Surface-mediated Electron Transfer Research Articles

Breaking hydrogen bond in metal free carbon nitride to induce peroxymonosulfate nonradical activation: Surface-mediated electron transfer

Breaking hydrogen bond in metal free carbon nitride (CN) to increase peroxymonosulfate (PMS) adsorption for surface-mediated electron transfer has been challenging. Herein, we incorporated N−CN monomer into CN to break hydrogen bond by the introduction of polyvinylpyrrolidone (PVP). The developed nonradical oxidation system achieved selective decomposition of highly hydrophilic contaminants and efficient removal of chemical oxygen demand (58.0%), total phosphorus (88.1%), ammonia nitrogen (100.0%) and total nitrogen (67.8%) from the secondary effluent in a continuous flow reactor. This was attributed to that the breakage of hydrogen bond accelerated the redistribution of electron densities at C and N sites, leading to an increase in the adsorption energy of N site to PMS. This promoted the formation of PVP/CN-PMS* complex for the selective and efficient removal of organics and inorganics. This work provided a new perspective for the selective and long-term treatment of practical wastewater.

Enhanced degradation of Rhodamine B through activating peroxydisulfate by humic acid-Fe incorporated biochar: Kinetics and mechanism studies

Sulfate radical advanced technologies (SR-AOPs) has been increasingly applied for chemical oxidation of toxic wastewater treatment, while the sluggish kinetics of Fe(III)/Fe(II) cycling remains a serious problem in its application. In this study, a novel co-catalyst of humic acid-Fe incorporated biochar (HA-Fe@BC) was successfully synthesized that can greatly accelerate the Fe(II) regeneration. More than 95 % organic pollutants could be degraded in HA-Fe@BC/peroxydisulfate (PDS) system, especially for the model pollutant Rhodamine B (an almost 8.0-fold kinetic increase). More excitingly, the superior catalytic activity of HA-Fe@BC for PDS activation may be attributed to the ample surface C-OH and C=O groups, accelerated electron transfer and formed surface HA-Fe@BC-PDS* complex with efficient oxidation activity. Radical capture, quenching experiments and electrochemical analysis confirmed that the HA-Fe@BC/PDS was caused by radicals (SO4•-+ O2•-+•OH = 68.13 %) and non-radical (surface-mediated electron transfer and 1O2 = 26.33 %) pathways. Besides, it also exhibited anti-interference performance against anions and organic matter (eg. Cl-, Br-, NO3–, and PO43-). Furthermore, the degradation pathways were proposed by LC-MS assisted with density functional theory (DFT) calculations, and the mechanism was proposed comprehensively. This work demonstrates the PDS activation by HA-Fe@BC through free radical and non-free radical pathways, which provides insight into application of biochar-based materials for wastewater treatment.

N-doped citrate-sludge-derived carbon (NCSC) effectively promotes peroxymonosulfate activation for perfluorooctanoic acid (PFOA) removal with surface-mediated electron transfer mechanism

In traditional wastewater treatment methods, the removal of emerging contaminants including perfluorooctanoic acid (PFOA) can be challenging. To address this, biochar is commonly utilized as an activator for peroxymonosulphate (PMS) to effectively eliminate organic pollutants. Sewage sludge has shown potential as a biochar precursor, but its complex composition and variable iron content, as well as the low specific surface area of the product limit the practical use of iron-dominated sludge-derived catalysts. To overcome this limitation, N-doped citrate-sludge-derived carbon (NCSC) was synthesized, possessing a low iron content (0.29 at%) and a large specific surface area (315.31 m2 g−1). As a comparison, Fe-/N-doped citrate-sludge-derived carbon (Fe-NCSC) was prepared by introducing exogenous iron, resulting in a higher iron content (2.12 at%) but a significantly reduced specific surface area (73.87 m2 g−1). In performance evaluation, the NCSC/PMS system achieved impressive removal efficiency, effectively eliminating 99.8% of PFOA (at an initial concentration of 2 mg L−1) within 60 min, while Fe-NCSC/PMS only achieved 84.6%. The slightly lower reaction rate per specific surface area of NCSC/PMS proved that large specific surface area was NCSC’s key advantage. The lower sensitivity of NCSC to pH and water substrates than FeNCSC suggested different activation mechanisms. Further analysis of reactive sites and species showed that the main oxidation mechanism of NCSC/PMS was forming the surface-bound PMS-NCSC complexes at the N sites, followed by PFOA donating electrons to the complexes to be oxidized, which was different from the Fe/N-dominated singlet oxygen mechanism of Fe-NSC/PMS. Furthermore, the reusability of the NCSC was demonstrated, with the removal rate decreasing to only 90.1% after four cycles and recovering to 94.8% after heated regeneration. In conclusion, this study provides a viable method for the elimination of emerging contaminants such as PFOA in water remediation.

Unraveling the intrinsic mechanism behind the selective oxidation of sulfonamide antibiotics in the Mn(II)/periodate process: The overlooked surface-mediated electron transfer process

Mn(II) exhibits a superb ability in activating periodate (PI) for the efficient degradation of aqueous organic contaminants. Nevertheless, ambiguous conclusions regarding the involved reactive species contributing to the removal of organic contaminants remain unresolved. In this work, we found that the Mn(II)/PI process showed outstanding and selective reactivity for oxidizing sulfonamides with the removal ranging from 57.1% to 100% at pH 6.5. Many lines of evidence suggest that the in-situ formed colloidal MnO2 (cMnO2) served as a catalyst to mediate electron transfer from sulfonamides to PI on its surface via forming cMnO2–PI complex (cMnO2−PI*) for the efficient oxidation of sulfonamides in the Mn(II)/PI process. Experimental results and density functional theory (DFT) calculations verify that the inclusive aniline moiety was the key site determining the electron transfer-dominated oxidation of sulfonamides. Furthermore, DFT calculation results reveal that the discrepancies in the removal of sulfonamides in the Mn(II)/PI process were attributed to different kinetic stability and chemical reactivity of sulfonamides caused by their heterocyclic substituents. In addition, a high utilization efficiency of PI was achieved in the Mn(II)/PI process owing to the surface-mediated electron transfer mechanism. This work provides deep insights into the surface-promoted mechanism in the cMnO2-involved oxidation processes.

Microbial community structure and potential function associated with poly-3-hydroxybutyrate biopolymer-boosted activation of peroxymonosulfate for waste-activated sludge decontamination

Excess waste-activated sludge (WAS) is a major biosolid management problem due to its biohazardous and recalcitrant content of phthalate esters (PAEs). This study aimed to assess the combined use of biopolymer, poly-3-hydroxybutyrate and peroxymonosulfate to degrade PAEs and decontaminate WAS. Poly-3-hydroxybutyrate was biosynthesized by Cupriavidus sp. L7L. The combined poly-3-hydroxybutyrate and peroxymonosulfate process removed 86 % of PAEs from WAS in 12 h. The carbonyl groups of poly-3-hydroxybutyrate were conducive to peroxymonosulfate activation leading to PAE degradation followed the radical pathway and surface-mediated electron transfer. Poly-3-hydroxybutyrate and peroxymonosulfate also enriched the PAE-biodegrading microbes in WAS. The microbial population and the functional composition in response to peroxymonosultate treatment was identified, with the genus Sulfurisoma being the most abundant. This synergistic treatment, i.e., advanced oxidation process, was augmented by highly promising microbial polyesters, exhibited important implications for WAS pretreatment toward circular bioeconomy that encompasses carbon–neutral biorefinery and mitigate pollution.

Transforming radical to non-radical pathway in peroxymonosulfate activation on nitrogen doped carbon sphere for enhanced removal of organic pollutants: Combined effect of nitrogen species and carbon structure

Carbon induced non-radical based peroxymonosulfate (PMS) activation has exhibited several advantages over radical process for eliminating organic pollutants in complicated water matrices. However, the relationship between configuration and composition of carbon and non-radical mechanism, which is important for designing high-efficiency carbon catalysts, remains elusive. In this work, the nitrogen doped carbon spheres (NCSs) with superior PMS activation ability were prepared from low-cost chitosan via two-step hydrothermal carbonization-pyrolysis method for organic pollutants removal. The carbon structure and nitrogen species of NCSs were tuned via changing hydrothermal temperature to explore their correlation with their PMS catalytic mechanism. Results showed hydrothermal carbonization boosted the content of pyridinic N, graphitic N and sp2-hybrided carbon (CC) in NCS, which significantly enhanced its catalytic performance and collaboratively promoted the catalytic mechanism switch from radical-dominant (SO4•− and •OH) to non-radical-dominant (surface-mediated electron transfer (SMET) and 1O2) process. The NCS with most pyridinic N and CC performed best, whose catalytic activity was 10.4 times higher than that without hydrothermal carbonization. The pyridinic N and CC enhanced the SMET process through strengthening PMS adsorption capacity and facilitating the electron migration from pollutant to PMS, respectively, while graphitic N triggered PMS oxidation on its neighboring electron-deficient C to produce 1O2.

Persulfate Oxidation of Sulfamethoxazole by Magnetic Iron-Char Composites via Nonradical Pathways: Fe(IV) Versus Surface-Mediated Electron Transfer.

Despite the vital roles of reactive radical species in the coupled iron-carbon composite/persulfate (PS) system for eliminating pollutants, nonradical contributions are typically overlooked. Herein, we developed two efficient magnetic iron-char composites via low-temperature (BCFe-400) and high-temperature (BCFe-700) pyrolysis. The two composites activated PS through nonradical pathways for sulfamethoxazole (SMX) degradation. In the BCFe-400/PS system, high-valent iron Fe(IV) was the dominant active species for the oxidation, evidenced by methyl phenyl sulfoxide-based probe tests, Mössbauer spectroscopy, and in situ Raman analyses with kinetic evaluation. In the BCFe-700/PS system, surface-mediated electron transfer dominated the oxidation, and the nonradical regime was probed by the electrochemical test and in situ Raman analysis. Furthermore, the BCFe-400/PS system maintained high efficiency in continuous degradation of SMX due to the feasible Fe2+generation toward Fe(IV) formation. In the BCFe-700/PS system, the stability of the system was limited due to the hindered electron transfer between the surface reactive complex (i.e., BCFe-700-PS*) and SMX, and thermal treatment would help recover the reactivity. Both BCFe-400/PS and BCFe-700/PS systems exhibited high performances for SMX removal in the presence of chloride and humic acid and in real water matrixes (e.g., seawater, piggery wastewater, and landfill leachate), exhibiting the great merits of the nonradical system. Overall, the study would provide new insights into PS activation by iron-loaded catalysts to efficiently degrade pollutants via nonradical pathways.

Enhanced reactivity and electron selectivity of GAC-Fe-Cu ternary micro-electrolysis system toward p-chloronitrobenzene under oxic conditions

A novel GAC-Fe-Cu ternary micro-electrolysis system was synthesized for the removal of p-chloronitrobenzene (p-CNB) under oxic conditions. p-CNB could be efficiently removed by GAC-Fe-Cu at a wide initial pH range of 1.0–9.0. In particular, the p-CNB removal efficiency of 96.96 % was obtained at initial pH of 7.2, and the degradation (44.96 %) was the major removal pathway. Additionally, reduction and oxidation simultaneously contributed to the degradation of p-CNB. The results indicated that OH was the prime reactive species under acidic conditions while O2− dominated the degradation of p-CNB under neutral conditions. Reduction reaction was remarkably enhanced in the presence of dissolved oxygen and the iron corrosion could be accelerated by in-situ generated H2O2. Furthermore, XPS analysis of GAC-Fe-Cu revealed the surface-mediated electron transfer and oxidant generation process. The excellent degradation efficiency of p-CNB at initial pH of 7.2 was attributed to the enhanced electron selectivity of GAC-Fe-Cu as well as the high selectivity of near-surface generated O2− toward p-CNB and its intermediate products.

Catalytic activation of peroxymonosulfate using CeVO4 for phenol degradation: An insight into the reaction pathway

In this study, a nanostructured CeVO4 material was synthesized using a simple co-precipitation technique and investigated as a catalyst for phenol degradation via peroxymonosulfate (PMS) activation. The structural and morphological properties of the CeVO4 catalyst were characterized using XRD, XPS, FTIR, Raman spectroscopy, TGA, SEM, and TEM. The results indicated that the prepared catalyst exhibited a pure crystalline phase with a rod-like structure. The CeVO4 catalyst exhibited high activity towards PMS activation, resulting in complete degradation of phenol within the first 80 min at room temperature. The degradation reaction was found to follow pseudo-first-order kinetics with a low activation energy of 23.74 kJ/mol. The degradation followed a catalytic surface-mediated electron transfer route as confirmed using ESR measurements. The effects of several experimental parameters on phenol degradation were investigated, including the catalyst loading, initial phenol concentration, PMS dosage, reaction temperature, and pH. The regeneration and reusability of the CeVO4 catalyst were also examined for 5 consecutive cycles. This study puts forward CeVO4 as a heterogeneous catalyst for the complete removal of persistent organic pollutants via PMS activation, indicating a favorable application in wastewater treatment.

Reduction Kinetics of Nitroaromatic Compounds by Titanium-Substituted Magnetite

Although there is a growing interest in environmentally friendly catalytic processes based on magnetic solids, the reactivity of titanomagnetite (Fe3–xTixO4) having a “tunable” solid-state Fe(II)/Fe(III) ratio for reductive transformation of nitroaromatic compounds has been never investigated. This study, for the first time, comprehensively examines the reduction kinetics of nitroaromatic compounds by titanium-substituted magnetite and compares with that of Fe(II) amended-unsubstituted magnetite at equal amounts of total Fe(II). First, we demonstrated that Ti substitution in magnetite increased considerably the ability of magnetite to reduce 4-nitrophenol (4-NP) as well as nitrobenzene (NB) in a surface-mediated electron transfer pathway. However, Fe(II) amendment of magnetite (x = 0) to have an equivalent amount of total Fe(II) as in the corresponding titanomagnetite (0.25 ≤ x ≤ 0.75) resulted in higher reduction rate constants for both substrates (4-NP and NB). Initial kobs was shown to increase exponen...

Graphene quantum dot functionalized by chitosan as an electrically conductive nano-material toward low potential detection: a new platform for interface science

Graphene quantum dot (GQD) with tunable sizes, (2–5 nm) especially displays unique optical and electrical properties due to the quantum confinement effect. Experiments have revealed that the band gap and HOMO/LUMO energy levels of GQDs can be independently controlled by either the lateral size or chemical functionalization. In the present work, a simple and controllable sequent electrodeposition method for the functionalization of graphene quantum dots with chitosan and preparation of a thin film on a glassy carbon electrode surface was proposed and further used for the construction of an electrochemical detector. This novel interface was constructed by two-step electrochemical deposition in solution containing graphene quantum dots and chitosan. This deposited interface possessed excellent stability by the characterization of cyclic voltammetry and wave voltammetry in phosphate buffer with pH 7.4. As the nanostructured sensing film provides plenty of active sites for the direct oxidation of vitamin C, the prepared sensor exhibited excellent electrochemical, catalytical and analytical performance at the pH 7.4. The kinetics of charge transfer and mass transport processes across the nanocomposite/solution interface were studied. The modified electrode showed an efficient electrocatalytic activity toward the oxidation of vitamin C through a surface mediated electron transfer. The catalytic rate constant and the vitamin C diffusion coefficient were reported.

Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium-Oxygen Batteries.

Fundamental understanding of growth mechanisms of Li2O2 in Li-O2 cells is critical for implementing batteries with high gravimetric energies. Li2O2 growth can occur first by 1e(-) transfer to O2, forming Li(+)-O2(-) and then either chemical disproportionation of Li(+)-O2(-), or a second electron transfer to Li(+)-O2(-). We demonstrate that Li2O2 growth is governed primarily by disproportionation of Li(+)-O2(-) at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li(+)-O2(-)-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li2O2 morphologies observed at different discharge overpotentials: formation of large (∼250 nm-1 μm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal-O2 batteries.

Graphene quantum dot functionalized by beta-cyclodextrin: a novel nanocomposite toward amplification of l-cysteine electro-oxidation signals

A novel nano-composite of graphene quantum dot-β-cyclodextrin was fabricated on the surface of glassy carbon electrode (GCE) using one step and green electrodeposition method. The redox of l-Cys behavior prepared nano-composite-modified of GCE was then characterized by cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Voltammograms of the modified electrode are recorded in physiological pH (phosphate buffer solution, pH 7.4), the kinetics of charge transfer and mass transport processes across the nano-composite/solution interface were studied. The modified electrode showed an efficient electrocatalytic activity toward the oxidation of l-Cysteine (l-Cys) through a surface-mediated electron transfer. The catalytic rate constant and the l-Cys diffusion coefficient were reported. A sensitive and time-saving method (differential pulse voltammetry) was developed for the analysis of l-Cys. The proposed voltammetric method was also applied to the determination of l-Cys using graphene quantum dot-β-cyclodextrin-GCE.

Cobalt hexacyanoferrate/graphene nanocomposite – Application for the electrocatalytic oxidation and amperometric determination of captopril

A nanocomposite of cobalt hexacyanoferrate/reduced graphene oxide was synthesized by a facile precipitation route. The nanocomposite morphology was characterized by scanning electron microscopy. A nanocomposite-modified carbon paste electrode was then fabricated and its redox behavior was characterized. In the voltammograms of the modified electrode recorded in phosphate buffer solution, pH 7.4, two quasi-reversible redox transitions appeared related to the Co(II)/Co(III) and Fe(II)/Fe(III) transitions. The diffusion of the counter cation into the nanocomposite and the charge transfer kinetics across the electrode/electrolyte interface were studied. The modified electrode showed an efficient electrocatalytic activity toward the electrooxidation of captopril. In the presence of captopril, the anodic peak current of the Fe(II)/Fe(III) transition increased and the cathodic one decreased, while, the peak currents of the Co(II)/Co(III) transition remained almost constant. This indicates that captopril was oxidized on the modified electrode surface through a surface mediated electron transfer (an electrocatalytic reaction). The catalytic rate constant and the electron transfer coefficient for the electrooxidation process and the captopril diffusion coefficient were reported. A sensitive and time-saving amperometric method was developed for the analysis of captopril. Based on the developed method, captopril was determined with a detection limit of 0.331μM. The proposed amperometric method was also applied to the analysis of captopril in tablets.

First-Principles Analysis of NOx Adsorption on Anhydrous γ-Al2O3 Surfaces

The interaction of nitrogen oxides NOx (x = 1−3) with γ-Al2O3 has been investigated using first-principles density functional theory calculations. NO and NO2 weakly physisorb on the clean, dehydrated (100) and (110) surfaces of γ-Al2O3, whereas the adsorption of the NO3 radical is rather strong. Only the basic-like O-down adsorption configurations were found to be stable. The interaction between NOx and γ-Al2O3 can be described as a surface-mediated electron transfer process. For single NOx adsorption, greater electron transfer from the surface to the adsorbate (negatively charged) yields stronger interaction between NOx and the surface. The adsorption of four combinations of NOx + NOy (x = 1−3, y = 2, 3) pairs on the (100) and the (110) facets of γ-Al2O3 were investigated. Except for the NO2 + NO2 pair, a strong cooperative effect that substantially enhances the stability of NOx on both γ-Al2O3 surfaces was found. This cooperative effect consists of surface-mediated electron transfer processes resulting in a favorable electrostatic interaction between two adsorbed NOx species. The NO+δNO3−δ pair was found to be the thermodynamically most stable state among the coadsorbed NOx + NOy pairs on both γ-Al2O3 surfaces. The results are used to analyze the experimentally observed NOx evolution during temperature programmed desorption from NO2-saturated γ-Al2O3 substrates.

Pyrroloquinoline quinone (PQQ) from methanol dehydrogenase and tryptophan tryptophylquinone (TTQ) from methylamine dehydrogenase

This chapter focuses on two well-characterized representative members of each class of these enzymes, the Pyrroloquinoline quinine (PQQ)-dependent methanol dehydrogenase (MEDH) and the tryptophan tryptophylquinone (TTQ)-dependent methylamine dehydrogenase (MADH). The chemical reaction mechanisms for catalysis by PQQ- and TTQ dependent enzymes are very similar to those used by other enzymes that possess carbonyl cofactors, such as pyridoxal phosphate-, topaquinone-, and pyruvoyl-dependent enzymes. The studies on these two classes of enzymes are facilitating a great deal of learning about mechanisms of carbonyl catalysis by enzymes, mechanisms of longrange interprotein electron transfer, and mechanisms of protein biosynthesis and posttranslational modification. PQQ and TTQ enzymes have an interesting common mechanistic feature of coupling active-site chemistry to surface-mediated electron transfer reactions. With the recent determinations of high-resolution crystal structures of more enzymes, and the development of systems for expression and site-directed mutagenesis of these enzymes, further insight into these fundamental biochemical processes is anticipated.

Mineral‐induced degradation of halogentated organic pollutants from aquatic systems

Several laboratory tests were conducted to examine Fe(II) as an enhancing agent in the dehalogenation of CHC13 in batch mode. The CHC13 degradation was found to be negligible when only Fe(II) is present as a reductant in the (aqueous) reaction system. However, in the presence of Fe(II) coated goethite, the rate of CHC13 degradation was enhanced to some extent. This observation can be explained by considering a surface mediated electron transfer step in the overall process. Substantial increase of the degradation rate was observed when the goethite particles first coated with fulvic acid were employed in the adsorption of Fe(II) for the degradation of CHC13.