Heterogeneous metals and metal-free carbon materials for oxidative degradation through persulfate activation: A review of heterogeneous catalytic activation of persulfate related to oxidation mechanism

Freezing-enhanced degradation of azo dyes in the chloride-peroxymonosulfate system

In this study, we investigated the freezing-induced acceleration of dye bleaching by chloride-activated peroxymonosulfate (PMS). It has been observed that the oxidation of chloride by PMS generates a free chlorine species, such as hypochlorous acid (HOCl), under mild acidic and circumneutral pH condition. This process is the major reason for the enhanced oxidation capacity for electron-rich organic compounds (e.g., phenol) in the chloride-PMS system. However, we demonstrated that the chloride-PMS system clearly reduced the total organic carbon concentration (TOC), whereas the HOCl system did not lead to decrease in TOC. Overall, the chemical reaction is negligible in an aqueous condition if the concentrations of reagents are low, and freezing the solution accelerates the degradation of dye pollutants remarkably. Most notably, the pseudo-first order kinetic rate constant for acid orange 7 (AO7) degradation is approximately 0.252 h-1 with 0.5 mM PMS, 1 mM NaCl, initial pH 3, and a freezing temperature of -20 °C. AO7 degradation is not observed when the solution is not frozen. According to a confocal Raman-microscope analysis and an experiment that used an extremely high dose of reactants, the freeze concentration effect is the main reason for the acceleration phenomenon. Because the freezing phenomenon is spontaneous at high latitudes and at mid-latitudes in winter, and the chloride is ubiquitous elsewhere, the frozen chloride-PMS system has potential as a method for energy-free and eco-friendly technology for the degradation of organic pollutants in cold environments.

MIL-101(Fe)/WS2 composites activated Na2S2O8 with visible light for removal of tetracycline

MIL-101(Fe)/WS2 catalyst was composited using a solvothermal method. To study the physical and chemical properties of the composite material, a series of characterizations such as scanning electron microscope (SEM), X-ray diffraction (XRD), and catalytic experiments were carried out. The photocatalysis of the prepared catalyst in the degradation of tetracycline was investigated using persulfate (PS, Na2S2O8) as a cocatalyst under visible light illumination. The above system can remove about 80% of tetracycline within 40 min. After three cyclic experiments, the material showed good recycling. According to material characterization and various experimental results, the enhanced performance of the material was attributed to the reduction of the recombination efficiency of photogenerated e- and h+, and activated persulfate to produce a large number of free radicals such as O2•-, SO4•- and 1O2 produced by the active sites provided by the catalyst's high specific surface area.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

The performance and mechanism of iron-mediated chemical oxidation: Advances in hydrogen peroxide, persulfate and percarbonate oxidation

Many studies have successfully built iron-mediated materials to activate or catalyze Fenton-like reactions, with applications in water and wastewater treatment being investigated. However, the developed materials are rarely compared with each other regarding their performance of organic contaminant removal. In this review, the recent advances of Fenton-like processes in homogeneous and heterogeneous ways are summarized, especially the performance and mechanism of activators including ferrous iron, zero valent iron, iron oxides, iron-loaded carbon, zeolite, and metal organic framework materials. Also, this work mainly compares three O-O bond containing oxidants including hydrogen dioxide, persulfate, and percarbonate, which are environmental-friendly oxidants and feasible for in-situ chemical oxidation. The influence of reaction conditions, catalyst properties and benefits are analyzed and compared. In addition, the challenges and strategies of these oxidants in applications and the major mechanisms of the oxidation process have been discussed. This work can help understand the mechanistic insights of variable Fenton-like reactions, the role of emerging iron-based materials, and provide guidance for choosing appropriate technologies when facing real-world water and wastewater applications.

Establishment of a novel system for photothermal removal of ampicillin under near-infrared irradiation: Persulfate activation, mechanism, pathways and bio-toxicology

One of the most effective ways to address the problems of low solar spectrum utilization in photocatalysis and the high cost of persulfate activation technology is to create a cost-effective synergistic photothermal persulfate system. In this work, a brand-new composite catalyst called ZnFe₂O₄/Fe₃O₄@MWCNTs (ZFC) was developed to activate PDS (K₂S₂O₈) from the aforementioned basis. ZFC's surface temperature could unbelievably reach 120.6 °C in 150 s together with the degrading synergistic system solution temperature could reach 48 °C under near-infrared light (NIR) in 30 min, thus accelerating the ZFC/PDS decolorization rate for reactive blue KN-R (150 mg/L) to 95% in 60 min. Furthermore, the ZFC's ferromagnetism bore it with good cycling performance, allowing it to maintain an 85% decolorization rate even after 5 cycles with OH·, SO₄^-·, ¹O₂, and O₂^-· dominating the degrading process. In the meantime, the DFT calculations of the kinetic constants for the entire process of S₂O₈^2- adsorption on Fe₃O₄ in dye degradation solution were in agreement with the outcomes of the experimental pseudo-first-order kinetic fitting. By analyzing the particular degradation route of ampicillin (50 mg/L) and the possible environmental impact of the intermediate using LC-MS and the toxicological analysis software (T.E.S.T.), respectively, it was shown that this system might function as an environmentally friendly method for removing antibiotics. This work may provide some productive research lines for the creation of a photothermal persulfate synergistic system and suggest fresh approaches to water treatment technology.

Fabrication of chitosan-MnO2‑iridium/nanoceria supported nanoparticles: Catalytic and anti-radical activities

Chitosan capped MnO₂‑iridium nanoparticles supported on nanoceria (Ch-MnO₂-Ir/CeO₂) were fabricated by using combination of colloidal solution and metal displacement galvanic methods. The oxidative degradation of acid orange 7 in aqueous solution by activated persulfate with the as-prepared nanoparticles was studied. The resulting Ch-MnO₂-Ir/CeO₂ with S₂O₈^2-, 80 % degraded 70.06 mg/L of acid orange 7 within 100 min, while at the same time, Ch-Ir, Ch-MnO₂, and Ch-Ir-MnO₂ remained inactive. CeO₂ increased the surface of the catalyst, and also improved the reactive oxygen species site of Ch-Ir-MnO₂ through the activation of S₂O₈^2- with CeO₂. The reversible redox cycle reaction, Ce (III) ↔ Ce (IV) and strong synergistic effect of MnO₂-Ir are responsible for the remarkable catalytic performance of Ch-MnO₂-Ir/CeO₂/S₂O₈^2- system. The degradation of acid orange 7 could be significantly retarded with inorganic (NO₃^- < Cl^- < SO₄^2- < H₂PO₄^- < HCO₃^-) and organic scavengers (ethanol < tertiary butanol < benzoquinone < phenol). Ch-MnO₂-Ir/CeO₂ exhibited excellent stability and reusability. Anti-radical activity of chitosan and Ch-MnO₂-Ir/CeO₂ was evaluated with 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical. The free radical properties increase with concentration of chitosan and Ch-MnO₂-Ir/CeO₂.

Electronic structure modulation of multi-walled carbon nanotubes using azo dye for inducing non-radical reaction: Effect of graphitic nitrogen and structural defect

Multiwalled carbon nanotube (MWCNT) have a great potential for advanced oxidation process as a metal free catalyst. However, there catalytic activity is very low and needs to be appropriately tuned. Herein, we demonstrate a novel synthesis method for tuning the defect and surface functionality of MWCNT using azo dyes and the catalytic performance was tested for the degradation of different organic contaminates using PMS as an oxidant. The content, type of heteroatom functional groups, and the defect parameters were optimized by varying the pH and concentration of the organic dye. The quenching effect showed that singlet oxygen (¹O₂) is the primary reactive species generated by graphitic nitrogen, which can be boosted by the degree of graphitic structure disruption in MWCNT. The Linear sweep voltammetry (LSV) also confirmed that extrinsic doping enhanced the non-radical degradation by increasing the direct charge transfer rate from MB to PMS. Moreover, the designed catalyst showed a fast degradation performance with 35.1 kJ/mol activation energy and achieved the highest dye degradation rate and even surpassed some state-of-the-art metal-based and metal-free catalysts. The effect of inorganic anions study has also confirmed its industrial applicability.

A comprehensive review on BPA degradation by heterogeneous Fenton-like processes

Synthetic organic pollutants emanating continuously in the ecosystem have become a global concern because of their toxicity and persistent nature. Bisphenol A (BPA) is one such pollutant which threatens public health and safety. It is a monomer used in manufacturing plastics, polycarbonate resins, epoxy resins and is a well-recognised endocrine disruptor mimicking estrogen. BPA leaches into food and beverages stored in containers causing contamination issues. Its widespread exposure and potential toxicity is an environmental health concern. In this review, a systematic investigation has been carried out on the heterogeneous catalysts used for Fenton-like processes for BPA degradation. The Fenton-like reaction is one such reaction that is used for wastewater remediation purposes. The reaction advances through the generation of powerful oxidizing radicals like •OH and SO₄^•- in the presence of a suitable catalyst. The application of various Fenton catalysts, with their distinguished morphological characteristics, oxidizing properties, toxicity analysis, and the present state of the art of BPA degradation by these catalysts, have been documented in the current work. This review also highlights a few challenges and prospects for analysing degradation products of landfill leachate.

Nanoarchitecturing TiO2/NiCr2O4 p-n heterojunction photocatalysts for visible-light-induced activation of persulfate to remove tetracycline hydrochloride

Herein, TiO₂/NiCr₂O₄ nanocomposites with p-n heterojunctions were synthesized via a refluxing method and used with peroxydisulfate (PDS) to produce extensive reactive radicals. Textural, optical, photoelectrochemical, structural, and morphological properties of the prepared materials were extensively investigated. After adding 1.48 mM PDS, the removal rate constant of tetracycline hydrochloride (TH) over the TiO₂/NiCr₂O₄ (20%)/PDS system was almost 20.5, 7.24, and 5.91-times as high as the pristine TiO₂, TiO₂/PDS, and TiO₂/NiCr₂O₄ (20%) samples, respectively. Therefore, the synergistic effect of PDS and heterogeneous photocatalysis remarkably impacted the degradation reaction of TH. It was proposed that all h⁺, ^•O₂^-, ^•OH, and SO₄^•- contributed to the degradation reaction. According to the formation of heterojunction between n-TiO₂, and p-NiCr₂O₄ semiconductors, a plausible mechanism for removal of different contaminants in the TiO₂/NiCr₂O₄/PDS system was discussed.

Persulfate-nitrogen doped graphene mixture as an oxidant for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters and nitrite

A series of 3-nitro-4-aryl-2H-chromen-2-ones in good yields have directly been obtained from aryl alkynoate esters and nitrite by employing a mixture of K₂S₂O₈-nitrogen doped graphene as an oxidant in a watery medium at room temperature. A plausible mechanism for the reaction is also reported. It reveals that the product is formed through a cascade of nitro radical addition, spirocyclization, and ester migration. When compared to known methods for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters, this protocol is environmentally friendly, sustainable, practical and energy efficient and does not use a harmful nitro source. Furthermore, nitrogen doped graphene used in this approach can be easily recovered and reused at least four times without losing its activity.

Research on modified blast furnace dust in demulsification: The synergistic effect of ferric oxide, hydrophobic carbon, and polysilicate

Current treatment processes for the cold rolling emulsion wastewater were in the dilemma that the cost was so high while the efficiency was not satisfied in China. In this work, a novel material with high-performance demulsification was obtained and used to treat the emulsion wastewater efficiently by modifying blast furnace dust (BFD), a steel industry waste. Firstly, the BFD was characterized by various analytical techniques and the results suggested that hydrophobic functional groups, positively charged iron oxide, and polysilicon in the BFD were contributed to removing stable oil droplets. Therefore, the BFD modification was conducted accordingly by optimizing the proportion and reaction conditions of these three components. The study demonstrated that the removal rate of oil and COD_cr reached 75.21% and 81.23% under the conditions of the carbon type of GWF-HAC-R90, the carbon content of 14.86%, and the n_(Fe)/n_(Si) of 1.55, respectively. Based on this, the effects of pH, demulsifier dosage, and average agitation rate were investigated. The emulsion components before and after demulsification were analyzed by GC-MS, and the demulsification mechanism was expounded combined with kinetics. Results showed that the Fe₂O₃ with positive charge played a dominant role in emulsions with mainly anionic surfactants, while hydrophobic carbon structures and polysilicate acted as the auxiliaries. Besides, comprehensive analysis and characterization results suggested that the demulsification effect was a combination of the synergic processes: 1) electrostatic interaction developed by the anionic surfactant oil droplets and the positively charged the BFD particles; 2) hydrophobic association among the oil droplet with nonionic surfactant and amphiphilic carbon-iron complexes; 3) adsorption bridging between the surfactant oil droplets and polysilicate. The results of comparative tests in treating the actual cold-rolling emulsion wastewater showed the MBFD could bring about significant technical, economic benefits and achieve utilization of metallurgical solid wastes.Implications: Blast furnace dust (BFD) is an industrial solid waste obtained by dry de-dusting from blast furnace gas during the blast furnace ironmaking process. The main components of BFD are iron oxide and carbon, and also contain small amounts of different recoverable non-ferrous metals zinc, secret, indium and lead, which have recovery value. In this study, we enhanced the three parts of BFD and used to demulsification. In this study, the BFD was modified in three aspects (hydrophobic functional groups, positively charged iron oxide, and polysilicon) to achieve the secondary utilization of waste in emulsion wastewater treatment. And investigated the effect of reaction conditions on the demulsification effect with modified blast furnace dust (MBFD). Furthermore, the GC-MS analyzes combined with kinetics interpret the demulsification mechanism with the MBFD as a secondary resource. Finally, the comparative tests in treating the actual cold-rolling emulsion wastewater showed the MBFD could bring about significant technical and economic benefits and achieve utilization of metallurgical solid wastes.

Iron-Based Dual Active Site-Mediated Peroxymonosulfate Activation for the Degradation of Emerging Organic Pollutants

It is still a challenge to synthesize highly efficient and stable catalysts for the Fenton-like reaction. In this study, we constructed an integrated catalyst with highly dispersed iron-based dual active sites, in which Fe₂N and single-atom Fe (SA-Fe) were embedded into nitrogen- and oxygen-co-doped graphitic carbon (Fe-N-O-GC-350). Extended X-ray absorption fine structure (EXAFS) confirmed the coordination structure of iron, and line combination fitting (LCF) demonstrated the coexistence of Fe₂N and SA-Fe with percentages of 75 and 25%, respectively. Iron-based dual active sites endowed Fe-N-O-GC-350 with superior catalytic activity to activate peroxymonosulfate (PMS) as evidenced by the fast degradation rate of sulfamethoxazole (SMX) (0.24 min^-1) in the presence of 0.4 mM PMS and 0.1 g/L Fe-N-O-GC-350. Unlike the reported singlet oxygen and high-valent iron oxo-mediated degradation induced by the SA-Fe catalyst, both surface-bound reactive species and singlet oxygen contributed to SMX degradation, while surface-bound reactive species dominated. Density functional theory (DFT) simulation indicated that Fe₂N and SA-Fe enhanced the adsorption of PMS, which played a key role in PMS activation. The Fe-N-O-GC-350/PMS system had resistance to the interference of common inorganic anions and high oxidation capacity to recalcitrant organic contaminants. This study elucidated the important role of Fe₂N in PMS activation and provide a clue to design rationally catalysts with iron-based dual active sites to activate PMS for the degradation of emerging organic pollutants.

Sulphate radical oxidation of benzophenone: kinetics, mechanisms and influence of water matrix anions

Benzophenone (BP) is an emerging contaminant that is widely distributed in soil, groundwater, sediment and surface water. In this study, the degradation kinetics, mechanisms, and influence of anions on thermally activated persulphate (TAP) oxidation of BP were systematically investigated. BP degradation was promoted by elevated temperature. The BP degradation data fitted well to the Arrhenius equation with calculated activation energy of 122.8 kJ/mol. BP degradation was also promoted by alkaline pH and high persulphate concentrations. Radical scavenging experiments suggested that both SO₄^•- and HO^• were involved in BP oxidation. Ultra-high-performance liquid chromatography coupled to Orbitrap mass spectrometry (UHPLC-Orbitrap-MS) identified six degradation intermediates. Based on these results, two possible reaction pathways were proposed. Water matrix anions had complex impacts on BP degradation by TAP. Cl^- had dual effects on the reaction: low concentration promoted it while high concentration inhibited it. Br^- strongly suppressed the reaction. SO₄^2- and NO₃^- did not affect the reaction. Overall, this study shows that thermally activated persulphate can effectively remove BP and water matrix anions greatly influence the reaction.

Activation of peroxymonosulfate by bicarbonate and acceleration of the reaction by freezing

This study demonstrates the positive effects of dissolved bicarbonate and carbonate anions on peroxymonosulfate (PMS) induced oxidation and the remarkable acceleration of the reaction by freezing. More than 90% of the initial 4-chlorophenol (4-CP) decomposed in the frozen case, whereas only less than 20% of the 4-CP was removed in the aqueous case in the same time period. This accelerated reaction is attributed to the freeze-concentration of the dissolved substrates (i.e., PMS, bicarbonate, and pollutants) in the quasi-liquid layer at the ice grain boundaries between ice crystals. The reaction between bicarbonate and PMS was found to be unique because none of the effects were observed in the phosphate and hydroxide cooperated system with freezing, although the base activation of PMS could participate under basic conditions (pH > 9). Based on electron paramagnetic resonance spectroscopy measurements and comparison with the photo-excited Rose Bengal system as a reference system for singlet oxygen (¹O₂) generation, ¹O₂ was found to have a minor effect on the oxidation of 4-CP in the frozen bicarbonate-PMS system. While, direct electron transfer from the target organic substrate to the PMS was suggested as a major mechanism of 4-CP oxidation, because the selected target organic substrates were decomposed with different tendencies, and the consumption of PMS was accelerated by the presence of an electron donating compound. The results show the potential applicability of the freezing phenomenon, which occurs naturally in the mid-latitude and polar area, to help a decomposition of water dissolved organic pollutants by the imitation of the natural purification process.

Persulfate-activated charcoal mixture: an efficient oxidant for the synthesis of sulfonated benzo[d][1,3]oxazines from N-(2-vinylphenyl)amides and thiols in aqueous solution

A series of 2,4-aryl-4-((arylsulfonyl)methyl)-4H-benzo[d][1,3]oxazines in good to excellent yields have directly been obtained from N-(2-vinylphenyl)amides and thiols by employing a mixture of K₂S₂O₈-activated charcoal in aqueous acetonitrile solution at 50 °C. A plausible mechanism for the reaction is reported. It reveals that the reaction follows a radical pathway and the persulfate has been the oxygen source for formation of the sulfone group in the products. It is worth mentioning that this protocol utilizes an easily accessible K₂S₂O₈-activated charcoal mixture and thiols, respectively, as an oxidant and sulfonylating precursors for the first time.

Activation of persulfate by heterogeneous catalyst ZnCo2O4–RGO for efficient degradation of bisphenol A

A nanocomposite, reduced graphene oxide (RGO) modified ZnCo2O4 (ZnCo2O4–RGO) was synthesized via one-step solvothermal method for activating persulfate (PS) to degrade bisphenol A (BPA). The morphology and structure of the nanocomposite were identified by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. RGO provides nucleation sites for ZnCo2O4 to grow and inhibits the agglomeration of the nanoparticles. The influence of different reaction conditions on the oxidation of BPA catalyzed by ZnCo2O4–RGO was investigated, including the content of RGO, the dosage of catalyst, the concentration of humic acid (HA), anions in the environment, the reaction temperature, and pH. BPA can be totally degraded within 20 min under optimized reaction conditions. The presence of HA, Cl−, and NO3− only has a slight effect on the oxidation of BPA, whereas the presence of either H2PO4− or HCO3− can greatly inhibit the reaction. ZnCo2O4–RGO shows good cycling stability and practical application potential. A reaction mechanism of the degradation of BPA was also explored.

Accelerated oxidation of microcystin-LR by Fe(II)-tetrapolyphosphate/oxygen in the presence of magnesium and calcium ions

Fe(II)-tetrapolyphosphate complexes are known to activate molecular oxygen (Fe(II)-TPP/O₂) to produce reactive oxidants (most likely, Fe(IV)-TPP complexes) that are capable of degrading refractory organic contaminants in water. This study found that magnesium and calcium ions (Mg²⁺ and Ca²⁺) accelerate the degradation of micfrocystin-LR (MC-LR), the most toxic and abundant cyanotoxin, by the Fe(II)-TPP/O₂ system. The addition of Mg²⁺ and Ca²⁺ increased the observed rate constant of MC-LR degradation by up to 4.3 and 14.8 folds, respectively. Mg²⁺ and Ca²⁺ accelerated the MC-LR degradation in the entire pH range, except for the alkaline region with pH > ca. 10. The addition of Mg²⁺ and Ca²⁺ also reshaped the pH-dependency of the MC-LR degradation, greatly increasing the rate of MC-LR degradation at neutral pH. It was found that Mg²⁺ and Ca²⁺ accelerate the reaction of Fe(II)-TPP complexes with oxygen, resulting in faster production of reactive oxidants. The findings from cyclic voltammetry and potentiometric titration suggest that Mg²⁺ and Ca²⁺ form ternary complexes with Fe(II)-TPP, which exhibit higher reactivity with oxygen. Due to the effects of Mg²⁺ and Ca²⁺, the rate of MC-LR degradation by the Fe(II)-TPP/O₂ system was even higher in natural water than in deionized water.

Fabrication of black TiO2−x/CuFe2O4 decorated on diatomaceous earth with enhanced sonocatalytic activity for ibuprofen mitigation

This study reports facile fabrication of black TiO_2−x/CuFe₂O₄ (Ti³⁺ self-doped titania coupled with copper ferrite), an efficient sonocatalyst for ibuprofen (IBP) mitigation.