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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.

[Display omitted] • A novel enhanced As(III) removal process, MIL-53(Fe)/sulfite, was developed. • As(III) removal was through a combination of oxidation and adsorption. • As(III) oxidation relied on the powerful Fe(IV) and SO 4 •− generated from SO 5 •−. • Oxidation product As(V) was adsorbed by forming monodentate complexes. • Procedure to realize the purification of real arsenic-spiked water was proposed. Iron based metal–organic frameworks (Fe-MOFs) have garnered international interest as a novel and promising category of adsorbent materials for the elimination of arsenic from neutral waters. Nevertheless, their practical implementation is constrained by their inadequate adsorption capacity for aqueous arsenite (As(III)), a prevalent arsenic species found in acidic environments such as acid mine drainage. Herein, a novel oxidation-adsorption process to effectively purify As(III)-contaminated water is constructed by combining the MIL-53(Fe) of high acid resistance with sulfite (S(IV)). Notably, the adsorption capacity of the MIL-53(Fe)/S(IV) process for As(III) at pH 3.5 reached 86.4 mg g−1, surpassing the values achieved by MIL-53(Fe) alone by 21.6 times for As(III) and 1.62 times for As(V) (oxidized As(III)). During the oxidation process, MIL-53(Fe) effectively activated S(IV), leading to the formation of the crucial intermediate SO 5 •−, which then underwent two distinct pathways to respectively generate Fe(IV) and SO 4 •−, accounting for rapid oxidation of As(III). In the adsorption process, the S(IV) activation (i.e., complexation and electron transfer) on the MIL-53(Fe) surface created additional adsorption sites by dredging the pores and altering the electron cloud distribution within the adsorbent. Furthermore, As(V) trended to form monodentate complexes with surface Fe (e.g., Fe O As bond) in the MIL-53(Fe)/S(IV) process. Notably, this oxidative removal process demonstrated reliable performance in complex water matrices and practical arsenic-contaminated waters. Overall, this study presents a promising strategy to enhance the efficacy of Fe-MOFs for the removal of As(III). [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.)

Graphical abstract Highlights • NiFe 2 O 4 –sulfite system is proposed as a novel sulfate-radical-based AOP. • Alkaline pH favored the activation of sulfite on NiFe 2 O 4 surface to produce SO x −. • Generation of >SO 3 2−–Ni(II)–OH complexes was the main factor for sulfite activation. • SO 5 − was the main oxidative species responsible for estriol degradation. • Main intermediates and pathways for the degradation of estriol were identified. Abstract We report a process using NiFe 2 O 4 nanocatalyst for the heterogeneous activation of sulfite ions for estriol (E3) oxidation. This NiFe 2 O 4 -sulfite system consisted of NiFe 2 O 4 as an catalyst, sulfite as a reductant, O 2 as an oxidant, and oxysulfur radicals (SO 3 −, SO 4 − and SO 5 −) and hydroxyl radicals (HO) as generated radicals. The experimental conditions for E3 removal were investigated, including the initial pH values, dissolved oxygen concentration, and NiFe 2 O 4 and sulfite ion dosages. The initial pH has a significant influence on the catalytic performance, and approximately 87.6% of E3 could be removed at pH 9.0. NiFe 2 O 4 nanocatalyst was characterized by high resolution and field emission transmission electron microscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. The catalytic reactivity and stability of the NiFe 2 O 4 nanocatalyst were investigated by consecutive repeated runs. Radicals generated in this NiFe 2 O 4 -sulfite system were identified via radical-scavenging and electron spin resonance experiments. SO 5 − was identified as major oxidative radicals for E3 removal, which is quite different from the previous studies on sulfite activation. The generation of >SO 3 2−-Ni(II)-OH on NiFe 2 O 4 surface played a critical role in sulfite auto-oxidation. Our results supplemented the mechanism of the sulfate radical generation with sulfite ion as a source. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A low concentration of Cu(II) can efficiently activate sulfite for the oxidation of contaminants. • Cu(OH) 2 particles are the major active species of Cu(II) in the catalysis of S(IV). • The role of oxysulfur radicals and/or hydroxyl radicals are demonstrated. • The rate constant of the reaction between SO 5 − and APAP was determined. Abstract In the recent years, sulfite-induced oxidation of various contaminants using oxysulfur radicals (SO 4 −, SO 5 − and SO 3 −) has been explored to establish novel advanced oxidation processes (AOPs). Although transition metals, such as cupric ions (Cu(II)), have been demonstrated to be capable of activating sulfite (S(IV)), the catalytic mechanism of the specific Cu(II) species is still not very clear. In this work, N-acetyl- para -aminophenol (APAP) was used as a basic substrate to investigate the process, mechanism and stability of the Cu(II)-S(IV) system with low dosages of Cu(II) (0.010 mmol L−1) in an alkaline solution (pH 10). The results show that 0.010 mmol L−1 Cu(II) can efficiently activate S(IV) for the oxidation of APAP and other contaminants (e.g., metoprolol, sulfanilamide, aniline, 4-chloroaniline, N,N-dimethylaniline, p -nitrophenol and 3-nitrophenol). Simulation calculations for the distribution of the Cu(II) species and pH-dependence of APAP oxidation in the Cu(II)-S(IV) system showed that Cu(OH) 2 particles are the major active species in the catalysis of S(IV). The surface complexation of S(IV) with the Cu(OH) 2 particles prior to the production of oxysulfur radicals is the basic mechanism of the alkaline Cu(II)-S(IV) system. Radical inhibition with scavengers (ethanol, tert-butyl alcohol or aniline) demonstrates that SO 5 −, SO 4 − and hydroxyl radicals (HO) are responsible for the contaminant oxidation. Moreover, SO 5 − is believed to be the main active species in the Cu(II)-S(IV) system, which has a second-order rate constant of 2.98 × 108 M−1 s−1 with APAP. This work renews our recognition of the Cu(II)-S(IV) system applications and further understanding of the catalytic role of the nascent Cu(OH) 2 particles in the activation of S(IV). [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A stable silicon-supported copper catalyst was synthesized using Cu-NH 3 complexes. • CuNSi/sulfite system is proposed as a promising system of SO 4 −-based AOPs. • Neutral pH favors the activation of sulfite on CuNSi surface for SO 4 − generation. • Oxidation of As(III) by CuNSi/sulfite system was achieved and optimized. • Possible mechanisms for the oxidation of As(III) were identified. Abstract Sulfite activation by metal is a promising SO 4 −-based advanced oxidation process (SR-AOPs) for pollutant removal. Homogeneous systems are limited by strict working pH and metallic oxide sludge generation. To overcome these drawbacks, a copper–silica composite modified by ammonia (CuNSi) was synthesized through a complexation–adsorption–combustion method and characterized by X-ray diffraction, X-ray photoelectron spectroscopies, transmission electronic microscopy etc. CuNSi demonstrates a stable and remarkable catalytic activation of sulfite for As(III) oxidation after five reuses in neutral wastewater. The negligible oxidation of As(III) under anoxic condition confirmed the importance of oxygen for the CuNSi/sulfite system. Bicarbonate significantly inhibited As(III) oxidation, whereas Cl− and NO 3 − slight restrained As(III) oxidation. The dosage of sulfite and As(III) affected initial rates of As(III) oxidation via the equation r 0 = k r × [Sulfite]2.11 × [As(III)]0.33 (k r = (3.68 ± 0.46) × 103 M−1.44 min−1). The apparent activation energy for As(III) oxidation was 60.14 ± 5.17 kJ mol−1. The SO 4 − was confirmed as the primary oxidative species, and the sulfite activation by CuNSi was through a Cu(II)–Cu(I)–Cu(II) cycle. Overall, this study suggests that the CuNSi/sulfite system can be a promising AOP for further application in wastewater treatment. [ABSTRACT FROM AUTHOR]

Sulfate radical (SO 4 − ) is believed to be one of the most highly reactive oxidants, as superior as hydroxyl radical (HO ), for various organic/inorganic contaminants removal in the field of pollution control chemistry. In the recent decade, sulfate radical-based advanced oxidation processes (SR-AOPs) have been developed quickly primarily due to the selective oxidation and high oxidative potential and therefore hold great promises. Although peroxydisulfate (PDS) and peroxymonosulfate (PMS) have been extensively utilized in various SR-AOPs, new attempts have been made to replace PDS/PMS with sulfite for the purpose of SO 4 − generation at lower cost. Indeed, some significant progresses have been achieved in driving SO 4 − generation from transient metal catalyzed sulfite auto-oxidation systems to oxidize contaminants. The background, basic mechanisms, and application of the transition metal catalyzed sulfite auto-oxidation systems in contaminants detoxification and microorganism inactivation are reviewed in this work. Meanwhile, we hereby also want to point out several important unresolved issues for future investigation. (1) How to realize quick reactions at near neutral pH? (2) How to achieve high rate of mineralization as equally as or at least close to the apparent complete elimination of substrates? (3) What are the relative contributions of various oxysulfur radicals to the transformation of contaminants. (4) Is it possible to control the extent of substrates oxidation so as to get target transformed products with desired properties? If so, SR-AOPs can be upgraded as product-oriented AOPs (PO-AOPs). This state-of-art minireview aims to discuss abovementioned issues and presents some recent progresses in this field. [ABSTRACT FROM AUTHOR]

Heterogeneous catalytic activation is important for potential application of new sulfate-radical-based advanced oxidation process using sulfite as source of sulfate radical. We report herein a heterogeneous system for sulfite activation by CoFe 2 O 4 nanocatalyst for metoprolol removal. Factors that influence metoprolol removal were investigated, including pH and initial concentrations of components. The CoFe 2 O 4 nanocatalyst was characterized by X-ray diffractometry (XRD) and transmission electron microscopy (TEM), and the catalytic stability was tested by consecutive runs. Radicals generated in the CoFe 2 O 4 S(IV) O 2 system were identified through radical quenching experiments and by electron spin resonance (ESR). The catalytic mechanism was elucidated further by X-ray photoelectron spectroscopy (XPS). The catalytic process was dependent on initial pH, and more than 80% of the metoprolol can be removed at pH 10.0 following the Langmubir-Hinshelwood equation. The generation of Co-OH complexes on the CoFe 2 O 4 surface was crucial for sulfite activation. SO 4 − was verified to be the main oxidative species responsible for metoprolol degradation. Other organic pollutants, such as sulfanilamide, sulfasalazine, 2-nitroaniline, sulfapyridine, aniline, azo dye X-3B and 4-chloroaniline, could also be removed in this CoFe 2 O 4 S(IV) O 2 system. The results suggest that the CoFe 2 O 4 S(IV) O 2 system has good application prospects in alkaline organic wastewater treatment. [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]

There is actually a need for efficient methods to clean waters and wastewaters from pollutants such as the bisphenol A endocrine disrupter. Advanced oxidation processes currently use persulfate or peroxymonosulfate to generate sulfate radicals. There are, however, few reports on the use of sulfite to generate sulfate radicals, instead of persulfate or peroxymonosulfate, except for dyes. Here we studied the degradation of the bisphenol A using iron(III) as catalyst and sulfite as precursor of oxysulfur radicals, at initial pH of 6, under UV irradiation at 395 nm. The occurrence of radicals was checked by quenching with tert-butyl alcohol and ethanol. Bisphenol A degradation products were analyzed by liquid chromatography coupled with mass spectrometry (LC-MS). Results reveal that iron(III) or iron(II) have a similar oxidation efficiency. Quenching experiments show that the oxidation rate of bisphenol A is 47.7 % for SO, 37.3 % for SO and 15 % for HO. Bisphenol A degradation products include catechol and quinone derivatives. Overall, our findings show that the photo-iron(III)-sulfite system is efficient for the oxidation of bisphenol A at circumneutral pH. [ABSTRACT FROM AUTHOR]

The oxidation of sulfite catalyzed by transition metal ions produces reactive oxysulfur species that can damage plasmid and isolated DNA in vitro. Among the four DNA bases, guanine is the most sensitive to one-electron oxidation promoted by the species formed in the autoxidation of sulfite (HSO5-, HO•, SO3•-, SO4•- and SO5•-) due to its low reduction potential and ability to bind transition metal ions capable to catalyze oxidative processes. Some oxidative DNA lesions are promutagenic and oxidative DNA damage is proposed to play a crucial role in certain human pathologies, including cancer.

Hexavalent chromium (Cr(VI)), a heavy-metal contaminant, can be easily reduced to less toxic trivalent chromium (Cr(III)) by sulfite ions (S(IV)). However, S(IV) has not drawn as much attention as the ferrous ion has. We report herein a novel Cr(VI)–S(IV)–O 2 system containing sulfite ions that rapidly and simultaneously reduces Cr(VI) and oxidize organic pollutants in the presence of oxygen in aqueous solutions. This Cr(VI)–S(IV)-O 2 system contains the initiator Cr(VI), the reductant S(IV), and the oxidant O 2 , which produce oxysulfur radicals (mainly SO 4 − and SO 5 − ) and hydroxyl radicals (OH ). The Cr(VI)/S(IV) molar ratio, pH, and oxygen content play important roles in the entire reaction system. Acidic conditions (pH 3.0) facilitated degradation of organic compounds and reduction of Cr(VI) as well. In addition, experiments of rapid degradation of several kinds of organic pollutants such as azo dye (acid orange 7, AO7), aniline, phenol, bisphenol A etc were also conducted. Preliminary results show that the removal rates of the analogs of phenols or aromatic amines in this Cr(VI)–S(IV)–O 2 system have a relationship with the electronic parameters (Hammett constant, σ) of the substituted groups. Thus, the Cr(VI)–S(IV)–O 2 system, provides an excellent strategy of “waste control by waste” for removing multiple industrial contaminants. [ABSTRACT FROM AUTHOR]

The oxidation of sulfite catalyzed by transition metal ions produces reactive oxysulfur species that can damage plasmid and isolated DNA in vitro. Among the four DNA bases, guanine is the most sensitive to one-electron oxidation promoted by the species formed in the autoxidation of sulfite (HSO5-, HO?, SO3?-, SO4?- and SO5?-) due to its low reduction potential and ability to bind transition metal ions capable to catalyze oxidative processes. Some oxidative DNA lesions are promutagenic and oxidative DNA damage is proposed to play a crucial role in certain human pathologies, including cancer.

Although sulfite-based advanced oxidation processes (AOPs) have received renewed attention due to the production of oxysulfur radicals, the feasibility of using ultrasound (US) to activate sulfite remains unknown. In this work, low frequency ultrasound has been applied for the first time to develop a novel sulfite activation process (US–S(IV)) for enhanced oxidation of arsenite (As(III)). Our results showed that the US–S(IV) process with 1 mM sulfite addition and 20 kHz 650 W ultrasound can achieve approximately 2.9-fold increase in As(III) oxidation rate compared to the US process at pH 7. The mechanisms underpinning the US–S(IV) process have been probed through radical-scavenging experiments and electron spin resonance (ESR) spectrometry. Direct ultrasonolysis of sulfite has been demonstrated to be the predominant pathway producing the primary sulfite radical (SO 3 ⁻) in the US–S(IV) process. Besides, the US-S(IV) process also works well in the treatment process of natural water, suggesting that this process could be promising in commercial scale application. This work not only provides a new application of ultrasound in sulfite-based AOP, but also provides further insights into how sulfite impacts the US process. [Display omitted] • Low frequency ultrasound (US) effectively activate sulfite (S(IV)). • SO 4 ⁻ played the most important role for arsenite oxidation in the US-S(IV) process. • A thermal activation process for S(IV) happened in the US-S(IV) process. • Dissolved oxygen (DO) and S(IV) consumption rate is low. • Environmental matrices took minor impact on arsenite oxidation. [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]

• 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]