co2+/Ni2+-SO 3 2−) was explored between sulfite and the metal sites on the NiCo 2 O 4 @BC surface. The redox cycle of the surface metal efficiently mediated sulfite activation and triggered the series radical chain reactions. The generated radicals, in particular the surface-bound radicals were involved in ATZ degradation. High performance liquid chromatography-tandem mass spectrometry (LC-MS) technique was used to detect the degradation intermediates. Density functional theory (DFT) calculations were performed to illustrate the possible degradation pathways of ATZ. Finally, an underlying mechanism for ATZ removal was proposed. The present study offered a low-cost and sustainable catalyst for sulfite activation to remove ATZ in an environmentally friendly manner from wastewater. [Display omitted] [ABSTRACT FROM AUTHOR]

Sulfite (SO 3 2−) activation is one of the most potential sulfate-radical-based advanced oxidation processes, and the catalysts with high efficiency and low-cost are greatly desired. In this study, the cobalt nanoparticles embedded in nitrogen-doped graphite layers (Co@NC), were used to activate SO 3 2− for removal of Methyl Orange in aqueous solution. The Co@NC catalysts were synthesized via pyrolysis of Co2+-based metal-organic framework (Co-MOF), where CoO was firstly formed at 400℃ and then partially reduced to Co nanoparticles embedded in carbon layers at 800℃. The Co@NC catalysts were more active than other cobalt-based catalysts such as Co2+, Co 3 O 4 and CoFe 2 O 4 , due to the synergistic effect of metallic Co and Co x O y. A series of chain reaction between Co species and dissolved oxygen was established, with the production and transformation of SO 3 •−, SO 5 2−, and subsequent active radicals SO 4 •− and HO•. In addition, HCO 3 − was found to play a key role in the reaction by complexing with Co species on the surface of the catalysts. The results provide a new promising strategy by using the Co@NC catalyst for SO 3 2− oxidation to promote organic pollutants degradation. [Display omitted] [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]

[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]

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.)

The easily synthesized, cost-effective, and stable photocatalysts for sulfite activation are always required for the enhancement of organic contaminants degradation. Herein, the facile coprecipitation synthesis of Bismuth oxybromide (BiOBr)/Montmorillonite (MMT) was reported, which could activate sulfite (SO 3 2−/HSO 3 −) under sunlight and accelerate the catalytic performance more effectively than pristine BiOBr. After adding sulfite to the photocatalysis system, the photodegradation efficiency of atrazine (ATZ) achieved 73.7% ± 1.5% after 5 min and 94.4% ± 1.6% after 30 min of sunlight irradiation with BiOBr/MMT. The BiOBr/MMT-sulfite system also presented remarkable photocatalytic performance to eliminate various contaminants, including ciprofloxacin, sulfadiazine, tetracycline, and carbamazepine. The various features of the photocatalyst materials were studied, including their surface morphology, structure, optical properties, and composition. The results illustrated that by adding MMT, the bandgap of the pristine BiOBr was reduced and the surface area was increased, which led to an increased ability to adsorb materials. Results of various influence factors showed this enhanced system had satisfactory and stable removal performance of ATZ in the pH range of 3.0–6.5, but HPO 4 2− had a strong negative effect on the system performance. Oxysulfur radicals (SO 5 ·- and SO 4 ·-), h+, and 1O 2 were discovered as the prevailing active species in the BiOBr/MMT-sulfite system. The proposed degradation mechanism of this photocatalyst-enhanced system revealed that sulfite adsorption on the surface of the photocatalyst played a vital role during the initial phase, and the degradation pathway of ATZ was discussed. This study provides a new synthesis strategy of a photocatalyst for sulfite activation and expands the potential uses of Bi-based photocatalysts in degrading difficult-to-remove organic pollutants. [Display omitted] • The BiOBr/MMT was successfully synthesized and activated sulfite. • BiOBr/MMT-HSO 3 - system exhibited satisfactory removal efficiency of various organics at pH of 3.0–6.5. • The key role of sulfite ion adsorption during the catalytic process was confirmed. • SO 4 ·-, h+, SO 5 ·-, and 1O 2 played a predominant role in the degradation of atrazine. [ABSTRACT FROM AUTHOR]

Sulfite (S(IV))-based advanced oxidation process has attracted widespread attention due to the generation of strongly oxidizing sulfate radicals (SO 4 •−) and hydroxyl radicals (HO•) for contaminants remediation. However, the contaminant degradation by singlet oxygen (1O 2) through a nonradical pathway was still unclear in the S(IV) activation system. Herein, a magnetic Fe 3 O 4 /g-C 3 N 4 (MCN) composite was synthesized for the utilization as a visible-light catalyst to activate S(IV) under visible-LED (Vis-LED) for the organic degradation. The incorporation of Fe 3 O 4 in g-C 3 N 4 up-regulated the photocatalytic performance in the S(IV) activation for X-3B degradation, and > 98% of X-3B (20 mg L−1) was degraded with the degradation rate constant (k obs) of 0.110 min−1 within 30 min. The SO 4 •− and 1O 2 produced in the MCN/S(IV)/Vis-LED system were identified as the primary reactive species through the quenching experiments and electron spin resonance. Interestingly, the light-induced generation of superoxide radical (O 2 •−) played a negligible role in the formation of 1O 2 , and most of 1O 2 was corroborated to be originated from SO 4 •− besides SO 5 •−, which was rarely reported in other S(IV) activation processes. The catalysts before and after utilization were characterized to further elucidate the mechanisms for the S(IV) activation. There were two possible pathways for the S(IV) activation: the electron transfer from S(IV) to the photo-generated holes and the ligand-to-metal charge-transfer (LMCT) within the surface Fe(III)−S(IV) complexes. Furthermore, the MCN/S(IV)/Vis-LED system was of high resistance to complex water matrixes (pH, inorganic anions, etc.), demonstrating its application perspective in the purification of wastewater containing organic pollutants. [Display omitted] • A novel sulfite-based AOP for organic degradation, MCN/S(IV)/Vis-LED, is developed. • Organic degradation relies on the SO 4 •− and 1O 2 generated via S(IV) activation. • Besides SO 5 •−, most of 1O 2 is originated from SO 4 •− instead of O 2 •−. • S(IV) activation is through electron transfer and ligand-to-metal charge-transfer. • Procedure to realize the purification of organic-contaminated water is proposed. [ABSTRACT FROM AUTHOR]

As sulfite has been employed currently in the catalytic degradation of various pollutants, the construction of more suitable and cheaper catalytic systems is required. Herein, a heterogeneous system for S(IV) activation by iron(III) titanium oxide (Fe 2 TiO 5) was proposed to proceed the removal of acid orange 7 (AO7) and seven typical pharmaceutical and personal care products (PPCPs). Typically, the AO7 (10 mg L−1) can be degraded up to 67% within 60 min ([Fe 2 TiO 5 ] = 0.8 g L−1, [S(IV)] = 2.0 mM, pH = 6.0). The effects of initial AO7 concentration, pH, Fe 2 TiO 5 dosage, S(IV) concentration, and co-existing anions (HCO 3 − and H 2 PO 4 −) were examined to explore the application conditions and resistance of Fe 2 TiO 5 −S(IV) system. Moreover, the stability of Fe 2 TiO 5 −S(IV) system was tested by sequential runs, while XRD was also performed for the robustness evaluation of catalyst. The scavenging experiment showed that the active species in the system mainly include SO 5 •−, HO•, and SO 4 •−, while SO 4 •− being the dominant one. Through the competitive experiments of EDTA, the density functional theory (DFT) computation and the XPS analysis, a sensible mechanism was proposed that the electron transfer within Fe(III)−SO 3 2− complex and the Fe(III)−Fe(II)−Fe(III) redox process were crucial for the sulfite activation. Meanwhile, a quantitative structural-activity relationship (QSAR) model for pseudo-first-order rate constants (k obs) was established with a high significance (R2 = 0.991, p = 0.006) by using four descriptors. This work provides an innovative strategy of sulfate radical-based advanced oxidation processes (SR−AOPs) for organic contaminants degradation using metal oxide catalysts. • Fe 2 TiO 5 –sulfite system is proposed as a novel sulfate-radical-based AOP. • Near-neutral pH favors sulfite activation by Fe 2 TiO 5 to produce oxysulfur radicals. • SO 4 •−, HO•, and SO 5 •− are main oxidative species for organic contaminants oxidation. • QSAR in degradation of seven PPCPs in Fe 2 TiO 5 –sulfite system is established. [ABSTRACT FROM AUTHOR]

Sulfate radical-based advanced oxidation process could efficiently remove many emerging contaminants from wastewater. Considering the unique properties of MnNb 2 O 6 , an efficient MnNb 2 O 6 /sulfite system was developed to remove doxycycline residue from the aqueous environment. In this research, MnNb 2 O 6 nanomaterial was successfully synthesized through a one-step hydrothermal route at 260 °C for 24 h. The paramagnetic behavior of MnNb 2 O 6 was observed at 300 K. Scanning electron microscopy and transmission electron microscopy images showed that the size of petal-like MnNb 2 O 6 crystals was 250–700 nm. The specific surface area of MnNb 2 O 6 was 24.84 m2/g. Optimization of MnNb 2 O 6 /sulfite process was achieved by Box-Behnken Design based on response surface methodology. Under the predicted optimal conditions (pH of 8.7, MnNb 2 O 6 dosage of 0.29 g/L, doxycycline concentration of 10.8 mg/L, and sodium sulfite concentration of 4.3 mM), the degradation efficiency of doxycycline could reach 85.1%. Mn species was found to be the dominant factor in MnNb 2 O 6 /sulfite system. Both SO 4 •− and •OH were identified as primary active radicals in MnNb 2 O 6 /sulfite system. Furthermore, six transformation products of doxycycline were elucidated. This work offers a simple water treatment process for efficiently removing doxycycline residue in the environment. [Display omitted] • MnNb 2 O 6 /sulfite system was developed for the removal of doxycycline. • The degradation process of doxycycline was optimized by RSM. • Mn species was the dominant factor. • SO 4 •− and •OH were identified as primary active radicals. • Six transformation products of doxycycline were provided. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • FeIII N-TiO 2 (FeN-TiO 2) was synthesized using an impregnation method. • FeN-TiO 2 showed a dual functionality for sulfite activation under visible LED. • The FeN-TiO 2 /sulfite/visible LED process was applied for diphenamid degradation. • A new degradation route of diphenamid was reported in this study. • A higher mineralization degree was achieved compared to previous sulfite studies. Abstract Degradation of diphenamid (DPA) was examined by a novel process through sulfite activation by Fe impregnated N-doped TiO 2 (FeN-TiO 2) under visible LED (Vis LED). The FeN-TiO 2 was synthesized using an impregnation method and characterized by various techniques. The mechanism of sulfite activation by FeN-TiO 2 under Vis LED was proposed. The reaction mechanisms were further elucidated by investigating the XPS spectra of the catalysts before and after the reaction. Thirty intermediates were identified and twenty-four of them are newly reported. A new pathway was reported for the first time in the DPA studies through the rupture of benzene ring linkage. A higher mineralization degree was achieved using the FeN-TiO 2 /sulfite/Vis LED process, which is not in accordance with previous reports on sulfite-based processes. The absence of sulfate adducts could provide a rational explanation of the higher mineralization degree during DPA degradation. Based on reusability test, the DPA degradation efficiency increased after successive usage of the catalyst. After the complete degradation of DPA, the leached Fe-ions were found to be negligible and sulfite was completely depleted. Considering several factors such as the cheap source of sulfite (an air pollutant waste from flue-gas desulfurization process), low cost of Fe, negligible leaching of Fe-ions, and high energy efficiency of Vis LED light, the FeN-TiO 2 /sulfite/Vis LED process could be a practical and green technology for the removal of wastewater contaminants. [ABSTRACT FROM AUTHOR]

Graphical abstract Efficient conversion of ammonium to dinitrogen was achieved by oxysulfur radicals oxidation that generated by In situ PEC activation of sulfite. Highlights • Oxysulfur radicals based AOPs for ammonia conversion was developed. • Oxysulfur radicals were produced by photoelectrochemical oxidation of sulfite on MoS 2. • Selective and efficient conversion of ammonia to dinitrogen was exclusively achieved. • Oxysulfur radicals are more powerful than OH for ammonia oxidation. • Sulfite serves as both hole scavenger and precursor of oxysulfur radicals. Abstract The advanced oxidation processes (AOPs) based on oxysulfur radicals (SO 3 −, SO 4 − and SO 5 −) has been receiving growing attention in wastewater treatment. In this study, we report the in situ photoelectrochemical activation of sulfite to produce oxysulfur radicals with MoS 2 nanosheets as a wide spectrum absorptive photoanode. At alkaline condition, the selective and efficient conversion of ammonia to dinitrogen was exclusively achieved in the presence of sulfite electrolyte under visible light irradiation. The sulfite plays multiple roles such as working as hole scavenger for improving stability of MoS 2 electrode by inhibiting photo-corrosion and serving as precursor of oxysulfur radicals in the meantime. The influences of radical scavenger, dissolved oxygen and electrolyte on the photoelectrochemical, electrochemical and photochemical conversion of ammonia verified that oxysulfur radicals are more powerful than hydroxyl radicals in terms of ammonia conversion. The proposed system appears to be applicable to in situ treatment of wastewater containing of ammonia and sulfite pollutants, such as wastewater from ammonia-absorption-desulfurization of combustion smoke. This work also provides a new protocol in the design of new AOPs, where oxysulfur radicals can work together with hydroxyl radicals for simultaneous pollutants degradation and detoxification. [ABSTRACT FROM AUTHOR]

Phenols are infamous pollutants in marine environments and present a grave danger to human health, which makes their efficient detection and removal serious issues. Colorimetry is a simple method for detecting phenols in water because phenols can be oxidized by natural laccase and generate a brown product. However, high cost and poor stability impede the wide-spread implementation of natural laccase in phenol detection. To reverse this adverse situation, a nanoscale Cu-S cluster, Cu 4 (MPPM) 4 (Cu 4 S 4 , MPPM = 2-mercapto-5-n-propylpyrimidine), is synthesized. As a stable and inexpensive nanozyme, Cu 4 S 4 shows excellent laccase-mimicking activity and prompts the oxidation of phenols. This characteristic makes Cu 4 S 4 a perfect option for phenol detection with colorimetry. In addition, Cu 4 S 4 also exhibits sulfite activation properties. It can degrade phenols and other pollutants with advanced oxidation processes (AOPs). Theoretical calculations show good laccase-mimicking and sulfite activation properties originating from appropriate interactions between Cu 4 S 4 and substrates. We anticipate that the phenol detection and degradation characteristics of Cu 4 S 4 make it a promising material to be used for practical phenol remediation in water environments. [Display omitted] • Cu 4 S 4 , a nanoscale Cu-S cluster with laccase-mimic activity, was developed. • As nanozyme, Cu 4 S 4 prompted phenols oxidation and realized their detection. • Cu 4 S 4 degraded phenols and other pollutants due to sulfite activation property. • Structure-activity relationship was studied in laccase-mimic and sulfite activation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The synthesized nitrogen-doped rGO (N-rGO) was firstly applied to activate sulfite. • Polarized carbon atom adjacent to the doped nitrogen primarily induced activation. • The generated HO• was mainly responsible for the oxidation of As(III) to As(V). • N-rGO with more pyridine nitrogen has higher catalytic activity. As a metal-free alternative, graphene-based catalysts have attracted increasing attention for minimizing the risk of heavy metal exposure. In this study, a nitrogen–doped reduced graphene oxide (N-rGO) material was synthesized by a one–pot hydrothermal method using dicyandiamide as the precursor. N-rGO has been adopted for sulfite activation for the first time, to our knowledge. The established N–rGO/sulfite system could effectively remove As(III) in water, and the solution pH and dissolved oxygen were the most important factors affecting the removal. The results of density functional theory calculations indicated that sulfite activation was more likely to occur on C atoms adjacent to the N dopant. It was inferred that the generated hydroxyl radical (HO•) played a vital role in As(III) oxidation. In addition, the conversion of pyridinic nitrogen to pyrrolic nitrogen was responsible for the poor long–term stability of N-rGO. These results demonstrated that N-rGO was a promising catalyst for water treatment. [ABSTRACT FROM AUTHOR]

The activation of sulfite by heterogeneous catalysts displays a great potential in the development of new sulfate radials based technologies for wastewater treatment. Herein, cobalt nanoparticles embedded in N-doped carbon nanotubes (Co@NC) were prepared by a simple pyrolysis method. Due to the synergistic effects of the cobalt nanoparticles and N-doped carbon nanotubes, the Co@NC catalyst intrinsically shows an outstanding efficiency, excellent reusability and high stability in the catalytic oxidation of methyl orange (MO) in the presence of sulfite and dioxygen. The structure and efficiency of the catalyst was significantly affected by the content of cobalt and pyrolysis temperature. Several quenching experiments and electron paramagnetic resonance were carried out to investigate the catalytic mechanism. It is found that hydroxyl and sulfate radicals worked together to degrade MO in the system. The formation and decomposition of peroxymonosulfate may be an important route of these reactive radicals production. The effect of different anions, bicarbonate concentration, initial solution pH and dye types on the performance of the catalyst was also studied. This study can open a new approach for design and preparation of encapsulated cobalt in carbon materials as effective catalysts for pollutants degradation via sulfite activation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Promoted removal of arsenic by ZIF-67/sulfite than ZIF-67 is achieved. • Fast As(III) removal is through an integration of oxidation and adsorption. • SO 4 − is responsible for As(III) oxidation in solution and on catalyst surface. • The sulfite activation by ZIF-67 provides extra sites for As(III,V) adsorption. • Operating parameters of ZIF-67/sulfite system for arsenic removal are optimized. Elimination of As(III) from water has gotten more attention in comparison with As(V) due to its higher mobility and toxicity. In this work, we provide an effective method by integrating a cobalt-based metal–organic framework ZIF-67 and sulfite (S(IV)) to purify As(III)-laden water. The coupled oxidation with adsorption process (i.e., As(III) removal by ZIF-67/S(IV)) surpasses a divided preoxidation-adsorption process (i.e., As(V) removal by ZIF-67) for arsenic removal from the multi-component water containing various coexisting anions. Herein, the mechanism investigation corroborated that the S(IV) plays dual roles as a complexing ligand and a precursor of oxysulfur radicals. The enhanced As(III) removal is attributed to the S(IV) activation on the ZIF-67 surface via various reactions (e.g., surface complexation and intramolecular electron transfer), which not only generates SO 4 − radicals for rapid oxidation of As(III) to As(V), but also provides more adsorption sites for the immobilization of the oxidized product (As(V)). Furthermore, the combined process can rapidly treat arsenic in the realistic contaminated water from 242 to 7 μg L−1 within 15 min, directly meeting the provisional maximum contaminant level (<10 μg L−1). Overall, the proposed work may aid the rational design of S(IV) mediated oxidation-adsorption process to remove arsenic from a variety of contaminated waters. [ABSTRACT FROM AUTHOR]

Recently, the development of efficient heterogenous catalysts for sulfite (SO 3 2-) activation as a green and cheap way of sulfate radical (SO 4 •−)-based advanced oxidation procedures (SR-AOPs) has attracted a great attention in wastewater treatment. Here, we describe an active catalyst, MnO x /g-C 3 N 4 nanocomposites (MnCN), for the photocatalytic activation of SO 3 2- for the degradation of methyl orange (MO) and other organic pollutants. Compared with pure g-C 3 N 4 , the MnCN catalyst exhibits much higher photocatalytic performance, with 94% MO removal accomplished in 20 min under ideal conditions. The catalyst performed well in ten recycle experiments without the change of structure. Comprehensive characterizations demonstrate that decoration of Mn oxides on g-C 3 N 4 surface not only facilitated the conversion of SO 3 2- to SO 4 •−, but also trapped the photogenerated electrons, and limited the reorganization of the photogenerated carrier and then generated additional electrons to reduce O 2 to O 2 •− radicals for the pollutant degradation. This goal of the study is to present a potentially effective strategy for treating refractory organic contaminants. [Display omitted] • This study successfully designed a novel complementary photocatalysis system. • The introduction of Mn in g-C 3 N 4 enhanced the activation ability of sulfite to sulfite radical. • Numerous O 2 •− radicals were the active factor to degrade the target pollutants. • The existence of Mn species promoted more photogenerated electrons to reduce O 2 to O 2 •− [ABSTRACT FROM AUTHOR]

[Display omitted] • The rod-shaped manganese oxides were obviously formed in Mn(II)/sulfite system. • SO 5 •- and SO 4 •- were responsible for the formation of manganese oxide. • The in situ -formed manganese oxide mainly contributed to the TCS degradation. • TCS oligomer intermediates were the primary products in Mn(II)/sulfite system. • The presence of Fe(II/III) and HA slightly inhibited TCS removal in natural water. Various earth-rich transition metal elements such as iron and manganese have been applied to activate sulfite for the degradation of pollutants. However, the redox process of manganese and its role in pollutants removal in the Mn(II)/sulfite system remain unclear. Batch experiment was performed in this study to investigate the manganese oxidation mechanisms and triclosan (TCS) degradation in a mixture solution with Mn(II) and sulfite under aerobic conditions. About 75 % removal of TCS was achieved in the Mn(II)/sulfite (200 μM/200 μM) system at pH 6.0 within 24 h aerobic reaction, and the conversion efficiency was positively related to the concentrations of Mn(II). The addition of Mn(II) led to the rapid depletion of sulfite within the initial two minutes and the rod-shaped MnO 2 was obviously generated in the subsequent reaction accompanied by the degradation of TCS. Quenching experiments revealed that manganese oxide, rather than sulfur radicals, was the major reactive oxidant for TCS degradation, and SO 5 •- and SO 4 •- were responsible for the formation of manganese oxide, directly verified by ABTS method and X-ray photoelectron spectroscopy (XPS) analysis. In addition, TCS oligomers (dimers and trimers) were mainly produced through a single electron process between manganese oxide and TCS. The results of this study proved the in-situ formation of active manganese oxide in Mn(II)/sulfite system under aerobic condition, which could induce the transformation of organic contaminants. [ABSTRACT FROM AUTHOR]

As a prospective member in the family of advanced oxidation processes (AOPs), heterogeneous sulfite activation shows low cost and high safety for poisonous organic pollutants' degradation. To obtain an efficient sulfite activator, sulfite oxidase (SuO x ), a molybdenum-based enzyme that can prompt oxidation and activation of sulfite, inspired us greatly. Based on the structure of SuO x , MoS 2 /BPE (BPE = 1, 2-bis-(4-pyridyl)-ethylene) is synthesized successfully. In MoS 2 /BPE , the BPE molecule is inserted between the MoS 2 layers as a pillar and the N atom links with Mo 4+ directly. MoS 2 /BPE generation and organic pollutants' degradation. At pH 7.0, its tetracycline degradation efficiency achieved is 93.9% in 30 min. Furthermore, its sulfite activation ability also endows x mimic activity. Theoretical calculation implies that BPE insertion optimizes the d-band center position of MoS 2 /BPE , which regulates the interaction between MoS 2 mimic activity and sulfite activation ability is clarified in detail.4 2- . This prompts • SO 4 - generation and organic pollutants' degradation. At pH 7.0, its tetracycline degradation efficiency achieved is 93.9% in 30 min. Furthermore, its sulfite activation ability also endows MoS 2 /BPE with excellent antibiofouling performance because • SO 4 - can kill the microorganisms in water effectively. This work develops a new sulfite activator based on SuO x . The connection between structure and SuO x mimic activity and sulfite activation ability is clarified in detail.

Sulfite(S(IV))-induced advanced oxidation processes (AOPs) have great prospect in the field of removing organic pollutants, yet developing highly efficient sulfite activation systems and optimizing active sites for favorable catalytic processes are important but still challenging. Herein, we have achieved a composite catalyst with modulated Co electron structure for efficient AOPs by decorating Co(OH) 2 on ultrathin graphitic carbon nitride (g-C 3 N 4) nanosheet through an adjustable strategy, which exhibits high catalytic performance in S(IV) activation system. At optimal pH 9, 92% of paracetamol (APAP) (0.005 mM) is removed with the degradation rate constant of k 1 = 0.193 min−1 within 30 min in presence of the composite material. The in-situ synthesis mode introduces strong heterogeneous interface interaction, resulting in directional electron transfer from cobalt hydroxide layer to g-C 3 N 4 sheet revealed by X-ray photoelectron spectroscopy and density functional theory (DFT) calculations. The underlying activity enhanced mechanisms for APAP in S(IV) activation system using Co(OH) 2 /g-C 3 N 4 are proposed: (i) The ultrathin g-C 3 N 4 nanosheets provide more anchoring centers for generating small Co(OH) 2 nanoparticles with abundant active sites which benefit to form metastable intermediates of Co(II)-SO 3 ; (ii) The strong interface interaction induces charge redistribution between Co(OH) 2 and g-C 3 N 4 conformed by DFT calculation, which modulates the d-band center of Co site and strengthens the bind of Co(II)-SO 3 , thereby giving rise to radicals (•OH, SO 4 • and O 2 • ) and nonradicals (1O 2 and electron transfer) oxidation for highly-efficient removal APAP. Our work will pave the way to build an environmentally friendly strategy for emerging organic pollutant degradation in water through building efficient catalysts in sulfite activation system. [Display omitted] • In-situ construction of Co(OH) 2 decorated ultra-thin g-C 3 N 4 was obtained, possessing strong heterogeneous interface interaction. • DFT calculations elaborated that the charge redistribution modulated d‐band center of Co and strengthened the bind of Co(II)-SO 3. • Synergistic effect of radical and nonradical oxidation for boosting the degradation of paracetamol via sulfite activation system. [ABSTRACT FROM AUTHOR]

A possible overall sulfite activation mechanism on Mn 2 O 3 @Mn 5 O 8 catalyst for organic contaminant degradation. [Display omitted] • Mn 2 O 3 @Mn 5 O 8 shows better catalytic performance than other S(IV) activators. • S(IV) is activated by Mn 2 O 3 @Mn 5 O 8 , generating SO 3 •−, SO 5 •−, SO 4 •−, and •OH radicals. • Experiments show phenol is mainly removed by •OH, and partially by SO 4 •− and SO 5 •−. • At an initial solution pH of 3.0–7.0, stable degradation of phenol is achieved. • Low toxicity and high S(IV) activation ability of Mn 2 O 3 @Mn 5 O 8 affords a green method. Less-toxic, cost-effective, stable, and highly efficient catalysts for sodium sulfite (S(IV)) activation are required to degrade organic pollutants from wastewater. Herein, we report the facile thermal synthesis of Mn 2 O 3 @Mn 5 O 8 that activates S(IV) more efficiently than other Mn and transition-metal oxides. Mn 2 O 3 @Mn 5 O 8 exhibits good performance and long-term stability for eliminating various contaminants from aqueous media, including phenol, bisphenol A, nitrobenzene, 2,4-dichlorophenol, and acetaminophen. Its high performance is attributed to its multivalency, unique architecture, surface hydroxyl groups (–OH), and high surface area. X-ray diffractometry and high-resolution transmission electron microscopy revealed that Mn 2 O 3 @Mn 5 O 8 comprises well-combined cubic Mn 2 O 3 and monoclinic Mn 5 O 8 crystalline structures, whereas electron paramagnetic resonance spectroscopy and scavenging tests showed that SO 5 •−, SO 4 •−, and •OH radicals are generated during S(IV) activation, with SO 3 •− as a precursor. The mixed-valence state provides effective and favorable electron transfer via Mn redox cycling (Mn(II) ↔ Mn(III) ↔ Mn(IV)), improving the S(IV) activation performance and catalytic activity. Mn 2 O 3 @Mn 5 O 8 /S(IV) system shows stable performance in the 3.0–7.0 pH range. Density functional theory calculations confirmed the higher catalytic activity as indicated by high –OH adsorption energy and significant inter-charge transformation. This study provides new insights and strategies for the activation of S(IV) using less-toxic metal oxides as catalysts and broadens the scope of heterogeneous Mn-based catalysts and S(IV) chemistry in real-world applications, particularly for the treatment of wastewater. [ABSTRACT FROM AUTHOR]

Sulfite has been used as a classic reductant for the dehalogenation and reduction of organic compounds for a long time, it is recently deemed as a promising alternative (for persulfate) to generate sulfate radical for wastewater treatment due to its low price and eco-toxicity. In contrast with the enormous work developed in the field of tetracycline (TC) degradation via PMS activization, sulfite activization could play a important role in TC degradation but there is only very few available reports in this area. Herein, the novel and efficient CoNHs nanocatalyst is designed and developed, via immobilization of hydrangea-shaped Co 3 O 4 nanoparticles onto graphitic carbon nanosheet (GCN), for the degradation of tetracycline via sulfite activation. The detailed characterizations have confirmed that CoNHs possesses a nanohydrangea-shaped structure with high microporosity. The comparison with other supports (such as CeO 2 and MoS 2), CoNHs provides the highest degradation efficiency in TC degradation, due to the synergistic effect between Co 3 O 4 and GCN. Free radical quenching experiments and EPR analysis confirm that SO 4 •- and O 2 •- are major reactive oxygen species in the CoNHs/sulfite system. This work could provide a simple, economical and durable cobalt-based catalyst for organic wastewater treatment via sulfite activation. [Display omitted] • CoNHs is developed by immobilization of hydrangea-shaped Co 3 O 4 nanoparticles onto GCN. • CoNHs is found to be an efficient catalyst for TC removal via sulfite activation. • SO 4 •- and O 2 •- are major reactive oxygen species in CoNHs/sulfite system. • This work provides a cobalt-based catalyst for wastewater treatment via sulfite activation. [ABSTRACT FROM AUTHOR]

[Display omitted] • FeO was first used as a ferrous source for sulfite (S(IV)) activation. • DFT and electrochemical analyses confirm the little role of interface catalysis. • The Fe(II/III)–S(IV) complex geometry was determined from thermodynamics. • The RSE decreased with the increase of S(IV) concentration. • The FeO/S(IV) process has an advantage over FeO/persulfate and FeO/peroxymonosulfate. In this study, we constructed an advanced oxidation process using FeO for sulfite (S(IV)) activation. By selecting antidepressant fluoxetine (FLX) as the target, the FeO/S(IV) system's performance was investigated. Up to 84.4% of FLX could be rapidly eliminated in 17.5 min under optimal reaction conditions. The results confirmed that the S(IV) activation was dominantly attributed to the released Fe2+ from FeO, rather than an interface reaction, supported by theoretical calculations and electrochemical characterization. Besides, the Fe–S(IV) complex geometry was determined using density functional theory (DFT), indicating a Fe–O–S ligand form. Energy files demonstrate the decomposition of FeSO 3 + complex might be endothermic. These findings provide new insight into the Fe(II)/S(IV) mechanism. The generated SO 4 •− primarily caused FLX removal, where 23 product types were detected. The degradation pathway was proposed, mostly containing hydroxylation and ring-opening, which decreased the toxicity of FLX. Furthermore, the FeO/S(IV) system had good reusability and applicability, maintaining high efficiency in real water. Compared to peroxydisulfate and peroxymonosulfate, S(IV) had obvious advantages in FeO-induced activation. This research discloses a promising technology for the FeO/S(IV) water treatment system. [ABSTRACT FROM AUTHOR]

Cobalt-based heterogeneous catalysts receive research interests in the sulfite activation to generate reactive radicals for abatement of organic contaminants in the aquatic environments, yet the sulfite activation efficacy is compromised by the limited utilization of available active cobalt sites. To address this problem, the atomically dispersed cobalt loaded on mesoporous silica SBA-15 (simplified as Co/SBA-15) is constructed for sulfite activation process regarding the incorporation of cobalt species into the confined space abundant with silicon hydroxyl groups in SBA-15. Co/SBA-15 exhibits excellent sulfite activation efficacy, in which iohexol can be completely degraded within 30 min. Sulfate radical (SO 4 •−) and hydroxyl radical (HO•) are the dominant reactive radicals responsible for iohexol abatement, in which 7.15 μM of SO 4 •− and 2.23 μM of HO• are produced. The excellent electron transfer efficiency of Co/SBA-15 is demonstrated by electrochemical impedance spectroscopy (EIS). Structural characterization and valence analysis suggest that the formation of atomically dispersed cobalt species facilitates the sulfite activation and Lewis acid sites on Co/SBA-15 can adsorb organic molecular fragments to realize the rapid conversion from >Co(III) to >Co(II) by electron transfer. The iohexol degradation pathways are also proposed based on UPLC-QTOF-MS analysis. In addition, the effects of various influencing factors including the reactant dosage, coexisting anions, humic acid and the different real water backgrounds on iohexol abatement are systematically investigated. This study is expected to provide an efficient heterogeneous catalyst for sulfite activation to achieve the rapid abatement of organic contaminants. [Display omitted] • Atomically dispersed Co/SBA-15 was prepared for efficient sulfite activation. • Co/SBA-15 with Lewis acid exhibited good electron transfer efficiency. • SO 4 •− and HO• were identified as the major radicals in Co/SBA-15/sulfite process. • Iohexol degradation products were identified and possible pathways were proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • VIS/sulfite/Fe/g-C 3 N 4 (N 2) system shows promise in contaminant removal and sterilization. • The introduction of sulfite enhances the formation of SO 4 •− and •O 2 –. • The presence of NVs accelerates Fe3+/Fe2+ redox cycle. • Fe/g-C 3 N 4-x with rich NVs exhibits stronger interaction towards HSO 3 −. • A mechanism of synergy between Fe, NVs, and g-C 3 N 4 in sulfite activation is proposed. A novel Fe-modified g-C 3 N 4 with rich N vacancies (NVs) under N 2 calcination was developed to active sulfite for the efficient removal of contaminants and bacterial inactivation under visible light (VIS). The degradation rates of VIS/sulfite/Fe/g-C 3 N 4 (N 2) for acetaminophen (ACT) were 10.94, 4.88, and 2.85 times higher than those of VIS/Fe/g-C 3 N 4 (N 2), VIS/sulfite/g-C 3 N 4 (N 2), and VIS/sulfite/Fe/g-C 3 N 4 (CO 2), respectively. Escherichia coli (E. coli) with an initial concentration of 2.5 × 108 CFU/mL decreased by 6.1 and 8.4 log CFU/mL within 60 min and 80 min of inactivation, respectively. The activation of sulfite led to the generation of oxysulfur radicals, and the consumption of hole (h+) by sulfite inhibited its recombination with electron (e−), promoting the formation of superoxide radical (•O 2 –). The synergistic effect of Fe, NVs, and g-C 3 N 4 in VIS/sulfite/Fe/g-C 3 N 4 (N 2) system promoted the generation of oxysulfur radicals. Density functional theory (DFT) calculations indicated that NVs facilitated charge transfer from Fe to g-C 3 N 4 substrate, accelerating the Fe3+/Fe2+ and oxysulfur radical cycles. Fe/g-C 3 N 4 with rich NVs exhibited lower adsorption binding energy, faster charge transfer, and longer S-O bond length than pristine Fe/g-C 3 N 4 during sulfite adsorption. This work unraveled the synergistic mechanism of Fe, NVs, and g-C 3 N 4 , which provides new insights into heterogeneous photocatalysts on sulfite activation for water treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co-TiO 2 exhibits a significant shift to visible light region. • Visible light is efficient to induce Metronidazole degradation in Co-TiO 2 /sulfite system. • Stepwise addition of sulfite could reduce futile consumption of radicals. • The Co2+–Co3+–Co2+ redox cycle on the surface of Co-TiO 2 generated SO 3 •- • The catalytic performance of Co-TiO 2 is very stable after consecutive reuses. The degradation of metronidazole (MNZ) by cobalt doped TiO 2 activation of sulfite has been studied in this work. The Co-TiO 2 was synthesized using a sol–gel method and characterized by TEM, EDS, DRS, XPS and EPR. The catalytic stability was tested by consecutive reuses. Up to 94% of MNZ was removed after 18 min reaction with Co-TiO 2 and sulfite dosage being 0.1 g/L and 5 mM, respectively. The influencing parameters, such as the initial concentration of MNZ (0.03–0.2 mM), sulfite dosage (1–10 mM), catalyst dosage (0.1–0.8 g/L) and pH (2–12) were examined. The MNZ removal was pH-dependent with an optimal pH of 7.0. The presence of halides and nitrite could inhibit the process effectively. The reaction mechanisms were explored by radical quenching experiments and EPR measurements, and investigated by the XPS spectra of the catalysts before and after reactions. The formation of Co-SO 3 complex and the Co2+–Co3+–Co2+ redox processes on the surface of the catalyst were crucial for sulfite activation. Compared to other Co species/persulfate processes, Co-TiO 2 /sulfite/LED combined the advantages of TiO 2 photocatalysis and activation of sulfite with transition metal. The study suggests a novel and promising process for the treatment of antibiotics in water. [ABSTRACT FROM AUTHOR]

This paper offers novel insights into the recently reported cobalt(II) sulfide (CoS) activated sulfite (S(IV)) process. After adding 0.5 g/L CoS and 0.4 mM S(IV) at pH 9.0, 83.1% of acetaminophen (ACP) was removed in 10 min. However, this effect was subject to the inhibition of coexisting chloride or humic acid. Dissolved O 2 played an important role in the reaction. The interactive impacts between S(IV) concentration and initial pH were found the most significant. The CoS/S(IV) system involved synchronous homogeneous and heterogeneous catalysis, which demonstrated sizable ACP degradation. The generated sulfate radical (SO 4 •−) and hydroxyl radical (HO•) were primarily responsible for ACP degradation. DFT-based first-principle calculations show the electrons transferred from sulfite to the CoS surface in heterogeneous activation. The pathway of ACP degradation was determined to primarily comprise addition and substitution reactions based on the detected intermediates. Besides, the CoS/S(IV) system demonstrated good reusability and was successful in degrading the other five organics: bisphenol A, metoprolol, estriol, atenolol, and imipramine. The ACP removal performance under real water background was also acceptable. As a result, the CoS/S(IV) system is expected to be a reliable approach for water treatment. [Display omitted] • A synergistic homogeneous-heterogeneous catalysis occurred in CoS/sulfite system. • The rapid conversion of SO 3 •− to HO• was clearly recorded in this process. • The interactive effect between sulfite dosage and initial pH were most significant. • Density functional theory was adopted to help infer the reaction mechanism. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sulfite was employed to enhance ZnFe 2 O 4 @PANI photocatalytic oxidation ability. • As(III) was rapid removed in wide pH range of 3–10 due to pH-dependent mechanism. • Free radicals and tetravalent iron [Fe(VI)] were responsible for As(III) removal. • Fe(VI) formation was attributed to bivalent iron [Fe(II)] and permonosulfate [HSO 5 –]. Inorganic arsenic (As) exists widely in industrial wastewater and poses a serious risk to human health. Here, sulfite was employed to enhance the photocatalytic oxidation ability, and As(III) containing wastewater was rapidly purified within 30 min. Furthermore, this photocatalytic system maintain excellent performance at wide pH range of 3–10. By measuring intermediate radicals and ions, pH-dependent mechanism of sulfite activation was proposed. Under acid conditions, sulfite radicals (•SO 3 -) were detected in photocatalytic system, and quickly convert into oxidizing radicals (•SO 4 - and •OH), which were responsible for As(III) oxidation. While at alkaline conditions, only tetravalent iron [Fe(IV)] was detected as oxidizing substance contributing to As(III) removal during photo reaction process. Moreover, Fe(IV) mainly existed on catalyst surface rather than solution, the limited Fe leaching could extend the service life of catalyst, which should be attributed to the core–shell structure of ZnFe 2 O 4 @PANI. These findings in this work advances the fundamental understanding of radicals conversion and Fe(IV) formation during sulfite activation process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag@CNG composites are successful fabricated by one-step hydrothermal reaction. • The composites show excellent visible light response and stability. • Enhanced degradation of dimethoate as sulfite activation and fast charge transfer. • Degradation mechanism and toxicity evolution of dimethoate were investigated. As pesticides are widely found in water, it is still a challenge to find an economical and effective technology to remove organophosphorus pesticides. Therefore, A composite with silver nanoparticles modified graphene oxide-supported graphitic carbon nitride microflowers ternary heterojunction (Ag@CNG) was synthesized by facile hydrothermal process. A novel composite system composed of photocatalysis and sulfite activation under visible light (Vis/Ag@CNG/sulfite) was constructed to realize the efficient removal of pesticides dimethoate (DT). Characterization results showed that the surface plasmon resonance (SPR) effect and ternary heterojunction could promote the photo-electrons transfer, and enhance the absorption of visible light. Moreover, the by-product sulfite added could be activated by holes to introduce novel SO 4 −, and obviously improve the decomposition of DT. Additionally, effects of operating parameters and supporting electrolyte on the degradation of DT were also detailed. Especially, the degradation pathways and mechanism of DT were revealed in depth through trapping experiments and GC–MS analysis combined with theoretical calculation. The toxicity evolution of products was also comprehensively evaluated. It was proved by degradation of different pollutants that the new composite system presented a great application potential for refractory and deleterious pollutants removal in water. [ABSTRACT FROM AUTHOR]

[Display omitted] • The photoelectrochemical performance of BiVO 4 photoanode was significantly improved by controlling morphology. • The PEC activation of sulfite simultaneously promoted PPCPs degradation, H 2 evolution, and E. coli disinfection. • SO 4 − contributed the most to BZP degradation. A new photoelectrochemical (PEC) system by means of adding sodium sulfite (Na 2 SO 3) was developed to treat sewage. The PEC system simultaneously promoted pharmaceuticals and personal care products (PPCPs) degradation, H 2 evolution and E. coli disinfection using an optimized, visible–light driven BiVO 4 photoanode. The PEC reactions were first carried out in 1.5 mM Na 2 SO 3 electrolyte. 92.3% of 2 ppm benzophenone-3 (BZP) was degraded, and 115.36 μmol of H 2 was produced in 90 min at 1.0 V vs. Ag/AgCl, which was a significant improvement over cases where the electrolyte was either Na 2 SO 4 or NaCl. The sulfite ions were first activated by holes and then converted into sulfate radicals, which played a dominant role in the degradation of BZP. As the holes were caught by the sulfite, charge separation was also enhanced, increasing H 2 evolution. The PEC reactions were also used to treat real sewage, in which case an 82% improvement in the rate constant of BZP degradation, 60% increase of H 2 evolution, and 0.78 log enhancement of E. coli disinfection were achieved by adding 1.5 mM Na 2 SO 3. The system was also feasible to degrade various PPCPs, and showed excellent reusability and stability, proving its great potential in sewage treatment. [ABSTRACT FROM AUTHOR]

Owing to the lower toxicity and mobility of inorganic As(V), the oxidative removal of As(III) is deemed as the optimal approach for arsenic elimination from water. Herein, a synthetic TiO 2 -supported CuO material (Cu-TiO 2) was coupled with sulfite (S(IV)) to remove As(III) at neutral pH. The combined process coupled oxidation with adsorption (i.e., As(III) removal by Cu-TiO 2 /S(IV)) was superior than a divided preoxidation-adsorption process (i.e., As(V) removal by Cu-TiO 2) for arsenic removal. Attractively, low concentration of As(III) (50−300 μg L−1) could be completely removed by Cu-TiO 2 (0.25 g L−1)/S(IV) (0.5 mM) within 60 min. Mechanism investigations revealed that the efficient As(III) removal was attributed to the continuous oxysulfur radicals (SO x •−) oxidation and Cu-TiO 2 adsorption. The surface-adsorbed and free sulfate radicals (SO 4 •−) were further identified as the crucial oxidizing species. The Cu-TiO 2 played the dual roles as a catalyst for S(IV) activation and an absorbent for arsenic immobility. The influence of operating parameters (i.e., As(III) concentration and sulfite dosage) and water chemistry (i.e., pH, inorganic anions, dissolved organic matters, and temperature) on As(III) removal were systematically investigated and optimized. Overall, the proposed process has potential application prospects in rehabilitating the As(III)-polluted water environment using industrial waste sulfite. [Display omitted] • As(III) removal was achieved in an integration of oxidation and adsorption. • Free and surface-adsorbed SO 4 •− were responsible for As(III) oxidation. • Sulfite drove As(III,V) forming more monodentate complexes on catalyst surface. • Possible synergy mechanism of Cu-TiO 2 /sulfite system was proposed. • The operation factors were investigated and optimized systematically. [ABSTRACT FROM AUTHOR]

• Co-MOFs were synthesized by a simple hydrothermal process. • Co-MOFs showed high activity for MO degradation via SO 3 2− activation. • The mechanism of pollutants degradation with SO 3 2− was illustrated. • The influence of several reaction parameters was studied. Activation of sulfite (SO 3 2−) by heterogeneous catalysts draws increasing attention due to their promising performance and environmental benignity. In this work, Co-based metal organic framework (Co-MOF) was synthesized and directly used as a catalyst to activate SO 3 2− for contaminants degradation. The Co-MOF catalyst exhibited high catalytic activity for Methyl Orange (MO) degradation in alkaline aqueous solution. Impressively, the catalytic performance of Co-MOF was more efficient than Co2+ and cobalt oxides of Co 3 O 4 , CoFe 2 O 4 and Co nanoparticles embedded in carbon tubes. Radical quenching experiments and electron paramagnetic resonance suggested that special coordination and redox of Co(II) and Co(III) in the pristine of Co-MOF initiated a series of chain reaction of SO 3 2− oxidation, with the formation of several oxysulfur radicals such as SO 3 −• and SO 4 −•. This work provides new insights into the stereoscopic cobalt based catalysts of sulfite catalysis and can guide new insights in future catalyst development in advanced oxidation process. [ABSTRACT FROM AUTHOR]

In this study, a series of glucose-derived carbon catalysts were developed and applied for the activation of sulfite for the oxidation of As(III). The process of sulfite activation with the carbon catalysts is based on the production of oxysulfur free radicals such as SO 3 •- , SO 5 . The factors responsible for the sulfite activation performance of carbon catalysts are conductivity and ketonic functional groups. A complex is formed between the sulfite and carbon catalysts, and the electron transfer that takes place within the complex leads to the generation of semiquinone and oxysulfur radicals, and finally, the oxysulfur radicals are converted into SO •- , and SO 4 •- . The factors responsible for the sulfite activation performance of carbon catalysts are conductivity and ketonic functional groups. A complex is formed between the sulfite and carbon catalysts, and the electron transfer that takes place within the complex leads to the generation of semiquinone and oxysulfur radicals, and finally, the oxysulfur radicals are converted into SO 4 •- by means of O 2 , which results in the As(III) oxidation. The efficiency of the sulfite/carbon system is enhanced under normoxia conditions due to the reversible transformation cycle occurring among C═O/C-O • /C-OH triads. The present study is of great environmental significance as sulfite is a source of SO 4 •- generated, and the activation is achieved by a metal-free carbon material, which makes the process viable and environmentally friendly.

• Supramolecular hydrogel assisted fabrication of single-atom Fe deposited bamboo-like porous g-C 3 N 4. • Superior sulfite activation over homogeneous Fe and g-C 3 N 4 due to the synergetic effect of single-atom Fe and g-C 3 N 4. • The contribution of synergetic effect depends on sulfite species. • DFT calculation favors the identification of diclofenac degradation pathways. • Successful dechlorination reduces the toxicity of diclofenac. Herein, through supramolecular gel assisted pre-configuration, a novel bamboo-like porous graphitic carbon nitride (g-C 3 N 4) deposited with single-atom Fe was successfully prepared and used for sulfite (S(IV)) activation. Unexpectedly, owing to the presence of single-atom Fe, hybrid material with only 2.5‰ of Fe exhibited 16 times higher S(IV) activation efficiency for diclofenac removal than pure g-C 3 N 4 under visible light irradiation. Moreover, a synergetic effect of Fe and g-C 3 N 4 was found to play the dominate role, and the synergetic factor was calculated to be 0.84. The synergetic mechanism mainly related to the generation of surface Fe-S(IV) complex, which could be affected by S(IV) species or the presence of H 2 PO 4 −. DCF removal was significantly enhanced at alkaline condition, but the enhancement was mainly attributed to photocatalysis but not synergetic effect. Decarboxylation, hydroxylation, chlorine abstraction and cleavages at bridging N atom were the main degradation pathways of DCF, which agreed well with Fukui index prediction. The toxicity of DCF was alleviated during the degradation process through successful mineralization of chlorine atoms. [ABSTRACT FROM AUTHOR]

In this study, biochar R550, obtained from rice husk charred at 550 °C, was used to detoxify Cr(VI) and organic pollutant p-nitrophenol (PNP) with the cooperation of sulfite, simultaneously. Cr(VI) was mainly reduced by sulfite, and the reduction was accelerated by biochar. Also, the reactive oxygen species formed in-situ as a result of enhanced oxidation of sulfite with Cr(VI)/R550 system and the activation of O 2 by R550, led to the degradation of PNP. Production of the radicals viz., SO 3 −, SO 4 − and OH was followed by Electron Paramagnetic Resonance (EPR) study, and the predominant role of SO 4 − towards PNP degradation was confirmed by radical quenching tests. The reaction completed biochar sample was undergone the X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) spectral analysis, which suggested that the carboxyl group of the biochar triggered and enhanced the reactivity of Cr(VI) via coordination linkage, which in turn activated the sulfite and converted the SO 3 2− into SO 4 − to a higher extent by overcoming the undesirable transformation of SO 3 2− to SO 4 2−. Such results could also be verified by the other three biochar (Oxi-R550, H-R550, and R800). The results of the present study shed light on the mechanism of biochar mediated sulfite activation process by Cr(VI) and also assured the viability and green approach of the technique in detoxifying the industrial effluents containing Cr(VI) and organic pollutants. Unlabelled Image • Sulfite could be activated by Cr(VI) to produce SO 3 −, SO 4 − and OH. • The activation of sulfite by Cr(VI) was accelerated by biochar. • Biochar could degrade organic pollutants by OH production from O 2 activation. • Both SO 4 − and OH contributed to the radical degradation of PNP. • Cr(VI) was reduced by sulfite. [ABSTRACT FROM AUTHOR]

• Captured SO 2 reutilization and water remediation were simultaneously achieved. • An EC/CoSi/sulfite system for efficient sulfate radical production was developed. • Insights for synergy mechanism of EC/CoSi/sulfite system were provided. • Influencing factors were investigated and operating conditions were optimized. The desulfurization by-product (e.g. sulfite) treatment and water pollution are two widespread concerns, while the sulfite activation by transition metals can be adopted to solve both of them simultaneously. In this study, a silica-supported cobalt catalyst (CoSi) was used to activate sulfite for the degradation of organic pollutants in an electrochemical system (the EC/CoSi/sulfite system), which demonstrated remarkable water treatment efficiencies due to sulfate radical oxidation. The synergy mechanisms of the EC/CoSi/sulfite system were revealed. (i) The as-prepared CoSi played the dual roles as an electro-adsorbent and a catalyst for sulfite activation. (ii) The electrolytic oxygenation supported the conversion of cobalt species and the chain reactions of oxysulfur radicals. (iii) The anodic oxidation of sulfite also generated oxysulfur radicals. The influencing factors and operating parameters were investigated and optimized systematically. Moreover, the efficient degradation of highly concentrated X-3B and non-selective oxidation of various organic compounds confirmed the promising application of the EC/CoSi/sulfite system in practical water treatment. Overall, the EC/CoSi/sulfite system created new value of the industrial waste (e.g. sulfite) for further application in the environmental remediation. [ABSTRACT FROM AUTHOR]

• Single Fe atoms confined in two-dimensional MoS 2 mimic sulfite oxidase enzyme. • MoS 2 not only consists of Mo catalytic sites but also provides the support for Fe catalytic sites. • Dual catalytic sites play collaboratively to obtain high reactivity for sulfite activation. • 2D catalyst allows interpretation of catalytic performances using theoretical calculation. Sulfite has been recently recognized as a source of radicals for oxysulfur radical-based advanced oxidation process due to its low cost and low toxicity. Inspired by sulfite oxidase enzyme, we demonstrate single Fe atoms confined in two-dimensional MoS 2 nanosheets (Fe x Mo 1- x S 2) as highly reactive catalysts for heterogeneous activation of sulfite, thereby promoting efficient oxidative degradation of propranolol in water. Highest propranolol degradation efficiency of ∼90 % was observed at pH 4.0. The Fe or Mo sites of Fe x Mo 1- x S 2 activate sulfite via the Fe2+/Fe3+ or Mo4+/Mo5+/Mo6+ redox cycle. More importantly, the synergistic catalysis involving the Fe and Mo sites play an essential role. SO 5 − is the major radical responsible for propranolol degradation with the 2nd-order rate constant of (1.1 ± 0.6)×107 M−1 s−1. Furthermore, the Fe x Mo 1- x S 2 /sulfite system is capable of degrading a wide range of recalcitrant organic pollutants and operating in a real water environment, suggesting its great potential in practical water treatment applications. [ABSTRACT FROM AUTHOR]