The mechanism and efficiency of MnO2 activated persulfate process coupled with electrolysis

Engineered biochar derived from food waste digestate for activation of peroxymonosulfate to remove organic pollutants

The anaerobic digestion of food waste, can result in large amounts of solid waste digestate, often without methods of disposal. In this study, a biochar was prepared from food waste digestate (FWD) by pyrolysis, and its potential to activate peroxymonosulfate (PMS) for the removal of pollutants from a simulated textile wastewater was evaluated. The results showed that the addition of biochar (0.5 g/L) and PMS (1 mM) to wastewater could remove >99% of a representative azo dye pollutant (reactive brilliant red X-3B, 1 g/L) within 10 min. The efficiency of this removal process was attributed to the catalytic sites in the biochar (graphitic carbon and nitrogen, pyridinium nitrogen and CO structures) which could activate PMS to produce reactive oxygen species (¹O₂, O₂^-, OH and SO₄^-). The results obtained in this study confirmed the activation potential of the biochar derived from FWD on PMS, providing an alternative utilization strategy for anaerobic FWD.

Evaluating perfluorooctanesulfonate oxidation in permanganate systems

Permanganate (PM) has shown to be able to oxidize a range of organic contaminants including perfluorooctane sulfonate (PFOS). However, mechanisms of PFOS removal by PM have been questioned. To provide clarity to what may be happening to PFOS in PM systems, here we evaluated the ability of PM on PFOS destruction by conducting studies similar to previous studies that reported PFOS destruction which included PM solutions and PM combined with persulfate (PS). We also evaluated if addition of various soluble catalysts could enhance PM's potential to breakdown PFOS. We observed no PFOS destruction by PM. We also show that the F^- and SO₄^2- generation reported in a published study as evidence that PM was breaking bonds in PFOS were found below or not significantly higher than reported limits of quantitation and that SO₄^2- impurities in technical PM approach the reported SO₄^2- levels. For PM-PS systems, heterogeneous PFOS distribution was observed when subsampling reaction vessels at different depths and "salting-out" of PFOS was evident. In addition, subsequent sonication and filtering of the samples led to the apparent disappearance of most of the PFOS, which was an artifact arising from the behavior of PFOS aggregates or potential hemi-micelle formation. Given these findings, addition of salts may have application for collecting or concentrating PFOS and other PFAAs in a remediation or water treatment strategy.

Simultaneous regeneration of cathodic activated carbon fiber and mineralization of desorbed contaminations by electro-peroxydisulfate process: Advantages and limitations

This study investigated the regeneration of phenol saturated activated carbon fiber (ACF) with a novel electro-peroxydisulfate (E-PDS) process. Compared with traditional electrochemical regeneration, E-PDS process could simultaneously regenerate the exhausted ACF and mineralize desorbed contaminants by activating PDS in water with a much lower energy consumption (1/6). According to the estimation of relative contributions involved in E-PDS process, reactive oxygen species (ROS), especially sulfate radical (SO₄^•-), played a dominant role in the degradation of phenol and its byproducts. It was worth noting that the accumulation of byproducts in solution increased significantly after SO₄^•- concentration decreased in aqueous solution. Further study proved that the regeneration efficiency of ACF could be improved by the application of multiple doses of PDS for the effective reduction of byproduct accumulation. However, application of multiple doses of PDS could not prevent ACF from being oxidized by ROS generated in the system, subsequently leading to loss of ACF adsorption capacity. This limitation is a significant concern in treatment technologies based on carbon materials activated by peroxides and such technologies should be studied further to obtain additional insights on their potential and applicability in industrial practice. Nevertheless, the adsorption capacity of ACF remained above 40% after three regeneration cycles in the E-PDS process. Therefore, E-PDS process showed promise for further evaluation as a potentially viable approach for the regeneration of carbons saturated with organic pollutants.

MnCeOX with high efficiency and stability for activating persulfate to degrade AO7 and ofloxacin

An efficient MnCeO_x composite was successfully synthesized for activation of persulfate to degrade acid orange 7 (AO7) and ofloxacin. Pollutants degradation efficiencies with different catalytic systems were investigated. Results showed the performance of MnCeO_x was better than MnO_x, CeO₂ and MnO_x + CeO₂. Thus, there was a clear synergistic effect (Se) between Mn and Ce in the composite, and the Se was 73.8% for AO7 and 39.6% for ofloxacin. In addition, AO7 removal fitted 1st order reaction while ofloxacin removal fitted 2nd order reaction in MnCeO_x/persulfate system. Moreover, MnCeO_x/persulfate system showed high efficiency in pH range of 5-9. Mechanism analysis showed that SO₄^- and OH on the surface of the catalyst were the main active species, and O₂^- also played an important role in pollutants degradation. Furthermore, MnCeO_x showed high activity in actual water. Finally, the possible degradation pathway of ofloxacin was proposed according to the high performance liquid chromatography-mass spectrometry result. Overall, this study provides an efficient and stable catalyst to activate persulfate to degrade refractory pollutants.

MnCeOx/diatomite catalyst for persulfate activation to degrade organic pollutants

Persulfate (PS)-based oxidation technologies are attracting increasing attentions in water treatment due to their high efficiency and stability. In this study, a novel diatomite supported MnCeO_x composite (MnCeO_x/diatomite) was prepared and characterized for activation of PS to degrade organic pollutants. Results indicated that diatomite not only dispersed MnCeO_x and increased the specific surface area of catalyst, but also improved the low-valence metal site (Mn²⁺ and Ce³⁺) and reactive oxygen species site (-OH) of MnCeO_x, thus enhancing the activities of MnCeO_x. MnCeO_x/diatomite/PS showed high efficiency for multiple dyes and pharmaceutical pollutants. Constant rate (k) of MnCeO_x/diatomite (k_{MnCeOx/diatomite}) was three times higher than the sum of constant rate of MnCeO_x (k_MnCeOx) and constant rate of diatomite (k_diatomite). In addition, MnCeO_x/diatomite showed wide pH application (5-9). Cl^- and NO₃^2- had no effect while SO₄^2- and humid acid had slightly negative effects on MnCeO_x/diatomite/PS system. Moreover, MnCeO_x/diatomite showed good reusability and stability. Mechanism analyses indicated that electron transfer of Mn and Ce attributed to the activation of PS and oxygen to produce free radicals. SO₄^-, OH and O₂^- on the surface of catalyst were the main active free radicals to attack pollutants.

Mineralization Enhancement of Pharmaceutical Contaminants by Radical-Based Oxidation Promoted by Oxide-Bound Metal Ions

While the use of transition metal oxides in catalyzing advanced oxidation reactions has been widely investigated, very few reports have focused on how the preliminary contact of oxides with target compounds may affect the succession of reaction. In this study, we examined the adsorption and electron transfer reactions of two fluoroquinolones, flumequine (FLU), and norfloxacin (NOR), with goethite (α-FeOOH) or manganese (Mn) oxide, and their impact on the subsequent mineralization of target compounds using H₂O₂ or S₂O₈^2- under UVA irradiation. Intriguingly, higher total organic carbon (TOC) removal was achieved when antibiotics and metal oxides were allowed for preequilibration before starting the oxidation reaction. The rate and extent of TOC removal are strongly dependent on the molecule structure and the redox-active mineral used, and much less on the preequilibration time. This high efficiency can be ascribed to the presence of reduced metal ions, chemically or photochemically generated during the first stage, onto oxide minerals. Oxide-bound Mn^II plays a crucial role in catalyzing oxidant decomposition and then producing greater amounts of radical species through a photoassisted redox cycle, regardless of the underlying surface, Mn^IVO₂ or Mn^IIIOOH. This finding will be of fundamental and practical significance to Mn-based oxidation reactions and wastewater treatment processes.

Heterogeneous degradation of organic contaminant by peroxydisulfate catalyzed by activated carbon cloth

An eco-friendly material, activated carbon cloth (ACC) was used as the heterogeneous catalyst in activation of peroxydisulfate (PDS) for the efficient degradation of organic pollutant in water. Besides, the effects of several parameters in the ACC/PDS process including initial pH, PDS concentration, reaction temperature, stirring speed and co-existing anions were investigated. Under optimum conditions, almost complete removal (98.6%) of AO7 in 60 min and 67.4% of total organic carbon (TOC) removal within 180 min were obtained, accompanied by the remarkable destruction of azo band and naphthalene ring on AO7. The electron paramagnetic resonance and radical quenching experiments were carried out to identify the reactive radicals in the ACC/PDS process. Surface characteristic techniques such as XRD, BET, SEM, FTIR, XPS were applied to analysis the change of crystal structure, surface area, surface morphology, functional groups on the surface of fresh and spent ACC samples. Hydroxyl groups (C‒OH) and π-π transitions significantly affected the catalytic activity of ACC. The intermediate products of AO7 oxidation were identified by LC-MS and the corresponding degradation pathway was proposed.

Promotion of short-chain fatty acids production and fermented sludge properties via persulfate treatments with different activators: Performance and mechanisms

This study explored the influences of peroxydisulfate (PDS) and peroxymonosulfate (PMS) activated with different catalysts on the anaerobic fermentation of waste activated sludge (WAS). All the treatments were effective in promoting short-chain fatty acids (SCFAs) production, particularly acetic acid, in the order of PMS/MnO₂ > PMS/Zn > PDS/Zn > PMS/Fe > PDS/Fe > PDS/MnO₂. Mechanistic investigations demonstrated that WAS disintegration was intensely induced by the free radicals (i.e., SO₄^- and OH) generated in PDS and PMS treating reactors. It significantly promoted the solubilization and hydrolysis processes and thereby provided sufficient bioavailable substrates for further acidogenic metabolisms. Additionally, it enlarged the abundance of functional bacteria responsible for SCFAs production. The simultaneous promotion of bioavailable substrates and fermentative microorganisms markedly contributed to the SCFAs enhancement. Moreover, the dewaterability and stabilization of fermented sludge were both improved with the PDS and PMS treatments, which were beneficial to the final disposal of WAS.

Mn-based catalysts for sulfate radical-based advanced oxidation processes: A review

Sulfate radical-based advanced oxidation processes (AOPs) have drawn increasing attention during the past two decades, and Mn-based materials have been proven to be effective catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS) to degrade many contaminants. This article presents a comprehensive review of various Mn-based materials to activate PMS and PDS. The activation mechanisms of different Mn-based catalysts (i.e., Mn oxides MnO_x, MnO_x hybrids, and MnO_x‑carbonaceous material composites) were first summarized and discussed in detail. Besides the commonly reported free radicals (SO₄^-• and ^•OH), non-radical mechanisms such as singlet oxygen and direct electron transfer have also been discovered for selected materials. The effects of pH, inorganic ions, natural organic matter (NOM), dissolved oxygen content, temperature, and the crystallinity of the materials on the catalytic reactivity were also discussed. Then, important instrumentations and technologies employed to characterize Mn-based materials and to understand the reaction mechanisms were concisely summarized. Three common overlooks in the experimental designs for examining the PMS/PDS-MnO_x systems were also discussed. Finally, future research directions were suggested to further improve the technology and to provide a guidance to develop cost-effective Mn-based materials to activate PMS/PDS.

Improved bioelectricity production using potassium monopersulfate as cathode electron acceptor by novel bio-electrochemical activation in microbial fuel cell

Potassium monopersulfate (PMS) without a catalyst as cathode electron acceptor was first established to improve the electricity generation performance of a microbial fuel cell (MFC) in this study. The work investigated the performance with pure PMS (PPMS) and compound PMS (CPMS). The concentration and initial pH of PMS had an effect on the electricity generation, which increased with higher PMS concentration and lower catholyte pH. In the PPMS-MFC system, the maximum voltage (0.972 V), power density (16.37 W/m³), optimal exchange current density (2.000 A/m³) and minimum polarization impedance (Rp: 97.33 Ω) were reached at 10 mM PMS and pH 3.0. However, the maximum power density (8.60 W/m³) was exhibited at 70 mM PMS and pH 3.0 in the CPMS system. Additionally, high COD removals of 99.41% and 98.71% in anode chambers were obtained in the two systems, respectively. Sulfate radicals (SO₄^-) and hydroxyl radicals (OH) played significant roles in the PPMS-MFC, while HClO was also a contributor in addition to SO₄^- and OH in the CPMS-MFC. Furthermore, SO₄^- and OH was generated in situ in the cathode to promote the reduction reaction. The inorganic anion had different effects on electricity generation. Finally, while energy was recovered, rhodamine B (RhB) was added to the cathode chamber and then removed successfully in PPMS-MFC system. This work confirmed that only PMS could be activated by bio-electrochemical method, which is an energy-saving, environmentally friendly and effective activation approach, and thus, it could be used as an efficient acceptor in a MFC.

Acid Orange 7 treatment and fate by electro-peroxone process using novel electrode arrangement

Electro-peroxone is a novel advanced oxidation process that surpasses ozonation or peroxone because of its advantages. In this technology, combining ozone and hydrogen peroxide generated electrochemically leads to the production of hydroxyl radicals, which are the strongest oxidizing agents. In this study, a cylindrical reactor with a continuous circular flow using novel arrangements of electrodes was used to examine the effects of variant parameters on dye removal efficiency. Acid Orange 7 (C₁₆H₁₁N₂NaO₄S) served as an indicator pollutant. Based on overall energy consumption and energy consumption per dye removed weight, electro-peroxone not only has proper efficiency at high dye concentrations, it also has the least energy consumption per dye removed weight; 53 KWh kg^-1 is achieved for 500 mg L^-1 initial dye concentration at 99% removal efficiency after 40 min. The results show that at the optimum condition of [Dye] = 500 mg L^-1, pH = 7.7, applied current = 0.5 A, O₃ rate = 1 L min^-1, and [Na₂SO₄] = 0.1 M, dye is removed completely after 90 min and COD and TOC removal is 99% and 90%, respectively. LC-MS results also showed that AO7 initially was converted to more toxic compounds than AO7 like benzoic acid but finally linear acidic intermediate with less toxicity such as fumaric acid was formed.

Persulfate activation by Fe(III) with bioelectricity at acidic and near-neutral pH regimes: Homogeneous versus heterogeneous mechanism

The combination of persulfate (PS) activation by iron ions with electrochemical process (electro/Fe³⁺/PS) is a promising advanced oxidation process. However, almost all these systems were performed in an unbuffered solution and actually under acidic pH condition, with the electricity being frequently supplied by external power. Considering the high buffering capacity of wastewater and energy saving, peroxydisulfate (PDS) activation by Fe(III) species with bioelectricity provided by microbial fuel cell (MFC) for bisphenol A (BPA) oxidation was investigated at fixed near-neutral pH as well as acidic pH. The results indicate that 90.8% of BPA could be removed at pH 2.5. Though the iron existed in the form of precipitate, BPA could still be efficiently removed at pH 6.0. The precipitate formed in the system at pH 6.0 was identified as the amorphous iron oxyhydroxides. Sulfate radicals in the bulk solution and that adsorbed on the precipitate were the dominant reactive species responsible for the oxidation of BPA in the homogeneous and heterogeneous MFC/Fe(III)/PDS processes, respectively. The mechanisms of BPA degradation at both pH values were proposed via EPR and quenching tests as well as XPS analysis. The effects of operating parameters, the mineralization, the mineralization current efficiency and energy consumption were also explored.

Mechanism and kinetics of catalytic ozonation for elimination of organic compounds with spinel-type CuAl2O4 and its precursor

CuAl₂O₄ based mixed oxides were used as heterogeneous catalysts for ozone activation to degrade organics in aqueous solution. The solids were thoroughly characterized by SEM/EDS, N₂ physisorption, XRD, FTIR, Pyridine-FTIR, TEM and XPS. We demonstrated that the solid precursor calcined at 300 °C exhibited the best catalytic ozonation activity with respect to CuAl₂O₄ spinel phase obtained at higher temperatures. Such performance was attributed to the better textural properties and a higher density of active sites (hydroxyl groups and Lewis acidity). Specifically, the mixed oxide/O₃ process allows to reach a near complete color removal of the dye solution (100 mg L^-1) in 25 min at neutral pH. Corresponding reaction rate value was measured at 0.112 min^-1 and was clearly higher compared with the single oxide ozonation process (0.071 min^-1 for CuO/O₃ and 0.074 min^-1 for Al₂O₃/O₃). Then, we proposed that such catalytic performance was related to a synergistic function between ≡Cu²⁺ and ≡Al³⁺, which took part of a mechanism of radical formation. In such mechanism, present ≡Al³⁺ could act as a reservoir for surface active sites such as hydroxyl groups and Lewis acid sites, while ≡Cu²⁺ could provide the possibility of electron transfer with ozone for the enhancement of radical generation. We suggested that the interaction between chemisorbed ozone and surface hydroxyl groups initially stabilized on ≡Al³⁺ initiated the generation of reactive radical species. This interaction led as well to the formation of surface adsorbed HO and few O₂^- on ≡Cu²⁺ Lewis acid sites. Besides, the interfacial redox reaction with ozone is favored by the presence of ≡Cu²⁺ following the sequence of ≡Cu²⁺/≡Cu⁺/≡Cu²⁺ redox cycle.

The efficiency and mechanism of dibutyl phthalate removal by copper-based metal organic frameworks coupled with persulfate

Copper-based metal organic framework (Cu-BTC) was prepared and used to remove dibutyl phthalate (DBP) in the presence of persulfate (PS). The surface characteristics, textural properties, and stability of activated Cu-BTC (denoted as Cu-BTC-A) were evaluated by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, N2 physical adsorption-desorption, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The effects of parameters such as initial pH, PS concentration, catalyst dosage, and free-radical quenchers have been investigated. The results showed that DBP could be removed in a wide pH range by Cu-BTC-A via mechanisms of adsorption and heterogeneous catalytic reaction. Unfortunately, the DBP removal was not completed because of radical scavenging reactions in Cu-BTC-A cages where PS can enter freely but DBP is blocked outside. Another explanation was that Cu-BTC-A showed a low adsorption capacity for DBP because the molecular size of DBP (15.84 × 11.00 × 7.56 Å) is larger than microporous cages (approximately 9 × 9 Å in diameter) of Cu-BTC-A.

Degradation of aqueous 2,4,4'-Trihydroxybenzophenone by persulfate activated with nitrogen doped carbonaceous materials and the formation of dimer products

In this work, we systematically investigated the persulfate (PS) activation potential of a series of nitrogen doped carbonaceous materials for the degradation of 2,4,4'-trihydroxybenzophenone (2,4,4'-HBP), an additive in polyvinyl acetate films and personal care products. Nitrogen originating from urea, NH₄NO₃, indole and polyaniline was doped into carbonaceous materials, including hydroxylated multi-walled carbon nanotubes (CNT-OH), large-inner thin-walled carboxylated carbon nanotubes (CNT-COOH) and graphite oxide (GO), to examine the catalytic effect. The NH₄NO₃-CNT-OH catalyst, which showed the best catalytic performance in 2,4,4'-HBP removal, was characterized by SEM, TEM, FT-IR, Raman, BET surface area, XRD and XPS, and pyrrolic nitrogen was found to play a highly important role in the activation of PS. Under the conditions of [2,4,4'-HBP]₀: [PS]₀ = 1: 500, T = 25 °C, pH₀ = 7.0, concentration of catalyst = 100 mg L^-1, 43.48 μM 2,4,4'-HBP was completely removed in 2 h. According to electron paramagnetic resonance (EPR) spectra and radical quenching experiments, hydroxyl and sulfate radicals on the surface of the catalyst contributed to the substrate oxidation. Cleavage of C-C bridge bond, hydroxylation and polymerization were mainly involved in the oxidation process, leading to the formation of 10 intermediates (e.g., dimers), as detected by the MS/MS spectra. To the best of our knowledge, this report is the first to describe the transformation mechanism of 2,4,4'-HBP in nitrogen doped carbonaceous materials catalyzed PS system.

Enhanced degradation of isoproturon in soil through persulfate activation by Fe-based layered double hydroxide: different reactive species comparing with activation by homogenous Fe(II)

Phenylurea herbicide residuals in soil may continuously contaminate surface water and groundwater due to unregulated and improper use. Herein, we reported a stable and active oxidation system including heterogeneous Fe-based layered double hydroxide materials as persulfate (PS) activators. Under mild conditions, 1% LDH in weight and 70 mM PS can completely degrade 500 mg/kg isoproturon in soil within 10 h, during which less than 0.1 ppm heavy metal leaching was detected. This remarkable performance was consistent in a broad pH range (3~11) and was resistant to various inorganic anions (Cl^-, Br^-, NO₃^-, HCO₃^-) and humic acid. Mechanism studies from scavenging tests, EPR, and fluorescence spectra collectively proved that besides •OH and •SO₄^-, singlet oxygen (¹O₂) and superoxide (•O₂^-) were also generated and were accounted for the oxidative degradation. This unique mechanism of generating diverse radicals was clearly distinguished from classic Fe(II)/PS system, significantly reduced the influence of varying parameters in water and soil matrix, and was suggestive to chemical oxidation system in soil remediation to avoid scavenging effects by background electrolytes or other components in water/soil matrix. Graphical abstract ᅟ.

Degradation of bisphenol A by electro-enhanced heterogeneous activation of peroxydisulfate using Mn-Zn ferrite from spent alkaline Zn-Mn batteries

Mn-Zn ferrite (Mn_0.6Zn_0.4Fe₂O₄) was prepared by a gel method using spent alkaline Zn-Mn batteries as raw materials and employed as catalyst to degrade bisphenol A (BPA) by electro-enhanced heterogeneous activation of peroxydisulfate (PDS). The effects of initial pH, current density, PDS concentration, and Mn-Zn ferrite dosage on BPA removal were investigated. The formation of reactive radicals was verified by electron paramagnetic resonance (EPR) spectroscopy. The results of radical quenching experiments indicate that surface-bound sulfate and hydroxyl radicals played an important role in BPA removal. The stability of Mn_0.6Zn_0.4Fe₂O₄ catalyst was investigated by cycling experiments, which indicates Mn_0.6Zn_0.4Fe₂O₄ is stable and can be reused. This work also provides an alternative way for the reutilization of spent alkaline Zn-Mn batteries.

Degradation of Acid Orange 7 using peroxymonosulfate catalyzed by granulated activated carbon and enhanced by electrolysis

Electrochemistry coupled with granulated activated carbon catalysis of peroxymonosulfate (electro/GAC/PMS) as a novel wastewater treatment process was performed for the degradation of Acid Orange 7 (AO7) in aqueous solution. The decolorization of AO7 was compared under different permutations and combinations of electro-oxidation, GAC and PMS. It showed that the electro/GAC/PMS process was the most effective and the decolorization of AO7 followed pseudo-first order kinetics. The surface chemistry of GAC samples was analyzed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Compared with the fresh samples, π-electron density and hydroxyl group content decreased under the GAC/PMS system, but kept the similar values under the electro/GAC/PMS system. Electron paramagnetic resonance and radical scavenger studies were used to verify the formation of sulfate radicals (SO₄^-) and hydroxyl radicals (OH). The optimized conditions were found to be: current density 8 mA cm^-2; PMS concentration 5 mM; GAC dosage 0.5 g L^-1; and initial pH value 5.0. GAC recycling experiments over 4 runs showed some decrease in reactivity. Overall, the results indicate that 100% color removal was readily achieved and 50.4% of TOC was removed which shows high efficiency of the electro/GAC/PMS process.