Advances in sustainable nano-biochar: precursors, synthesis methods and applications.

Nano-biochar, characterized by its environmentally friendly nature and unique nanostructure, offers a promising avenue for sustainable carbon materials. With its small particle size, large specific surface area, abundant functional groups and tunable pore structure, nano-biochar stands out due to its distinct physical and chemical properties compared to conventional biochar. This paper aims to provide an in-depth exploration of nano-biochar, covering its sources, transformation mechanisms, properties, applications, and areas requiring further research. The discussion begins with an overview of biomass sources for nano-biochar production and the conversion processes involved. Subsequently, primary synthesis methods and strategies for functionalization enhancement are examined. Furthermore, the applications of nano-biochar in catalysis, energy storage, and pollutant adsorption and degradation are explored and enhanced in various fields.

New insight into the spatio-temporal patterns of functional groups of hotspot inside the composting aggregates by synchrotron-based FTIR in hyperthermophilic composting.

Hyperthermophilic composting (HTC) is a recently developed and highly promising organic fraction of municipal solid waste (OFMSW) treatment technology. Investigation of organic matter (OM) dynamics in compost particle is thus crucial for the understanding of humification of HTC process. Herein, this work aimed to study the chemical and structural changes of OM at the molecular level during HTC of OFMSW using EEM and SR-FTIR analyses. Additionally, two-dimensional correlation spectroscopy (2D-COS) was also utilized to probe and identify the changes in chemical constituents and functional groups of organic compounds on the surface of compost particles during different composting periods. Results show that SR-FTIR can detect fine-scale (~µm) changes in functional groups from the edges to the interior of compost particles during different composting periods by mapping the particles in situ. In the hyperthermophilic stage (day 9), the extracted µ-FTIR spectrum reveals a distinct boundary between anaerobic and aerobic regions within the compost particle, with a thickness of anaerobic zone (1460 cm-1) of approximately 30 µm inside the particle's core. This provides direct evidence of anaerobic trends at compost microscales level within compost particles. 2D-COS analysis indicated that organic functional groups gradually agglomerated in the order of 1330 > 2930 > 3320 > 1600 > 1030 > 895 cm-1 to the core skeleton of cellulose degradation residues, forming compost aggregates with well physicochemical properties. Overall, the first combination of SR-FTIR and EEM provides complementary explanations for the humification mechanism of HTC, potentially introducing a novel methodology for investigating the environmental behaviors and fates of various organic contaminants associated with OM during the in-situ composting biochemical process.

Biochar applications for treating potentially toxic elements (PTEs) contaminated soils and water: a review.

Environmental pollution with potentially toxic elements (PTEs) has become one of the critical and pressing issues worldwide. Although these pollutants occur naturally in the environment, their concentrations are continuously increasing, probably as a consequence of anthropic activities. They are very toxic even at very low concentrations and hence cause undesirable ecological impacts. Thus, the cleanup of polluted soils and water has become an obligation to ensure the safe handling of the available natural resources. Several remediation technologies can be followed to attain successful remediation, i.e., chemical, physical, and biological procedures; yet many of these techniques are expensive and/or may have negative impacts on the surroundings. Recycling agricultural wastes still represents the most promising economical, safe, and successful approach to achieving a healthy and sustainable environment. Briefly, biochar acts as an efficient biosorbent for many PTEs in soils and waters. Furthermore, biochar can considerably reduce concentrations of herbicides in solutions. This review article explains the main reasons for the increasing levels of potentially toxic elements in the environment and their negative impacts on the ecosystem. Moreover, it briefly describes the advantages and disadvantages of using conventional methods for soil and water remediation then clarifies the reasons for using biochar in the clean-up practice of polluted soils and waters, either solely or in combination with other methods such as phytoremediation and soil washing technologies to attain more efficient remediation protocols for the removal of some PTEs, e.g., Cr and As from soils and water.

Progress in electrocatalytic hydrogen evolution of transition metal alloys: synthesis, structure, and mechanism analysis.

At present, the problems of high energy consumption and low efficiency in electrocatalytic hydrogen production have limited the large-scale industrial application of this technology. Constructing effective catalysts has become the way to solve these problems. Transition metal alloys have been proved to be very promising materials in hydrogen evaluation reaction (HER). In this study, the related theories and characterization methods of electrocatalysis are summarized, and the latest progress in the application of binary, ternary, and high entropy alloys to HER in recent years is analyzed and studied. The synthesis methods and optimization strategies of transition metal alloys, including composition regulation, hybrid engineering, phase engineering, and morphological engineering were emphatically discussed, and the principles and performance mechanism analysis of these strategies were discussed in detail. Although great progress has been made in alloy catalysts, there is still considerable room for applications. Finally, the challenges, prospects, and research directions of transition metal alloys in the future were predicted.

Technologies for pollutant removal and resource recovery from blackwater: a review.

Blackwater (BW), consisting of feces, urine, flushing water and toilet paper, makes up an important portion of domestic wastewater. The improper disposal of BW may lead to environmental pollution and disease transmission, threatening the sustainable development of the world. Rich in nutrients and organic matter, BW could be treated for resource recovery and reuse through various approaches. Aimed at providing guidance for the future development of BW treatment and resource recovery, this paper presented a literature review of BWs produced in different countries and types of toilets, including their physiochemical characteristics, and current treatment and resource recovery strategies. The degradation and utilization of carbon (C), nitrogen (N) and phosphorus (P) within BW are underlined. The performance of different systems was classified and summarized. Among all the treating systems, biological and ecological systems have been long and widely applied for BW treatment, showing their universality and operability in nutrients and energy recovery, but they are either slow or ineffective in removal of some refractory pollutants. Novel processes, especially advanced oxidation processes (AOPs), are becoming increasingly extensively studied in BW treatment because of their high efficiency, especially for the removal of micropollutants and pathogens. This review could serve as an instructive guidance for the design and optimization of BW treatment technologies, aiming to help in the fulfilment of sustainable human excreta management.

Pollution characteristics and health risks of heavy metals in road dust in Ma'anshan, China.

Road dust contains various heavy metals, which are re-suspension in the air under the action of wind and other external forces, threatening people's health all the time. Road dust was collected in the industrial heavy traffic area (IHT), non-industrial heavy traffic area (HT), urban area (UA), and study recreation area (SR) of Ma'anshan. The pollution degree of heavy metals in the four areas was calculated and demonstrated IHT > HT > UA > SR. In addition to the Ni (24.24 mg kg-1)metals, the metals concentrations of Cr (74.14 mg kg-1), Cu (91.8 mg kg-1), Zn (393.03 mg kg-1), Cd (9.93 mg kg-1), and Pb (72.85 mg kg-1) were all higher than the local soil background values. Cu comes from traffic emissions, Pb, Cd, and Zn mainly come from industrial emissions, as well as traffic emissions. While Cr and Ni mainly come from industrial emissions and local soil re-suspension. The non-carcinogenic risk of each heavy metal to children is 10 times higher than that of adults. Among them, the non-carcinogenic risk of Cr, Cd, and Pb to children is close to 1, so great attention should be paid to it. According to the study of enrichment factor (EF) and geo-accumulation index (Igeo), Cd is extremely polluted and it is imperative to reduce Cd pollution.

New insight in algal cell adhesion and cake layer evolution in algal-related membrane processes: Size-fractioned particles, initial foulant seeds and EDEM simulation.

A clear understanding of algal cell adhesion and cake layer evolution in algal-related membrane processes (ARMPs) is urgently required to mitigate the membrane fouling. In this study, the effect of microparticles (10 µm-30 µm), subvisible particles (0.45 µm-10 µm), and ultrafine particles (50 kDa-0.45 µm) on the membrane fouling were explored based on the filtration performance through Hermia models, thermodynamic analysis, and simulation of extended discrete element method (EDEM). The results illustrated that microparticles played an important role in algal cell aggregation and the formation of initial clusters. Intermediate blocking fouling occurred when filtrating the subvisible particle, which facilitated internal adhesion and enhanced biofilm formation. In addition, the interfacial attractive force for the initial algal adhesion was obviously increased when the membrane surfaces were in high concentration of protein and polysaccharide. Moreover, the EDEM simulation demonstrated that subsequent particles, particularly the particles with small sizes, preferred to occupy the spaces among the previously deposited particles. This study provided new insights into the contributions of size-fractioned particles to initial fouling and their influence on the successive adhesion of other contaminants.

Microalgal wastewater recycling: Suitability of harvesting methods and influence on growth mechanisms.

Wastewater recycling helps address the challenge of microalgae biomass commercialization by allowing for efficient resource recovery. In this study, three conventional harvesting methods, including centrifugation, microfiltration, and flocculation sedimentation, were investigated to explore the effects of harvesting methods on the characteristics of recycled wastewater and the growth of microalgae to select a suitable harvesting method for the microalgal wastewater recycling system. During the wastewater recycling process, the least amount of accumulated substances was exhibited in the wastewater recycled by microfiltration, followed by centrifugation, and the most by flocculation sedimentation. After 4 batches of cultivation, microalgal biomass harvested from centrifugation wastewater and microfiltration wastewater was 21.26 % and 13.54 % higher than that from flocculation wastewater, respectively. Lipids, carbohydrates and pigments were all increased by varying degrees. Additionally, flocculation sedimentation was not suitable for the microalgal wastewater recycling process since the low residual nutrients, high salinity, and excessive algal organic matter severely inhibited the growth of microalgae. Under the regulation of phytohormones, microalgae increased their energy reserves, enhanced photosynthesis, and improved their defense capability to resist the increasing abiotic stress. This study provides scientific support for the selection of suitable harvesting technology during the microalgal wastewater recycling process.

Biogenic synthesis of reduced graphene oxide decorated with silver nanoparticles (rGO/Ag NPs) using table olive (olea europaea) for efficient and rapid catalytic reduction of organic pollutants.

In this work, graphene oxide (GO) sheets were prepared via a facile electrochemical exfoliation of graphite in acidic medium and subsequent oxidation with potassium permanganate. The GO sheets were employed for preparation of reduced GO adorned with nanosized silver (rGO/Ag NPs) using green reduction of GO and Ag(I) via olive fruit extract as a reducing and immobilizing agent. The crystal phase, morphology, and nanostructure of the prepared catalyst were characterized by XRD, SEM, EDX, UV-Vis and Raman spectroscopy techniques. The as-prepared rGO/Ag NPs showed superior catalytic performance towards the complete reduction (up to 99%) of 4-nitrophenol (4-NPH) to 4-aminophenol (4-APH) and rhodamine B (RhB) to Leuco RhB within 180 s using NaBH4 at ambient condition. The rate constant (k) values were found to be 0.021 and 0.022 s-1 for 4-NPH and RhB reduction, respectively. In addition, the regenerated catalyst could be reused after seven cycles without losing any apparent catalytic efficiency. Accounting for the excellent catalytic capability, chemical stability and environment-friendly synthesis protocol, the rGO/Ag NPs has great potential working as a heterogeneous catalyst in the transforming harmful organic contaminants into less harmful or harmless compounds.

Application of sulfur-coated magnetic carbon nanotubes for extraction of some polycyclic aromatic hydrocarbons from water resources.

In the present work, novel sulfur-coated magnetic carbon nanotubes (MCNTs-S) material was fabricated by S coating on the MCNTs using a simple heating procedure. TGA, EDX, XRD, TEM, and VSM were employed to characterize the as-prepared composite. Using HPLC-UV system, the produced superparamagnetic sorbent was employed for the extraction and measurement of trace levels of five polycyclic aromatic hydrocarbons (PAHs) in environmental waters. The synergistic effect of the sulfur layer and CNTs substrate is primarily responsible for the remarkable extraction efficiency of the MCNTs-S sorbent towards PAHs. The experimental factors including MCNTs-S dosage, sorption time, elution solvent, ionic strength and solution pH were explored and optimized. Considering that the ionic strength and pH do not have any impact on the PAHs extraction, as a result, there is no need the unnecessary adjustment of the water samples. The linear dynamic ranges and detection limits under optimal conditions were in the range of 0.05-0.11 ng mL-1 and 0.2-150 ng mL-1, respectively. The analysis of PAHs in the real samples (sea water and river water) using this approach was successfully assessed with appropriate recovery values (94.6%-99.0%).

PAC-UF Process Improving Surface Water Treatment: PAC Effects and Membrane Fouling Mechanism.

In this study, the water purification effect and membrane fouling mechanism of two powdered activated carbons (L carbon and S carbon) enhancing Polyvinylidene Fluoride (PVDF) ultrafiltration (UF) membranes for surface water treatment were investigated. The results indicated that PAC could effectively enhance membrane filtration performance. With PAC addition, organic removal was greatly enhanced compared with direct UF filtration, especially for small molecules, i.e., the S-UF had an additional 25% removal ratio of micro-molecule organics than the direct UF. The S carbon with the larger particle size and lower specific surface area exhibited superior performance to control membrane fouling, with an operation duration of S-UF double than the direct UF. Therefore, the particle size and pore structure of carbon are the two key parameters that are essential during the PAC-UF process. After filtration, acid and alkaline cleaning of UF was conducted, and it was found that irreversible fouling contributed the most to total filtration resistance, while the unrecoverable irreversible resistance ratio with acid cleaning was greater than that with alkaline cleaning. With PAC, irreversible UF fouling could be relieved, and thus, the running time could be extended. In addition, the membrane foulant elution was analyzed, and it was found to be mainly composed of small and medium molecular organic substances, with 12% to 21% more polysaccharides than proteins. Finally, the hydrophilicity of the elution was examined, and it was observed that alkaline cleaning mainly eluted large, medium, and small molecules of hydrophilic and hydrophobic organic matter, while acid cleaning mainly eluted small molecules of hydrophilic organic matter.

Effect of Ammonia on Laminar Combustion Characteristics of Methane-Air Flames at Elevated Pressures.

In this paper, laminar combustion characteristics of methane/ammonia/air flames are numerically investigated using the Chemkin/Premix code. The initial temperature is set as 298 K; the initial pressures are set as 1, 2, 5, 10, and 20 atm; and the equivalence ratios are set as 0.8-1.6. Laminar burning velocity (LBV); adiabatic flame temperature (AFT); net heat release rate (NHRR); and the mole fractions of H, NH2, NO, NO2, and HCN at stoichiometric ratio are studied with ammonia (NH3) addition. Meanwhile, temperature sensitivity and rate of production (ROP) are analyzed. The results show that with the increase of the initial pressures, LBV decreases and AFT and NHRR increase. With the increase of ammonia doping ratios, LBV, AFT, and NHRR decrease. From temperature sensitivity analyses, the main reactions that promote temperature rise are R39 (H + O2 < = > O + OH), R100 (OH + CH3 < = > CH2(S) + H2O), R102 (OH + CO < = > H + CO2), and R122 (HO2 + CH3 < = > OH + CH3O). The main reactions that inhibit temperature rise are R53 (H + CH3(+M) < = > CH4(+M)), R36 (H + O2 + H2O < = > HO2 + H2O), and R46 (H + HO2 < = > O2 + H2). For the rate of production of the free radical pool, the trends of H and NO are consuming first and then producing, and the trends of NH2, NO2, and HCN are the opposite. The pathway from methane to carbon dioxide is CH4 → CH3 → CH3O → CH2O → HCO → CO → CO2, and the pathway from ammonia to nitrogen is NH3 → NH2 → NH/HNO → NO/NO2 → N2.

Evaluation of the performance of different membrane materials for microalgae cultivation on attached biofilm reactors.

Attached microalgae production in wastewater is a promising method to further develop biofilm reactors by reducing economic costs associated with biomass separation and harvesting. However, the reliability of materials to support such adherence needs further investigation. Five common microfiltration membranes were evaluated in this study to assess their influence on the efficacy of harvesting Chlorella pyrenoidosa. The material-to-material, algae-to-algae, and algae-to-material interactions were studied based on the Extended Derjaguin, Landau, Verwey, Overbeek (XDLVO) theory. The results showed that Chlorella pyrenoidosa was hydrophobic and that the algae particles derived from this algae type tended to agglomerate. Furthermore, the algae-membrane adhesion free energy further validated the accumulation of biomass in the experiments - the cellulose acetate nitrate (CACN) membrane and the cellulose acetate (CA) membrane obtained an optical biomass production of 59.93 and 51.27 g m-2. The presence of these interactions promoted the adhesion of more microalgae particles to the membrane. Moreover, the relationship between the algae-membrane and the distance at which the microalgae approached the membrane surface was simulated. The study indicated that the XDLVO theory could be successfully applied to the mechanism for the adhesion of the attached culture of Chlorella pyrenoidosa to the membrane material.

A promising microalgal wastewater cyclic cultivation technology: Dynamic simulations, economic viability, and environmental suitability.

The microalgal wastewater cyclic cultivation technology (AWC2T) proposed in this study helps address the challenges surrounding water scarcity and ecological sustainability in a clean, resource-efficient, and affordable manner. A novel microalgae growth model (AGM) elucidating the growth mechanisms of microalgae in the AWC2T system was established for dynamic simulations and design optimization. The recycled wastewater accelerated the growth rate of microalgae, and increased biomass and lipids content by 11% and 37.65%, respectively, after 8 batches of cultivation. The accumulated soluble algae products (SAPs) enhanced microalgae growth by providing nutrients and regulating metabolism. In addition, scenario simulations illustrated the excellent long-term performance of the AWC2T system. 100% recycling of microalgal wastewater could save 0.3% N and 54.36% P. The techno-economic analysis (TEA) and life cycle assessment (LCA) explored how economic and sustainability principles can be embedded into the life cycle of microalgae production. The AWC2T led to outcomes vastly superior to non-cyclic technology by enabling the high-level recovery of resources, providing substantial benefits, enhancing contingency and risk resistance, and offsetting a host of unintended environmental effects.

Diatomite Dynamic Membrane Fouling Behaviour during Dewatering of Chlorella pyrenoidosa in Aquaculture Wastewater.

Combined microalgal and membrane filtration could effectively treat aquaculture wastewater; however, the membrane fouling induced by extracellular organic matter (EOM) during the dewatering process is an issue. This study investigated diatomite dynamic membrane (DDM) fouling behaviour during the dewatering of Chlorella pyrenoidosa under the influence of copper ions. The results indicate that copper ion heavy metals in aquaculture wastewater significantly affected purification and algae dewatering by DDM. Aquaculture wastewater with a high copper concentration (1 and 0.5 mg/L) could induce serious DDM fluxes and cake layer filtration resistance (Rc), whereas fewer filtration fluxes were induced when aquaculture wastewater had a low copper concentration, particularly that of 0.1 mg/L, at which the Rc was lowest and the concentration effect was highest. Macromolecular organics of EOM, such as biopolymers, polysaccharides, and proteins, were responsible for DDM fouling and accumulated mostly in the slime layer, whereas only a small amount of them accumulated in the diatomite layer. The DDM rejected more protein-like organics of EOM in the slime layer when dewatering algae at low copper concentrations (<0.1 mg/L); however, when using the DDM to dewater algae at high copper concentrations, more polysaccharides of EOM were rejected (0.5 < Cu2+ < 5 mg/L). This result has significant ramifications for aquaculture wastewater treatment as well as algae separation and concentration by the DDM.

Application of Coagulation-Membrane Rotation to Improve Ultrafiltration Performance in Drinking Water Treatment.

The combination of conventional and advanced water treatment is now widely used in drinking water treatment. However, membrane fouling is still the main obstacle to extend its application. In this study, the impact of the combination of coagulation and ultrafiltration (UF) membrane rotation on both fouling control and organic removal of macro (sodium alginate, SA) and micro organic matters (tannic acid, TA) was studied comprehensively to evaluate its applicability in drinking water treatment. The results indicated that membrane rotation could generate shear stress and vortex, thus effectively reducing membrane fouling of both SA and TA solutions, especially for macro SA organics. With additional coagulation, the membrane fouling could be further reduced through the aggregation of mediate and macro organic substances into flocs and elimination by membrane retention. For example, with the membrane rotation speed of 60 r/min, the permeate flux increased by 90% and the organic removal by 35% in SA solution, with 40 mg/L coagulant dosage, with an additional 70% increase of flux and 5% increment of organic removal to 80% obtained. However, too much shear stress could intensify the potential of fiber breakage at the potting, destroying the flocs and resulting in the reduction of permeate flux and deterioration of effluent quality. Finally, the combination of coagulation and membrane rotation would lead to the shaking of the cake layer, which is beneficial for fouling mitigation and prolongation of membrane filtration lifetime. This study provides useful information on applying the combined process of conventional coagulation and the hydrodynamic shear force for drinking water treatment, which can be further explored in the future.

Natural organic matter separation by forward osmosis: Performance and mechanisms.

The purification performance of a forward osmosis (FO) membrane on natural organic matter (NOM) contained in real surface water by was investigated systematically. FO could reject the natural dissolved organic matter (DOM) effectively with removal efficiencies of approximately 99.0%. When the natural water samples (e.g., raw surface water) had lower fouling tendencies, the active layer facing the draw solution (AL-facing-DS mode) provided a higher water flux than that in the alternative membrane orientation because the isoflux point occurred later in the process. It was found that the concentration of calcium ions had a more severe effect on decreasing the fouling flux of the FO membrane than that of the organic foulant. Furthermore, the concentrated feed solution had a more significant effect on the fouling flux decline of the natural DOM containing more small molecules than natural DOM containing more macromolecules. Additionally, the fouling that occurred in the AL-facing-DS orientation was compensated by the reduced internal concentration polarization (ICP) level based on the occurrence of the critical compensation point. It was also revealed that the permeation drag caused by the water flux and the chemical interactions induced by the feed solution pH and the calcium ion concentration played a significant role in the adsorption of small natural DOM molecules in the porous structure of the FO membrane. Based on the analysis of the interfacial free energies, the interactions between the natural DOM and the surface of the support layer dominated the initial fouling of the FO membrane, while subsequent fouling was controlled by the interaction between the approaching DOM molecules and the already adsorbed DOM.

Remarkable phosphate recovery from wastewater by a novel Ca/Fe composite: Synergistic effects of crystal structure and abundant oxygen-vacancies.

Calcium-based materials are considered to be promising adsorbents for phosphate removal in the water environment due to their environmental friendliness and low price. However, improving the efficiency and rate of P adsorption of calcium-based materials still needs further exploration. In this study, a high-efficiency and eco-friendly Ca/Fe composite was rationally designed and fabricated by a co-precipitated method. Batch adsorption experiments showed that Ca/Fe composites with a Ca: Fe molar ratio of 3: 1 exhibited a remarkable phosphate sorption capacity of 161.4 mg P/g. Furthermore, the phosphate adsorption capacity of Ca/Fe-3/1 composite was maintained relatively high at pH 3-11 due to the ligand exchange, electrostatic and chemical precipitation. In addition, the experiment performed to determine the effect of coexisting ions shows that only carbonate ions slightly inhibit the phosphate adsorption effect of the Ca/Fe-3/1 composite. The newly prepared Ca/Fe composites have a fast phosphate removal efficiency. The XPS and EPR analysis showed that a large number of oxygen vacancies were formed on Ca/Fe composites due to the introduction of magnetic Fe. This is the first time to introduction oxygen vacancies into Ca/Fe composites by co-precipitation. The existence of oxygen vacancies can promote electron transfer rate and reduce the bonding energy barrier for phosphate adsorption, thereby increasing the phosphate absorption rate of the Ca/Fe composites. The enhanced phosphate removal by Ca/Fe composites with abundant oxygen vacancies provides a new strategy for the preparation of commercial phosphate -controlling materials.

Can flow-electrode capacitive deionization become a new in-situ soil remediation technology for heavy metal removal?

In this study, we present a novel soil electrochemical remediation technology (called S-FCDI), which is based on flow-electrode capacitive deionization (FCDI), for Cd removal from kaolin while under continuous operation mode. The results demonstrated that Cd can be effectively removed from kaolin with reasonable energy consumption and minimal macroelement loss. The carboxylic (OOH) functional groups on the surface of activated carbon (AC) facilitated the transfer of Cd from kaolin onto carbon surface. A stable acidic environment, which is advantageous for continuous Cd desorption, was achieved as a result of the balance between H+ generation and transmembrane migration. Once these net negative charges on the particle were eliminated or reversed, the adsorbed Cd could be released easily and driven in concentrated stream by electrostatic repulsion. Under the optimal operating conditions (i.e., carbon =50 g/L, j = 3.47 A/m2, pHi = 3.2, [NaCl]a =8.6 mmol/L), more than 80 % Cd was removed from (200 g) kaolin after continuous 19 h operation at a relatively low electricity consumption of 22.7 kW h/kg Cd and a limited Al loss of 0.06 wt‰. These results from this work demonstrated that S-FCDI could be an alternative soil electrochemical remediation technology for heavy metal removal with low soil damage.

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.