Yolk-shell ZIFs@SiO2 and its derived carbon composite as robust catalyst for peroxymonosulfate activation

Activity and recyclability enhancement of pH-dependent Fe0@BC-mediated heterogeneous sodium percarbonate (SPC)-reducing agents (RA) system

Dyes pose great threats to the aquatic environment and human health. Fe0-based Fenton-like systems have been widely employed for the degradation of organic dyes. However, the regulation of degradability and recyclability was still unclear. In this study, Rhodamine B (RhB) was served as the model pollutant, hydroxylamine hydrochloride was selected as the RA, the natural photocatalysis system demonstrated stable operation. RA, as performance enhancement agent, was firstly reported in micro/nano-Zero-Valent Iron@Biochar (m/nZVI@BC) based SPC-RA system. Carrier size-fractionated m/nZVI@BC was fabricated by one-step carbothermal method. As a result, RA synergistically interacted with SPC, and the reaction time reduced from 15 min to 4 min. In the 0.010 g m/nZVI@BC-mediated SPC-RA system, over 95% of RhB (100 mg·L-1, 1041.667 mg·g-1) was successfully degraded. The maximum degradation ability could still exceed 1g·g-1 via 5 times repeated applications. Meanwhile, the loss of degradability, caused by halving SPC concentration could be compensated by RA dosage measurement. The entire degradation process was predominantly dominated by free radicals (•OH> 1O2> •O2-> •CO3-). Reactive oxidizing species (ROSs) were primarily excited by α-Fe0, Fe3C and N sites of biochar (BC). Light and BC carrier dedicated slight influence. These discoveries shed a light on the activity and recyclability regulation of catalytic material, aligning with the principles of green chemistry and cleaner production. This study demonstrates a novel approach to efficient management of solid waste disposal, reuse of waste biomass, advanced treatment of dye-containing wastewater, pollution control in aquatic environments.

Mechanisms of interaction between metal-organic framework-based material and persulfate in degradation of organic contaminants (OCs): Activation, reactive oxygen generation, conversion, and oxidation

Metal-organic frameworks (MOFs)-based materials have been of great public interest in persulfate (PS)-based catalytic oxidation for wastewater purification, because of their excellent performance and selectiveness in organic contaminants (OCs) removal in complex water environments. The formation, fountainhead and reaction mechanism of reactive oxygen species (ROSs) in PS-based catalytic oxidation are crucial for understanding the principles of PS activation and the degradation mechanism of OCs. In the paper, we presented the quantitative structure-activity relationship (QSAR) of MOFs-based materials for PS activation, including the relationship of structure and removal efficiency, active sites and ROSs as well as OCs. In various MOFs-based materials, there are many factors will affect their performances. We discussed how various surface modification projects affected the characteristics of MOFs-based materials used in PS activation. Moreover, we revealed the process of ROSs generation by active sites and the oxidation of OCs by ROSs from the micro level. At the end of this review, we putted forward an outlook on the development trends and faced challenges of MOFs for PS-based catalytic oxidation. Generally, this review aims to clarify the formation mechanisms of ROSs via the active sites on the MOFs and the reaction mechanism between ROSs and OCs, which is helpful for reader to better understand the QSAR in various MOFs/PS systems.

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.

A persulfate oxidation system for removing acid orange from aqueous solution: Evaluation and degradation mechanism

Peroxymonosulfate-based advanced oxidation (PMS-AOP) is a promising technology for the degradation of environmental pollutants. PMS can be activated by various transition metals, especially cobalt-based catalysts, but pure cobalt catalyst suffers from severe metal leakage and poor cyclicality. This study synthesized NiCo₂O₄ using a co-precipitation hydrothermal method. The structures, morphologies, and chemical states of the prepared catalysts were hexagonal sheet structures. The activation of PMS by catalyst (NiCo₂O₄) is investigated in a PMS/carbonate (PC) system for Orange II degradation. The observed pseudo-first-order rate constants (k₁) were assessed by the effects of different water matrices and operation conditions. The results show that k_obs with NiCo₂O₄ were increased by 13 times than that of treatment without NiCo₂O₄. This was mainly due to Co³⁺ and Ni³⁺ act as electron acceptors to capture electrons from the PMS/PC system, forming a good redox cycle with HSO₅^-/SO₅^- and oxidizing Co²⁺/Ni²⁺ to produce a large amount of more active components (e.g., ¹O₂ and SO₄^⋅-). The good reusability and high stability of NiCo₂O₄ were demonstrated by five recycle tests. These results suggest that the NiCo₂O₄/PC system is an efficient and stable method of pollution remediation.

A simple and green method to prepare non-typical yolk/shell nanoreactor with dual-shells and multiple-cores: Enhanced catalytic activity and stability in Fenton-like reaction

Nowadays, non-typical yolk/shell structure has drawn much attentions due to the better catalytic performance than traditional counterparts (one yolk/one shell). In this study, ZIF-67 @Co₂SiO₄/SiO₂ yolk/shell structure was prepared in one-step at room temperature, in which ZIF-67 was served as the hard-template, H₂O was served as etchant and tetraethyl orthosilicat was served as the raw material for Co₂SiO₄/SiO₂. After calcination, the non-typical Co_xO_y @Co₂SiO₄/SiO₂ yolk/shell nanoreactor with Co₂SiO₄/SiO₂ dual-shells and Co_xO_y multiple-cores was obtained. On the one hand, more active sites were exposed on multiple-cores surface and better protection were provided by dual-shells. On the other hand, the sheet-like Co₂SiO₄ inner shell not only extended the travel path and retention time of pollutants trapped in cavity, but also separated the multiple-cores from aggregation. Therefore, the nanoreactor displayed the outstanding catalytic activity and recyclability in Fenton-like reaction. Metronidazole (20 mg/L) was completely degraded after 30 min, rhodamine B (50 mg/L) and methyl orange (20 mg/L) were removed even within 5.0 min. Catalytic mechanism indicated that ¹O₂ greatly contributed to the pollutant degradation. This paper presented a simple, versatile, green and energy-saving method for non-typical yolk/shell nanoreactor, and it could inspire to prepare other catalysts with high activity and stability for environmental remediation.

PMS activation by magnetic cobalt-N-doped carbon composite for ultra-efficient degradation of refractory organic pollutant: Mechanisms and identification of intermediates

Refractory organic pollutant effluent has led to severe water pollution. In this study, magnetic Co-N-doped carbon hybrid catalysts (Co-NC-x) were fabricated using a facile cation exchange combined pyrolysis and self-reduction technique to activate peroxymonosulfate (PMS) for rehabilitation of the water environment. Factors affecting the catalytic activity of the Co-NC-850 were comprehensively examined. 100% of RhB degradation efficiency within 20 min was achieved in the Co-NC-850/PMS system at the optimum conditions (C₀ = 80 mg L^-1, catalyst loading 0.025 g L^-1, PMS concentration 0.8 mM, native pH and 25 °C). The electron paramagnetic resonance measurements and competitive quenching tests demonstrated that a sulfate radical (SO₄•^-) and singlet oxygen (¹O₂) account for RhB degradation in the Co-NC-850/PMS system, and ¹O₂ contributed ~86.2% to RhB removal. The synergistic effect of Co⁰ nanoparticles (NPs) and NC on Co-NC-850 might induce a predominant non-radical route to trigger PMS activation for RhB degradation. Direct oxidation of O₂•^- by a hydroxyl radical (•OH) might be the crucial process for forming ¹O₂. Magnetic response and successive cycles verified that Co-NC-850 has superior separable performance and reusability. This innovative magnetic Co-NC-850 hybrid catalyst for PMS activation delivered vast potential for disintegration of refractory organic contaminants.

Magnetic porous carbons derived from iron-based metal-organic framework loaded with glucose for effective extraction of synthetic organic dyes in drinks

The conversion of metal-organic frameworks (MOFs) to porous carbon has attracted extensive attention for developing multifunctional adsorbent materials. Herein, we demonstrated a facile method to prepare magnetic porous carbon via calcinating MIL-101(Fe) precursor loaded with glucose at 700 °C in an N₂ atmosphere. The obtained magnetic porous carbon (MPCG) contained plenty of oxygen-containing functional groups and exhibited an enlarged specific surface area (177.7 m²/g) compared with its precursor (41.2 m²/g). In addition, MPCG can be easily separated from the matrix by a magnet. Benefitting from these advantages, the magnetic porous carbon exhibited high affinity toward four synthetic organic dyes (amaranth, ponceau 4R, sunset yellow, and lemon yellow) in an aqueous solution. Moreover, the adsorbent can be applied to quantitatively detect synthetic organic dyes in drinks coupled with chromatography. A new magnetic solid-phase extraction method for dye analysis yielded reasonable linearity (r □ 0.99), low limits of detection (0.047-0.076 μg/L), and good precision within the analyte concentration range of 0.25-50 μg/L.

Preparation of Mn2O3/MIL-100(Fe) composite and its mechanism for enhancing the photocatalytic removal of rhodamine B in water

In this study, Mn₂O₃/MIL-100(Fe) composite was successfully synthesized by the hydrothermal method and applied for photocatalytic removal of rhodamine B (RhB) in water. The physical and chemical properties of the synthesized materials were characterized by XRD, FTIR, SEM, UV-visible, and BET analyses. Experimental results showed a great enhancement in the photocatalytic ability of the Mn₂O₃/MIL-100(Fe) composite as compared to individual Mn₂O₃ or MIL-100(Fe) under visible light and persulfate activation. The affecting factors such as pH, photocatalyst dose, RhB concentration, and Na₂S₂O₈ concentration were investigated to find out the best conditions for efficient photocatalysis. By conducting a radical quenching test, all radicals of HO˙, SO₄˙^-, ¹O₂, and O₂˙^- were found to be important in photocatalytic decomposition. The mechanism was proposed for the enhancement of photocatalytic RhB removal via band potential calculation, charge separation, surface redox reaction, and key reactive oxidation species. With its durability, reusability, and high efficiency, the Mn₂O₃/MIL-100(Fe) composite emerges as a potential photocatalyst working under visible light for application in wastewater treatment.

An innovative, highly stable Ag/ZIF-67@GO nanocomposite with exceptional peroxymonosulfate (PMS) activation efficacy, for the destruction of chemical and microbiological contaminants under visible light

In this work, Ag nanoparticles were loaded on ZIF-67 covered by graphene oxide (Ag/ZIF-67@GO), and its catalytic performance was studied for the heterogeneous activation of peroxymonosulfate (PMS) under visible-light. The catalyst surface morphology and structure were analyzed by FT-IR, XRD, XPS, DRS, FE-SEM, EDX, TEM, BET, ICP-AES and TGA analysis. The efficacy of PMS activation by the Ag/ZIF-67@GO under visible light was assessed by phenol degradation and E. coli inactivation. Phenol was completely degraded within 30 min by HO^•, SO₄^•- and O₂^•- generated through the photocatalytic PMS activation. In addition, total E. coli inactivation was attained in 15 min that confirmed the highly efficient catalytic activation of PMS by the as-made nanocomposite under visible light. The reaction mechanism was elucidated and the importance of the generated reactive species followed the order of: HO^• > SO₄^•- > O₂^•- > h⁺, implying a radical-pathway dominated process.

Enhanced adsorption and catalytic peroxymonosulfate activation by metal-free N-doped carbon hollow spheres for water depollution

Rational design of metal-free carbon-based heterogeneous catatlyst for wastewater remediation via peroxymonosulfate (PMS) activation is highly desirable. Here, hollow structured porous carbon with abundant N, a high graphitization degree, and a large specific surface area and pore volume (1301 m²/g and 1.12 cm³/g) was synthesized by the pyrolysis of core-shell structured composites consisting of polystyrene (PS) cores and Zeolitic imidazolate frameworks-8 (ZIF-8) shells. The hollow structured carbon (CPS@ZIF-8) was characterized thoroughly and applied for phenol degradation by the activation of PMS. The effects of operation conditions such as the catalyst and PMS dose, phenol concentration, initial pH, and temperature on phenol removal were investigated comprehensively. Moreover, the main reactive species involved in phenol oxidation were investigated, and a plausible mechanism for the degradation of phenol is proposed. The results show that CPS@ZIF-8 exhibited an excellent phenol adsorption and degradation performance, which can be mainly ascribed to its large surface area, abundance of nitrogen and hollow porous structure. Moreover, both the nonradical pathway (involving ¹O₂) and the radical pathway (involving SO₄^- and O₂^-) were found to be involved in the decomposition of phenol.