Production Of Sulfate Radicals Research Articles

Comparative evaluation of α-Bi2O3/CoFe2O4 and ZnO/CoFe2O4 heterojunction nanocomposites for microwave induced catalytic degradation of tetracycline

Two microwave (MW) responsive heterojunction nanocomposite catalysts, i.e., α-Bi2O3/CoFe2O4 (BO/CFO) and ZnO/CoFe2O4 (ZO/CFO), with weight% ratio of 70/30, 50/50, 30/70 were synthesized by sequential thermal decomposition and co-precipitation methods, and used for the degradation of tetracycline (TC) under MW irradiation. The formation of desired catalysts was confirmed through the characterization results of XRD, FT-IR, SEM, VSM, UV-DRS, XPS, BET, etc. Using batch MW experiments, the catalyst dose, pH, initial TC concentration, reaction temperature, and MW power were optimized for TC removal. Under the following reaction conditions: catalyst dose ∼1 g/L, initial TC concentration ∼1 mg/L, temperature ∼90 °C, MW ∼450 W, BO/CFO, and ZO/CFO showed ∼97.55% and 88.23% TC degradation, respectively, after 5 min. The difference in the catalytic response against TC degradation indicated the difference in reflective loss (RL) between these two catalysts. The presence of other competitive anions has affected the removal efficiency of TC due to the scavenging effect. The radical trapping study revealed the significant contribution of TC degradation by hydroxyl radicals in the case of ZO/CFO, whereas for BO/CFO, superoxide (●O2−) and hydroxyl radicals (●OH) both played influential roles. The Z-scheme heterojunction of BO/CFO allowed the formation of ●O2− but the same was inhibited in type-II heterojunction of ZO/CFO due to the valance band position. The dielectric loss, magnetic loss, interfacial polarization, and high electrical conductivity, ‘hotspots’ were produced over the catalyst surface alongside electron-hole separation at heterojunctions, which were responsible for the generation of reactive oxygen species. In addition, Co3+/Co2+ and Fe3+/Fe2+ redox cycles have promoted ●O2− and sulfate radical production during persulfate application. Among the two MW responsive catalysts, BO/CFO could be a potential material for rapidly destroying emerging organic pollutants from wastewater without applying other oxidative chemicals under MW irradiation.

Wireless pulsed nanophotoelectrochemical cell for the ultrafast degradation of organic pollutants

An urgent demand exists for advanced-technologies to efficiently remove persistent organic pollutants from water, while minimizing energy consumption. Here, we introduce an innovative wireless nanophotoelectrochemical (nPEC) cell using pulsed light for the ultrafast degradation/mineralization of organic pollutants. The nPEC cell comprises a nanostructured Si-pn photodiode that monolithically integrates: (i) a Si-n/Au nanowire-based-photocathode for effective light absorption and photovoltage generation, and (ii) a Si-p/mesoporous-NiPt photoanode serving as catalyst to wirelessly amplify the sulfate radical production by low-intensity light without any bias voltage. The efficacy of the nPEC cell was shown by ultrafast degradation (>99 %) and mineralization (>98 %) of three emerging pollutants (tetracycline, levofloxacin and anatoxin-A). Notably, reaction kinetics were boosted by more than one order of magnitude when exposed to light intensities ca. 5-fold lower than sunlight. Remarkably, pulsed light beams in the 100–500 Hz range provided an additional enhancement in the degradation/mineralization efficiencies, reducing energy-input by half, while enhancing the catalyst's oxidation state and durability.

Ultrasound‐Activated Piezocatalyst Triggers Sulfate Radical Production for Augmented Oxidative Tumor Suppression

Sonopiezoelectric therapy (SPT) has gained ever‐increasing prominence in tumor therapy as a combination of ultrasound (US) and piezoelectric nanomaterials to generate cytotoxic reactive oxygen species (ROS) for tumor suppression. In this study, the facile hydrothermal method is used to construct the distinct 2D piezocatalyst Bi2Fe4O9 nanosheets (BFO NSs). The engineered BFO NSs can create a piezoelectric field when exposed to US irradiation, facilitating the separation of electron–hole (e−–h+) pairs to play a crucial role in Fe3+/Fe2+ circulation, which catalyzes the activation of peroxymonosulfate (PMS), resulting in an amplified production of cytotoxic sulfate radical (SO4•−). The produced SO4•− combines with ROS generated by SPT including hydroxyl radicals (•OH), and singlet oxygen (1O2), leading to the augmented oxidative tumor suppression. The BFO NSs and PMS are further integrated into a thermal hydrogel (BFO/PMS‐Gel) for in vitro and in vivo investigations on their catalytic antitumor effects. In both in vitro and in vivo studies, it is validated that BFO/PMS‐Gel, under US irradiation, significantly enhances radicals generation efficiency, which achieves superior antitumor performance while maintaining high therapeutic biocompatibility and biosafety, providing a representative paradigm of US‐activated multiple radicals production as enabled by piezoelectric nanomedicine for antitumor treatment.

Sulfate Radical in (Photo)electrochemical Advanced Oxidation Processes for Water Treatment: A Versatile Approach.

The search for a simple and clean approach toward the production of sulfate radicals for water treatment gave rise to electrochemical and photoelectrochemical activation techniques. The photoelectrochemical activation method does not just distinguish itself as a promising activation method, it is also used as an efficient water treatment method with the ability to treat a myriad of pollutants due to the complementary effects of highly reactive oxidizing species. This perspective highlights some merits that distinguish sulfate monoanion radicals from hydroxyl radicals. It highlights the electrochemical, photoelectrochemical, and in situ photoelectrochemical routes of generating sulfate radicals for advanced oxidation process approach to water treatment. We provide a detailed account of the few known applications of sulfate radical enhanced photoelectrochemical treatments of water laden with organics. Finally, we placed this area of research in perspective by providing outlooks and conclusive remarks.

New insight into highly efficient removal of tetracycline by calcined hydroxyapatite activated peroxymonosulfate: The role of calcium carbonate and phosphate group

Tetracycline (TC) is an important antibiotic extensively used in livestock breeding and aquaculture, and so an effective removal strategy is urgently needed. This study explores the use of a series of hydroxyapatite calcinated at different temperatures (300, 400, 500, 600, and 700 °C) to activate peroxymonosulfate (PMS) for the removal of TC. The optimal conditions for the removal of 10 mg/L TC using hydroxyapatite calcinated at 600 °C (CHAP600) as a catalyst were determined to be: pH of 6.0, a catalyst dosage of 0.2 g/L and a PMS concentration of 0.3 mM. Under these conditions, the removal efficiency of TC reached 94 %. The potential degradation pathway of TC was proposed by identifying the intermediates using High Performance Liquid Chromatography/Triple Quad mass spectrometry. The characterization analysis revealed that the presence of calcium carbonate on the surface of CHAP600 promoted the generation of singlet oxygen (1O2). Additionally, the phosphate group in CHAP600 served as a catalyst for the production of sulfate radicals (SO4·−) from PMS, which subsequently react with hydroxyl or surface hydroxyl group to generate hydroxyl radicals (·OH). And the active species 1O2 is the most significant contributor to the catalytic degradation process of TC by CHAP600 activated PMS, followed by ·OH, and then SO4·−. This study provides new insight for the mechanism of removing organic contaminants using PMS activated with calcined hydroxyapatite.

Synergetic-effect-enhanced electrochemiluminescence of zein-protected Au–Ag bimetallic nanoclusters for CA15-3 detection

In this work, a sandwich-type electrochemiluminescence (ECL) system was constructed for the detection of CA15-3. Gold-silver bimetallic nanoclusters (Au–Ag BNCs) with zein as a protective ligand were synthesized, and the excellent ECL performance of this material was demonstrated for the first time. Zein carrying a variety of groups that ligated with Au–Ag BNCs, forming a protective shell of zein, effectively prevented clusters from aggregating or growing into larger nanoparticles. The synergistic effect of the bimetal promotes the ECL emission, making this nanoscale material an ideal ECL probe. GO-PANI, which effectively promoting the production of sulfate radicals of the co-reactant and significantly increasing the ECL strength, was a good sensing platform for antibody immobilization. Consequently, we constructed an ECL sensor with GO-PANI as the sensing platform and Au–Ag BNCs@zein as the ECL probe, with a detection range of 0.001–100 U mL−1 and a detection limit of 0.0003 U mL−1, provided a strong support for the sensor for future CA15-3 detection applications.

Thermal assisted heterogeneous activation of peroxymonosulfate by activated carbon to degrade perfluorooctanoic acid in soil

The chemical degradation of perfluorooctanoic acid (PFOA), an –environmentally persistent contaminant, has recently attracted much attention. Still, very few reports have focused on the degradation of PFOA in soil due to the process’s non-liquid, complex system and uneasy operation. In this study, thermal treatment was applied in the remediation of PFOA contaminated soil. Complementary to thermal activation of peroxymonosulfate (PMS), fresh activated carbon (FAC) and modified activated carbon (NAC), which were used as heterogeneous catalysts, were incorporated into the system to promote the production of sulfate radicals (SO4•−) and the diminution of PFOA in soil. Two portions of soils, both homogenous but one with and the other without natural organic carbon (OC), were provided to study the impact of OC on the elimination of PFOA in soil. The degradation of PFOA was initiated using the combined methods in cohesion with heat activation. During the process, the effect of temperature, the dosing amount of catalyst and oxidant, the reaction time and the volume ratio between soil and solution on the degradation of PFOA in soil were fully studied and optimized. The results showed that over a 12 h period, a PFOA (1.0 mg/kg) removal of 76% and 70% by using FAC/PMS and NAC/PMS respectively could be achieved at 60 °C. Furthermore, density functional theory (DFT) calculations were combined with experimental data and verify that the adsorption ability of both FAC and NAC for PFOA are 3 times stronger than that of soil grain. This greater adsorption can attract PFOA molecules from within soil and provide opportunities for interactions with oxidative radicals. This work sheds light on a much over-looked aspect of advanced oxidation of PFOA in soil systems, which might provide a potential option for the in-situ remediation of PFOA contaminated soil in the future.

Recent advances in persulfate-based advanced oxidation processes for organic wastewater treatment

In recent years, with the emergence of new pollutants, the effective treatment of wastewater has become very important. Persulfate-based advanced oxidation processes have been successfully applied to the treatment of wastewater, such as wastewater containing antibiotics, pharmaceuticals and personal care products, dyes, endocrine-disrupting chemicals, chlorinated organic pollutants, and phenolics, for the degradation of refractory organic contaminants. This paper summarizes the production of sulfate radicals, which can be generated by the activation of persulfate via conventional and emerging approaches. The existing problems of persulfate-based advanced oxidation processes were analyzed in detail, including residual sulfates, coexisting factors (coexisting inorganic anions and natural organic matter), and energy consumption. This paper proposes corresponding possible solutions to the problems mentioned above, and this paper could provide a reference for the application of persulfate-based advanced oxidation processes in actual wastewater treatment.

Role of nitrite ligands in enhancing sulfate radical production via catalytic peroxymonosulfate activation by cobalt complexes

• A Co(III) complex with six nitrite ligands outperformed free Co 2+ in PMS activation. • Complexation with nitrite facilitated Co(II)/Co(III) cycling. • Radical-induced oxidation occurred in parallel with the Co(IV)-induced oxidation. • Complexation of Co(II) with nitrites resulted in an improved PMS activation capacity. • Nitrite addition enabled catalytic PMS activation by Co 2+ and Co 3 O 4 . In this study, hexanitrocobaltate (HN-Co(III)) was compared against Co 2+ with respect to the capability for peroxymonosulfate (PMS) activation. The role of NO 2 − ligands in enhancing Co(II)/Co(III) cycling was confirmed based on the two-fold increase in the benzoic acid (BA) degradation efficiency of HN-Co(III)/PMS vs. Co 2+ /PMS and the negligible reduction in the activation performance of HN-Co(III) over repeated use. This was in line with accelerated BA degradation and catalytic activity recovery upon NO 2 − addition to Co 2+ /PMS. A negative shift in the anodic peak potential of the Co(II)/Co(III) couple and an increase in the anodic current over time imply that NO 2 − coordination promoted the capacity of Co(II) for PMS activation and facilitated the Co(III)-to-Co(II) conversion through hydrolysis. Along with sulfate radical as the primary oxidant, high-valent cobalt acted as the non-radical secondary oxidant based on the transformation of phenyl methyl sulfoxide to the corresponding sulfone in the HN-Co(III)/PMS mixtures.

A Review on the Degradation of Pollutants by Fenton-Like Systems Based on Zero-Valent Iron and Persulfate: Effects of Reduction Potentials, pH, and Anions Occurring in Waste Waters.

Among the advanced oxidation processes (AOPs), the Fenton reaction has attracted much attention in recent years for the treatment of water and wastewater. This review provides insight into a particular variant of the process, where soluble Fe(II) salts are replaced by zero-valent iron (ZVI), and hydrogen peroxide (H2O2) is replaced by persulfate (S2O82−). Heterogeneous Fenton with ZVI has the advantage of minimizing a major problem found with homogeneous Fenton. Indeed, the precipitation of Fe(III) at pH > 4 interferes with the recycling of Fe species and inhibits oxidation in homogeneous Fenton; in contrast, suspended ZVI as iron source is less sensitive to the increase of pH. Moreover, persulfate favors the production of sulfate radicals (SO4•−) that are more selective towards pollutant degradation, compared to the hydroxyl radicals (•OH) produced in classic, H2O2-based Fenton. Higher selectivity means that degradation of SO4•−-reactive contaminants is less affected by interfering agents typically found in wastewater; however, the ability of SO4•− to oxidize H2O/OH− to •OH makes it difficult to obtain conditions where SO4•− is the only reactive species. Research results have shown that ZVI-Fenton with persulfate works best at acidic pH, but it is often possible to get reasonable degradation at pH values that are not too far from neutrality. Moreover, inorganic ions that are very common in water and wastewater (Cl−, HCO3−, CO32−, NO3−, NO2−) can sometimes inhibit degradation by scavenging SO4•− and/or •OH, but in other cases they even enhance the process. Therefore, ZVI-Fenton with persulfate might perform unexpectedly well in some saline waters, although the possible formation of harmful by-products upon oxidation of the anions cannot be ruled out.

Cobalt-Enhanced Mass Transfer and Catalytic Production of Sulfate Radicals in MOF-Derived CeO2 • Co3 O4 Nanoflowers for Efficient Degradation of Antibiotics.

Antibiotics discharge has been a critical issue as the abuse in clinical disease treatment and aquaculture industry. Advanced oxidation process (AOPs) is regarded as a promising approach to degrade organic pollutants from wastewater, however, the catalysts for AOPs always present low activities, and uncontrollable porosities, thus hindering their further wider applications. In this work, an aliovalent-substitution strategy is employed in metal-organic framework (MOF) precursors assembly, aiming to introduce Co(II/III) into Ce-O clusters which could modify the structure of the clusters, then change the crystallization, enlarge the surface area, and regulate the morphology. The introduction of Co(II/III) also enlarges the pore size for mass transfer and enriches the active sites for the production of sulfate radicals (SO4• - ) in MOF-derived catalysts, leading to excellent performance in antibiotics removal. Significantly, the CeO2 •Co3 O4 nanoflowers could efficiently enhance the generation of sulfate radical SO4• - and promote the norfloxacin removal efficiency to 99% within 20min. The CeO2 •Co3 O4 nanoflowers also present remarkable universality toward various antibiotics and organic pollutants. The aliovalent-substitution strategy is anticipated to find wide use in the exploration of high-performance MOF-derived catalysts for various applications.

A novel persulfate-photo-bioelectrochemical hybrid system promoting the degradation of refractory micropollutants at neutral pH

Bio-electro-Fenton is emerging as an alternative technology for the efficient and cost-effective removal of refractory micropollutants. Though promising, there are still several challenges that limit its wide application, including acidic operating conditions (pH at 2–3), the addition of supporting electrolytes (e.g., Na2SO4), and the issue of iron sludge generation. To address these challenges, a novel hybrid persulfate-photo-bioelectrochemical (PPBEC) system is proposed to remove model micropollutants (carbamazepine and clorfibric acid), from secondary effluent at low persulfate (PS) dosage and neutral pH. The effect of crucial operating parameters on the process was studied, including input voltage, cathodic aeration velocity, and PS dose. Under optimal conditions (0.6 V, 0.005 mL min−1 mL−1 and 1 mM), the PPBEC system achieved approx. 0.56–1.71 times greater micropollutant removal with 93% lower energy consumption when compared to the individual processes (UV/PS and PBEC). The improved performance was attributed to a faster production of sulfate radicals by UV irradiation, hydrogen peroxide activation and single-electron reduction, and hydroxyl radicals generated by UV irradiation. Furthermore, the transformation products of carbamazepine and clorfibric acid were identified and the probable pathways are proposed. Finally, the ecotoxicity of the PPBEC treated effluent was assessed by using Vibrio Fischeri, which exhibited a non-toxic effect.

Degradation of acetaminophen by activated peroxymonosulfate using Co(OH)2 hollow microsphere supported titanate nanotubes: Insights into sulfate radical production pathway through CoOH+ activation

Pharmaceuticals and personal care products (PPCPs) are of great concern due to their increasing health effects, so advanced treatment technologies for PPCPs removal are urgently needed. In this study, titanate nanotubes decorated Co(OH)2 hollow microsphere (CoM/TNTs) composites were synthesized by a two-step solvothermal method, and used to activate peroxymonosulfate (PMS) through heterogenous catalysis for acetaminophen (ACE) degradation in water. The optimum material (CoM/TNTs0.5) activated PMS system exhibited high ACE removal efficiency and quick kinetic, as 93.0% ACE was degraded even within 10 min. The two components in CoM/TNTs showed a synergetic effect on PMS activation for radicals production: Co(OH)+ from CoM was the primary active species to active PMS, while TNTs could offer abundant –OH groups for Co(OH)+ formation. Density functional theory (DFT) calculation further interpreted the mechanism of Co(OH)+ for PMS activation by means of reaction potential energy surface (PES) analysis. Both the scavenger quenching tests and electron paramagnetic resonance analysis revealed that the sulfate radical (SO4-·) played a dominant role in ACE degradation. Moreover, DFT calculation also suggested that the ACE atoms with high Fukui index (f-) represented the active sites for electrophilic attack by SO4-·. The toxicity analysis based on quantitative structure-activity relationship (QSAR) verified the reduced toxicity of transformation products. Furthermore, CoM/TNTs also had good reusability and stability over five cycles. This work provides deep insights into the reaction mechanisms of radical production and organics attack in cobalt-based PMS activation system.

Continuous fixed-bed oxidation of metronidazole by the sulfate radical based process over nitric acid treated granular activated carbon

Activation of persulfate in the presence of nitric acid treated granular activated carbon was studied in order to investigate the degradation of metronidazole from aqueous medium in the continuous up-flow fixed-bed reactor. A tubular glass reactor with internal diameter of 2 cm and height of 15 cm was filled with desirable mass of rod shaped activated carbon with diameter of 4 mm and height of 6 mm. The synthetic wastewater containing persulfate and metronidazole was entered into the reactor in up-flow operation. At steady state condition and under the optimum values of operating parameters, about 88 % of influent persulfate was decomposed and decreased to 73 % after 7 h. Under this condition, the metronidazole removal reached to 93 % and gradually decreased to 74 % at the end of process due to the saturation of the catalyst and occupation of available active sites for sulfate radical production. The results indicated promising performance of sulfate radical based advanced oxidation process in continuous fixed-bed reactor for degradation of resistant pollutants from aqueous medium.

Degradation of ciprofloxacin using a low-grade titanium ore, persulfate, and artificial sunlight.

In this study, the magnetic fraction (MF) of a low-grade titanium ore (TO) was successfully used as an alternative Fe2+ source in five reuse cycles, in combination with persulfate (PS) and simulated sunlight (SSL) for the degradation of ciprofloxacin (CIP). The best response of the CIP initial concentration, irradiation time, and doses of MF and PS to degrade completely this pollutant were determined based on an experimental design. However, the individual application of MF, PS, or SSL fails to achieve this goal at the optimal experimental condition. Furthermore, the MF-PS-SSL system showed a higher production of sulfate radicals and a concentration of dissolved Fe2+ ions compared with data obtained for the MF-PS system. The best performance attained by the former system is due to the synergy produced between the photo-generated electrons, and the reaction of PS with the Fe2+ ions leached gradually from the MF, which increased sulfate radical production. After five reuse cycles of the MF, the oxidation system showed a CIP degradation of 100% in 100 min, no residual content of PS, a CIP mineralization of 6%, a marginal increase in the biodegradability (BOD5/COD ratio), a MF loss of 7.5%, and a twofold increase in toxicity; however, this parameter was lower than the effective concentration at 50% inhibition (EC50). The substitution of MF with an iron salt decreased the degradation efficiency of the antibiotic by 14%, probably owing to the immediate excess of Fe2+ in the solution, which can be oxidized to Fe3+ ions, and as a consequence of this, the production rate of the sulfate radical was also reduced.

Thermally activated persulfate for the chemical oxidation of chlorinated organic compounds in groundwater

Chlorinated pesticides were extensively produced in the XX century, generating high amounts of toxic wastes often dumped in the surroundings of the production sites, resulting in hot points of soil and groundwater pollution worldwide. This is the case of Bailín landfill, located in Sabiñánigo (Spain), where groundwater is highly polluted with chlorobenzenes (mono, di, tri and tetra) and hexachlorocyclohexanes. This study addresses the abatement of chlorinated organic compounds (COCs) present in the groundwater coming from the Bailín landfill by thermally activated persulfate, PS (TAP). The influence of temperature (30–50 °C) and oxidant concentration (2–40 g L−1) on the efficiency of COCs (initial concentration of COCs = 57.53 mg L−1, determined by the solubility of the pollutants in water) degradation has been investigated. Raising the reaction temperature and PS concentration the degradation of COCs significantly accelerates, as a result of higher production of sulfate radicals. The thermal activation of PS implies side reactions, involving the unproductive decomposition of this oxidant. The activation energy calculated for this reaction (128.48 kJ mol−1) reveals that is slightly more favored by temperature than the oxidation of COCs by sulfate radicals (102.4–115.72 kJ mol−1). At the selected operating conditions (PS = 10 g L−1, 40 °C), the almost complete conversion of COCs and a dechlorination and mineralization degree above 80% were obtained at 168 h reaction time. A kinetic model, able to adequately predict the experimental concentration of COCs when operating at different temperatures and initial concentration of PS has been proposed.

TW80 and GLDA-enhanced oxidation under electrokinetic remediation for aged contaminated-soil: Does it worth?

Soil from a former chemical plant was found to be contaminated with organochlorine compounds (OCs) from aged pesticide residues and underwent surfactant-enhanced electrokinetic remediation (EKR). Tween 80 (TW80) and N,N-Dicarboxymethyl glutamic acid tetrasodium (GLDA) were used to increase the availability of OCs and to enhance oxidation efficiency in the soil under EKR. The results indicate that flushing soil with TW80 increased the removal of persistent OCs from the soil, with 40%–80% removal efficiency. Further, incorporating TW80 with sodium persulfate significantly advanced oxidation and removed 60%–82% of OCs (in both absence and presence of the nZVI reactive barrier) within 10 days. Compared to control treatment, the addition of TW80 improved EKR by 20%–30%. The improvement was attributed to the ability of TW80 to solubilize hardly-soluble compounds and to sustain the activation of persulfate ions (S2O82−) by dissolving metal ions in the soil that would stimulate the production of sulfate radicals (SO4•−) and thus the degradation of pollutants (in the absence of nZVI reactive barrier). Differing from TW80, GLDA’s capacity to solubilize and extract OCs from the soil was much lower and less reliable, averaging around 10% compared to 60% OC-removal for TW80. However, in the absence of oxidant activators from an external source, GLDA was important for sustaining the Fenton reaction. Its inclusion increased pollutant removal from an average of 50%–60%. The addition of the nZVI reactive barrier did not significantly improve remediation within 7 days. These findings suggest that TW80 combined with EKR and advanced oxidation in the presence of the nZVI reactive barrier is the best in situ approach to remediate soils contaminated with OCs from chemical waste of industrial processes.

Evidences of the Electrochemical Production of Sulfate Radicals at Cathodically Polarized TiO2 Nanotubes Electrodes

This work presents a novel approach for cathodically polarized TiO2 nanotube electrodes in electrochemical advanced oxidation processes generating sulfate radicals. Although TiO2 do not offer electrochemical response at the anodic region in the dark, its simple self-doping can enable oxidation capacity. Cathodically polarized TiO2 electrode (CP-TNT) was applied in the electrolysis of p-nitrosodimethylaniline dye in sulfate and nitrate aqueous electrolytes, showing much better decolorization efficiency in the presence of sulfate ions. Another evidence of SO4•− generation was the detection of persulfate ions (SO4•− + SO4•− → S2O82−) formed during bulk electrolysis of a sulfate aqueous solution (38 μmol L−1 S2O82− after 30 min). These results open the way for application of CP-TNT electrodes in EAOPs as an alternative for highly cost anodes for electrochemical generation of active sulfate species for degradation of organic contaminants.

Electrochemical degradation of perfluorooctanoic acid by macro-porous titanium suboxide anode in the presence of sulfate

The present study investigated the electrochemical degradation of perfluorooctanoic acid (PFOA) by macro-porous titanium suboxide (p-TiSO) anode in the presence of sulfate at pH-neutral condition (pH 6.9 ± 0.3). Based on initial PFOA concentration of 0.1 mmol L−1, current density of 10 mA cm−2 and electrolysis of 2 h, the removal efficiencies of PFOA were 97.1% and 90.8% for Na2SO4 electrolyte (kapp = 2.1 h−1) and NaNO3 electrolyte (k = 0.84 h−1), corresponding to defluorination efficiency of 61.4% and 54.2%, respectively. The 2.3-fold enhancement of oxidation rate should be originated from the production of sulfate radicals (SO4−) by activation of sulfate via direct electron transfer with anode and indirect OH-mediated oxidation under the potential of 3.01–3.41 V versus standard hydrogen electrode (SHE). The production of OH and SO4− could be verified on a qualitative basis by using electron spin resonance (ESR) technology. Localized pH measurement achieved with a Pt/IrOx ultra-micro electrode revealed significantly lower pH inside the pores of p-TiSO anode (pH 2.6) compared to that in bulk electrolyte (pH 6.1). The formation of pH gradient should be attributed to surface interaction and diffusion restriction in the porous structure, and the resulting low pH was favorable for enhancing oxidizability of OH toward activation of bisulfate and oxidation of PFOA pollutant. This study offers a proof-in-concept demonstration of electrochemical activation of sulfate and PFOA degradation without need for chemical addition and pH adjustment, which can be realized by simply creating macro-porous structure of anode materials in electrochemical advanced oxidation process.

Activation of Persulfates Using Siderite as a Source of Ferrous Ions: Sulfate Radical Production, Stoichiometric Efficiency, and Implications

Ferrous ions (Fe2+) rapidly activate persulfates to produce sulfate radicals. However, the high reactivity of Fe2+ toward sulfate radicals means that they are easily scavenged, which reduces the stoichiometric efficiency of persulfates. To improve the stoichiometric efficiency, siderite was used to activate peroxydisulfate (PDS) and peroxymonosulfate (PMS), with phenol as a model contaminant. Near-100% degradation of phenol was achieved by siderite-activated PDS or PMS. In contrast, only 34% and 25% of the phenol was degraded by Fe2+- and nanoscale-magnetite-activated persulfates, respectively. The stoichiometric efficiencies of PMS and PDS activated by siderite were more than 4.4 and 3.6 times higher, respectively, than those activated by Fe2+. Electron paramagnetic resonance recorded both sulfate radicals and hydroxyl radicals. The effects of pH, iron dissolution, and scavenging were characterized, and the results indicated that siderite mainly activated persulfates by acting as a source of Fe2+ and tha...