The consortium of heterogeneous cobalt phthalocyanine catalyst and bicarbonate ion as a novel platform for contaminants elimination based on peroxymonosulfate activation

Size-Dependent Catalysis in Fenton-like Chemistry: From Nanoparticles to Single Atoms

State-of-the-art Fenton-like reactions are crucial in advanced oxidation processes (AOPs) for water purification. This review explores the latest advancements in heterogeneous metal-based catalysts within AOPs, covering nanoparticles (NPs), single-atom catalysts (SACs), and ultra-small atom clusters. A distinct connection between the physical properties of these catalysts, such as size, degree of unsaturation, electronic structure, and oxidation state, and their impacts on catalytic behavior and efficacy in Fenton-like reactions. In-depth comparative analysis of metal NPs and SACs is conducted focusing on how particle size variations and metal-support interactions affect oxidation species and pathways. The review highlights the cutting-edge characterization techniques and theoretical calculations, indispensable for deciphering the complex electronic and structural characteristics of active sites in downsized metal particles. Additionally, the review underscores innovative strategies for immobilizing these catalysts onto membrane surfaces, offering a solution to the inherent challenges of powdered catalysts. Recent advances in pilot-scale or engineering applications of Fenton-like-based devices are also summarized for the first time. The paper concludes by charting new research directions, emphasizing advanced catalyst design, precise identification of reactive oxygen species, and in-depth mechanistic studies. These efforts aim to enhance the application potential of nanotechnology-based AOPs in real-world wastewater treatment.

Regulating electronic structure of Fe single-atom site by S/N dual-coordination for efficient Fenton-like catalysis

The activity of single-atom catalysts in peroxymonosulfate activation process is bound up with the local electronic state of metal center. However, the large electronegativity of N atoms in Metal-N4 restricts the electron transfer between center metal atom and peroxymonosulfate. Herein, we constructed Fe-SN-C catalyst by incorporating S atom in the first coordination sphere of Fe single-atom site (Fe-S1N3) for Fenton-like catalysis. The Fe-SN-C with a low valent Fe is found to exhibit excellent catalytic activity for bisphenol A degradation, and the corresponding rate constant reaches 0.405 min-1, 11.9-fold higher than the original Fe-N-C. Besides, the Fe-SN-C/PMS system exhibits ideal catalytic stability under the effect of wide pH range and background substrates by the fast generation of high-valent Fe species. Experimental results and theoretical calculations reveal that the dual coordination of S and N atoms notably increases the local electron density of Fe atoms and electron filling in eg orbital, causing a d band center shifting close to the fermi level and thereby optimizes the activation energy for peroxymonosulfate decomposition via Fe 3d-O 2p orbital interaction. This work provides further development of promising SACs for the efficient activation of peroxymonosulfate based on direct regulation of the coordination environment of active center metal atoms.

Efficient and sustainable photocatalytic inactivation of E. coli by an innovative immobilized Ag/TiO2 photocatalyst with peroxymonosulfate (PMS) under visible light

A novel catalytic system for effective photocatalytic inactivation of Escherichia coli (E. coli) was constructed by anchoring Ag nanoparticles (AgNPs) on silane coupling agent (SCA) pretreated TiO2 nano-tube arrays (Ag/SCA/TiO2NTAs). Morphology and structural analyses revealed that SCA could disperse AgNPs evenly on TiO2NTAs, thus inducing a superior surface plasmon resonance (SPR) effect. Ag/SCA/TiO2NTAs catalyst exhibited excellent inactivation performance when in the presence of peroxymonosulfate (PMS) and visible light (VL), with 6-log E. coli was completely inactivated within 60 min, which was 5.3, 12.5 and 13.2 times higher than that of Ag/SCA/TiO2NTAs/VL, PMS/VL and Ag/SCA/TiO2NTAs/PMS/dark systems, respectively. Additionally, the photocatalyst exhibited a highly reusable property, with the inactivation performance almost unchanged after ten cycles of uses with minimal Ag leaching. The inactivation mechanism analysis demonstrated that both radical (SO4•-, OH) and non-radical (h+, 1O2) pathways involved in E. coli inactivation, and SCA played a pivotal role in the production of reactive species. Chloride ions (Cl-) greatly enhanced the inactivation efficiency, while bicarbonate (HCO3-) and phosphate (H2PO4-) showed an inhibitory effect. Humic acid (HA) displayed a dual effect on inactivation performance, where the low concentration of HA facilitated the bacteria inactivation, while the higher dose suppressed bacteria inactivation. Moreover, the system exhibited excellent inactivation performance in tap water. This work first used SCA as the binder to fix AgNPs on TiO2NTAs for VL photocatalytic inactivation of bacteria with the assistance of PMS, which was expected to provide some insights into the practical treatment of drinking water.

Efficient One-Pot Synthesis of 2,5-Furandicarboxylic Acid from Sugars over Polyoxometalate/Metal-Organic Framework Catalysts

Converting extensive sugars into value-added 2,5-furandicarboxylic acid (FDCA) has been considered to be a promising approach to developing sustainable substitutes for chemicals from fossil resources. The complicated conversion processes involved multiple cascade reactions and intermediates, which made the design of efficient multifunction catalysts challenging. Herein, we developed a catalyst by introducing phosphotungstic acid (PW) and Co sites into the UiO-66, which achieved a one-pot cascade conversion of fructose-to-FDCA with high conversion (>99 %) and yield (94.6 %) based on the controllable Lewis/Brønsted acid sites and redox sites. Controlled experiments and detailed characterizations show that the multifunctional PW/UiO(Zr, Co) catalysts successfully affords the direct synthesis of FDCA from fructose via dehydration and selective oxidation in the one-pot reaction. Additionally, the MOF catalysts could also efficiently convert various sugars into FDCA, which has broad application prospects. This study provides new strategies for designing multifunctional catalysts to achieve efficient production of FDCA from biomass in the one-pot reaction.

Efficacy and mechanism of peroxymonosulfate activation by single-atom transition metal catalysts for the oxidation of organic pollutants: Experimental validation and theoretical calculation

Single-atom catalysts can activate peroxymonosulfate (PMS) to enhance its oxidation of organic pollutants in water treatment. We synthesized a series of carbon-supported single-atom transition metal catalysts (MnN@C, FeN@C, CoN@C, NiN@C, and CuN@C) with similar compositions and structures. Their catalytic activity toward PMS activation and oxidation mechanisms were investigated using acid orange 7 (AO7) as a model pollutant. The degradation rate (min^-1·mol^-1·g·m^-2) of AO7 followed order: FeN@C/PMS (7.576 × 10³) > MnN@C/PMS (5.104 × 10³) > CoN@C/PMS (1.919 × 10³) ≫ NiN@C/PMS (0.058 × 10³) > CuN@C/PMS (0.035 × 10³). Electron transfer mediated by surface-activated PMS was found to be the main regime of AO7 oxidation in the catalytic systems. Density functional theory calculations indicated that the degradation of AO7 was promoted by the intense adsorption of PMS and the electron transfer between AO7 and the surface-activated PMS on the catalyst. The cleavage of the naphthalene ring and the azo group was the primary degradation pathway. The toxicity of the products was significantly reduced. This research provides valuable findings for preparing highly efficient single-atom transition metal catalysts for PMS-based degradation of toxic and refractory organic pollutants from water.

Rapid activation of peroxymonosulfate with iron(Ⅲ) complex for organic pollutants degradation via a non-radical pathway

Developing high-performance catalytic systems for eliminating contaminants effectively in water has received a lot of attention. However, the complexity of practical wastewater poses a challenge for degrading organic pollutants. Non-radical active species with strong resistance to interference have shown great advantages in degrading organic pollutants under complex aqueous conditions. Herein, a novel system was constructed by Fe(dpa)Cl₂ (FeL, dpa = N, N'-(4-nitro-1,2-phenylene) dipicolinamide) activating peroxymonosulfate (PMS). The mechanism study verified that the FeL/PMS system had high efficiency in producing high-valent iron-oxo and singlet oxygen (¹O₂) to degrade various organic pollutants. In addition, the chemical bonding between PMS and FeL was elucidated by the density functional theory (DFT) calculations. The FeL/PMS system could remove 96% Reactive Red 195 (RR195) in 2 min, which was much higher than other systems involved in this study. More attractively, the FeL/PMS system demonstrated general resistance to interference from common anions (Cl^-, HCO₃^-, NO₃^- and SO₄^2-), humic acid (HA) and pH changes and were thus compatible with various natural waters. This work provides a new approach for producing non-radical active species, which is a promising catalytic system for water treatment.

Robust polystyrene resin-supported nano-CoFe2O4 mediated peroxymonosulfate activation for efficient oxidation of 1-hydroxyethane 1,1-diphosphonic acid

Nanosized spinel cobalt ferrite (CoFe₂O₄) shows high performance in peroxymonosulfate (PMS) activation for decontamination in water, but is yet challenged by the easily leached Co(II) with high toxicity. Herein, macroporous polystyrene resin is used as the support to improve the stability of CoFe₂O₄ nanoparticles during PMS activation. CoFe₂O₄@S201 exerted high catalytic activity toward PMS activation for oxidation of 1-hydroxyethane 1,1-diphosphonic acid (HEDP), with the apparent rate normalized by Co content 38.2 times higher than that of the unsupported CoFe₂O₄. Meanwhile, one order of magnitude lower Co leaching (< 2.1 μg L^-1) was detected during the catalytic oxidation. The Co(II)-PMS complex was the primary oxidant responsible for the oxidation of HEDP. The catalytic durability and stability of CoFe₂O₄@S201 for degradation of HEDP in actual wastewater were systematically evaluated in both batch and continuous-flow mode. It is found that the organic resin, which is often considered to be intolerant to oxidation, is rather stable during the non-radical process. The total cobalt leaching of the fresh CoFe₂O₄@S201 cannot be ignored in the 100-h continuous-flow run. In contrast, much lower cobalt leaching and slightly higher oxidation efficiency were observed for the regenerated CoFe₂O₄@S201, which might be due to the removal of unreactive and unstable Co sites on the surface in the first trial. The findings shed light on the potential of organic supports for improving the stability and activity of nanosized CoFe₂O₄ and other nano-catalysts toward practical application.

Review on the degradation of chlorinated hydrocarbons by persulfate activated with zero-valent iron-based materials

Chlorinated hydrocarbons (CHCs) are often used in industrial processes, and they have been found in groundwater with increasing frequency in recent years. Several typical CHCs, including trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), carbon tetrachloride (CT), etc., have strong cytotoxicity and carcinogenicity, posing a serious threat to human health and ecological environment. Advanced persulfate (PS) oxidation technology based on nano zero-valent iron (nZVI) has become a research hotspot for CHCs degradation in recent years. However, nZVI is easily oxidized to form the surface passivation layer and prone to aggregation in practical application, which significantly reduces the activation efficiency of PS. In order to solve this problem, various nZVI modification solutions have been proposed. This review systematically summarizes four commonly used modification methods of nZVI, and the theoretical mechanisms of PS activated by primitive and modified nZVI. Besides, the influencing factors in the engineering application process are discussed. In addition, the controversial views on which of the two (SO₄·^- and ·OH) is dominant in the nZVI/PS system are summarized. Generally, SO₄·^- predominates in acidic conditions while ·OH prefers neutral and alkaline environments. Finally, challenges and prospects for practical application of CHCs removal by nZVI-based materials activating PS are also analyzed.

Bicarbonate enhanced heterogeneous activation of peroxymonosulfate by copper ferrite nanoparticles for the efficient degradation of refractory organic contaminants in water

Nowadays, the treatment of residual refractory organic contaminants (ROCs) is a huge challenge for environmental remediation. In this study, a potential process is provided by copper ferrite catalyst (CuFe₂O₄) activated peroxymonosulfate (PMS, HSO₅^-) in the bicarbonate (HCO₃^-) enhanced system for efficient removal of Acid Orange 7 (AO7), 2,4-dichlorophenol, phenol and methyl orange (MO) in water. The impact of key reaction parameters, water quality components, main reactive oxygen species (ROS), probable degradation mechanism, rational degradation pathways and catalyst stability were systematically investigated. A 95.0% AO7 (C₀ = 100 mg L^-1) removal was achieved at initial pH (pH₀) of 5.9 ± 0.1 (natural pH), CuFe₂O₄ dosage of 0.15 g L^-1, PMS concentration of 0.98 mM, HCO₃^- concentration of 2 mM, and reaction time of 30 min. Both sulfate radical (SO₄^-•) and hydroxyl radical (^•OH) on the surface of catalyst were proved as the predominant radical species through radical quenching experiments and electron paramagnetic resonance (EPR) analysis. The buffer nature of HCO₃^- was partially contributed for the enhanced degradation of AO7 under CuFe₂O₄/PMS/HCO₃^- system. Importantly, according to ¹³C nuclear magnetic resonance (NMR) and EPR analysis, the positive effect of bicarbonate may be mainly attributed to the formation of peroxymonocarbonate (HCO₄^-), which may enhance the generation of ^•OH. The magnetic CuFe₂O₄ particles can be well recycled and the leaching concentration of Cu was acceptable (<1 mg L^-1). Considering the widespread presence of bicarbonate in water environment, this work may provide a safe, efficient, and sustainable technique for the elimination of ROCs from practical complex wastewater.

Ball milling-assisted preparation of sludge biochar as a novel periodate activator for nonradical degradation of sulfamethoxazole: Insight into the mechanism of enhanced electron transfer

The non-radical pathway of periodate (PI) activation for the removal of persistent organic contaminants has received increasing attention due to its higher stability and oxidative advantages. In this study, the degradation of sulfamethoxazole (SMX) by ball mill treated magnetic sludge biochar (BM-MSBC) through activation of PI by electron transfer mechanism was reported. Experimental and characterization results showed that the ball milling treatment resulted in a better pore and defect structure, which also significantly enhanced the electron transfer capacity of the sludge biochar. The BM-MSBC/PI system exhibited notable dependence of activator concentration and initial pH, while the effect of PI concentration was not significant. The coexisting substances (common anions and natural organic matters) hardly affect the degradation of SMX in the BM-MSBC/PI system. The phytotoxicity experiments suggested that the treatment of BM-MSBC/PI system could significantly reduce the biological toxicity of SMX solution. This study provides a novel, economical, and facile modification method for the application of sludge biochar in advanced oxidation processes.

Single-atom site catalysts for environmental remediation: Recent advances

Single-atom site catalysts (SACs) can maximize the utilization of active metal species and provide an attractive way to regulate the activity and selectivity of catalytic reactions. The adjustable coordination configuration and atomic structure of SACs enable them to be an ideal candidate for revealing reaction mechanisms in various catalytic processes. The minimum use of metals and relatively tight anchoring of the metal atoms significantly reduce leaching and environmental risks. Additionally, the unique physicochemical properties of single atom sites endow SACs with superior activity in various catalytic processes for environmental remediation (ER). Generally, SACs are burgeoning and promising materials in the application of ER. However, a systematic and critical review on the mechanism and broad application of SACs-based ER is lacking. Herein, we review emerging studies applying SACs for different ERs, such as eliminating organic pollutants in water, removing volatile organic compounds, purifying automobile exhaust, and others (hydrodefluorination and disinfection). We have summarized the synthesis, characterization, reaction mechanism and structural-function relationship of SACs in ER. In addition, the perspectives and challenges of SACs for ER are also analyzed. We expect that this review can provide constructive inspiration for discoveries and applications of SACs in environmental catalysis in the future.

Unraveling spongy Co3O4 mediated activation of peroxymonosulfate: Overlooked involvement of instantaneously produced high-valent-cobalt-oxo

Peroxymonosulfate (PMS) activation induced by tricobalt tetroxide spinel (Co₃O₄) has been confirmed as a typical Haber-Weiss reaction, while free radicals were once considered as the dominated reactive species in the previous studies. However, the catalytic mechanism of the spongy Co₃O₄ driven PMS activation was surprisingly found as a radical/nonradical mixed process rather than a pure radical process in the present work. The important role of sulfate radical (SO₄^-) was confirmed through the quenching experiments. Despite the inhibition of furfuryl alcohol (FFA) and 1,4-benzoquinone (BQ) on degradation was generally accepted as the evidence to support the existence of ¹O₂ and O₂^-, additional experiments using methyl phenyl sulfoxide (PMSO) as the indicator indeed verified high-valent-cobalt-oxo rather than ¹O₂ and O₂^- dominated the very early reaction stage. Notably, instead of homogeneous Co³⁺, heterogeneous Co(IV) = O on catalyst surface was believed to be responsible for the oxidation of organics. Spongy Co₃O₄ not only possessed stronger catalytic ability than commercial Co₃O₄ (k_{[spongy Co3O4]} = 0.74 min^-1, k_[Co3O4] = 0.08 min^-1), but also owned preferable stability. The performance of catalytic system was barely affected by the solution pH under the near neutral condition. Besides, little suppression of the widely existing anions on the degradation indicated the potential application of spongy Co₃O₄/PMS system. This study provides a reliable oxidation technology for the removal of organic pollutants, and sheds new light on the cobalt oxide triggered PMS activation process.

Cobalt Single Atoms Anchored on Oxygen-Doped Tubular Carbon Nitride for Efficient Peroxymonosulfate Activation: Simultaneous Coordination Structure and Morphology Modulation

Simultaneous regulation of the coordination environment of single-atom catalysts (SACs) and engineering architectures with efficient exposed active sites are efficient strategies for boosting peroxymonosulfate (PMS) activation. We isolated cobalt atoms with dual nitrogen and oxygen coordination (Co-N₃ O₁ ) on oxygen-doped tubular carbon nitride (TCN) by pyrolyzing a hydrogen-bonded cyanuric acid melamine-cobalt acetate precursor. The theoretically constructed Co-N₃ O₁ moiety on TCN exhibited an impressive mass activity of 7.61×10⁵  min^-1  mol^-1 with high ¹ O₂ selectivity. Theoretical calculations revealed that the cobalt single atoms occupied a dual nitrogen and oxygen coordination environment, and that PMS adsorption was promoted and energy barriers reduced for the key *O intermediate that produced ¹ O₂ . The catalysts were attached to a widely used poly(vinylidene fluoride) microfiltration membrane to deliver an antibiotic wastewater treatment system with 97.5 % ciprofloxacin rejection over 10 hours, thereby revealing the suitability of the membrane for industrial applications.

Ultrahigh Peroxymonosulfate Utilization Efficiency over CuO Nanosheets via Heterogeneous Cu(III) Formation and Preferential Electron Transfer during Degradation of Phenols

In persulfate activation by copper-based catalysts, high-valent copper (Cu(III)) is an overlooked reactive intermediate that contributes to efficient persulfate utilization and organic pollutant removal. However, the mechanisms underlying heterogeneous activation and enhanced persulfate utilization are not fully understood. Here, copper oxide (CuO) nanosheets (synthesized with a facile precipitation method) exhibited high catalytic activity for peroxymonosulfate (PMS) activation with 100% 4-chlorophenol (4-CP) degradation within 3 min. Evidence for the critical role of surface-associated Cu(III) on PMS activation and 4-CP degradation over a wide pH range (pH 3-10) was obtained using in situ Raman spectroscopy, electron paramagnetic resonance, and quenching tests. Cu(III) directly oxidized 4-CP and other phenolic pollutants, with rate constants inversely proportional to their ionization potentials. Cu(III) preferentially oxidizes 4-CP rather than react with two PMS molecules to generate one molecule of ¹O₂, thus minimizing this less efficient PMS utilization pathway. Accordingly, a much higher PMS utilization efficiency (77% of electrons accepted by PMS ascribed to 4-CP mineralization) was obtained with CuO/PMS than with a radical pathway-dominated Co₃O₄/PMS system (27%) or with the ¹O₂ pathway-dominated α-MnO₂/PMS system (26%). Overall, these results highlight the potential benefits of PMS activation via heterogeneous high-valent copper oxidation and offer mechanistic insight into ultrahigh PMS utilization efficiency for organic pollutant removal.

Unveiling the role of cobalt species in the Co/N-C catalysts-induced peroxymonosulfate activation process

In this study, three Co/N-C catalysts were prepared by pyrolysis of bimetallic zeolitic imidazole frameworks with different Co/Zn ratio, and the critical active Co species in peroxymonosulfate (PMS) activation was investigated. The three catalysts had distinct cobalt species but similar N configuration and graphitization degree. The Co species were distributed as single atoms (Co SAs) at a Co/Zn molar ratio of 1:8, while Co nanoclusters (Co NCs) and Co nanoparticles (NPs) would be formed with further increase in Co content. The degradation efficiency of BPA did not show correlation with the increasing of Co content in catalyst. Based on the catalytic performance comparison and reactive species detection, Co SAs was identified as active sites, which could interact with PMS to generate ¹O₂ via path of PMS→HOO*→O*→¹O₂. However, the role of NCs and NPs in directly activating PMS was negligible. In addition, the increase of Co content in Co/N-C catalyst would result in mass cobalt leaching, which enhanced the BPA degradation via homogeneous catalytic reactions with Co^IV as reactive species. It is an effective way to design the Co/N-C catalyst with high catalytic activity and stability via regulating the formation of Co SAs.

Efficient pH-universal degradation of antibiotic tetracycline via Co2P decorated Neosinocalamus affinis biochar

Considering the complexity of traditional cobalt phosphide (Co₂P) loaded biochar synthesis research on a simple and efficient synthesis method has practical significance. In this study, after phosphoric acid activation, Neosinocalamus affinis biochar (NAB) and nanoplate Co₃O₄ quickly formed a Co₂P-NAB composite material with high Co₂P crystallinity and was uniformly dispersed on the surface of NAB in a microwave reactor. Co₂P-NAB has an excellent catalytic degradation effect in the activation of peroxymonosulfate (PMS) to degrade tetracycline (TC). The optimal TC degradation efficiency was achieved with the addition of 50 mg L^-1 TC concentration, 0.2 g L^-1 catalysts, 0.406 mM PMS and pH = 6.02. In addition, according to the pseudo-first-order reaction rate constant calculation, the composite of Co₂P-NAB and PMS the synergy efficiency is 81.55 %. Compared with Co₂P-NAB (10.83 %) and PMS (7.62 %) alone, the Co₂P-NAB/PMS system has a significant promotion effect on the degradation of TC molecules. Additionally, the Co₂P-NAB/PMS system had a TC mineralization rate of 68 % in 30 min. Furthermore, after a series of characterization, detection and analysis, and influencing factor experiments, we proposed a potential mechanism for the Co₂P-NAB/PMS reaction system to degrade TC and found that singlet oxygen (¹O₂) plays an essential role in the non-radical degradation process. Finally, according to the liquid chromatography-mass spectrometry (LC-MS) detection of TC degradation intermediates, a possible degradation route was proposed. Therefore, this work uses microwave technology to present a novel and simple synthesis method for transition metal phosphides, which provides potential application value for the treatment of actual wastewater with heterogeneous catalysts.

Activation of peroxymonosulfate by iron oxychloride with hydroxylamine for ciprofloxacin degradation and bacterial disinfection

Iron oxychloride (FeOCl) is a known effective iron-based catalyst and has been used in advanced oxidation processes (AOPs). This study intends to achieve more facile free radicals generation from peroxymonosulfate (PMS) activation by exploring the Fe(III)/Fe(II) cycle of FeOCl in the presence of hydroxylamine (HA). With 0.2 g/L FeOCl, 1.5 mM PMS, and 1 mM HA, the PMS/FeOCl/HA system could effectively achieve 98.88% of the oxidative degradation of 5 mg/L ciprofloxacin (CIP) in 15 min and quickly inactivate 99.99% of E. coli (10⁸ CFU/mL) in 5 min at near-neutral pH. HA played an important role in promoting the Fe(III)/Fe(II) cycle, thereby greatly improving the oxidation activity of the system. The reactive oxygen species (ROS) such as HO, SO₄^- and O₂^- were identified as the dominated free radicals produced in the system. The intermediate products of CIP detected by liquid chromatograph-mass spectrometer (LC-MS) and three possible degradation pathways of CIP were proposed. The presence of common anions in the PMS/FeOCl/HA system, including HCO₃^-, Cl^-, SO₄^2-, and NO₃^-, enhanced the degradation efficiency of CIP to varying degrees at the concentrations of 10 mM. Moreover, FeOCl maintained a high degradation capability for CIP after several recycles. This work offers a new promising means of catalyzing the PMS-based AOPs in the degradation of refractory organics.

Accelerating the peroxidase-like activity of Co2+ by quinaldic acid: Mechanism and its analytical applications

Although enzyme mimics have been widely developed, limited catalytic efficiency is still a bottleneck, especially under neutral condition. Herein, we reported the bioactive quinaldic acid (QA) significantly boosted the peroxidase-like activity of Co²⁺ in the presence of bicarbonate (HCO₃^-). With 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS) as the substrate, the catalytic activity of Co²⁺ (1 μM) was increased by over 300 times upon adding 100 μM QA. The formed Co²⁺ complex had much higher turnover number (5.52 min^-1) than that of cobalt-based nanozymes (0.011-0.51 min^-1) in decomposing H₂O₂. Based on this system, ultrasensitive colorimetric methods for the detection of Co²⁺, bicarbonate and urease activity were achieved with limits of detection of 4.6 nM, 40 μM and 0.00125 U/mL, respectively. For the first time, this work established an ultrasensitive method for the detection of urease activity by activating a peroxidase-like mimic with the produced HCO₃^-.

Ca(OH)2-mediated activation of peroxymonosulfate for the degradation of bisphenol S

Alkaline substances could activate peroxymonosulfate (PMS) for the removal of organic pollutants, but relatively high alkali consumption is generally required, which can cause too high pH of the solution after the reaction and lead to secondary pollution. Within this study, PMS activated by a relatively low dosage of Ca(OH)₂ (1 mM) exhibited excellent efficiency in the removal of bisphenol S (BPS). The pH of the solution declined to almost neutral (pH = 8.2) during the reaction period and conformed to the direct emission standards (pH = 6–9). In a typical case, BPS was completely degraded within 240 min and followed the kinetics of pseudo-first-order. The degradation efficiency of BPS depended on the operating parameters, such as the Ca(OH)₂, PMS and BPS dosages, initial solution pH, reaction temperature, co-existing anions, humic acid (HA), and water matrices. Quenching experiments were performed to verify that singlet oxygen (¹O₂) and superoxide radicals (O₂˙⁻) were the predominant ROS. Degradation of BPS has been significantly accelerated as the temperature increased. Furthermore, degradation of BPS could be maintained at a high level across a broad range of pH values (5.3–11.15). The SO₄⁻, NO₃⁻ did not significantly impact the degradation of BPS, however, both HCO₃⁻ and HA inhibited oxidation of BPS by the Ca(OH)₂/PMS system, and Cl⁻ had a dual-edged sword effect on BPS degradation. In addition, based on the 4 identified intermediates, 3 pathways of BPS degradation were proposed. The degradation of BPS was lower in domestic wastewater compared to other naturals waters and ultrapure; nevertheless, up to 75.86%, 77.94% and 81.48% of BPS was degraded in domestic wastewater, Yaohu Lake water and Poyang Lake water, respectively. Finally, phenolic chemicals and antibiotics, including bisphenol A, norfloxacin, lomefloxacin hydrochloride, and sulfadiazine could also be efficiently removed via the Ca(OH)₂/PMS system.

Ca(OH)₂ can activate PMS to effectively remove BPS, and it can meet the requirements of direct discharge after reaction.

Activation of peroxymonosulfate by bicarbonate and acceleration of the reaction by freezing

This study demonstrates the positive effects of dissolved bicarbonate and carbonate anions on peroxymonosulfate (PMS) induced oxidation and the remarkable acceleration of the reaction by freezing. More than 90% of the initial 4-chlorophenol (4-CP) decomposed in the frozen case, whereas only less than 20% of the 4-CP was removed in the aqueous case in the same time period. This accelerated reaction is attributed to the freeze-concentration of the dissolved substrates (i.e., PMS, bicarbonate, and pollutants) in the quasi-liquid layer at the ice grain boundaries between ice crystals. The reaction between bicarbonate and PMS was found to be unique because none of the effects were observed in the phosphate and hydroxide cooperated system with freezing, although the base activation of PMS could participate under basic conditions (pH > 9). Based on electron paramagnetic resonance spectroscopy measurements and comparison with the photo-excited Rose Bengal system as a reference system for singlet oxygen (¹O₂) generation, ¹O₂ was found to have a minor effect on the oxidation of 4-CP in the frozen bicarbonate-PMS system. While, direct electron transfer from the target organic substrate to the PMS was suggested as a major mechanism of 4-CP oxidation, because the selected target organic substrates were decomposed with different tendencies, and the consumption of PMS was accelerated by the presence of an electron donating compound. The results show the potential applicability of the freezing phenomenon, which occurs naturally in the mid-latitude and polar area, to help a decomposition of water dissolved organic pollutants by the imitation of the natural purification process.

Degradation of phosphonates in Co(II)/peroxymonosulfate process: Performance and mechanism

The increased release of phosphonates to natural waters causes global concern due to their potential threat to the aquatic environment. It is curial to mineralize phosphonates to orthophosphate (PO₄^3-) before they are thoroughly removed from wastewater via conventional biological treatment. In this study, we systematically investigated the performance and mechanism of degradation of phosphonates in Co(II)-triggered peroxymonosulfate (PMS) activation process. The degradation efficiency of various phosphonates is highly dependent on their coordination with Co(II). Using 1-hydroxyethane 1,1-diphosphonic acid (HEDP) as a target pollutant, the Co(II)/PMS process is effective in a broad solution pH range from 5.0 to 10.0. Multiple experimental results imply that Co(II)-PMS complex is the primary reactive species, while hydroxyl radicals (HO^•), sulfate radicals (SO₄^•-), singlet oxygen (¹O₂) and Co(III) play as the secondary reactive species for the degradation of HEDP. The presence of Cl^-, HCO₃^-, and natural organic matters (NOM) inhibits the degradation of HEDP. However, in real water samples, the selectivity and efficiency for HEDP removal in the Co(II)/PMS process are higher than that in free radicals-mediated advanced oxidation processes. This study not only sheds new lights on the mechanism of Co(II)-triggered PMS activation process, but also provides feasible technology for the degradation of phosphonates in wastewater.

Recent advances in cobalt-activated sulfate radical-based advanced oxidation processes for water remediation: A review

Sulfate radical-based advanced oxidation processes (SR-AOPs) have attracted increasing attention for the degradation of organic contaminants in water. The oxidants of SR-AOPs could be activated to generate different kinds of reactive oxygen species (ROS, e.g., hydroxyl radicals (OH), sulfate radicals (SO₄^-), singlet oxygen (¹O₂), and superoxide radicals (O₂^-)) by various catalysts. As one of the promising catalysts, cobalt-based catalysts have been extensively investigated in catalytic activity and stability during water remediation. This article mainly summarizes recent advances in preparation and applications of cobalt-based catalysts on peroxydisulfate (PDS)/peroxymonosulfate (PMS) activation since 2016. The review covers the development of homogeneous cobalt ions, cobalt oxides, supported cobalt composites, and cobalt-based mixed metal oxides for PDS/PMS activation, especially for the latest nanocomposites such as cobalt-based metal-organic frameworks and single-atom catalysts. This article also discussed the activation mechanisms and the influencing factors of different cobalt-based catalysts for activating PDS/PMS. Finally, the future perspectives on the challenges and applications of cobalt-based catalysts are presented at the end of this paper.

Atomically Dispersed Cobalt Sites on Graphene as Efficient Periodate Activators for Selective Organic Pollutant Degradation

Pollutant degradation via periodate (IO₄^-)-based advanced oxidation processes (AOPs) provides an economical, energy-efficient way for sustainable pollution control. Although single-atomic metal activation (SMA) can be exploited to optimize the pollution degradation process and understand the associated mechanisms governing IO₄^--based AOPs, studies on this topic are rare. Herein, we demonstrated the first instance of using SMA for IO₄^- analysis by employing atomically dispersed Co active sites supported by N-doped graphene (N-rGO-CoSA) activators. N-rGO-CoSA efficiently activated IO₄^- for organic pollutant degradation over a wide pH range without producing radical species. The IO₄^- species underwent stoichiometric decomposition to generate the iodate (IO₃^-) species. Whereas Co²⁺ and Co₃O₄ could not drive IO₄^- activation; the Co-N coordination sites exhibited high activation efficiency. The conductive graphene matrix reduced the contaminants/electron transport distance/resistance for these oxidation reactions and boosted the activation capacity by working in conjunction with metal centers. The N-rGO-CoSA/IO₄^- system exhibited a substrate-dependent reactivity that was not caused by iodyl (IO₃^·) radicals. Electrochemical experiments demonstrated that the N-rGO-CoSA/IO₄^- system decomposed organic pollutants via electron-transfer-mediated nonradical processes, where N-rGO-CoSA/periodate* metastable complexes were the predominant oxidants, thereby opening a new avenue for designing efficient IO₄^- activators for the selective oxidation of organic pollutants.

Nonradicals induced degradation of organic pollutants by peroxydisulfate (PDS) and peroxymonosulfate (PMS): Recent advances and perspective

Nonradical persulfate oxidation processes have emerged as a new wastewater treatment method due to production of mild nonradical oxidants, selective oxidation of organic pollutants, and higher tolerance to water matrixes compared with radical persulfate oxidation processes. Since the case of the nonradical activation of peroxydisulfate (PDS) was reported on CuO surface in 2014, nonradical persulfate oxidation processes have been extensively investigated, and much achievement has been made on realization of nonradical persulfate activation processes and understanding of intrinsic reaction mechanism. Therefore, in the review, nonradical pathways and reaction mechanisms for oxidation of various organic pollutants by PDS and peroxymonosulfate (PMS) are overviewed. Five nonradical persulfate oxidation pathways for degradation of organic pollutants are summarized, which include surface activated persulfate, catalysts-free or catalysts mediated electron transfer, ¹O₂, high-valent metals, and newly derived inorganic oxidants (e.g., HOCl and HCO₄^-). Among them, the direct oxidation processes by persulfate, nonradical based persulfate activation by inorganic/organic molecules and in electrochemical methods is first overviewed. Moreover, nonradical based persulfate activation mechanisms by metal oxides and carbon materials are further updated. Furthermore, investigation methods of interaction between persulfate and catalyst surface, and nature of reactive species are also discussed in detail. Finally, the future research needs are proposed based on limited understanding on reaction mechanism of nonradical based persulfate activation. The review can offer a comprehensive assessment on nonradical oxidation of organic pollutants by persulfate to fill the knowledge gap and provide better guidance for future research and engineering application of persulfate.

Activation of peroxymonosulfate by phosphite: Kinetics and mechanism for the removal of organic pollutants

In this study, phosphite (HPO₃^2-) was used as a novel activator to activate peroxymonosulfate (PMS) for acid orange 7 (AO7) removal. Under the optimized conditions, the decolorization efficiency of AO7 was 82.1% within 60 min with rate constant values (k_obs) of 0.0301 min^-1. Besides, effects of the solution pH and the co-existing inorganic anions including Cl^-, HCO₃^-, HPO₄^2- and SO₄^2- on AO7 removal were also investigated. Except for SO₄^2-, other examined co-existing inorganic anions displayed favorable effects on the removal of AO7. Furthermore, the mechanism for PMS activation by the HPO₃^2- was deeply elucidated by radical scavenger including ethanol (EtOH), tert-butanol (TBA), l-histidine and tiron, and electron spin resonance (ESR) studies. It was proposed that singlet oxygen (¹O₂) would be the dominant reactive oxygen species (ROS) in the HPO₃^2-/PMS system for contamination degradation at neutral pH condition. The findings of this study provided useful information for the application of the substances in industrial wastewaters to activate PMS for organic contaminants degradation and in particular for HPO₃^2--rich electroplating wastewater treatment.

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.

Unraveling the Overlooked Involvement of High-Valent Cobalt-Oxo Species Generated from the Cobalt(II)-Activated Peroxymonosulfate Process

Sulfate radical (SO₄^•-) is widely recognized as the predominant species generated from the cobalt(II)-activated peroxymonosulfate (PMS) process. However, in this study, it was surprisingly found that methyl phenyl sulfoxide (PMSO) was readily oxidized to the corresponding sulfone (PMSO₂) with a transformation ratio of ∼100% under acidic conditions, which strongly implied the generation of high-valent cobalt-oxo species [Co(IV)] instead of SO₄^•- in the Co(II)/PMS process. Scavenging experiments using methanol (MeOH), tert-butyl alcohol, and dimethyl sulfoxide further suggested the negligible role of SO₄^•- and hydroxyl radical (^•OH) but favored the generation of Co(IV). By employing ¹⁸O isotope-labeling technique, the formation of Co(IV) was conclusively verified and the oxygen atom exchange reaction between Co(IV) and H₂O was revealed. Density functional theory calculation determined that the formation of Co(IV) was thermodynamically favorable than that of SO₄^•- and ^•OH in the Co(II)/PMS process. The generated Co(IV) species was indicated to be highly reactive due to the existence of oxo-wall and capable of oxidizing the organic pollutant that is rather recalcitrant to SO₄^•- attack, for example, nitrobenzene. Additionally, the degradation intermediates of sulfamethoxazole (SMX) in the Co(II)/PMS process under acidic conditions were identified to further understand the interaction between Co(IV) and the representative contaminant. The developed kinetic model successfully simulated PMSO loss, PMSO₂ production, SMX degradation, and/or PMS decomposition under varying conditions, which further supported the proposed mechanism. This study might shed new light on the Co(II)/PMS process.

Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks

Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

Reactive Cobalt⁻Oxo Complexes of Tetrapyrrolic Macrocycles and N-based Ligand in Oxidative Transformation Reactions

High-valent cobalt–oxo complexes are reactive transient intermediates in a number of oxidative transformation processes e.g., water oxidation and oxygen atom transfer reactions. Studies of cobalt–oxo complexes are very important for understanding the mechanism of the oxygen evolution center in natural photosynthesis, and helpful to replicate enzyme catalysis in artificial systems. This review summarizes the development of identification of high-valent cobalt–oxo species of tetrapyrrolic macrocycles and N-based ligands in oxidation of organic substrates, water oxidation reaction and in the preparation of cobalt–oxo complexes.

Efficient degradation of imipramine by iron oxychloride-activated peroxymonosulfate process

Synthesized iron oxychloride (FeOCl) was firstly applied to activate peroxymonosulfate (PMS) to degrade imipramine (IMI), a tricyclic antidepressant. Compared to some other Fe-based materials including zero valent iron, Fe₂O₃, Fe₃O₄ and ferric ions, FeOCl presented an impressive catalytic activity on PMS at near-neutral condition due to its unique structure containing abundant unsaturated iron atoms and oxo-bridged configuration. With an increase of FeOCl dose, PMS dose or initial pH in ranges of 0.02 - 0.5 g/L, 0.1 - 2.5 mM and 4.0 - 8.0, the degradation efficiency of IMI was effectively raised by 64.0%, 48.5% and 50.6%, respectively. The presence of either bicarbonate or chloride stimulated the removal of IMI. Moreover, 70.4% of IMI was degraded under the background of real water with 2 mM PMS. The possible reactive species were identified as sulfate and hydroxyl radicals. The formed hypochlorite through the reaction of PMS and the released chloride ions may also contribute to the degradation of IMI. Among the oxidants, sulfate radical was proven to be the dominate one in the system. Additionally, the FeOCl/PMS system can overall effectively degrade six other organic compounds including amitriptyline, desipramine, propranolol, nitrobenzene, methyl-paraben and ethyl-paraben, further suggesting the possible application of this system in treatment of vast aquatic micro-organic pollutants.