Lewis acids promoted organic pollutants degradation in aqueous solution with peroxymonosulfate and MnO2: New insights into the activation mechanism

Homogenous Oxidizing Oligomerization Coupled with Coagulation for Water Purification

Natural manganese oxides could induce the intermolecular coupling reactions among small-molecule organics in aqueous environments, which is one of the fundamental processes contributing to natural humification. These processes could be simulated to design novel advanced oxidation technology for water purification. In this study, periodate (PI) was selected as the supplementary electron-acceptor for colloidal manganese oxides (Mn(IV)aq) to remove phenolic contaminants from water. By introducing polyferric sulfate (PFS) into the Mn(IV)aq/PI system and exploiting the flocculation potential of Mn(IV)aq, a post-coagulation process was triggered to eliminate soluble manganese after oxidation. Under acidic conditions, periodate exists in the H4IO6- form as an octahedral oxyacid capable of coordinating with Mn(IV)aq to form bidentate complexes or oligomers (Mn(IV)-PI*) as reactive oxidants. The Mn(IV)-PI* complex could induce cross-coupling process between phenolic contaminants, resulting in the formation of oligomerized products ranging from dimers to hexamers. These oligomerized products participate in the coagulation process and become stored within the nascent floc due to their catenulate nature and strong hydrophobicity. Through coordination between Mn(IV)aq and H4IO6-, residual periodate is firmly connected with manganese oxides in the floc after coagulation and could be simultaneously separated from the aqueous phase. This study achieves oxidizing oligomerization through a homogeneous process under mild conditions without additional energy input or heterogeneous catalyst preparation. Compared to traditional mineralization-driven oxidation techniques, the proposed novel cascade processes realize transformation, convergence, and separation of phenolic contaminants with high oxidant utilization efficiency for low-carbon purification.

Carbide lime as substrates to boost energy recuperation and dyestuff removal in constructed wetland-microbial fuel cell integrated with copper oxide/carbon cloth cathode

Methylene blue (MB) was regarded as a highly toxic and hazardous substance owing to its irreparable hazard and deplorable damage on the ecosystem and the human body. The treatment of this colorant wastewater appeared to be one of the towering challenges in wastewater treatment. In this study, a microbial fuel cell coupled with constructed wetland (CW-MFC) with effective MB elimination and its energy recuperation concurrently based on the incorporation of carbide lime as a substrate in a new copper oxide-loaded on carbon cloth (CuO/CC) cathode system was studied. The crucial influencing parameters were also delved, and the MB degradation and chemical oxygen demand (COD) removal efficiencies were correspondingly incremented by 97.3% and 89.1% with maximum power output up to 74.1 mW m-2 at optimal conditions (0.2 g L-1 carbide lime loading and 500 Ω external resistance). The carbide lime with high calcium ion content was greatly conducive for the enrichment of critical microorganism and metabolic activities. The relative abundances of functional bacteria including Proteobacteria and Actinobacteriota were vividly increased. Moreover, the impressive results obtained in printed ink wastewater treatment with a COD removal efficiency of 81.3% and a maximum power density of 58.2 mW m-2, which showcased the potential application of CW-MFC.

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.

Manganese oxides activated peroxymonosulfate for ciprofloxacin removal: Effect of oxygen vacancies and chemical states

Ciprofloxacin (CIP) as an anti-inflammatory drug is frequently detected in various water resources. Recently, Sulfate Radical-based advanced oxidation processes with manganese oxides have been recognized as a highly effective method for CIP degradation. Herein, ε-MnO₂ was obtained through a convenient drying process. After different atmospheric treatments, MnO and Mn₂O₃ were fabricated for subsequent degradation experiments. The results show that MnO exhibits better catalytic activity than Mn₂O₃, with high removal efficiency of almost 84.3% for CIP. Quenching test and electron paramagnetic resonance spectra confirm that ¹O₂ is the dominant species during reaction, while ·OH and SO₄^·- play a supporting role. A related discussion about the role of valence states of Mn and oxygen vacancies is presented, which can provide a theoretical basis for further development of Mn/peroxymonosulfate system.

Mn2O3 as an Electron Shuttle between Peroxymonosulfate and Organic Pollutants: The Dominant Role of Surface Reactive Mn(IV) Species

The environmentally benign Mn oxides play a crucial role in the transformation of organic contaminants, either through catalytically decomposing oxidants, e.g., peroxymonosulfate (PMS), or through directly oxidizing the target pollutants. Because of their dual roles and the complex surface chemical reactions, the mechanism involved in Mn oxide-catalyzed PMS activation processes remains obscure. Here, we clearly elucidate the mechanism involved in the Mn₂O₃ catalyzed PMS activation process by means of separating the PMS activation and the pollutant oxidation process. Mn₂O₃ acts as a shuttle that mediates the electron transfer from organic substrates to PMS, accompanied by the redox cycle of surface Mn(IV)/Mn(III). Multiple experimental results indicate that PMS is bound to the surface of Mn₂O₃ to form an inner-sphere complex, which then decomposes to form long-lived surface reactive Mn(IV) species, without the generation of sulfate radicals (SO₄^•-) and hydroxyl radicals (HO^•). The surface reactive Mn(IV) species are proposed to be responsible for the degradation of organic contaminants (e.g., phenol) and the formation of singlet oxygen (¹O₂), followed by the regeneration of the surface Mn(III) sites on Mn₂O₃. This study advances the fundamental understanding of the underlying mechanism involved in transition metal oxide-catalyzed PMS activation processes.

Brønsted-acid sites promoted degradation of phthalate esters over MnO2: Mineralization enhancement and aquatic toxicity assessment

Advanced oxidation processes (AOPs) are important technologies for aqueous organics removal. Despite organic pollutants can be degraded via AOPs generally, high mineralization of them is hard to achieve. Herein, we synthesized a manganese oxide nanomaterial (H2-OMS-2) with abundant Brønsted-acid sites via ion-exchange of cryptomelane-type MnO₂ (OMS-2), and tested its catalytic performance for the degradation of phthalate esters via peroxymonosulfate (PMS) activation. About 99% of dimethyl phthalate (DMP) at a concentration of 20 mg/L could be degraded within 90 min and 82% of it could be mineralized within 180 min over 0.6 g/L of catalyst and 1.8 g/L of PMS. The catalyst could activate PMS to generate SO₄^-˙ and ·OH as the dominant reactive oxygen species to reach complete degradation of DMP. Especially, the higher TOC removal rate was obtained due to the rich Brønsted-acid sites and surface oxygen vacancies on the catalyst. Kinetics and mechanism study showed that Mn^II/Mn^III might work as the active sites during the catalytic process with a lower reaction energy barrier of 55.61 kJ/mol. Furthermore, the catalyst could be reused for many times through the regeneration of the catalytic ability. The degradation and TOC removal efficiencies were still above 98% and 65% after seven consecutive cycles, respectively. Finally, H2-OMS-2-catalyzed AOPs significantly reduced the organismal developmental toxicity of the DMP wastewater through the investigation of zebrafish model system. The present work, for the first time, provides an idea for promoting the oxidative degradation and mineralization efficiencies of aqueous organic pollutants by surface acid-modification on the catalysts.

Enhanced degradation of organic compounds through the interfacial transfer of electrons in the presence of phosphate and Nitrogen-cobalt doped graphitic carbon

Peroxymonosulfate (PMS) has been activated for the generation of reactive oxygen species by nitrogen-doped carbonaceous material. However, the influence of phosphate on the degradation performance has not been reported. In this study, phosphate ions accelerate PMS decomposition and degradation of target organic compounds such as carbamazepine, atrazine, sulfamethoxazole, and benzoic acid. It was revealed that the physical mixture of phosphate with Co and N doped graphitic carbon (GcN/Co) demonstrates the occurrence of P C, P N, and P O - C bonds. Essentially, the graphitic N or graphitic N P increased in the presence of phosphate. This was correlated with the lower electrical transfer resistance, improved electrical conductivity, and higher electron morbidity confirmed by different electrochemical tests. Moreover, due to the strong buffering capacity of phosphate at neutral pH, bicarbonate was used to confirm the negligible influence of pH. The presence of phosphate helps to recover the scavenging effect of Cl^- but has no effect on the presence of HCO₃^- and CO₃^2-. Nevertheless, GcN/Co demonstrates good reusability for three reaction cycles, however, in order to maintain a high catalytic performance phosphate needs to be replenished after each cycle.

Copper in LaMnO3 to promote peroxymonosulfate activation by regulating the reactive oxygen species in sulfamethoxazole degradation

Perovskites with flexible texture structures and excellent catalytic properties have attracted considerable attention in peroxymonosulfate (PMS) activation for addressing organic contaminants in water. In this study, the role of copper to promote PMS activation performance of LaMnO₃ was investigated. 100% sulfamethoxazole (SMX) degradation and 34% mineralization were achieved over copper doped LaMnO₃ while only 60% SMX was removed without TOC removal by LaMnO₃. Especially, compared with LaMnO₃, the pseudo-first-order reaction rate constant was increased by 8.30 times when the atomic ratio of Cu/Mn was 1:3. It proved that only ¹O₂ was generated in LaMnO₃ while ¹O₂, especially •OH and SO₄•^- were all detected in Cu-LaMnO₃/PMS system. The characterization results showed that Cu induced the formation of LaMnO₃ and La₂CuO₄ heterostructure with enhanced content of relatively low-valence Mn and Cu and abundant oxygen vacancies (OVs). Hence, the efficient PMS activation by Cu-LaMnO₃ was due to regulating the produced reactive oxygen species (ROS). A radical dominant instead of ¹O₂ involved PMS activation mechanism over LaMnO₃-La₂CuO₄ was proposed for efficient degradation of SMX. Finally, the possible degradation pathways of SMX were discussed based on HPLC-MS analysis. This study provided a new insight of improving the catalytic activity of perovskites in PMS activation in water treatment.

Degradation of antibiotics in aqueous media using manganese nanocatalyst-activated peroxymonosulfate

ε-MnO₂ effectively activates peroxymonosulfate (PMS) for the efficient degradation of emerging pollutants. ε-MnO₂ was synthesized by a facile thermal-treatment method and its long-term stability and efficiency for the elimination of emerging pollutants, including sulfamethoxazole (SMX), sulfachloropyridazine (SCP), sulfamethazine (SMT), ciprofloxacin (CIP), and azithromycin (AZI), from aqueous media were evaluated. ε-MnO₂ was found to activate PMS more efficiently than α-MnO₂, β-MnO₂, or δ-MnO₂, owing to its high - OH-group content, unique structure, and high surface area. Sulfate (SO₄^•-), hydroxyl (•OH), and superoxide (O₂^•-) radicals, as well as singlet oxygen (¹O₂) were generated, with O₂^•- acting as the ¹O₂ precursor. The ε-MnO₂/PMS system proved to be effective in the pH range of 3.5-9.0 and the rate of SMX degradation was not significantly affected by the presence of inorganic anions or natural organic matter. The proposed pathway for the activation of PMS by ε-MnO₂ includes inner-sphere interactions between ε-MnO₂ and PMS, and electron transfer to PMS via the MnIII ↔ MnIV redox cycle, which generates reactive oxygen species. These findings provide new insight into PMS activation by less-toxic metal oxides as catalysts and demonstrate that Mn-based materials can be used to effectively treat water matrices containing emerging pollutants.

High performance of the A-Mn2O3 nanocatalyst for persulfate activation: Degradation process of organic contaminants via singlet oxygen

In this study, the catalytic activation of persulfate (PS) via metal oxides was investigated, and the A-Mn₂O₃ nanocatalyst was found to have the highest efficiency among other PS activators for the degradation of organic contaminants. Additionally, A-Mn₂O₃ exhibited a remarkable efficiency in activating PS for the degradation of phenol compared to both B-Mn₂O₃ and C-Mn₂O₃. This was attributed to the longer bonds between edge-sharing MnO₆ octahedra, the unique structure, the high content surface -OH groups, and the average oxidation states. This indicated that all these properties played an important role in an efficient PS activation. Electron paramagnetic resonance (EPR) spectroscopy, scavenger tests, and chemical probes were conducted to investigate the reactive oxygen species (ROS). Singlet oxygen (¹O₂) was determined to be the main ROS generated from PS activation. A plausible mechanism study was proposed, which involved inner-sphere interactions. An electron transfer of the Mn species facilitated the decomposition of PS to generate HO₂^•/O₂^{• -} radicals, which were utilized as a precursor for ¹O₂ generation via direct oxidation or the recombination of HO₂^•/O₂^{• -}. Finally, the phenol and Sulfachloropyridazine (SCP) degradation pathways were proposed by ¹O₂ over the A-Mn₂O₃/PS system according to HPLC and LC-MS results.