Nonradical electron transfer-based peroxydisulfate activation by a Mn-Fe bimetallic oxide derived from spent alkaline battery for the oxidation of bisphenol A

Roles of naturally occurring biogenic iron-manganese oxides (BFMO) in PMS-based environmental remediation: A complete electron transfer pathway

Bisphenol A (BPA) is a pervasive endocrine disruptor that enters the environment through anthropogenic activities, posing significant risks to ecosystems and human health. Advanced oxidation processes (AOPs) are promising methods for the removal of organic microcontaminants in the environment. Biogenic manganese oxides (BMO) are reported as catalysts due to their transition metal nature, and are also readily generated by manganese-oxidizing microorganisms in the natural environment, and therefore their roles and effects in AOPs-based environmental remediation should be investigated. However, biogenic iron-manganese oxides (BFMO) are actually generated rather than BMO due to the coexistence of ferrous ions which can be oxidized to iron oxides. Therefore, this study produced BFMO originating from a highly efficient manganese-oxidizing fungus Cladosporium sp. XM01 and chose peroxymonosulfate (PMS) as a typical oxidant for the degradation of bisphenol A (BPA), a model organic micropollutant. Characterization results indicate that the formed BFMO was amorphous with a low crystallinity. The BFMO/PMS system achieved a high degradation performance that 85 % BPA was rapidly degraded within 60 min, and therefore the contribution of BFMO cannot be ignored during PMS-based environmental remediation. Different from the findings of previous studies (mostly radicals and singlet oxygen), the degradation mechanism was first proven as a 100 % electron-transfer pathway mediated by high-valence Mn under acidic conditions provided by PMS. The findings of this study provide new insights into the degradation mechanisms of pollutants using biogenic metal oxides in PMS activation and the contribution of their coexistence in AOPs-based environmental remediation.

Enhancement of peroxymonosulfate activation with nickel foam-supported CuCo2O4 for tetracycline degradation: Performance and mechanism insights

The modulation of bimetallic oxide structures and development of efficient, easily recoverable catalysts are expected to effectively overcome the limitations associated with powdered catalysts in activating peroxymonosulfate (PMS). In this study, CuCo2O4 was successfully immobilized on the surface of nickel foam (NF) via an electrodeposition-calcination procedure, with highly efficient activation of PMS for tetracycline (TC) degradation (0.55 min-1). Besides acting as a support carrier and providing ample active sites, NF mediated electron transport, prevented the leaching of metal ions and enhanced the efficiency of recycling. Density functional theory (DFT) calculations and experimental tests illustrated that Cu/Co dual-sites can efficiently adsorb PMS, enabling simultaneous reduction and oxidation reactions. The dual-site synergy substantially decreased the adsorption barrier and increased the electron transfer rate. Especially, the Cu+/Cu2+ redox couple acted as an electron donor and facilitated rapid charge transfer, leading to the conversion of Co3+ to Co2+. Moreover, the CuCo2O4@NF + PMS system effectively eliminated TC by employing radical pathways (SO4•-, •OH) and nonradical processes (1O2, e-). Therefore, this study introduces a new approach to overcome the limitations of powdered bimetallic oxides, providing a promising solution for practical applications.

Catalytic Degradation of Bisphenol A with a Magnetically Recoverable Geopolymer Composite Using Coal Gangue

The widespread presence and use of Bisphenol A (BPA) in aquatic environments has caused significant ecological damage. Coal gangue (CG), a byproduct of coal mining, poses a major environmental concern due to its vast land occupation and potential for pollution. A magnetic recyclable geopolymer (MnFe2O4-CGP) using coal gangue geopolymer (CGP) as the carrier was successfully synthesized and was evaluated for its ability to Fenton-like degrade BPA. The characterization techniques revealed the successful incorporation of spherical MnFe2O4 onto the CGP surface and that CGP serves as an excellent platform for the immobilization and dispersion of MnFe2O4. The degradation rate reached 100% within 60 min at pH = 5, 15 mmol/L H2O2, 0.6 g/L catalyst, and 50 mg/L BPA, significantly higher than MnFe2O4 and CGP alone. It was indicated that the degradation rate of BPA in MnFe2O4-CGP composites was 0.1121 min-1, which was consistent with the first-order kinetic model. The saturation magnetization of MnFe2O4-CGP was measured to be 10.96 emu/g, enabling convenient recovery. MnFe2O4-CGP exhibited excellent stability, as the degradation rate of BPA remained above 95% even after five reaction cycles. This efficiency may be due to the MnFe2O4-CGP induced generation of reactive radicals. Quenching and EPR radical trapping experiments unequivocally confirmed that the reactive radical was hydroxyl radical (•OH). These results indicate that MnFe2O4-CGP has potential application prospects as a magnetic recyclable geopolymer composite in Fenton-like catalysis.

Dual Nonradical Catalytic Pathways Mediated by Nanodiamond-Derived sp2/sp3 Hybrids for Sustainable Peracetic Acid Activation and Water Decontamination

Peracetic acid (PAA) oxidation catalyzed by metal-free carbons is promising for advanced water decontamination. Nevertheless, developing reaction-oriented and high-performance carbocatalysts has been limited by the ambiguous understanding of the intrinsic relationship between carbon chemical/molecular structure and PAA transformation behavior. Herein, we comprehensively investigated the PAA activation using a family of well-defined sp2/sp3 carbon hybrids from annealed nanodiamonds (ANDs). The activity of ANDs displays a volcano-type trend, with respect to the sp2/sp3 ratio. Intriguingly, sp3-C-enriched AND exhibits the best catalytic activity for PAA activation and phenolic oxidation, which is different from persulfate chemistry in which the sp2 network normally outperforms sp3 hybridization. At the electron-rich sp2-C site, PAA undergoes a reduction reaction to generate a reactive complex (AND-PAA*) and induces an electron-transfer oxidation pathway. At the sp3-C site adjacent to C═O, PAA is oxidized to surface-confined OH* and O* successively, which ultimately evolves into singlet oxygen (1O2) as the primary reactive species. Benefiting from the dual nonradical regimes on sp2/sp3 hybrids, AND mediates a sustainable redox recycle with PAA to continuously generate reactive species to attack water contaminants, meanwhile maintaining structural/chemical integrity and exceptional reusability in cyclic runs.

Heteroatom substitution enhances generation and reactivity of surface-activated peroxydisulfate complexes for catalytic fenton-like reactions

Peroxydisulfate (PDS)-based Fenton-like reactions are promising advanced oxidation processes (AOPs) to degrade recalcitrant organic water pollutants. Current research predominantly focuses on augmenting the generation of reactive species (e.g., surface-activated PDS complexes (PDS*) to improve treatment efficiency, but overlooks the potential benefits of enhancing the reactivity of these species. Here, we enhanced PDS* generation and reactivity by incorporating Zn into CuO catalyst lattice, which resulted in 99% degradation of 4-chlorophenol within only 10 min. Zn increased PDS* generation by nearly doubling PDS adsorption while maintaining similar PDS to PDS* conversion efficiency, and induced higher PDS* reactivity than the common catalyst CuO, as indicated by a 4.1-fold larger slope between adsorbed PDS and open circuit potential of a catalytic electrode. Cu-O-Zn formation upshifts the d-band center of Cu sites and lowers the energy barrier for PDS adsorption and sulfate desorption, resulting in enhanced PDS* generation and reactivity. Overall, this study informs strategies to enhance PDS* reactivity and design highly active catalysts for efficient AOPs.

Development of natural attapulgite derived ferromanganese spinel oxides as heterogeneous catalysts for persulfate activation of tetracycline degradation

Ferromanganese spinel oxides (MnFe2O4, MFO) have been proven effective in activating persulfate for pollutants removal. However, their inherent high surface energy often leads to agglomeration, diminishing active sites and consequently restricting catalytic performance. In this study, using Al-MCM-41 (MCM) mesoporous molecular sieves derived from natural attapulgite as a support, the MFO/MCM composite was synthesized through dispersing MnFe2O4 nanoparticles on MCM carrier by a simple hydrothermal method, which can effectively activate persulfate (PS) to degrade Tetracycline (TC). The addition of Al-MCM-41 can effectively improve the specific surface area and adsorption performance of MnFe2O4, but also reduce the leaching amount of metal ions. The MFO/MCM composite exhibited superior catalytic reactivity towards PS and 84.3% removal efficiency and 64.7% mineralization efficiency of TC (20 mg/L) was achieved in 90 min under optimized conditions of 0.05 mg/L catalyst dosage, 5 mM PS concentration, room temperature and no adjustment of initial pH. The effects of various stoichiometric MFO/MCM ratio, catalyst dosage, PS concentration, initial pH value and co-existing ions on the catalytic performance were investigated in detail. Moreover, the possible reaction mechanism in MFO-MCM/PS system was proposed based on the results of quenching tests, electron paramagnetic resonance (EPR) and XPS analyses. Finally, major degradation intermediates of TC were detected by liquid chromatography mass spectrometry technologies (LC-MS) and four possible degradation pathways were proposed. This study enhances the design approach for developing highly efficient, environmentally friendly and low-cost catalysts for the advanced treatment process of antibiotic wastewater.

Efficient degradation of benzalkonium chloride by FeMn-CA300 catalyst activated persulfate process: Surface hydroxyl potentiation mechanism and degradation pathway

The consumption of disinfectants increased dramatically with the outbreak of the COVID-19 epidemic. Benzalkonium chloride (DDBAC), a cationic surfactant disinfectant for import and export cargoes, is used for effective degradation method. For DDBAC effective degradation, polyhedral Fe-Mn bimetallic catalyst of Prussian blue analogue (FeMn-CA300) was novelty developed for rapid peroxymonosulfate (PMS) activation. Results showed that the Fe/Mn redox and surface hydroxyl groups in the catalyst played an important role in the DDBAC-enhanced degradation. The removal effectiveness of 10 mg L-1 DDBAC was up to 99.4% in 80 min under the initial pH = 7, catalyst dosage of 0.4 g L-1, and PMS concentration of 15 mmol L-1. In addition, FeMn-CA300 had a wide pH applicability range. The results indicated that hydroxyls, sulfate radicals, and singlet oxygen could effectively improve the degradation efficiency, where sulfate radicals played a crucial role. Finally, the corresponding degradation path of DDBAC was further provided according to GC-MS results. The results of this study provide new insights into the degradation of DDBAC, thereby highlighting the great potential of FeMnca300/PMS to control refractory organic compounds in the aqueous phase.

Stable and recyclable FeS-CMC-based peroxydisulfate activation for effective bisphenol A reduction: performance and mechanism

In this study, a novel material, iron sulfide modified by sodium carboxymethyl cellulose (FeS-CMC), was successfully synthetized for peroxydisulfate (PDS) activation to remove bisphenol A (BPA). Characterization results showed that FeS-CMC had more attachment sites for PDS activation due to its higher specific surface area. A stronger negative potential contributed to preventing nanoparticles from reuniting in the reaction and improving the interparticle electrostatic interactions of the materials. Fourier transform infrared spectrometer (FTIR) analysis of FeS-CMC suggested that the coordination of the ligand for combining sodium carboxymethyl cellulose (CMC) with FeS was monodentate. A total of 98.4% BPA was decomposed by the FeS-CMC/PDS system after 20 min under optimized conditions (pH = 3.60, [FeS-CMC] = 0.05 g/L and [PDS] = 0.88 mM). The isoelectric point (pH_pzc) of FeS-CMC is 5.20, and FeS-CMC contributed to reducing BPA under acidic conditions but showed a negative effect under basic conditions. The presence of HCO₃^-, NO₃^- and HA inhibited BPA degradation by FeS-CMC/PDS, while excess Cl^- accelerated the reaction. FeS-CMC exhibited excellent performance in oxidation resistance with a final removal degree of 95.0%, while FeS was only 20.0%. Furthermore, FeS-CMC showed excellent reusability and still reached 90.2% after triple reusability experiments. The study confirmed that the homogeneous reaction was the primary part of the system. Surface-bound Fe(II) and S (-II) were found to be the major electron donors during activation, and the reduction of S (-II) contributed to the cycle of Fe(III)/Fe(II). Sulfate radicals (SO₄^•-), hydroxyl radicals (•OH), superoxide radicals (O₂^•-) and singlet oxygen (¹O₂) were produced at the surface of FeS-CMC and accelerated the decomposition of BPA. This study offered a theoretical basis for improving the oxidation resistance and reusability of iron-based materials in the presence of advanced oxidation processes.

Persulfate promoted visible photocatalytic elimination of bisphenol A by g-C3N4-CeO2 S-scheme heterojunction: The dominant role of photo-induced holes

In the last few years, coupling heterogeneous photocatalysis with persulfate (PDS) activation process is an efficient approach to generate abundant reactive oxidative species towards organic contaminant removal in water, however, the key role of PDS in photocatalytic process remains ambiguous. Herein, a novel g-C₃N₄-CeO₂ (CN-CeO₂) step-scheme (S-scheme) composite was constructed to photo-degrade bisphenol A (BPA) with the presence of PDS under visible irradiation. At 2.0 mM PDS, 0.7 g/L CN-CeO₂ and natural pH 6.2, 94.2% of BPA could be eliminated in 60 min under visible light (Vis) illumination. Aside from the previous view of free radical generation, it tends to assume that most of PDS molecules acted as electron sacrificial agents for capturing photo-induced e^- to form sulfate ions, greatly improving the charge carrier separation and thus enhancing the oxidation capacity of nonradical hole (h⁺) for the removal of BPA. Good correlations are further found between the rate constant and descriptor variables (i.e., Hammett constant σ^-/σ⁺ and half-wave potential E_1/2), exhibiting selective oxidation for organic pollutants in the Vis/CN-CeO₂/PDS system. The study brings more insights into mechanistic understanding of persulfate-enhanced photocatalytic process for addressing water decontamination.

Modified birnessite MnO2 as efficient Fenton-like catalysts through electron transfer process between the simultaneously surface-activated peroxymonosulfate and pollutants

The development of efficient and eco-friendly Mn-based hybrids for the degradation of biorefractory organic pollutants via peroxymonosulfate (PMS) activation is highly desired. In this study, a novel graphite nanosheet (GNs)-based Fe-Mn bimetallic oxide (Fe doped birnessite MnO₂, FeMn/GNs) was synthesized under mild conditions. Compared with monometallic Fe or Mn oxide on GNs, FeMn/GNs exhibited a higher surface area, decreased Mn oxidation states, stronger interaction with GNs, and more active sites for PMS adsorption. Among different Fe/Mn ratios, Fe₂Mn₁/GNs showed the optimum performance for bisphenol A (BPA) degradation with the first-order rate constant of 0.22 min^-1, which was about 8.5 and 12.9 times higher than that of Mn/GNs and Fe/GNs, respectively. Different from the pollutant-catalyst-PMS electron transfer mechanism for Mn/GNs, the direct two-electron transfer in FeMn/GNs+PMS system, was mainly processed between the simultaneously activated BPA and PMS. This was probably based on the double adsorption sites of Fe and Mn species on the same catalyst: PMS was adsorbed by Fe species through hydroxyl groups, while BPA was mainly coordinated with Mn species due to the layered structure and hydrophobicity of the Mn oxide. This study is expected to provide the rational design of efficient Mn-based hybrids for PMS activation.

Simultaneous alleviation of Cd availability in contaminated soil and accumulation in rice (Oryza sativa L.) by Fe-Mn oxide-modified biochar

Fe-Mn oxide-modified biochar (BC-FM) was used to remediate Cd-contaminated soil and mitigate Cd accumulation in rice. The roles of Fe and Mn in soil Cd immobilization and in controlling Cd uptake by rice were investigated via X-ray photoelectron spectroscopy (XPS) characterization and chemical analysis. Fe and Mn loaded on BC-FM increased the removal efficiencies of CaCl₂ extractable Cd in soil and Cd in pore water compared to those in only biochar (BC)-treated soil, with maximum removal rates at 67.9 % and 77.8 %, respectively. The XPS results indicated that the redox reactions of the Fe-Mn oxides on BC-FM surface affected Cd immobilization in the soil. The Fe (II/III) components on BC-FM were primarily converted to Fe₃O₄ in the soil system, which may form stable complexes with Cd²⁺ (Fe-O-Cd) during the entire rice growth period, and Cd may be bound to MnO or Mn₂O₃ in the form of CdMn₂O₄. The excellent adsorption performance of BC-FM enhanced by Fe-Mn oxides reduced the available Cd in the soil and stimulated Fe and Mn transport in rice, thereby inhibiting Cd accumulation in the aerial parts of rice. Cd concentrations in brown rice under BC-FM treatments reached the national safety standard (0.2 mg/kg, GB2762-2017). And BC-FM significantly increased the biomass of brown rice with a maximum rate of 26.8 %. These findings suggest that BC-FM could be used as an efficient material for Cd-contaminated soil remediation, and Fe-Mn plays important role in immobilizing Cd in soil and reducing Cd transport in rice.

Insights into the mechanism of carbocatalysis for peracetic acid activation: Kinetic discernment and active site identification

Peracetic-acid-based advanced oxidation processes (PAA-AOPs) on metal-free catalysts have emerged as charming strategies for water contaminant removal. However, the involved reactive species and their corresponding active sites are ambiguous. Herein, using carbon nanotube (CNT) as a model carbocatalyst, we demonstrated that, under neutral conditions, the CNT-PAA* complex was the dominant reactive species to oxidize phenolic compounds via electron-transfer process (ETP), whereas the surface-bound hydroxyl radicals (^·OH_surface) played a minor role on the basis of quenching and electrochemical tests as well as Raman spectroscopy. More importantly, the experimental and density functional theory (DFT) calculation results collaboratively proved that the active site for ETP was the sp²-hybridized carbon on the CNT bulk, while that for radical generation was the edge-located hydroxyl group (C-OH), which lowered the energy barrier for cleaving the O-O bond in CNT-PAA* complex. We further discerned the oxidation kinetic constants (k_oxid) of different pollutants from the apparent kinetic constants in CNT/PAA system. The significant negative linear correlation between lnk_oxid and half-wave potential of phenolic compounds suggests that the pollutants with a lower one-electron oxidation potential (i.e., stronger electron-donating ability) are more easily oxidized. Overall, this study scrutinizes the hybrid radical and non-radical mechanism and the corresponding active sites of the CNT/PAA system, providing insights into the application of PAA-AOPs and the development of ETP in the remediation of emerging organic pollutants.