Rapid oxidation of paracetamol by Cobalt(II) catalyzed sulfite at alkaline pH

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

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

Mesoporous cobalt-manganese layered double hydroxides promote the activation of calcium sulfite for degradation and detoxification of metronidazole

Metronidazole (MNZ), a commonly used antibiotic, poses risks to water bodies and human health due to its potential carcinogenic, mutagenic, and genotoxic effects. In this study, mesoporous cobalt-manganese layered double hydroxides (CoxMny-LDH) with abundant oxygen vacancies (Ov) were successfully synthesized using the co-precipitation method and used to activate calcium sulfite (CaSO3) with slight soluble in water for MNZ degradation. The characterization results revealed that Co2Mn-LDH had higher specific areas and exhibited good crystallinity. Co2Mn-LDH/CaSO3 exhibited the best catalytic performance under optimal conditions, achieving a remarkable MNZ degradation efficiency of up to 98.1 % in only 8 min. Quenching experiments and electron paramagnetic resonance (EPR) tests showed that SO4•- and 1O2 played pivotal roles in the MNZ degradation process by activated CaSO3, while the redox cycles of Co2+/Co3+ and Mn3+/Mn4+ on the catalyst surface accelerated electron transfer, promoting radical generation. Three MNZ degradation routes were put forward based on the density functional theory (DFT) and liquid chromatography-mass spectrometer (LC-MS) analysis. Meanwhile, the toxicity analysis result demonstrated that the toxicity of intermediates post-catalytic reaction was decreased. Furthermore, the Co2Mn-LDH/CaSO3 system displayed excellent stability, reusability, and anti-interference capability, and achieved a comparably high removal efficiency across various organic pollutant water bodies. This study provides valuable insights into the development and optimization of effective heterogeneous catalysts for treating antibiotic-contaminated wastewater.

Insight into the Discriminative Efficiencies and Mechanisms of Peroxy Activation via Fe/Cu Bimetallic Catalysts for Wastewater Purification

Fe/Cu bimetallic catalysts have a synergistic effect that can effectively enhance catalytic activity, so Fe/Cu bimetallic catalysts have been extensively studied. However, the efficacy and mechanisms of Fe/Cu bimetallic catalysts' peroxidation activation have rarely been explored. In this study, Fe/Cu bimetallic materials were fabricated to catalyze different oxidizing agents, including peroxymonosulfate (PMS), peroxydisulfate (PDS), peroxyacetic acid (PAA), and hydrogen peroxide (H2O2), for the degradation of sulfamethoxazole (SMX). The Fe/Cu/oxidant systems exhibited an excellent degradation efficiency of sulfamethoxazole (SMX). In the Fe/Cu/PMS, Fe/Cu/PDS, and Fe/Cu/PAA systems, the main reactive oxygen species (ROS) responsible for SMX degradation were hydroxyl radical (•OH) and singlet oxygen (1O2), while the main ROS was only •OH in the H2O2 system. The differences in the surface structure of the materials before and after oxidation were examined, revealing the presence of a large amount of flocculent material on the surface of the oxidized PMS material. Anion experiments and actual body experiments also revealed that the PMS system had a strong anti-interference ability. Finally, a comprehensive comparison concluded that the PMS system was the optimal system among the four oxidation systems. Overall, this work revealed that the PMS oxidant has a better catalytic degradation of SMX compared to other oxidizers for Fe/Cu, that PMS generates more ROS, and that the PMS system has a stronger resistance to interference.

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

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

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

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

Rapid degradation and detoxification of metronidazole using calcium sulfite activated by CoCu two-dimensional layered bimetallic hydroxides: Performance, mechanism, and degradation pathway

In this study, cobalt copper-layered double hydroxides (CoCu-LDHs) were prepared by coprecipitation as catalysts to activate CaSO3 for metronidazole (MNZ) degradation. This is the first report on layered double hydroxides activating sulfite for the degradation of organic pollutants. Meanwhile, to address the issue of self-quenching reactions readily occurring in conventional sulfite advanced oxidation systems and resulting in low oxidant efficiency, CaSO3 with slightly soluble in water was used instead of commonly used Na2SO3, to improve the limitations of traditional systems. The results showed that in the CoCu-LDHs/CaSO3 system, the degradation rate of MNZ reached 98.7% within 5 min, representing a 23.0% increase compared to the CoCu-LDHs/Na2SO3 system. Owing to the excellent catalytic performance exhibited by CoCu-LDHs, characterizations including XRD, FTIR, SEM, TEM, BET and XPS were carried out to investigate this further. The results confirmed the successful synthesis of CoCu-LDH, and the activation mechanism study revealed that Co and Cu were considered to the main elements in activating CaSO3, demonstrating good synergistic effects. In addition, the oxygen vacancies on the catalyst surface also played a positive role in generating radicals and promoting electron transfer. Subsequently, the effects of Co/Cu ratio, catalyst dosage, oxidant concentration, pollutant concentration, pH and coexisting substances on MNZ degradation were investigated. Additionally, based on the LC-MS analysis of degradation products and toxicity tests, MNZ was transformed into different intermediates with low toxicity through four pathways, eventually mineralizing into inorganic small molecules. After six cycles, the MNZ degradation rate still reached 82.1%, exhibiting excellent stability and recyclability. In general, this study provides new ideas for activating sulfite, while providing theoretical support for subsequent research on sulfite advanced oxidation system.

Modulating the >Co(II)/Co(III) redox cycling via confinement of cobalt with WS2 for the ultrafast sulfite activation

Sulfite autoxidation in combination with the cobalt-based heterogeneous activators, has recently emerged as the efficient sulfate radical (SO4•-) generation process for organic micropollutant abatement in the water and wastewater treatment, yet the sluggish >Co(II)/Co(III) redox cycling currently compromises the efficacy of radical generation and the potential applications. Herein, regarding that the reductive W(IV) species in WS2 can modulate the >Co(II)/Co(III) redox cycling in the advanced oxidation processes, confinement of cobalt with WS2 (Co-WS2) is designed and characterized. The Co-WS2/sulfite process achieves an ultrafast tetracycline (TC) abatement (~100 % abatement of TC within 1 min) under circumneutral conditions with lower dosage of sulfite and activator, outperforming the current cobalt-based heterogeneous counterparts. The dominant reactive radicals are identified as SO4•- and hydroxyl radical (HO•), which are quantified to be 9.7 μM and 4.5 μM, respectively. The superior radical generation efficiency and the concomitant TC abatement rely on the excellent redox properties and electron transfer capability of Co-WS2. The inter-transformation of >Co(II)/>Co(III) can be accelerated via the involvement of the reductive W(IV) species with the redox-reversibility of the W(IV)/W(VI) couple in the presence of sulfite. The TC degradation intermediates and the corresponding pathways are also proposed according to the ultra-performance liquid chromatography and quadrupole-time of flight mass spectrometry (UPLC-QTOF-MS) analysis. In addition, the influences of the reactant dosage, coexisting anions (HCO3-, HPO42-, Cl- and NO3-), humic acid and the various real water matrices on TC abatement are thoroughly explored. Especially, the Co-WS2/sulfite process is advantageous owing to the negligible effect of the coexisting anions on the TC abatement. This study provides a novel heterogeneous activator for significantly improving sulfite activation efficacy to achieve the efficient organic micropollutant abatement.

Generation and engineering applications of sulfate radicals in environmental remediation

Sulfate radical (SO4•-)-based advanced oxidation processes (AOPs) have become promising alternatives in environmental remediation due to the higher redox potential (2.6-3.1 V) and longer half-life period (30-40 μs) of sulfate radicals compared with many other radicals such as hydroxyl radicals (•OH). The generation and mechanisms of SO4•- and the applications of SO4•--AOPs have been examined extensively, while those using sulfite as activation precursor and their comparisons among various activation precursors have rarely reviewed comprehensively. In this article, the latest progresses in SO4•--AOPs were comprehensively reviewed and commented on. First of all, the generation of SO4•- was summarized via the two activation methods using various oxidant precursors, and the generation mechanisms were also presented, which provides a reference for guiding researchers to better select two precursors. Secondly, the reaction mechanisms of SO4•- were reviewed for organic pollutant degradation, and the reactivity was systematically compared between SO4•- and •OH. Thirdly, methods for SO4•- detection were reviewed which include quantitative and qualitative ones, over which current controversies were discussed. Fourthly, the applications of SO4•--AOPs in various environmental remediation were summarized, and the advantages, challenges, and prospects were also commented. At last, future research needs for SO4•--AOPs were also proposed consequently. This review could lead to better understanding and applications of SO4•--AOPs in environmental remediations.

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

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

Cobalt-doped molybdenum disulfide for efficient sulfite activation to remove As(III): Preparation, efficacy, and mechanisms

The sulfite(S(IV))-based advanced oxidation process has attracted significant attention in removing As(III) in the water matrix for its low-cost and environmental-friendly. In this study, a cobalt-doped molybdenum disulfide (Co-MoS₂) nanocatalyst was first applied to activate S(IV) for As(III) oxidation. Some parameters including initial pH, S(IV) dosage, catalyst dosage, and dissolved oxygen were investigated. The experiment results show that >Co(II) and >Mo(VI) on the catalyst surface promptly activated S(IV) in the Co-MoS₂/S(IV) system, and the electron transfer between Mo, S, and Co atoms accelerated the activation. SO₄^•- was identified as the main active species for As(III) oxidation. Furthermore, DFT calculations confirmed that Co doping improved the MoS₂ catalytic capacity. This study has proven that the material has broad application prospects through reutilization test and actual water experiments. It also provides a new idea for developing bimetallic catalysts for S(IV) activation.

Activation of sulfite by metal-organic framework-derived cobalt nanoparticles for organic pollutants removal

Sulfite (SO₃^2-) 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 SO₃^2- 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⁺, Co₃O₄ and CoFe₂O₄, due to the synergistic effect of metallic Co and Co_xO_y. A series of chain reaction between Co species and dissolved oxygen was established, with the production and transformation of SO₃^•-, SO₅^2-, and subsequent active radicals SO₄^•- and HO•. In addition, HCO₃^- 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 SO₃^2- oxidation to promote organic pollutants degradation.

Treasuring industrial sulfur by-products: A review on add-value to reductive sulfide and sulfite for contaminant removal and hydrogen production

Reductive sulfur-containing by-products (S-BPs) released from industrial process mainly exist in the simple form of sulfide and sulfite. In this study, recent advances to remove and make full use of reductive S-BPs to achieve efficient contaminant removal and hydrogen production are critically reviewed. Sulfide, serves as both reductant and nucleophile, can form intermediates with the catalyst surface functional group through chemical interaction, efficiently promoting the catalytic reduction process to remove contaminants. Sulfite assisted catalytic process could be classified to the advanced reduction processes (ARPs) and advanced oxidation processes (AOPs), mainly depending on the presence of dissolved oxygen (DO) in the solution. During ARPs, sulfite could generate reductive active species including hydrated electron (e_aq^-), hydrogen radical (H·), and sulfite radical (SO₃^•-) under the irradiation of UV light, leading to the efficient reduction removal of a variety of contaminants. During AOPs, sulfite could first produce SO₃^•- under the action of the catalyst or energy, initiating a series of reactions to produce oxysulfur radicals. Various contaminants could be effectively removed under the role of these oxidizing active species. Sulfides and sulfites could also be removed along with promoting hydrogen production via photocatalytic and electrocatalytic processes. Besides, the present limitations and the prospects for future practical applications of the process with these S-BPs are proposed. Overall, this review gives a comprehensive summary and aims to provide new insights and thoughts in promoting contaminant removal and hydrogen production through taking full advantage of reductive S-BPs.

Degradation of sulfamethazine in water by sulfite activated with zero-valent Fe-Cu bimetallic nanoparticles

In this work, zero-valent Fe-Cu bimetallic nanoparticles were synthesized using a facile method, and applied to activate sulfite for the degradation of sulfamethazine (SMT) from the aqueous solution. The key factors influencing SMT degradation were investigated, namely the theoretical loading of Cu, Fe-Cu catalyst dosage, sulfite concentration and initial solution pH. The experimental results showed that the Fe-Cu/sulfite system exhibited a much better performance in SMT degradation than the bare Fe⁰/sulfite system. The mechanism and possible degradation pathway of SMT in Fe-Cu/sulfite system were revealed. The reactive radicals that played a dominant role in the SMT degradation process were •OH and SO₄^•-, while the loading of Cu induced the synergistic effect between Fe and Cu. The redox cycle between Cu(I)/Cu(II) remarkably contributed to the conversion of Fe(III) to Fe(II), greatly enhancing the catalytic performance of Fe-Cu bimetal. In real groundwater applications, the Fe-Cu/sulfite system also exhibited satisfactory SMT degradation. The 30-day aging tests of Fe-Cu particles demonstrated that the aging of catalyst was not obviously affecting the removal of SMT. Furthermore, the reusability of catalyst was evidenced by the recycling experiments. This study provides a promising application of bimetal activated sulfite for enhanced contaminant degradation in groundwater.

CuS Nanoparticles Trigger Sulfite for Fast Degradation of Organic Dyes under Dark Conditions

CuS nanoparticles (CuS NPs) were synthesized by a simple precipitation method using rice starch water as a capping and reducing agent. The phase composition, morphology, absorbance, chemical bonds, and chemical states of the CuS NPs were investigated systematically and then examined for dye degradation catalytic activity with or without sulfite (SO3 2-) under dark conditions. Herein, we observed two reaction trends after the addition of SO3 2- in a CuS NPs/dye system, first substantially enhanced dye degradation and second greater degradation activity between reaction time interval "t" 0-12 min. The redox cycling of Cu(II)/Cu(I) and oxidized sulfur (SO x 2-) species on the surface of CuS NPs played a major role for the activation of SO3 2- and generation and transformation of a sulfite radical (•SO3 -) into a sulfate radical (•SO4 -). Scavenging studies of reactive oxygen species (ROS) revealed that •SO4 - was major reactive species involved in dye degradation. Our study showed that SO3 2- acted as a source and CuS NP surface acted as an SO3 2- activating agent for the generation of •SO4 -, which degrades the dyes. The activation pathway of SO3 2- and generation pathway of relevant ROS were proposed.

Roles of Sulfites in Reverse Osmosis (RO) Plants and Adverse Effects in RO Operation

More than 60 years have passed since UCLA first announced the development of an innovative asymmetric cellulose acetate reverse osmosis (RO) membrane in 1960. This innovation opened a gate to use RO for commercial use. RO is now ubiquitous in water treatment and has been used for various applications, including seawater desalination, municipal water treatment, wastewater reuse, ultra-pure water (UPW) production, and industrial process waters, etc. RO is a highly integrated system consisting of a series of unit processes: (1) intake system, (2) pretreatment, (3) RO system, (4) post-treatment, and (5) effluent treatment and discharge system. In each step, a variety of chemicals are used. Among those, sulfites (sodium bisulfite and sodium metabisulfite) have played significant roles in RO, such as dechlorination, preservatives, shock treatment, and sanitization, etc. Sulfites especially became necessary as dechlorinating agents because polyamide hollow-fiber and aromatic thin-film composite RO membranes developed in the late 1960s and 1970s were less tolerable with residual chlorine. In this review, key applications of sulfites are explained in detail. Furthermore, as it is reported that sulfites have some adverse effects on RO membranes and processes, such phenomena will be clarified. In particular, the following two are significant concerns using sulfites: RO membrane oxidation catalyzed by heavy metals and a trigger of biofouling. This review sheds light on the mechanism of membrane oxidation and triggering biofouling by sulfites. Some countermeasures are also introduced to alleviate such problems.

Strengthening arsenite oxidation in water using metal-free ultrasonic activation of sulfite

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 (SO₃⁻) 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.

Insight into the synergetic effect of photocatalysis and transition metal on sulfite activation: Different mechanisms for carbamazepine and diclofenac degradation

Sulfite [S(IV)] is a promising alternative for sulfate radical-based advanced oxidation processes (SR-AOPs). Transition metal and photocatalysis are generally considered to have a synergetic effect for S(IV) activation. However, the study shows that the synergetic effect is target specific. Herein, an ultra-small Fe₂O₃ clusters deposited graphitic carbon nitride is synthesized and used for S(IV) activation. For carbamazepine (CBZ) degradation, photogenerated holes can transform S(IV) into sulfate radical and photogenerated electrons can accelerate Fe(II)/Fe(III) cycle, which account for the synergetic effect. In contrast, the degradation of diclofenac (DCF) depends on the excitation of DCF rather than photocatalyst. Instead of radical precursor, S(IV) acts as the electron transfer bridge between excited DCF and photocatalyst. Thus, the deposition of Fe₂O₃ negatively affects DCF degradation. Density Functional Theory calculation shows that the first excited state rather than the ground state of diclofenac is more suitable for reactive site prediction, which confirms the photosensitization-like degradation mechanism. Moreover, the effects of pH and coexisted anions varies for CBZ and DCF. The study shed light on the synergetic effect of transition metal and photocatalysis for S(IV) activation, and also open an avenue for the study of target specific mechanisms for AOPs.

Transition Metal Ions as Ozonation Catalysts: An Alternative Process of Heterogeneous Catalytic Ozonation

The aim of this study is to elucidate the mechanism of micropollutants’ removal in drinking water by the application of catalytic ozonation, using transition metals as appropriate catalysts. For that purpose, the degradation of 500 μg/L of p-chlorobenzoic acid (p-CBA) and benzotriazole with the addition of 2 mg/L of ozone in the presence of 1 mg/L of Co(II) or Fe(II) and at pH 7.8 were examined. It was found that in distilled water experiments, both metal ions can be characterized as catalysts, enhancing the ozonation process; however, in the natural water matrix, only iron presented higher removal rates of examined organic pollutants, when compared to single ozonation. The metal ions present catalytic activity, when they can form precipitates, hence converting the initially homogeneous process of catalytic ozonation towards a heterogeneous one. However, when 2 mg/L of ozone was applied in natural water experiments, Co(II)—unlike Fe(II)—could not be oxidized into its trivalent form, hence it cannot precipitate as Co(OH)3. Therefore, under these experimental conditions, this metal was not found to present any catalytic activity. Nevertheless, the addition of phosphates (PO43−) in concentrations higher than 100 mg/L can increase the oxidation ability of the Co(II)/O3 system, due to the resulting sufficient formation of Co3(PO4)2 precipitates. Although cobalt can enhance the •OH production (and therefore, the ozonation procedure) under these conditions, the relatively highly added concentration of phosphate ions makes the treated water non-potable, resulting in the application of further treatment to remove the excess phosphates. Therefore, only Fe(II) can be considered as a sufficient catalyst to enhance the ozonation processes.

Sulfite Activation by Glucose-Derived Carbon Catalysts for As(III) Oxidation: The Role of Ketonic Functional Groups and Conductivity

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 SO₃^•-, SO₅^•-, and SO₄^•-. 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 SO₄^•- by means of O₂, 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 SO₄^•- generated, and the activation is achieved by a metal-free carbon material, which makes the process viable and environmentally friendly.

Efficient destruction of emerging contaminants in water by UV/S(IV) process with natural reoxygenation: Effect of pH on reactive species

UV/sulfite systems with oxygen have recently been considered as advanced oxidation processes in view of the participation of oxysulfur radicals. However, the contribution of •OH and the efficiency of destructing emerging contaminants (ECs) in water remain largely unclear. Here, the UV/S(IV) process was applied with natural reoxygenation to degrade two typical ECs, diethyl phthalate (DEP) and bisphenol A (BPA) showing different properties. Solution pH played the key role in determining the reactive species, and both DEP and BPA were more favorably degraded at more alkaline conditions with higher utilization efficiency of SO₃^2-. Specifically, the H•, O₂^•-, •OH and SO₃^•- were identified at acidic condition, but the amount of •OH accumulated significantly with the elevation of pH. Competitive quenching experiments showed that e_aq^- and •OH dominated the degradation of DEP and BPA at alkaline condition, respectively. Besides, DEP showed higher quantum efficiency for the indirect photolysis and mineralization degree than that of BPA at pH 9.2 mainly due to the direct use of the primary photoproduct. The possible transformation mechanisms of S(IV) and mineralization routes of both pollutants were proposed. This study may provide new insights into the mechanisms involved in UV/S(IV) process and a promising alternative for efficient removal of ECs in water.

Fabrication of Bi1.81MnNbO6.72/sulfite system for efficient degradation of chlortetracycline

The design of eco-friendly Bi_1.81MnNbO_6.72/sulfite system for efficient degradation of chlortetracycline was achieved. The feasibility of synthesizing Bi_1.81MnNbO_6.72 by hydrothermal method was determined by X-ray diffraction. The magnetic test suggested that Bi_1.81MnNbO_6.72 possessed paramagnetic properties, indicating unpaired electrons were present. Scanning electron microscope and transmission electron microscopy images revealed that Bi_1.81MnNbO_6.72 octahedra exhibited exposed [1,1,1] crystal plane containing high density of Bi, Mn and Nb metal atoms. Large numbers of metal atoms will facilitate heterogeneous catalytic process. In a batch system with aeration, Bi_1.81MnNbO_6.72 could be used as sulfite activator for the disposal of chlortetracycline. The reaction kinetics of the degradation process conformed to the pseudo-second-order kinetic model. In Bi_1.81MnNbO_6.72/sulfite process, initial pH, Bi_1.81MnNbO_6.72 dosage, sulfite and chlortetracycline concentrations, as well as inorganic salt ions had great effect on chlortetracycline degradation. Under optimal conditions, the efficiency of Bi_1.81MnNbO_6.72/sulfite system for degradation of chlortetracycline could reach 76.2%. Moreover, Mn (II) plays a key role in the initiation of the catalytic reaction in Bi_1.81MnNbO_6.72/sulfite process. Generated SO₃^●‒ could act as main reactive species in Bi_1.81MnNbO_6.72/sulfite process, while HO^● was also involved. Three new degradation products were detected by UHPLC/MS/MS and the possible degradation pathways in this system were proposed. Based on this, we believe that Bi_1.81MnNbO_6.72/sulfite is a type of process for degradation of organic contaminants with research significance and application prospects.

Single-atom catalysis in advanced oxidation processes for environmental remediation

This review presents the recent advances in synthetic strategies, characterisation, and computations of carbon-based single-atom catalysts, as well as their innovative applications and mechanisms in advanced oxidation technologies.

Activation of peroxymonosulfate by cobalt-impregnated biochar for atrazine degradation: The pivotal roles of persistent free radicals and ecotoxicity assessment

Cobalt-mediated activation of peroxymonosulfate (PMS) has been extensively investigated for the degradation of emerging organic pollutants. In this study, PMS activation via cobalt-impregnated biochar towards atrazine (ATZ) degradation was systematically examined, and the underlying reaction mechanism was explicated. It was found that persistent free radicals (PFRs) contained in biochar play a pivotal role in PMS activation process. The PFRs enabled an efficient transfer electron to both cobalt atom and O₂, facilitating the recycle of Co(III)/Co(II), and thereby leaded to an excellent catalytic performance. In contrast to oxic condition, the elimination of dissolved oxygen significantly retarded the ATZ degradation efficiency from 0.76 to 0.36 min^-1. Radical scavenging experiments and electron paramagnetic resonance (EPR) analysis confirmed that the ATZ degradation was primarily due to SO₄^·- and, to a lesser extent, ·OH. In addition, dual descriptor (DD) method was carried out to reveal reactive sites on ATZ for radicals attacking and predicted derivatives. Meanwhile, the possible ATZ degradation pathways were accordingly proposed, and the ecotoxicity evaluation of the oxidation intermediates was also conducted by ECOSAR. Consequently, the cobalt-impregnated biochar could be an efficient and environmentally friendly catalyst to activate PMS for abatement and detoxication of ATZ.

Paracetamol degradation by photo-activated peroxydisulfate process (UV/PDS): kinetic study and optimization using central composite design

In this study, peroxydisulfate (PDS) was successfully activated by UV-irradiation for the degradation of paracetamol (PCT) frequently detected in the environment. Results showed that increasing the initial PDS concentration from 5 to 20 mM promote the removal of PCT from 49.3% to 97.5% after 240 min of reaction time. As the initial PCT concentration increased from 0.066 to 0.132 mM, the degradation efficiency of PCT decreased from 98% to 73% after 240 min of reaction time, while the optimal pH was found to be 6. It is apparent that the degradation rate of PCT was favored by the lamp power regardless of the initial PCT concentration, for 0.132 mM of PCT, the degradation efficiency increased from 73% to 95% when the lamp power increased from 9 to 30 W, respectively. The kinetic of degradation of the PCT was described by a pseudo-second order kinetic model. The model obtained by central composite design led to the following optimal conditions for PCT degradation: 0.132 mM initial PCT concentration, 20 mM PDS dose, pH solution 6 and lamp power 30 W led to the removal of 92% of PCT at 25 °C within 240 min of reaction time.

Diphenamid photodegradation using Fe(III) impregnated N-doped TiO2/sulfite/visible LED process: Influence of wastewater matrix, kinetic modeling, and toxicity evaluation

Sulfite-based photocatalysis has been recently employed as a promising technology for the treatment of organic pollutants via the generation of reactive radicals. In this contribution, the effect of wastewater matrix constituents and toxicity evaluation were systematically investigated in the Fe^III impregnated N-doped TiO₂ (FeN-TiO₂)/sulfite/visible LED (Vis LED) process in the presence of diphenamid (DPA) as a model organic pollutant. The results showed that the presence of HCO₃^-, SO₄^2-, NO₃^-, and F^- had no detrimental effect on DPA degradation. Conversely, the presence of Cr(VI), NO₂^-, Cl^-, and Br^- caused a stronger retardation effect. The effect of natural organic matter such as humic acid (HA) was inert at normal concentrations. Interestingly, the retardation effect of inorganic ions can be quantified at any given ion concentration based on the linear correlations between the DPA decay (first-order kinetic constants) and concentration of ion species. Toxicity tests on Synechocystis sp., Microcystis flos-aquae, and Nostoc sp. algae revealed that higher toxicity was noticed at 240 min treatment time accompanied by lower toxicity with prolonging the treatment time for all selected algae except for Microcystis flos-aquae. In addition, novel two-phase mathematical models were successfully proposed to predict the accumulation of intermediates depending on their evolution profile.

Multifunctional magnetic MgMn-oxide composite for efficient purification of Cd2+ and paracetamol pollution: Synergetic effect and stability

A multifunctional magnetic composite (0.3Ma-MgMnLDO-a) with the function of Cd²⁺ adsorption and paracetamol (PAM) degradation was successfully fabricated. Surface morphology showed that Fe₃O₄ agglomeration was overcome on composite. The composite had high specific surface area of 105.32 m² g^-1 and saturation magnetization of 40 emu∙g^-1. 0.3Ma-MgMnLDO-a could reach Cd²⁺ adsorption equilibrium within 5 min with 99 % removal rate. The maximum adsorption capacity was 3.76 mmol·g^-1 (422.62 mg g^-1), which apparently higher than that of Fe₃O₄-a and MgMnLDO-a, indicating that the Fe/Mn synergism results in excellent ability for Cd²⁺ adsorption. Moreover, the composite could efficiently activate peroxymonosulfate (PMS) to rapid degrade PAM with the highest first-order rate constants (k_obs = 0.116 min^-1) and total organic carbon (TOC) removal rate (67.7 %), which also due to the contribution of Fe/Mn synergism in PMS activation. The cycling of Mn^III/Mn^IV and Fe^II/Fe^III played an important role in activating PMS to generateO₂^-•, ¹O₂ and OH for degradation. The composite exhibited both stable adsorption and catalytic performance on wide pH (3-9) and five reuse cycles. Notably, there was mutual promotion between Cd²⁺ and PAM adsorption, while the coexistence of Cd²⁺ had slight inhibition on PAM degradation. Overall, the magnetic composite had promising application for purifying heavy metals and pharmaceuticals.

Efficient activation of sulfite autoxidation process with copper oxides for iohexol degradation under mild conditions

Sulfite has been recently emerging as an appealing sulfate radical (SO₄^•-) precursor for efficient treatment of organic contaminants. Due to the negligible autoxidation of sulfite, activators are often introduced to accelerate sulfite autoxidation and the concomitant generation of SO₄^•-. Present heterogeneous activators are mostly not very effective under mild conditions (pH 7.0-8.0). In this work, efficient activation of sulfite with copper oxides including Cu₂O and CuO for iohexol degradation under mild pH conditions is proposed. In a comparison of iohexol degradation efficiency by sulfite autoxidation activated with different metal oxides (Co₃O₄, CoO, α-Fe₂O₃, γ-Fe₂O₃, CuO and Cu₂O), CuO and Cu₂O with lower toxicity are efficient activators and removal efficiencies of ~95% can be obtained at pH 8.0. SO₄^•- is identified to be the major species contributing to the removal of iohexol by electron paramagnetic resonance (EPR) spectroscopy and quenching experiment. Based on the effect of ionic strength and copper leaching, sulfite is proposed to interact with copper oxides via inner-sphere coordination. Effect of critical influencing parameters and efficacy of copper oxides in real water matrixes are investigated. The results suggest that using copper oxides as activators is a new alternative to promote sulfite autoxidation process for rapid contaminants degradation.

Efficient abatement of an iodinated X-ray contrast media iohexol by Co(II) or Cu(II) activated sulfite autoxidation process

Efficient abatement of an iodinated X-ray contrast media iohexol by an emerging sulfite autoxidation advanced oxidation process is demonstrated, which is based on transition metal ion-catalyzed autoxidation of sulfite to form active oxidizing species. The efficacy of the combination of sulfite and transition metal ions (Ag(I), Mn(II), Co(II), Fe(II), Cu(II), Fe(III), or Ce(III)) was tested for iohexol abatement. Co(II) and Cu(II) are proven to show more pronounced catalytic activity than other metals at pH 8.0. According to the quenching studies, sulfate radical (SO₄^•-) is identified to be the primary species for oxidation of iohexol. Increasing dosages of metal ion or sulfite and higher pH values are favorable for iohexol abatement. Inhibition of iohexol abatement is observed in the absence of dissolved oxygen, which is vital for the production of SO₅^•- and subsequent formation of SO₄^•-. Overall, activation of sulfite to produce reactive radicals with extremely low Co(II) or Cu(II) concentrations (in the range of μg L^-1) in circumneutral conditions is confirmed, which offers a potential SO₄^•--based advanced oxidation process in treatment of aquatic organic contaminants.

Rate constants of sulfate radical anion reactions with organic molecules: A review

The rate constants of sulfate radical anion reaction (k_SO4-) with about 230 organic molecules of environmental interest are tabulated and discussed, together with both the methods of rate constant determinations and the reaction mechanisms. k_SO4-'s were collected from the original publications. The highest values in the ∼10⁹ M^-1 s^-1 range are published for aromatic molecules. There is a tendency that electron donating substituents increase and electron withdrawing substituents decrease these values. There are just a few compounds with rate constants established using different techniques in different laboratories. k_SO4-'s determined in different laboratories by the direct techniques, pulse radiolysis or laser flash photolysis, in most cases agree reasonably. The values determined by competitive experimental techniques, by complex kinetics calculations, or by modelling show a large scatter. Some of these techniques seem to be questionable for k_SO4- determination. The sulfate radical anion reacts with ketone and amine moieties of molecules by electron transfer. The same mechanism is also suggested for the reaction with aromatic rings. However, in a few cases addition to the double bond and sulfate anion elimination reactions were distinguished. A typical reaction with the aliphatic parts of the molecule is H-abstraction.

Role of Ferrate(IV) and Ferrate(V) in Activating Ferrate(VI) by Calcium Sulfite for Enhanced Oxidation of Organic Contaminants

Although the Fe(VI)-sulfite process has shown great potential for the rapid removal of organic contaminants, the major active oxidants (Fe(IV)/Fe(V) versus SO₄^•-/^•OH) involved in this process are still under debate. By employing sparingly soluble CaSO₃ as a slow-releasing source of SO₃^2-, this study evaluated the oxidation performance of the Fe(VI)-CaSO₃ process and identified the active oxidants involved in this process. The process exhibited efficient oxidation of a variety of compounds, including antibiotics, pharmaceuticals, and pesticides, at rates that were 6.1-173.7-fold faster than those measured for Fe(VI) alone, depending on pH, CaSO₃ dosage, and the properties of organic contaminants. Many lines of evidence verified that neither SO₄^•- nor ^•OH was the active species in the Fe(VI)-CaSO₃ process. The accelerating effect of CaSO₃ was ascribed to the direct generation of Fe(IV)/Fe(V) species from the reaction of Fe(VI) with soluble SO₃^2- via one-electron steps as well as the indirect generation of Fe(IV)/Fe(V) species from the self-decay of Fe(VI) and Fe(VI) reaction with H₂O₂, which could be catalyzed by uncomplexed Fe(III). Besides, the Fe(VI)-CaSO₃ process exhibited satisfactory removal of organic contaminants in real water, and inorganic anions showed negligible effects on organic contaminant decomposition in this process. Thus, the Fe(VI)-CaSO₃ process with Fe(IV)/Fe(V) as reactive oxidants may be a promising method for abating various micropollutants in water treatment.