Analyzing the active sites of carbocatalyst for peroxydisulfate activation: Specific surface area or electrochemical surface area?

Persulfates activation by various nanomaterials has been intensively reported for advanced oxidation processes (AOPs), and substantial progress has been made in understanding the mechanism. However, most of the published articles only present the unnormalized catalytic properties, which generated confusion to compare different catalysts and identify the active sites. Herein, we presented electrochemical surface area (ECSA) as a practical normalized method and confirmed the primary active sites in N-doped graphene. By controlling the aggregation state of graphene sheets to adjust the activity of doped graphite-N species, the active sites for peroxydisulfate (PDS) activation were accurately estimated. In further experiments, specific surface area (SSA, by N2-physisorption and methylene blue adsorption) and ECSA were adopted to conclude the normalized oxidation rate constant and graphitic-N was confirmed as the primary site in nitrogen-doped graphene for the carbocatalyst/PDS system. The normalized results revealed that SSA derived from inert gas on materials could not reflect the true active sites at solid-liquid interface, while ECSA considering the operated solid-liquid situation can be used for accurate estimation of the active sites. Therefore, this study suggests that ECSA integrates the properties of both kinetics and thermodynamics, which can be adopted as a useful methodology for analyzing nano-sized environmental catalysts performance.

Origins of Selective Oxidation in Carbon-Based Nonradical Oxidation Processes toward Organic Pollutants: Quantitative Structure-Activity Relationships (QSARs).

Metal-free carbon material-mediated nonradical oxidation processes (C-NOPs) have emerged as a research hotspot due to their excellent performance in selectively eliminating organic pollutants in aqueous environments. However, the selective oxidation mechanisms of C-NOPs remain obscure due to the diversity of organic pollutants and nonradical active species. Herein, quantitative structure-activity relationship (QSAR) models were employed to unveil the origins of C-NOP selectivity toward organic pollutants in different oxidant systems. QSAR analysis based on adsorption and oxidation descriptors revealed that C-NOP selectivity depends on the oxidation potentials of organic pollutants rather than on adsorption interactions. However, the dominance of electronic effects in selective oxidation decreases with increasing structural complexity of organic pollutants. Moreover, the oxidation threshold solely depends on the inherent electronic nature of organic pollutants and not on the reactivity of nonradical active species. Notably, the accuracy of substituent descriptors (Hammett constants) and theoretical descriptors (e.g., highest occupied molecular orbital energy, ionization potential, and single-electron oxidation potential) is significantly influenced by the complexity and molecular state of organic pollutants. Overall, the study findings reveal the origins of organic pollutant-oriented selective oxidation and provide insight into the application of descriptors in QSAR analysis.

Mineralization versus polymerization pathways in heterogeneous Fenton-like reactions.

Fenton reaction has been widespread application in water purification due to the excellent oxidation performances. However, the poor cycle efficiency of Fe(III)/Fe(II) is one of the biggest bottlenecks. In this study, graphite (GP) was used as a green carbon catalyst to accelerate Fenton-like (H2O2/Fe3+ and persulfate/Fe3+) reactions by promoting ferric ion reduction and intensifying diverse peroxide activation pathways. Significantly, the carboxyl group on GP anchors iron ions to form GP-COOFe(III) which promote persulfate adsorption to form surface complexes and induce an electron transfer pathway (ETP). While the electron-rich hydroxyl and carbonyl groups will combine to from GP-COFe(II), a reductive intermediate to activate peroxide to generate free radicals (from H2O2 and PDS) or high-value iron [Fe(IV)] (from PMS). Consequently, different pathways lead to distinct degree of oxidation: i) radicals in H2O2/Fe3+/GP prefer to mineralize bisphenol A (BPA) with no selectivity; ii) Fe(IV) in PMS/Fe3+/GP partially oxidizes BPA but cannot open the aromatic ring; iii) ETP in PMS/ or PDS/Fe3+/GP drives coupling reactions to form polymeric products covered on catalyst surface. Thus, rational engineering surface functionality of graphite and selecting proper peroxides can realize on-demand selectivity and oxidation capacity in Fenton-like systems.

Insights into the mechanism of carbocatalysis for peracetic acid activation: Kinetic discernment and active site identification.

Peracetic-acid-based advanced oxidation processes (PAA-AOPs) on metal-free catalysts have emerged as charming strategies for water contaminant removal. However, the involved reactive species and their corresponding active sites are ambiguous. Herein, using carbon nanotube (CNT) as a model carbocatalyst, we demonstrated that, under neutral conditions, the CNT-PAA* complex was the dominant reactive species to oxidize phenolic compounds via electron-transfer process (ETP), whereas the surface-bound hydroxyl radicals (·OHsurface) played a minor role on the basis of quenching and electrochemical tests as well as Raman spectroscopy. More importantly, the experimental and density functional theory (DFT) calculation results collaboratively proved that the active site for ETP was the sp2-hybridized carbon on the CNT bulk, while that for radical generation was the edge-located hydroxyl group (C-OH), which lowered the energy barrier for cleaving the O-O bond in CNT-PAA* complex. We further discerned the oxidation kinetic constants (koxid) of different pollutants from the apparent kinetic constants in CNT/PAA system. The significant negative linear correlation between lnkoxid and half-wave potential of phenolic compounds suggests that the pollutants with a lower one-electron oxidation potential (i.e., stronger electron-donating ability) are more easily oxidized. Overall, this study scrutinizes the hybrid radical and non-radical mechanism and the corresponding active sites of the CNT/PAA system, providing insights into the application of PAA-AOPs and the development of ETP in the remediation of emerging organic pollutants.

Multimetallic CuCoNi Oxide Nanowires In Situ Grown on a Nickel Foam Substrate Catalyze Persulfate Activation via Mediating Electron Transfer.

In situ growth of nanostructures on substrates is a strategy for designing highly efficient catalytic materials. Herein, multimetallic CuCoNi oxide nanowires are synthesized in situ on a three-dimensional nickel foam (NF) substrate (CuCoNi-NF) by a hydrothermal method and applied to peroxydisulfate (PDS) activation as immobilized catalysts. The catalytic performance of CuCoNi-NF is evaluated through the degradation of organic pollutants such as bisphenol A (BPA) and practical wastewater. The results indicate that the NF not only plays an important role as the substrate support but also serves as an internal Ni source for material fabrication. CuCoNi-NF exhibits high activity and stability during PDS activation as it mediates electron transfer from BPA to PDS. CuCoNi-NF first donates electrons to PDS to arrive at an oxidized state and subsequently deprives electrons from BPA to return to the initial state. CuCoNi-NF maintains high catalytic activity in the pH range of 5.2-9.2, adapts to a high ionic strength up to 100 mM, and resists background HCO3- and humic acid. Meanwhile, 76.6% of the total organic carbon can be removed from packaging wastewater by CuCoNi-NF-catalyzed PDS activation. This immobilized catalyst shows promising potential in wastewater treatment, well addressing the separation and recovery of conventional powdered catalysts.

Insights into the role of dual reaction sites for single Ni atom Fenton-like catalyst towards degradation of various organic contaminants.

The trade-off of Fenton-like catalysts in activity and stability remains a challenge in practical remediation applications. In this work, we successfully synthesized an efficient and stable catalyst comprised of single nickel (Ni) atoms dispersed on N-doped porous carbon (named Ni-SAs@CN) through a simple micropore confinement strategy. The catalyst exhibited outstanding catalytic performance with 25.8 min-1 turnover frequency for peroxymonosulfate (PMS) activation toward degradation of various organic pollutants (e.g., antibiotics, dyes, and plasticizers) in a wide pH range (4.5-10.8). Electron paramagnetic resonance and in situ Raman analyses demonstrated that both radical (including SO4•- and •OH) and Ni-PMS* dominated nonradical (via electron transfer) pathways played pivotal role in the decomposition of organics. The X-ray adsorption fine structure analysis and computational pieces of evidence demonstrate that the atomically dispersed NiN4 coordination is the intrinsic catalytic site for PMS activation. Meanwhile, pyrrolic N acts as a functional site to anchor target contaminants to the surface region for oxidation. In this process which is benefited from the dual active sites, the target contaminants were degraded via combined radical and nonradical pathways, which significantly boost the overall oxidation and mineralization kinetics.

Monodispersed CuO nanoparticles supported on mineral substrates for groundwater remediation via a nonradical pathway.

Nonradical oxidation based on singlet oxygen (1O2) has attracted great interest in groundwater remediation due to the selective oxidation property and good resistance to background constituents. Herein, recoverable CuO nanoparticles (NPs) supported on mineral substrates (SiO2) were prepared by calcination of surface-coated metal-plant phenolic networks and explored for peroxymonosulfate (PMS) activation to generate 1O2 for degrading organic pollutants in groundwater. CuO NPs with a close particle size (40 nm) were spatially monodispersed on SiO2 substrates, allowing highly exposure of active sites and consequently leading to outstanding catalytic performance. Efficient removal of various organic pollutants was obtained by the supported CuO NPs/PMS system under wide operation conditions, e.g., working pH, background anions and natural organic matters. Chemical scavenging experiments, electron paramagnetic resonance tests, furfuryl alcohol decay and solvent dependency experiments confirmed the formation of 1O2 and its dominant role in pollutants removal. In situ characterization with ATR-FTIR and Raman spectroscopy and computational calculation revealed that a redox cycle of surface Cu(II)-Cu(III)-Cu(II) was responsible for the generation of 1O2. The feasibility of the supported CuO NPs/PMS for actual groundwater remediation was evaluated via a flow-through test in a fixed-bed column, which manifested long-term durability, high mineralization ratio and low metal ion leaching.

Role of sulfide-modified nanoscale zero-valent iron on carbon nanotubes in nonradical activation of peroxydisulfate.

Sulfamethoxazole (SMX) is highly persistent and difficult to remove, making it urgent to find an efficient method for alleviating the enormous environmental pressure of SMX. In this study, sulfide-modified nanoscale zero-valent iron on carbon nanotubes (S-nZVI@CNTs) was prepared to activate peroxydisulfate (PDS) for the degradation of SMX. The results showed that SMX was completely removed within 40 min (kobs=0.1058 min-1) in the S-nZVI@CNTs/PDS system. By analyzing quenching experiments and electron paramagnetic resonance (EPR), singlet oxygen (1O2) was the main active species of the S-nZVI@CNTs/PDS system. 1O2 might be mediated by the abundant carbonyl groups (CO) on carbon nanotubes through spectroscopic analyses. In addition, sulfur doping transitioned the activation pathway to a nonradical pathway. Spectroscopic analyses and electrochemical experiments confirmed that the formation of CNTs-PDS complexes and S-nZVI could promote electron transfer on the catalyst surface. Furthermore, the main degradation intermediates of SMX were identified, and five possible transformation pathways were proposed. The S-nZVI@CNTs/PDS system possessed advantages including high anti-interference (Cl-, NO3-, HA), a strong applicability, recyclability and a low PDS consumption, offering new insight into the degradation of antibiotic wastewater.

Origins of Electron-Transfer Regime in Persulfate-Based Nonradical Oxidation Processes.

Persulfate-based nonradical oxidation processes (PS-NOPs) are appealing in wastewater purification due to their high efficiency and selectivity for removing trace organic contaminants in complicated water matrices. In this review, we showcased the recent progresses of state-of-the-art strategies in the nonradical electron-transfer regimes in PS-NOPs, including design of metal and metal-free heterogeneous catalysts, in situ/operando characterization/analytical techniques, and insights into the origins of electron-transfer mechanisms. In a typical electron-transfer process (ETP), persulfate is activated by a catalyst to form surface activated complexes, which directly or indirectly interact with target pollutants to finalize the oxidation. We discussed different analytical techniques on the fundamentals and tactics for accurate analysis of ETP. Moreover, we demonstrated the challenges and proposed future research strategies for ETP-based systems, such as computation-enabled molecular-level investigations, rational design of catalysts, and real-scenario applications in the complicated water environment. Overall, this review dedicates to sharpening the understanding of ETP in PS-NOPs and presenting promising applications in remediation technology and green chemistry.

Peroxymonosulfate activation by algal carbocatalyst for organic dye oxidation: Insights into experimental and theoretical.

Preparation of nitrogen-doped algal carbocatalyst (NC) for peroxymonosulfate (PMS) activation to oxidative degrade methylene blue (MB), and the mechanism of radical and nonradical pathway in N-C/PMS system are investigated. Firstly, a series of N-doped carbonaceous materials (NC) were prepared using nitrogen-rich Taihu blue algae biomass as precursor at different annealing temperatures. It was found that the NC prepared by annealing at 800 °C (N-C-8) showed an optimal MB degradation performance of over 99% after 60 min. Confirmed by electron paramagnetic resonance (EPR) analyses and radical quenching experiments, radical and nonradical pathway (1O2 oxidation and electron-transfer) are both involved in MB degraded process. Moreover, both graphitic N derived from the intrinsic Taihu blue algae, and nitrogen vacancy evolved from nitrogen dopants decomposition exhibited high correlation with the MB removal rate in the N-C/PMS system. Finally, three possible degradation pathways of MB were proposed based on the Density Functional Theory (DFT) calculation and identified intermediates. Overall, this work provides a new insight into the intrinsic roles of nitrogen-dopants and nitrogen vacancies on the as-prepared carbocatalyst for PMS activation, and advances the understanding of the resource utilization of algal biomass.

Novel Nonradical Oxidation of Sulfonamide Antibiotics with Co(II)-Doped g-C3N4-Activated Peracetic Acid: Role of High-Valent Cobalt-Oxo Species.

Herein, we report that Co(II)-doped g-C3N4 can efficiently trigger peracetic acid (PAA) oxidation of various sulfonamides (SAs) in a wide pH range. Quite different from the traditional radical-generating or typical nonradical-involved (i.e., singlet oxygenation and mediated electron transfer) catalytic systems, the PAA activation follows a novel nonradical pathway with unprecedented high-valent cobalt-oxo species [Co(IV)] as the dominant reactive species. Our experiments and density functional theory calculations indicate that the Co atom fixated into the nitrogen pots of g-C3N4 serves as the main active site, enabling dissociation of the adsorbed PAA and conversion of the coordinated Co(II) to Co(IV) via a unique two-electron transfer mechanism. Considering Co(IV) to be highly electrophilic in nature, different substituents (i.e., five-membered and six-membered heterocyclic moieties) on the SAs could affect their nucleophilicity, thus leading to the differences in degradation efficiency and transformation pathway. Also, benefiting from the selective oxidation of Co(IV), the established oxidative system exhibits excellent anti-interference capacity and achieves satisfactory decontamination performance under actual water conditions. This study provides a new nonradical approach to degrade SAs by efficiently activating PAA via heterogeneous cobalt-complexed catalysts.

Identifying the Persistent Free Radicals (PFRs) Formed as Crucial Metastable Intermediates during Peroxymonosulfate (PMS) Activation by N-Doped Carbonaceous Materials.

A nonradical mechanism involved in peroxymonosulfate (PMS) activation in carbonaceous materials (CMs) is still controversial. In this study, we prepared N-doped CMs, including hollow carbon spheres (NHCSs) and carbon nanotubes (N-CNTs), to probe the crucial intermediates during PMS activation. The results suggested that the higher efficiency and lower activation energy (13.72 kJ mol-1) toward phenol (PN) degradation in an NHCS/PMS system than PMS alone (∼24.07 kJ mol-1) depended on a typical nonradical reaction. Persistent free radicals (PFRs) with a g factor of 2.0033-2.0045, formed as crucial metastable intermediates on NHCS or N-CNT in the presence of PMS, contribute largely to the organic degradation (∼73.4%). Solid evidence suggested that the formation of PFRs relied on the attack of surface-bonded •OH and SO4•- or peroxides in PMS, among which surface-bonded SO4•- was most thermodynamically favorable based on theoretical calculations. Electron holes within PFRs on NHCSs shifted the Fermi level to the positive energy with the valance band increasing from 1.18 to 1.98 eV, promoting the reactivity toward nucleophilic substances. The degradation intermediates of aromatic compounds (e.g., PN) and electron rearrangement triggered the evolution of PFRs from oxygen-centered to carbon-centered radicals. Moreover, due to the specific electron configuration, graphitic N on NHCS was critical for stabilizing the PFRs. This study provides insightful understanding of the fate of organic contaminants and the structure-activity relationship of reactivity of CMs toward PMS activation.

Insights into the oxidation of organic contaminants by Co(II) activated peracetic acid: The overlooked role of high-valent cobalt-oxo species.

The combination of Co(II) and peracetic acid (PAA) is a promising advanced oxidation process for the abatement of refractory organic contaminants, and acetylperoxy (CH3CO3•) and acetoxyl (CH3CO2•) radicals are generally recognized as the dominant and selective intermediate oxidants. However, the role of high-valent cobalt-oxo species [Co(IV)] have been overlooked. Herein, we confirmed that Co(II)/PAA reaction enables the generation of Co(IV) at acidic conditions based on multiple lines of evidences, including methyl phenyl sulfoxide (PMSO)-based probe experiments, 18O isotope-labeling technique, and in situ Raman spectroscopy. In-depth investigation reveals that the PAA oxidation mechanism is strongly pH dependent. The elevation of solution pH could induce major oxidants converting from Co(IV) to oxygen-centered radicals (i.e., CH3CO3• and CH3CO2•). The presence of H2O2 competitively consumes both Co(IV) and reactive radicals generated from Co(II)/PAA process, and thus, leading to an undesirably decline in catalytic performance. Additionally, as a highly reactive and selective oxidant, Co(IV) reacts readily with organic substances bearing electron-rich groups, and efficiently attenuating their biological toxicity. Our findings enrich the fundamental understanding of Co(II) and PAA reaction and will be useful for the application of Co(IV)-mediated processes.

Multipath elimination of bisphenol A over bifunctional polymeric carbon nitride/biochar hybrids in the presence of persulfate and visible light.

Polymeric carbon nitride (PCN) has become a star material either in photocatalysis or in persulfate (PS) activation. In this work, we synthesized bifunctional biochar (BC)-doped PCN through a facile one-pot thermal treatment process. The PCN/BC hybrid (CNBC) with an optimized proportion could not only activate PS directly, but also possessed improved optical properties. Amorphous BC domains generated from the carbonization of external corncob provided attachments for the in-situ growth of PCN and upgraded its catalytic ability including electron transport property, visible light (VIS) utilization, and oxidation power. Mechanism studies demonstrated that in the CNBC/PS system without VIS, a nonradical electron transfer route was responsible for the degradation of bisphenol A (BPA), while in the CNBC/PS/VIS system, radical/nonradical mixing mechanisms including mediated electron transfer, radical oxidation, and hole oxidation were unveiled. Degradation pathways of BPA were deduced including direct oxidation at the aromatic ring, ß-scission of isopropyl, and ring cleavage. Most of the intermediates were less toxic than BPA as assessed by the ECOSAR software. The CNBC/PS/VIS system showed satisfactory resistance to environmental interferences except for HCO3-. This work provides a simple but effective strategy for the synthesis of PCN-based bifunctional catalysts and deepens mechanistic insights into hybrid advanced oxidation technologies.

Activation of persulfate by MnOOH: Degradation of organic compounds by nonradical mechanism.

Advanced oxidation processes (AOPs) based on persulfate (PS) has attracted great attention due to its high efficiency for degradation of organic pollutants. Manganese-based materials have been considered as the desirable catalysts for in-situ chemical oxidation since they are abundant in the earth's crust and environment-friendly. In this study, manganese oxyhydroxide (MnOOH) was used as an activator for PS to degrade p-chloroaniline (PCA) from wastewater. The effects of MnOOH dosage, PS dosage and initial pH on PCA degradation performance were studied. Experimental results showed that PCA degradation efficiency was enhanced by higher MnOOH and PS addition, and the degradation efficiency was slightly inhibited as the initial pH increased from 3 to 9. MnOOH showed excellent stability and reusability when used as the activator of PS. In addition, a comprehensive study was conducted to determine the PS activation mechanism. The results revealed that PS activation by MnOOH followed a nonradical mechanism. No 1O2 was generated, and the main active substance in the reaction was the activated PS molecule on the surface of MnOOH. The hydroxyl group on the catalyst surface acted as a bridge connecting PS and the catalyst, leading to the activation of PS. The intermediates during PCA degradation were also analyzed, and three possible degradation pathways of PCA were proposed. This study expects to deepen the understanding of the PS activation mechanism by manganese oxide, and provides technical support for the practical application of AOPs of manganese-based materials for wastewater treatment.

The duet of surface and radical-based carbocatalysis for oxidative destructions of aqueous contaminants over built-in nanotubes of graphite.

Metal-free mesoporous graphitic frameworks with built-in nanotubes (CPGs) were synthesized via facile co-pyrolysis of cyclodextrin and a cobalt salt with subsequent acid pickling to remove the embedded metal species. Due to the high graphitic degree and built-in few-layer nanotubes, the as-synthesized carbonaceous materials possess a higher catalytic ozonation activity than that of the state-of-the-art carbon nanotubes (CNTs) and LaMnO3 perovskite catalysts for the destruction of different aqueous contaminants. For recalcitrant oxalic acid removal, 50 mg L-1 oxalic acid was completely degraded in 20 min. Compared with other nanocarbons, the as-synthesized materials also demonstrated robust structural stability and reusability. The electron paramagnetic resonance (EPR) and selective radical quenching tests revealed that the destruction of the aqueous organics predominantly relied on surface-adsorbed complexes (O*ad and O2*) from activated ozone molecules. Owing to the occurrence of this surface oxidation pathway, the compatibility of the CPGs/O3 system was significantly enhanced for treatment of real wastewater, where the inorganic anions and organic natural organic matter would inhibit radical oxidation as radical scavengers. Furthermore, by employing organics with different ionization potentials (IPs) as the target pollutants, the CPGs/O3 system was discovered to obtain a high oxidation potential.

New insight into the substituents affecting the peroxydisulfate nonradical oxidation of sulfonamides in water.

The large consumption and discharge of sulfonamides (SAs) have potentially induced antibiotic resistance genes, posing inestimable threats to humans and ecosystems. In the present study, five SAs with different substituents were regarded as target compounds to be degraded using the nonradical dominated peroxydisulfate (PDS) activation process by the combination of 1O2 oxidation and direct electron transfer. The degradation rates, toxicities and pathways of SAs largely varied with their substituents. For instance, sulfathiazole with five-membered substituent had the highest degradation rate of 0.19 min-1, which was 3.8 times as the rate of sulfanilamide (0.05 min-1) without substituent. Then the theoretical calculation was adopted to further confirm that different substituents on the SAs could influence the molecular orbital distribution and their stability, thus resulting in the different removal rate of SAs. Finally, the products of different SAs were concisely deduced to take insight into the effects of different substituents on SAs degradation pathways. It was demonstrated that the geometrical differences among various SAs caused by the different substituents contributed to the different degradation pathways of SAs. Representatively, the special Smiles-type rearrangement pathway was occurred in the six-membered SAs instead of in the five-membered SAs, which inversely resulted in the slower degradation rate of six-membered SAs than the five-membered SAs. Thus, the present study provides a valuable insight into the effects of substituents on the degradation rate and transformation pathways of SAs in the nonradical PDS activation process.

Degradation of aniline by electrochemical activation of peroxydisulfate at MWCNT cathode: The proofed concept of nonradical oxidation process.

Enhanced elimination of aniline in aqueous solution was achieved by coupling electrosorption of aniline and electrochemical activation of peroxydisulfate (PDS) at multi-walled carbon nanotube (MWCNT) cathode, in which a synergistic effect occurred. It was found that PDS could be effectively activated under a small voltage at MWCNT cathode owing to the specific pore structures of MWCNTs. A nonradical oxidation pathway instead of radical-based oxidation was proposed from the cathodic activation of PDS, wherein PDS molecules with a modified electronic structure was suggested to be the principal reactive species. Meanwhile, the influences of various operation parameters such as electrode potential, PDS concentration, presence of chloride ions on the elimination efficiency, and the stability of MWCNT electrode were also attempted. Therefore, the electrochemical activation of PDS by MWCNT cathode is a promising energy-saving method for the treatment of organic pollutants in wastewater.

Electrochemical activation of persulfates at BDD anode: Radical or nonradical oxidation?

The combination of persulfates (peroxydisulfate (PDS) and peroxymonosulfate (PMS)) and electrolysis using boron-doped diamond (BDD) anode is a promising green advanced oxidation process. In comparison with electrolysis alone, electrochemical activation of persulfates at BDD anode considerably enhanced the degradation of carbamazepine (CBZ). The experimental results indicate that the surface-adsorbed hydroxyl radical (HO) played the dominant role. The generally proposed nonradical oxidation mechanism ignored hydroxyl radical (HO) oxidation because low concentration of radical scavenger (<10 M methanol or 5 M tertbutanol) could not effectively scavenge the surface-adsorbed HO. The quasi steady-state concentration of HO was estimated to be about 5.0-9.1 × 10-12 M for electrolysis with BDD anode, and it was increased to 1.1-1.6 × 10-11 M and 3.2-5.0 × 10-11 M for addition of 5 mM PDS and PMS, respectively. The results of cyclic voltammetry (CV) and chronoamperometry as well as evolution of dissolved oxygen (DO) reveal that the electrochemically activated persulfates molecule (PDS∗/PMS∗) promoted the production of HO via water dissociation at BDD anode and enhanced the direct electron transfer (DET) reaction, which otherwise inhibited the oxygen evolution side reaction. Therefore, higher current efficiency was achieved in electrochemical activation of persulfates process compared with electrolysis process. Additionally, the transformation products of CBZ were also investigated and their formation pathways were proposed.

Nonradical oxidation from electrochemical activation of peroxydisulfate at Ti/Pt anode: Efficiency, mechanism and influencing factors.

Electrochemical activation of peroxydisulfate (PDS) at Ti/Pt anode was systematically investigated for the first time in this work. The synergistic effect produced from the combination of electrolysis and the addition of PDS demonstrates that PDS can be activated at Ti/Pt anode. The selective oxidation towards carbamazepine (CBZ), sulfamethoxazole (SMX), propranolol (PPL), benzoic acid (BA) rather than atrazine (ATZ) and nitrobenzene (NB) was observed in electrochemical activation of PDS process. Moreover, addition of excess methanol or tert-butanol had negligible impact on CBZ (model compound) degradation, demonstrating that neither sulfate radical (SO4-) nor hydroxyl radical (HO) was produced in electrochemical activation of PDS process. Direct oxidation (PDS oxidation alone and electrolysis) and nonradical oxidation were responsible for the degradation of contaminants. The results of linear sweep voltammetry (LSV) and chronoamperometry suggest that electric discharge may integrate PDS molecule with anode surface into a unique transition state structure, which is responsible for the nonradical oxidation in electrochemical activation of PDS process. Adjustment of the solution pH from 1.0 to 7.0 had negligible effect on CBZ degradation. Increase of either PDS concentration or current density facilitated the degradation of CBZ. The presence of chloride ion (Cl-) significantly enhanced CBZ degradation, while addition of bicarbonate (HCO3-), phosphate (PO43-) and humic acid (HA) all inhibited CBZ degradation with the order of HA >> HCO3- > PO43-. The degradation products of CBZ and chlorinated products were also identified. Electrochemical activation of PDS at Ti/Pt anode may serve as a novel technology for selective oxidation of organic contaminants in water and soil.