Bicarbonate-activated persulfate oxidation of acetaminophen

The pH-dependent degradation of sulfadiazine using natural siderite activating PDS: The role of singlet oxygen

Occurring naturally siderite (FeCO₃) was used as the heterogeneous catalyst to activate peroxodisulfate (PDS) for the degradation of sulfadiazine under different initial pH values. The findings of this system exhibited various ROS (e.g. ¹O₂, SO₄^- and OH) present during a wide range of pH values. Among them, ¹O₂ could significantly facilitate the initial degradation rate, and the increased pH enhanced the role of ¹O₂. The factors including initial pH values, siderite dosage, PDS concentration, initial contaminants concentration, and water matrix were discussed. The role of each ROS was investigated through quenching test and electron paramagnetic resonance (EPR). Furthermore, the comprehensive degradation process was proposed based on the LC-MS results. And the cycle test demonstrates the reusability of siderite at a pH of 3. Accordingly, this study is of great significance for understanding the degradation of such sulfonamide pollutants in the siderite/PDS system.

Application of percarbonate and peroxymonocarbonate in decontamination technologies

Sodium percarbonate (SPC) and peroxymonocarbonate (PMC) have been widely used in modified Fenton reactions because of their multiple superior features, such as a wide pH range and environmental friendliness. This broad review is intended to provide the fundamental information, status and progress of SPC and PMC based decontamination technologies according to the peer-reviewed papers in the last two decades. Both SPC and PMC can directly decompose various pollutants. The degradation efficiency will be enhanced and the target contaminants will be expanded after the activation of SPC and PMC. The most commonly used catalysts for SPC activation are iron compounds while cobalt compositions are applied to activate PMC in homogenous and heterogeneous catalytical systems. The generation and participation of hydroxyl, superoxide and/or carbonate radicals are involved in the activated SPC and PMC system. The reductive radicals, such as carbon dioxide and hydroxyethyl radicals, can be generated when formic acid or methanol is added in the Fe(II)/SPC system, which can reduce target contaminants. SPC can also be activated by energy, tetraacetylethylenediamine, ozone and buffered alkaline to generate different reactive radicals for pollutant decomposition. The SPC and activated SPC have been assessed for application in-situ chemical oxidation and sludge dewatering treatment. The challenges and prospects of SPC and PMC based decontamination technologies are also addressed in the last section.

Removal of acetaminophen through direct electron transfer by reactive Mn2O3: Efficiency, mechanism and pathway

Mn₂O₃ with certain oxidative reactivity, was fabricated via sol-gel method and applied for the removal of acetaminophen (APAP). The mechanism of APAP oxidation was revealed in depth through electrochemical tests and X-ray photoelectron spectroscopy (XPS). Moreover, the selective abatement of various organic contaminants contained different functional groups by Mn₂O₃ was investigated through linear free energy relationship (LFER) estimated with peak potentials (E_op) of these organic contaminants. Under acidic condition, APAP could be effectively eliminated by Mn₂O₃. The open circuit potential (OCP) and zeta potential tests illustrate that the oxidative reactivity of Mn₂O₃ is associated to the surface acid-base behavior of Mn₂O₃ and its surface charge situation. The XPS experiments and Mn leaching results imply that Mn(III) could capture electron from APAP and release Mn²⁺ to aqueous phase. The intermediates could be ascribed to fragmentation of acetamido radicals and phenoxy radicals, both of which were formed through electron transfer from APAP to Mn₂O₃. The reactive Mn₂O₃ shows selective oxidation of different contaminants in the electron transfer process. LFER analysis indicates good negative linear correlation between lnk₁ and E_op of various pollutants. The efficiency of Mn₂O₃ in the elimination of APAP and selective oxidation of different contaminants suggest some new insights for transformation of APAP and other electron-rich pollutants in the environments.

Nonradicals induced degradation of organic pollutants by peroxydisulfate (PDS) and peroxymonosulfate (PMS): Recent advances and perspective

Nonradical persulfate oxidation processes have emerged as a new wastewater treatment method due to production of mild nonradical oxidants, selective oxidation of organic pollutants, and higher tolerance to water matrixes compared with radical persulfate oxidation processes. Since the case of the nonradical activation of peroxydisulfate (PDS) was reported on CuO surface in 2014, nonradical persulfate oxidation processes have been extensively investigated, and much achievement has been made on realization of nonradical persulfate activation processes and understanding of intrinsic reaction mechanism. Therefore, in the review, nonradical pathways and reaction mechanisms for oxidation of various organic pollutants by PDS and peroxymonosulfate (PMS) are overviewed. Five nonradical persulfate oxidation pathways for degradation of organic pollutants are summarized, which include surface activated persulfate, catalysts-free or catalysts mediated electron transfer, ¹O₂, high-valent metals, and newly derived inorganic oxidants (e.g., HOCl and HCO₄^-). Among them, the direct oxidation processes by persulfate, nonradical based persulfate activation by inorganic/organic molecules and in electrochemical methods is first overviewed. Moreover, nonradical based persulfate activation mechanisms by metal oxides and carbon materials are further updated. Furthermore, investigation methods of interaction between persulfate and catalyst surface, and nature of reactive species are also discussed in detail. Finally, the future research needs are proposed based on limited understanding on reaction mechanism of nonradical based persulfate activation. The review can offer a comprehensive assessment on nonradical oxidation of organic pollutants by persulfate to fill the knowledge gap and provide better guidance for future research and engineering application of persulfate.

Central composite design applied to paracetamol degradation by heat-activated peroxydisulfate oxidation process and its relevance as a pretreatment prior to a biological treatment

In this study, the degradation of paracetamol (PCT) by thermo-activated peroxydisulfate (PDS) and the feasibility of coupling thermo-activated peroxydisulfate to activated sludge culture were examined. The effect of the relevant parameters on the thermo activated peroxydisulfate process, namely temperature, PDS concentration, initial pH and initial PCT concentration was investigated. As observed, the solution pH did not have a significant effect on the PCT degradation. The temperature increased the degradation of PCT, while an increase of the initial PCT concentration impacted negatively its degradation yield. The PDS concentration showed an optimal value of 8 mM. The operating parameters were then optimized by using a central composite design (CCD). After performing a screening of the various factors, response surface analysis led to the following optimal conditions for the yield of PCT degradation: 0.33, 5 mM, pH = 6 and 68°C for the initial PCT concentration, the initial peroxydisulfate concentration and the temperature respectively, leading to the removal of 94.2% of PCT. Under these conditions, the BOD₅/COD ratio increased from 0.008 initially to 0.34 after 10 h. Showing a significant improvement of the biodegradability; consequently and even if the limit of biodegradability (0.4) was not achieved, a biological treatment could be promisingly considered.

Degradation of benzotriazole by sulfate radical-based advanced oxidation process

Benzotriazole (BTA) is a recalcitrant contaminant that is widely distributed in aquatic environments. This study explored the effectiveness of sulfate radical-based advanced oxidation process in degrading BTA (SR-AOP). The sulfate radical was generated by heat activation of persulfate (PS). Our results show alkaline pH promoted the BTA degradation. The solution pH also affected the speciation of total radicals. Sulfate radical ( S O 4 ⋅ - ) predominated at acidic pH while hydroxyl radical (HO^•) predominated at basic pH. High temperature, high PS concentration and low BTA concentration promoted the BTA degradation. Influence of water matrix constituents on the reaction kinetics was assessed. We found that ≤10 mM of Cl^- promoted the reaction, but 100 mM Cl^- inhibited it. H C O 3 - was similar to Cl^-. Br^- and C O 3 2 - inhibited the reaction while S O 4 2 - did not affect the reaction. N O 3 - of ≤10 mM did not affect the reaction, but 100 mM of N O 3 - inhibited it. Eleven degradation intermediates were identified using ultra-high solution Orbitrap mass spectrometry. Based on the intermediates identified, possible reaction pathways were proposed. Overall, SR-AOP can effectively mineralize BTA, but water matrix constituents greatly influenced the reaction kinetics and thus should be carefully considered for its practical application. Abbreviations: BTA, benzotriazole; PS, persulfate; PMS, peroxymonosulfate; SPC, sodium percarbonate; AOP, advanced oxidation process; PS-AOP, persulfate-based advanced oxidation process; SR-AOP, sulfate radical-based advanced oxidation process; TAP, thermally activated persulfate; TOC, total organic carbon; TBA, tert-butyl alcohol.

Application of biochar in advanced oxidation processes: supportive, adsorptive, and catalytic role

The advanced oxidation processes (AOPs), especially sulphate radical (SO₄^•-)-based AOPs (SR-AOPs), have been considered more effective, selective, and prominent technologies for the removal of highly toxic emerging contaminants (ECs) due to wide operational pH range and relatively higher oxidation potential (2.5-3.1 V). Recently, biochar (BC)-based composite materials have been introduced in AOPs due to the dual benefits of adsorption and catalytic degradation, but the scientific review of BC-based catalysts for the generation of reactive oxygen species (ROSs) through radical- and non-radical-oriented routes for EC removal was rarely reported. The chemical treatments, such as acid/base treatment, chemical oxidation, surfactant incorporation, and coating and impregnation of minerals, were applied to make BC suitable as supporting materials (SMs) for the loading of Fenton catalysts to boost up peroxymonosulphate/persulphate/H₂O₂ activation to get ROSs including ^•OH, SO₄^•-, ¹O₂, and O₂^•- for targeted pollutant degradation. In this review, all the possible merits of BC-based catalysts including supportive, adsorptive, and catalytic role are summarised along with the possible route for the development prospects of BC properties. The limitations of SR-AOPs especially on production of non-desired oxyanions, as well as disinfection intermediates and their potential solutions, have been identified. Lastly, the knowledge gap and future-oriented research needs are highlighted.

Change of disinfection byproducts formation potential of natural organic matter after exposure to persulphate and bicarbonate

Activated persulphate (PS) oxidation is a promising in situ remediation technology for groundwater and soils. Application of this technology to contaminated zones may result in a large quantity of PS residue in the subsurface environment due to inefficient activation and repeated injection. In this study, we demonstrated that natural organic matter (NOM) molecules could be reconfigured due to exposure to unactivated PS and bicarbonate, resulting in reduced disinfection byproducts formation potential in post chlorination process. Fourier transformed inferred spectrometry (FTIR), size exclusive chromotraghy (SEC), and mass spectrometry (MS) analysis revealed that hydroxylation and carboxylation of NOM occurred, followed by inter-molecular coupling via ether bonds. The change of both the reactivity toward free chlorine and molecular structure of NOM during PS/bicarbonate treatment was well mimicked by 3,5-dihydroxylbenzoic acid, suggesting that phenolic moieties in NOM molecules were the main sites underwent transformation in the PS/bicarbonate system. We propose that peroxymonocarbonate (HCO₄^-) formed upon the reaction between PS and bicarbonate was the main reactive species responsible for the reconfiguration of NOM. It selectively attacked the phenolic moieties via single-electron abstracting mechanism, leading to phenoxy radical intermediates which couple to each other via C-O-C bonds. The findings of this study shed light on the environmental behaviors and impacts of PS in groundwater environment.

Fe3+-sulfite complexation enhanced persulfate Fenton-like process for antibiotic degradation based on response surface optimization

To improve the cycle between Fe³⁺ and Fe²⁺ in persulfate (PS) Fenton-like system, sulfite (Na₂SO₃) was used as the iron complexing agent to enhance the degradation of sulfamethoxazole (SMX) antibiotic in water. Response surface methodology (RSM) was applied to regulate the operation parameters for the Fe³⁺/Na₂SO₃/PS synergistic system. Based on the RSM, the SMX could be completely degraded when the concentration of Fe³⁺, Na₂SO₃, and PS were 0.4, 0.5, and 2.5 mM, respectively. The result showed that the synergistic process represented a high Fe³⁺ utilization rate and SMX degradation efficiency. After 1 h reaction, 100.00% of SMX and 27.80% of total organic carbon were removed under the ambient conditions containing the initial SMX concentration of 10 μM and initial pH of 5.96. Free radical masking and electron spin-resonance tests proved that hydroxyl radical (HO) and oxysulfur radicals (SO_x^-, x = 3, 4, 5) were all played the significant role in the antibiotic removal, and the primary active radical was HO. The SMX decomposition pathways based on the formed intermediates was proposed through the high-performance liquid chromatography and mass spectrum analyses. The toxicity assessment prediction indicated that the toxicities of decomposed SMX byproducts were reduced after the coupling treatment.

Crucial roles of oxygen and superoxide radical in bisulfite-activated persulfate oxidation of bisphenol AF: Mechanisms, kinetics and DFT studies

Though natural reducing agents have been demonstrated as desirable catalysts for environmental remediation, the mechanism of catalytic activation of persulfate (PS) by bisulfite (S(IV)) remains unclear. In this study, an emerging contaminant bisphenol AF (BPAF) was employed as the target compound to examine the activation and degradation mechanism in PS/S(IV) system. Sulfate radical (SO₄•-) was evidenced as the dominant radical accounting for BPAF degradation via quantitative analysis, while hydroxyl radical (•OH) and singlet oxygen (¹O₂) were minor contributors. Superoxide radical (O₂•-) was identified as an intermediate radical in promoting BPAF removal through quenching experiments and electron paramagnetic resonance analysis. Tests in oxygen-rich and oxygen-deficient systems were conducted and the results were contrasted to elucidate the important role of oxygen in BPAF degradation and SO₄•--formation. In addition, the effect of Dissolved Oxygen (DO) was simulated using two separate kinetic models. Decomposition mechanism of BPAF was afterwards clarified via the density-functional theory calculations using Fukui index to predict the vulnerable sites and the intermediate products. This study provides a mechanistic understanding of the activation of PS/S(IV) system on the BPAF removal, especially the critical role of DO and O₂•- in SO₄•- generation.

Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks

Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

Tuning of Persulfate Activation from a Free Radical to a Nonradical Pathway through the Incorporation of Non-Redox Magnesium Oxide

Nonradical-based advanced oxidation processes for pollutant removal have attracted much attention due to their inherent advantages. Herein we report that magnesium oxides (MgO) in CuOMgO/Fe₃O₄ not only enhanced the catalytic properties but also switched the free radical peroxymonosulfate (PMS)-activated process into the ¹O₂ based nonradical process. CuOMgO/Fe₃O₄ catalyst exhibited consistent performance in a wide pH range from 5.0 to 10.0, and the degradation kinetics were not inhibited by the common free radical scavengers, anions, or natural organic matter. Quantitative structure-activity relationships (QSARs) revealed the relationship between the degradation rate constant of 14 substituted phenols and their conventional descriptor variables (i.e., Hammett constants σ, σ^-, σ⁺), half-wave oxidation potential (E_1/2), and pK_a values. QSARs together with the kinetic isotopic effect (KIE) recognized the electron transfer as the dominant oxidation process. Characterizations and DFT calculation indicated that the incorporated MgO alters the copper sites to highly oxidized metal centers, offering a more suitable platform for PMS to generate metastable copper intermediates. These highly oxidized metals centers of copper played the key role in producing O₂^•- after accepting an electron from another PMS molecule, and finally ¹O₂ as sole reactive species was generated from the direct oxidation of O₂^•- through thermodynamically feasible reactions.

Degradation of bisphenol A by persulfate coupled with dithionite: Optimization using response surface methodology and pathway

The degradation efficiency of bisphenol A (BPA) was investigated in the process of persulfate (PS) coupled with dithionite (DTN) as a function of concentration of BPA, PS, DTN and solution pH. A simple response surface methodology (RSM) based on central composite design (CCD) was employed to determine the influence of individual and interaction of above variables and the optimum processing parameters. It is satisfactory of a quadratic model with low probabilities (<0.0001) at a confidence level of 95% to predict the BPA degradation efficiency. The model was well fitted to the actual data and the correlation coefficients of R² and R²-adj were 0.9270 and 0.8885, respectively. In addition, the obtained optimum conditions for BPA degradation were 1.79 μM, 131.77 μM, 93.64 μM for BPA, PS, DTN and pH = 3.62, respectively. It achieved a degradation efficiency >90% within 150 min. Moreover, the trapping experiment of active species demonstrated that SO₄^·- and ·OH were the dominant species and natural water matrix showed an obvious inhibition effect on BPA degradation. The BPA degradation pathway was predicted based on GC-MS results in this study.

Enhancement of iron-mediated activation of persulfate using catechin: From generation of reactive species to atenolol degradation in water

Persulfate (PS) activation reaction, which forms sulfate radical (SO₄^-), can be used to degrade organic pollutants in water. However, a drawback of this reaction is that the regeneration of ferrous ions requires a high concentration of hydrogen peroxide (Fenton-like reaction) or use of UV radiation. Catechin (CAT), a non-toxic antioxidant of natural origin from tea, is used in this work to improve the sulfate radical-mediated degradation of atenolol (ATL, a model pollutant) in water using relatively low concentrations of reactive chemical species (less than 100 μM). To the best of the author's knowledge, the direct effect of CAT on the oxidation state of iron, which is promoted by the reduction of ferric into ferrous ions with the enhancement of SO₄^- formation in the presence of PS, is demonstrated for the first time. The enhancement versus inhibition effect of CAT and the chemical mechanism of the iron-based activation process are explained. Results show that UVA radiation, which is representative of solar light, accelerates the initial degradation of ATL by more than 30% through ferric iron photolysis. Finally, a reaction mechanism leading to the formation of hydroxyl radicals (HO) and SO₄^- is proposed considering the implication of different activation/reaction chemical steps.

The impact of wastewater matrix on the degradation of pharmaceutically active compounds by oxidation processes including ultraviolet radiation and sulfate radicals

Ultraviolet radiation (UV)-activated peroxydisulfate (PDS) and peroxymonosulfate (PMS) advanced oxidation processes were examined for their capacity to remove nine pharmaceutically active compounds (PhACs) from secondary effluent. The effect of operational parameters (initial oxidant concentration, UV exposure time, pH, common coexisting anions and effluent organic matter (EfOM)) on UV/PDS and UV/PMS treatment efficiency was investigated in a collimated beam device housing a low-pressure mercury UV lamp emitting light at 253.7 nm. Both AOPs achieved high removals (>90%) when applied to pure water. Under otherwise similar conditions the removal percentage fell by 20-30% due to the scavenging of effluent organic matter (EfOM) in secondary effluent. Finally, eliminating EfOM but maintaining the inorganic composition, the radical scavenging effect was reduced and 98.3% and 85.6% average removals were obtained by UV/PDS and UV/PMS, respectively. Increasing pH improved degradation of several PhACs containing amine groups. Higher oxidant dosages created only a significant benefit in UV/PDS. The chloride anion produced a negligible effect on both processes, while higher nitrate concentrations increased removal percentage but did not affect degradation rate constants. Finally and surprisingly, the addition of bicarbonate had the strongest positive impact on the degradation kinetics observed, even stronger than the elimination of EfOM from secondary effluent.

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.

Peroxydisulfate activation by positively polarized carbocatalyst for enhanced removal of aqueous organic pollutants

Multi-walled carbon nanotubes (MWCNTs)/peroxydisulfate (PDS) is a green oxidative system for abatement of aqueous organic pollutants, while the powder form and poor cycling performance of the catalyst limit its practical application. To solve these problems, fabricating a MWCNT cathode (negative polarization) to coupling carbocatalysis-driven PDS activation with electrosorption of organic pollutant was previously demonstrated to be a possible solution to these problems. To further improve the activation efficiency of PDS, positive polarization of MWCNT electrode (anode) was adapted to activate PDS for removing acyclovir and phenol in this work. Under a working voltage of 1.2 V, the MWCNT anode was more efficient than the MWCNT cathode and the non-polarized MWCNT electrode for PDS activation and removal of organic pollutants, owing to the enhanced attraction between S₂O₈^2- anions and anode. Although the positive/negative polarization of MWCNT electrode doesn't alter the nonradical mechanism involved in MWCNTs/PDS system, theoretical calculations suggest that different polarization affect the electron configuration and oxidative capacity of activated S₂O₈^2- bounded to MWCNTs differently, and that the adsorbed S₂O₈^2- with stretched S-O bond and much higher oxidative capacity than that in the case of non-polarized MWCNT electrode is responsible for the MWCNTs anode, while adsorbed S₂O₈^2- with stretched O-O bond and slightly higher oxidative capacity is responsible for the MWCNTs cathode. Finally, implications of operation parameters including electrode potential, energy cost, pH, etc. on the elimination efficiency by MWCNT anode/PDS system were investigated and the results suggest that the MWCNT anode/PDS is an efficient and economical metal-free electrochemical oxidative system for organic contaminants remediation.

Insights into the mechanism of nonradical reactions of persulfate activated by carbon nanotubes: Activation performance and structure-function relationship

This study aimed to elucidate the intrinsic mechanisms of PS activation by carbon nanotubes (CNTs). Singlet oxygen generation (¹O₂) and direct CNTs-mediated electron transfer were hypothesized to be two major pathways of the oxidation of 2,4-dichlorophenol (2,4-DCP) by PS in the presence of both unmodified and modified CNTs. For the first time, roles of CNT active sites responsible for PS activation were determined using CNT derivatization and structural characterization. By selectively deactivating the carbonyl, hydroxyl or carboxylic groups on CNTs surface and linear sweep voltammetry (LSV) analysis, CO groups were determined to be the main active sites contributing to the direct electron transfer oxidation, while singlet oxygen was generated at CNTs defects. Subsequent UV irradiation was shown to cause the recovery of surface defects with I_D/I_G of CNTs increasing by 21%. This resulted in the regeneration of the performance for the coupled system and allowed for multi-cycle activation of PS by CNTs. These results suggest that CNTs/PS system combined with regeneration based on UV irradiation can be used as an effective alternative process for continuous degradation of recalcitrant aqueous contaminants through the non-radical mechanism.

Enhanced activation of persulfate by nitric acid/annealing modified multi-walled carbon nanotubes via non-radical process

This study aim to explore an effective approach of persulfate (PS) activation based on multi-walled carbon nanotubes (CNTs) for the phenol degradation. The nitric acid/annealing modified CNTs were obtained by oxidation with concentrated nitric acid followed by annealing at various high temperatures (400-1000 °C). The modified CNTs catalysts were characterized and their catalytic degradation performances of phenol with PS were investigated. The results reveal that the modified CNTs can obviously enhance the phenol removal. The catalytic activity is improved by nitric acid modification and obviously influenced by the annealing temperature. PS concentration and solution pH could also affect the catalytic degradation of phenol. The mechanism of enhanced PS activation by the nitric acid/annealing modified CNTs was proposed that the nitric acid modification resulted in more defective edges and oxygen-containing groups, and subsequent annealing at high temperate facilitated the conversion of sp³ to sp² carbon, then the catalytic activity for PS activation was enhanced by the active sites of CO group and sp²-hybridized carbon at the defective edges. In addition, a non-radical process of phenol degradation was demonstrated in terms of the radical quenching tests and electron paramagnetic resonance (EPR) spectra analyses. It is suggested that ¹O₂ is likely the main reactive specie generated in PS activation by the modified CNTs for the phenol removal.

A mini review of activated methods to persulfate-based advanced oxidation process

Persulfate-based advanced oxidation has been widely applied in environmental remediation for degrading contaminants. In recent years, numerous kinds of organic analytes including pesticides, dyes and pharmaceuticals, have emerged and related researches on the activation methods, the mechanism and the application have been performed. The activation is critical in persulfate-based advanced oxidation because the persulfate alone has a weaker oxidation potential to degrade these organic pollutants. Hence various activation methods have been extensively investigated to achieve a higher oxidation efficiency. These novel methods are gradually expanding the applicability and practicality. This review focuses on the classification of the different activation methods based on whether it is related to the substances or not. The effect of the environmental conditions (solution pH, dosage and the co-existing substance) on the oxidation capacity are also discussed.

Activation of sodium percarbonate by vanadium for the degradation of aniline in water: Mechanism and identification of reactive species

Sodium percarbonate (SPC)-based advanced oxidation process has been applied to the wastewater treatment in recent years. In the present study, a novel catalyst utilization of vanadium(V) was investigated for the activation of SPC for aniline degradation. The mechanism of SPC activation by V(IV) was demonstrated, and the major free radicals were identified through scavenging tests and electron paramagnetic resonance (EPR) analysis. The performance of aniline degradation was evaluated in the V(IV)/H₂O₂ and V(IV)/H₂O₂/Na₂CO₃ systems and compared with that of the V(IV)/SPC system. The influences of initial pH and effects of inorganic anions were also studied. The results show that aniline could be decomposed efficiently by SPC activated with V(IV) and the degradation efficiency increased with the increase in V(IV) and SPC dosage. O₂^-, CO₃^-, and OH were found to participate in aniline degradation, and O₂^- and CO₃^- were confirmed to be the predominant species. The decomposition of aniline was enhanced when equal amount of H₂O₂ was utilized instead of SPC under acidic condition while a comparative behavior was achieved in the V(IV)/H₂O₂/Na₂CO₃ system. Furthermore, aniline degradation was not impacted significantly by the initial pH and addition of Cl^-, SO₄^2- and NO₃^-, while HCO₃^- led to a betterment of aniline removal. These results indicate that the V(IV)/SPC system is an effective and promising approach for the removal of aniline from water for its feasibility and stability, which achieves eliminating contaminants by another waste.

Degradation of acetaminophen with ferrous/copperoxide activate persulfate: Synergism of iron and copper

To enhance the advanced oxidation process based on persulfate, CuO was introduced into the Fe²⁺/PS system to achieve a synergistic effect between Fe and Cu. Results showed that Fe²⁺ was able to adsorb onto the CuO surface to form Fe(II) and further reduced Cu(II) into Cu(I), which can further release into the solution to participate in oxidation reactions. In this case, SO₄^·- can be generated via Fe²⁺ and Cu⁺ reactions with PS and ·OH from Cu⁺ reaction with dissolved oxygen (DO). The degradation efficiency of APAP was studied under the optimal condition (initial pH 6.5, PS = 0.8 g L^-1, Fe²⁺ = 0.7 mM, CuO = 0.3 g L^-1), and the results indicated that the Fe²⁺/CuO/PS system can achieve a higher degradation rate of APAP (92% within 90 min) rather than Fe²⁺/PS and CuO/PS system (79% and 10%). Quenching experiment was performed to verify the active radicals in the Fe²⁺/CuO/PS system. Sulfate and hydroxyl radicals were generated in the Fe²⁺/CuO/PS system. Besides, some critical factors, such as Fe²⁺ concentration, catalyst dosage, PS concentration, initial pH (buffers and nonbuffers), and dissolved oxygen were evaluated in bath experiments. Results indicated that dissolved oxygen was essential in the Fe²⁺/CuO/PS system. APAP degradation experiments were conducted in surface water, and the intermediates were detected via GC-MS. The results indicated that the Fe²⁺/CuO/PS system is effective in the treatment of APAP in natural waters.

New insight into the mechanism of peroxymonosulfate activation by sulfur-containing minerals: Role of sulfur conversion in sulfate radical generation

Peroxymonosulfate (PMS) or persulfate activation by sulfur-containing minerals has been applied extensively for the degradation of contaminants; however, the role of sulfur conversion in this process has not been fully explored. In this study, pyrite (FeS₂)-based PMS activation process was developed for diethyl phthalate (DEP) degradation, and its underlying mechanisms were elucidated. PMS was found to be efficiently activated by FeS₂ for DEP degradation and mineralization, achieving 58.9% total organic carbon removal using 0.5 g/L FeS₂ and 2.0 mM PMS. Sulfides were the dominant electron donor for PMS activation, and mediated Fe(II) regeneration to activate PMS on the surface of FeS₂ particles. Meanwhile, different sulfur conversion intermediates, such as S₅^2-, S₈⁰, S₂O₃^2-, and SO₃^2-, were formed from the oxidation of sulfides by Fe(III) and PMS, and determined by X-ray photoelectron spectroscopy and in-situ attenuated total reflectance Fourier transform infrared spectroscopy analysis. SO₃^2- was the dominant sulfur species responsible for sulfate radicals (SO₄^-) generation by activating PMS directly or activating Fe(III) to initiate a radical chain reaction, which was supported by the electron paramagnetic resonance results. This study highlights the important role of sulfur conversion in PMS activation by pyrite and provides new insights into the mechanism of oxidant activation by sulfur-containing minerals.

Efficient degradation of imipramine by iron oxychloride-activated peroxymonosulfate process

Synthesized iron oxychloride (FeOCl) was firstly applied to activate peroxymonosulfate (PMS) to degrade imipramine (IMI), a tricyclic antidepressant. Compared to some other Fe-based materials including zero valent iron, Fe₂O₃, Fe₃O₄ and ferric ions, FeOCl presented an impressive catalytic activity on PMS at near-neutral condition due to its unique structure containing abundant unsaturated iron atoms and oxo-bridged configuration. With an increase of FeOCl dose, PMS dose or initial pH in ranges of 0.02 - 0.5 g/L, 0.1 - 2.5 mM and 4.0 - 8.0, the degradation efficiency of IMI was effectively raised by 64.0%, 48.5% and 50.6%, respectively. The presence of either bicarbonate or chloride stimulated the removal of IMI. Moreover, 70.4% of IMI was degraded under the background of real water with 2 mM PMS. The possible reactive species were identified as sulfate and hydroxyl radicals. The formed hypochlorite through the reaction of PMS and the released chloride ions may also contribute to the degradation of IMI. Among the oxidants, sulfate radical was proven to be the dominate one in the system. Additionally, the FeOCl/PMS system can overall effectively degrade six other organic compounds including amitriptyline, desipramine, propranolol, nitrobenzene, methyl-paraben and ethyl-paraben, further suggesting the possible application of this system in treatment of vast aquatic micro-organic pollutants.

Oxidation of Organic Compounds in Water by Unactivated Peroxymonosulfate

Peroxymonosulfate (HSO₅^- and PMS) is an optional bulk oxidant in advanced oxidation processes (AOPs) for treating wastewaters. Normally, PMS is activated by the input of energy or reducing agent to generate sulfate or hydroxyl radicals or both. This study shows that PMS without explicit activation undergoes direct reaction with a variety of compounds, including antibiotics, pharmaceuticals, phenolics, and commonly used singlet-oxygen (¹O₂) traps and quenchers, specifically furfuryl alcohol (FFA), azide, and histidine. Reaction time frames varied from minutes to a few hours at pH 9. With the use of a test compound with intermediate reactivity (FFA), electron paramagnetic resonance (EPR) and scavenging experiments ruled out sulfate and hydroxyl radicals. Although ¹O₂ was detected by EPR and is produced stoichiometrically through PMS self-decomposition, ¹O₂ plays only a minor role due to its efficient quenching by water, as confirmed by experiments manipulating the ¹O₂ formation rate (addition of H₂O₂) or lifetime (deuterium solvent isotope effect). Direct reactions with PMS are highly pH- and ionic-strength-sensitive and can be accelerated by (bi)carbonate, borate, and pyrophosphate (although not phosphate) via non-radical pathways. The findings indicate that direct reaction with PMS may steer degradation pathways and must be considered in AOPs and other applications. They also signal caution to researchers when choosing buffers as well as ¹O₂ traps and quenchers.

Efficient decolorization of typical azo dyes using low-frequency ultrasound in presence of carbonate and hydrogen peroxide

The aims of this study as to evaluate and understand the decolorization of azo dyes using carbonate and hydrogen peroxide under low-frequency ultrasonic irradiation. Under optimal conditions, the decolorization ratio of acid orange 8 (AO 8), a typical azo dye, was > 90% after 2 h of irradiation. The decolorization rate of AO 8 was 0.023 min^-1 under ultrasonic irradiation, which was about two times that without ultrasound. Different from the results of other published studies, OH played a minor role, while CO₃^- played the most important role in AO 8 ultrasonic decolorization in the presence of CO₃^2- and H₂O₂, with a contribution of 56.52%, followed by CO₄^2- (32.61%) and ¹O₂ (10.87%). Another difference is that CO₃^- formed through the cleavage of peroxymonocarbonate or peroxydicarbonate under ultrasonic irradiation rather than through reaction between hydroxyl radical and carbonate. Investigations for different azo dyes revealed that the decolorization rate decreased in the order AO 8 ≈ orange II > acid red 9 > acid yellow 11, probably because of molecular differences among the azo dyes.