Introduction of cluster-to-metal charge transfer in UiO-66-NH2 for enhancing photocatalytic degradation of bisphenol a in the existence of peroxymonosulfate

Role of Interface of Metal-Organic Frameworks and Their Composites in Persulfate-Based Advanced Oxidation Process for Water Purification

The persulfate activation-based advanced oxidation process (PS-AOP) is an important technology in wastewater purification. Using metal-organic frameworks (MOFs) as heterogeneous catalysts in the PS-AOP showed good application potential. Considering the intrinsic advantages and disadvantages of MOF materials, combining MOFs with other functional materials has also shown excellent PS activation performance and even achieves certain functional expansion. This Review introduces the classification of MOFs and MOF-based composites and the latest progress of their application in PS-AOP systems. The relevant activation/degradation mechanisms are summarized and discussed. Moreover, the importance of catalyst-related interfacial interaction for developing and optimizing advanced oxidation systems is emphasized. Then, the interference behavior of environmental parameters on the interfacial reaction is analyzed. Specifically, the initial solution pH and coexisting inorganic anions may hinder the interfacial reaction process via the consumption of reactive oxygen species, affecting the activation/degradation process. This Review aims to explore and summarize the interfacial mechanism of MOF-based catalysts in the activation of PS. Hopefully, it will inspire researchers to develop new AOP strategies with more application prospects.

Oxygen-modified graphitic carbon nitride with nitrogen-defect for metal-free visible light photocatalytic H2O2 evolution

Photocatalytic oxygen reduction is regarded as the cleanest approach for the production of hydrogen peroxide (H2O2). Herein, oxygen-modified graphite carbon nitride (g-C3N4) with nitrogen-defect (namely g-C3N4-ND4-OM3) was synthesized by a feasible method. Owing to the existence of nitrogen vacancy and oxygen-containing functional group, the absorption bands derived from n → π* and π → π* electronic transitions were enhanced, thereby enlarging the visible light response range of catalysts. Interestingly, nitrogen-defect can capture electron and effectively suppress the recombination of photoinduced electrons and holes. More importantly, the introduction of oxygen-containing functional groups can improve the hydrophilicity of g-C3N4, which was beneficial for the adsorption of dissolved oxygen. The electrostatic potential distributions of g-C3N4-based photocatalyst structural unit were also changed after introducing nitrogen vacancy and oxygen-containing functional group, and the electron-donating ability of g-C3N4 was improved. As a result, the evolution rate of H2O2 catalyzed by g-C3N4-ND4-OM3 was as high as 146.96 μmol/g/L under visible light irradiation. The photocatalytic H2O2 generation was completed through the direct 2-e- oxygen reduction. In short, current work will share novel insights into photocatalytic H2O2 generation over g-C3N4-based catalyst.

A study on the performance of a novel adsorbent UiO-66 modified by a nickel on removing tetracycline in wastewater

In the present study, Ni-UiO-66 was synthesized to improve the adsorption efficiency of tetracycline (TC) in wastewater treatment. To this end, nickel doping was performed in the preparation process of UiO-66. The synthesized Ni-UiO-66 was characterized by XRD, SEM and EDS, BET, FTIR, TGA, and XPS for obtaining the lattice structure, surface topography, specific surface area, surface functional groups, and thermostability. More specifically, Ni-UiO-66 has a removal efficiency and adsorption capacity of up to 90% and 120 mg g-1, respectively, when used to treat TC. The presence of ions HCO3-, SO42-, NO3- and PO43- slightly affects the TC adsorption. A 20 mg L-1 humic acid reduces the removal efficiency from 80% to 60%. The performed analyses revealed that Ni-UiO-66 had similar adsorption capacity in wastewater with different ion strengths. The variation of adsorption capacity with the adsorption time was fitted using a pseudo-second-order kinetic equation. Meanwhile, it is found that the adsorption reaction occurs only on the monolayer of the UiO-66 surface so the adsorption process can be simulated using the Langmuir isotherm model. The thermodynamic analysis indicates that the adsorption of TC is an endothermic reaction. Electrostatic attraction, hydrogen-bond interaction, and π-π interaction might be the main reasons for the adsorption. The synthesized Ni-UiO-66 has well adsorption capacity and stable structure. Accordingly, it is expected to achieve a good prospect in industrial applications and wastewater treatment plants.

Sulfate Radical-Based Degradation of Organic Pollutants: A Review on Application of Metal-Organic Frameworks as Catalysts

The degradation of organic pollutants present in domestic and industrial effluents is a matter of concern because of their high persistence and ecotoxicity. Recently, advanced oxidation processes (AOPs) are being emphasized for organic pollutant removal from effluents, as they have shown higher degradation efficiencies when compared to conventional activated sludge processes. Sulfate radical-based methods are some of the AOPs, mainly carried out using persulfate (PS) and peroxymonosulfate (PMS), which have gained attention due to the ease of sulfate radical generation and the effective degradation of organic molecules. PMS is gaining more popularity because of its high reactivity and ability to generate excess sulfate radicals. PMS has been the major focus; therefore, its mechanism has been explained, and limitations have been elaborated. The involvement of metal-organic frameworks for PMS/PS activation applied to organic pollutant removal and recent advances in the application of biochar and hydrogel-assisted metal-organic frameworks have been discussed.

The calcium alginate-immobilized Co-g-C3N4 composite microspheres as an efficient mediator to activate peroxymonosulfate for degrading organic pollutants

Poor water stability and difficult separation severely limited the application of Co-based catalysts in persulfate activation. Herein, for the first time, the calcium alginate-immobilized Co-g-C₃N₄-2 composite microspheres were prepared by a feasible method. Notably, embedding Co ion into g-C₃N₄ can improve its specific surface area and electrochemical activities. More significantly, as-prepared Co-g-C₃N₄-2 microsphere presented excellent catalytic performance in PMS activation for the degradation of TC. For the activation mechanisms of PMS over Co-g-C₃N₄-2 microspheres, the calcium alginate microspheres could mediate the direct electron transfer between TC and PMS, while both radical and nonradical pathways were involved in the activation of PMS over Co-g-C₃N₄-2. Meanwhile, SO₄^•-, OH^•, O₂^•- and ¹O₂ were major reactive oxygen species formed in the Co-g-C₃N₄-2 microsphere/PMS system. Proposed Co-g-C₃N₄-2 microsphere/PMS system still exhibited great degradation ability towards TC over a wide pH range, and co-existing anions had weak influence on TC degradation over Co-g-C₃N₄-2 microsphere/PMS system. Moreover, the construction of Co-g-C₃N₄-2 microspheres not only avoided the release of metal ion from catalyst, but also provided convenience for the recovery of catalyst. In short, current work shared some novel insights into the application of heterogeneous catalysis in persulfate activation for wastewater treatment.

A newly-designed free-standing NiCo2O4 nanosheet array as effective mediator to activate peroxymonosulfate for rapid degradation of emerging organic pollutant with high concentration

Nowadays effective treatment of high concentration organic wastewater is still a formidable task facing human beings. Herein, for the first time, a well-defined ZIF-67-derived NiCo₂O₄ nanosheet array was successfully prepared by a feasible method. In comparison with ordinary NiCo₂O₄ nanosphere, the formation of nanosheet structure could offer more opportunities to exposure internal active sites of NiCo₂O₄, thereby resulting in smaller interface resistance and higher charge transfer efficiency. As expected, ZIF-67-derived NiCo₂O₄ nanosheet array displayed great performance in peroxymonosulfate (PMS) activation. More importantly, recyclable redox couples of Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ endowed the stable catalytic activity of NiCo₂O₄ nanosheet. Interestingly, developed NiCo₂O₄-1/PMS oxidation system could achieve the effective degradation of antibiotics with high concentration in a short time. Both radical and nonradical pathways were involved into PMS activation, wherein SO₄^-, OH, O₂^- and ¹O₂ were major reactive oxygen species. The formation paths of reactive oxygen species and effects of inorganic anions were also investigated. Electrochemical analyses revealed that NiCo₂O₄-1 with nanosheet structure mediated the electron transfer between PMS and tetracycline (TC), which played a vital role in TC degradation. Furthermore, developed NiCo₂O₄-1/PMS oxidation system displayed great removal ability towards TC in actual water samples, and degradation products were low toxicity or no toxicity. In short, current work not only developed an effective oxidation system for completing the rapid degradation of antibiotic with high concentration, but also shared some novel insights into the activation mechanism of SR-AOPs.