Direct Electron Transfer Process Research Articles

Nitrogen and sulfur co-doped mesoporous carbon as an efficient peracetic acid activator for the degradation of 4-chlorophenol: A nonradical mechanism dominated by singlet oxygen

Metal-free peracetic acid (PAA) activators are urgently required due to their low toxicity. Heteroatom doping is beneficial to improve the catalytic performance of carbon materials, but the activation performance of doped carbon materials on PAA and the underlying mechanism have not been reported. Herein, nitrogen and sulfur co-doped mesoporous carbons (NS-MC-x, with x representing the mass ratio of thiourea:mesoporous carbon) were fabricated and used as efficient PAA activators for 4-chlorophenol (4-CP) degradation. The NS-MC-1.0/PAA system can remove > 95 % of 4-CP within 10 min, far more than the MC/PAA system. Co-existing anions show no effect on the degradation of 4-CP, whereas humic acid has a certain degree of inhibition. Singlet oxygen (1O2) primarily drives the degradation of 4-CP, with a secondary contribution from a direct electron transfer process mediated by reactive surface complexes, while the contribution of radicals is minimal. In addition, NS-MC-1.0 exhibits excellent reusability for PAA activation, which is conductive to its practical application. This study facilitates further research on metal-free catalysts for PAA activation and offers fresh perspectives on the utilization of PAA in degrading emerging pollutants.

Reactive species identification in KCl activated biochar/persulfate system under different pH condition: Theoretical calculation and column experiments

The challenges of remediating polluted water in mine areas are exacerbated by low emission dispersion and long-lasting pollution, particularly the significant fluctuations in pH value. Notably, the treatment of flotation reagent polluted water with variable pH scenarios remains a challenge. In this study, metal-free biochar-based materials (KBBC)/persulfate system was used for flotation reagent contaminated wastewater treatment with a wide pH range. Regarding the intrinsic nature of the biochar-based materials, ball milling enhanced the degree of graphitization, while introducing a hard template of KCl increased the proportion of carbonyl functional groups on their surface, thereby facilitating persulfate activation. Experimental findings revealed that the KBBC/PS system displayed efficient degradation of aniline aerofloat (AAF) as a representative flotation reagent across a wide pH range (pH = 3.6–10.4 with buffer). Investigations on the mechanism indicated that, under acidic and neutral conditions, superoxide radicals and direct electron transfer processes constituted primary pathways for pollutant degradation. Meanwhile, various active species contributed under alkaline conditions with increased involvement of free radicals (•OH and SO4•−) and singlet oxygen. Furthermore, continuous flow experiments were set up to prove the potential applicability of the KBBC/PS process. This study not only deepens the understanding of persulfate activation by metal-free carbon-based materials across a wide pH range but also expands their potential applications in mine-polluted water purification.

Energy-efficient Treatment of Refractory Industrial Effluent Using Flow-through Electrochemical Processes: Oxidation Mechanisms and Reduction of Chlorinated Byproducts

While flow-through anodic oxidation (FTAO) technique has demonstrated high efficiency to treat various refractory waste streams, there is an increasing concern on the secondary hazard generation thereby. In this study, we developed an integrated system that couples FTAO and cathodic reduction processes (termed FTAO-CR) for sustainable treatment of chlorine-laden industrial wastewater. Among four common electrode materials (i.e., Ti4O7, β-PbO2, RuO2, and SnO2-Sb), RuO2 flow-through anode exhibited the best pollutant removal performance and relatively low ClO3 and ClO4 yields. Because of the significant scavenging effect of Cl− in real wastewater treatment, the direct electron transfer process played a dominant role in contaminant degradation for both active and nonactive anodes though active species (i.e., active chlorine) were involved in the subsequent transformation of the organic matter. A continuous FTAO-CR system was then constructed for simultaneous COD removal and organic and inorganic chlorinated byproduct control. The quality of the treated effluent could meet the national discharge permit limit at low energy cost (~4.52 kWh m3 or ~0.035 kWh g1-COD). Results from our study pave the way for developing novel electrochemical platforms for the purification of refractory waste streams whilst minimizing the secondary pollution. Environmental ImplicationsIndustrial wastewater usually features high organic matter content and complex composition, posing a significant environmental risk. While anodic oxidation has demonstrated high efficiency, there is increasing concern on the secondary hazard generation thereby. An energy-efficient system coupling flow-through anodic oxidation and cathodic reduction processes is therefore proposed, achieving simultaneous organic contaminant removal and chlorinated byproduct control. The direct electron transfer process imparts an advantage in treating chlorine-laden wastewater compared to the •OH-based advanced oxidation counterparts. The results of this study pave the way for developing novel electrochemical platforms for the purification of refractory waste streams whilst minimizing the secondary pollution.

Ball Milling of La2O3 Tailors the Crystal Structure, Reactive Oxygen Species, and Free Radical and Non-Free Radical Photocatalytic Pathways.

Non-free radical photocatalysis with metal oxide catalysts is an important advanced oxidation process that enables the removal of various emerging environmental pollutants, such as tetracycline. Here, four hexagonal La2O3 photocatalysts with different densities of oxygen vacancy and crystalline features are synthesized and then further treated by ball milling. Ball milling of these La2O3 photocatalysts is found to increase the amount of oxygen vacancies on their surfaces and thereby the amount of 1O2 species produced by them. The photocatalytic degradation of TC by these La2O3 photocatalysts depends on the oxygen vacancies present on them. Furthermore, the ones with a strong (101) diffraction peak remove tetracycline from water systems largely with 1O2 and •OH species, whereas those with a weak (101) diffraction peak do so mainly via 1O2 and direct electron transfer (DET) process. Their overall catalytic properties are also studied by density functional theory calculations. Moreover, the organic products produced from tetracycline by La2O3 photocatalysts containing a strong (101) diffraction peak are found to be less toxic than those produced by La2O3 photocatalysts containing a weak (101) diffraction peak. This study also provides convincing evidence that the structures of La2O3 determine the species that is produced by it and that end up mediating photocatalytic reaction pathways (i.e., free radical versus non-free radical) to degrade an emerging environment pollutant.

Effect of zero-valent iron (ZVI) and biogas slurry reflux on methane production by anaerobic digestion of waste activated sludge.

This study aimed to improve anaerobic digestion (AD) efficiency through the addition of zero-valent iron (ZVI) and biogas slurry. This paper demonstrated that methane production was most effectively promoted at a biogas slurry reflux ratio of 60%. The introduction of ZVI into anaerobic systems does not enhance its bioavailability. However, both biogas slurry reflux and the combination of ZVI with biogas slurry reflux increase the relative abundance of microorganisms involved in the direct interspecific electron transfer (DIET) process. Among them, the dominant microorganisms Methanosaeta, Methanobacterium, Methanobrevibacter, and Methanolinea accounted for over 60% of the total methanogenic archaea. The Tax4Fun function prediction results indicate that biogas slurry reflux and the combination of ZVI with biogas slurry reflux can increase the content of key enzymes in the acetotrophic and hydrotrophic methanogenesis pathways, thereby strengthening these pathways. The corrosion of ZVI promotes hydrogen production, and the biogas slurry reflux provided additional alkaline and anaerobic microorganisms for the anaerobic system. Their synergistic effect promoted the growth of hydrotrophic methanogens and improved the activities of various enzymes in the hydrolysis and acidification phases, enhanced the system's buffer capacity, and prevented secondary environmental pollution. PRACTITIONER POINTS: Optimal methane production was achieved at a biogas slurry reflux ratio of 60%. Biogas slurry reflux in anaerobic digestion substantially reduced discharge. ZVI addition in combination with biogas slurry reflux facilitates the DIET process.

Enhancing Anaerobic Biodegradation of Phenanthrene in Polluted Soil by Bioaugmentation and Biostimulation: Focus on the Distribution of Phenanthrene and Microbial Community Analysis

The remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soils has received much attention in recent years, and most of the contaminated sites are in anaerobic environments, such as deep soils and flooded soils. We simulated the natural flooded soil environment, selected phenanthrene (PHE) as a model PAH contaminant, and designed batch experiments run for 63 days to comprehensively investigate the effects of the combined addition of anaerobic sludge and granular biochar on microbial community and function and the anaerobic biodegradation of PHE. Firstly, the residue, distribution, and removal of PHE in the flooded soil environment were quantified for each group. Secondly, the effects of bioaugmentation of soil indigenous microorganisms by the addition of anaerobic activated sludge and biostimulation of biochar on the removal of PHE from the soil were analyzed against each other. Lastly, the changes in the structure of the microbial community under the effect of bioaugmentation and biostimulation were illustrated by sequencing analyses. The results of this study showed that the removal efficiency of PHE reached 72.0% after the addition of anaerobic activated sludge. The incorporation of anaerobic activated sludge and biochar resulted in a 25.3% increase in PHE removal compared to a single soil, suggesting that the combination of bioaugmentation and biostimulation can have a synergistic effect on the anaerobic biodegradation of PHE in contaminated soils. The results of sequencing analysis further indicated that the introduction of an exogenous microbial community changed the dominant genera associated with PHE degradation and introduced methanogenic archaea, which enriched the metabolic pathways of the carbon cycle in the system. On this basis, the addition of biochar resulted in higher anaerobic microbial community diversity, functional dominant species were enriched, and the direct interspecies electron transfer (DIET) process between electroactive bacteria (Bacteroides, f_Geobacteraceae) and Methanosaeta was facilitated, which accelerated the degradation of PHE by anaerobic microbial communities. The results of this study provide regulatory tools and basic data support for enhanced bioremediation of PAHs in flooded soils.

Enhanced methane production via anaerobic digestion assisted with Fe3O4 nanoparticles supported on microporous granular activated carbon

Granular activated carbon (GAC) was used as supporting material for different loading of magnetite (Fe3O4) nanoparticles using the wet impregnation method, for its use as biocatalysts during the anaerobic digestion (AD). The highest responses in terms of methane productivity were achieved with the two modified materials using 1.5% and 3% of Fe3O4. With (1.5%)Fe3O4/GAC, the conversion efficiency to methane was 97.1%, with a methane yield of 388.1 mL CH4/g COD, which was higher than the control without GAC, achieving only 72.1% of conversion and a yield of 286.6 mL CH4/g COD, representing an increment of ∼ 26% for these two parameters with the modified material. Also, the (1.5%)Fe3O4/GAC promotes the highest consumption of volatile fatty acids (VFAs) and ethanol compared to other cultures. The improvement in the AD by the modified materials could be attributable to its surface chemistry composed of a greater extent of Fe2+ and Fe3+, the external surface area, and the electron-accepting capacity. In addition, the close association between microorganisms and the (1.5%)Fe3O4/GAC biocatalyst within the anaerobic medium could be evidence that the direct interspecies electron transfer (DIET) process is occurring.

Effect of semiconductor materials on anaerobic methanogenesis of terephthalic acid: Promotion and inhibition mechanisms

AbstractThe treatment of purified terephthalic acid wastewaters via anaerobic methods poses a significant challenge due to the slow degradation rate and extended reaction time required for the most recalcitrant terephthalic acid (TA). To address this challenge, this study investigated the efficacy of various semiconductors in promoting TA degradation and determined their respective functional roles. Results showed that the addition of g‐C3N4 increased methane production by 28.38%, indicating its potential as a replacement for TiO2 in TA degradation. The improvement was attributed to the intensified direct interspecies electron transfer (DIET) process, facilitated by microorganisms that directly utilize conductive TiO2/g‐C3N4 as intermediaries to accelerate the electron transfer rate. The unfavorable methanogenesis performances of CeO2 and Fe2O3 were accredited to agglomeration phenomena and toxic effects. This study provides insights into the roles of exogenous materials in promoting or limiting the DIET process, and offers an environmentally friendly and highly‐effective strategy for treating industrial purified‐terephthalic acid wastewater.

Shorten migration distance of ROS and accelerate electron transfer by engineering dual Lewis acid sites for synergistic activation of PMS toward rapid TC degradation

Shortening the migration distance of reactive oxygen species (ROS) and accelerating electron transfer are promising strategies for boosting the performance of catalysts to activate peroxymonosulfate (PMS). Herein, metallic Lewis acid and non-metallic Lewis acid co-doped graphitic carbon nitride (Co,B-C3N4) are successfully prepared by one-step calcination for synergistically enhanced PMS activation. Cobalt sites significantly accelerated the electron transfer process. Boron acts as an adsorption site for the contaminants, extensively reducing the distance that ROS needs to migrate. Therefore, Co,B-C3N4 demonstrates exceptional PMS activation capability, successfully degrading 99% of tetracycline (TC) in just 6 min with a rate constant reaching 0.407 min−1. Moreover, Co,B-C3N4 still displayed outstanding catalytic performance in complex water environments and long-term cycling conditions. Density Functional Theory (DFT) calculation and mechanism studies demonstrated that non-free radical path (direct electron transfer process and 1O2) plays a dominant role. This work provides a feasible strategy for designing PMS activation catalysts.

Iron(III)-(1,10-Phenanthroline) Complex Can Enhance Ferrate(VI) and Ferrate(V) Oxidation of Organic Contaminants via Mediating Electron Transfer.

Recent research has primarily focused on the utilization of reductants as activators for Fe(VI) to generate high-valent iron species (Fe(IV)/Fe(V)) for the degradation of emerging organic contaminants (EOCs). However, a significant drawback of this approach arises from the reaction between reductants and ferrates, leading to a decrease in oxidation capacity. This study introduces a novel discovery that highlights the potential of the iron(III)-(1,10-phenanthroline) (Fe(III)-Phen) complex as an activator, effectively enhancing the degradation of EOCs by Fe(VI) and augmenting the overall oxidation capacity of Fe(VI). The degradation of EOCs in the Fe(VI)/Fe(III)-Phen system is facilitated through two mechanisms: a direct electron transfer (DET) process and electron shuttle action. The DET process involves the formation of a Phen-Fe(III)-Fe(VI)* complex, which exhibits a stronger oxidation ability than Fe(VI) alone and can accept electrons directly from EOCs. On the other hand, the electron shuttle process utilizes Fe(III)-Phen as a redox mediator to transfer electrons from EOCs to Fe(VI) through the Fe(IV)/Fe(III) or Fe(IV)/Fe(II)/Fe(III) cycle. Moreover, the Fe(III)-Phen complex can improve the utilization efficiency of Fe(V) by preventing its self-decay. This study's findings may present a viable option for utilizing an effective catalyst to enhance the oxidation of EOCs by Fe(VI) and Fe(V).

Unveiling the spatially confined oxidation processes in reactive electrochemical membranes

Electrocatalytic oxidation offers opportunities for sustainable environmental remediation, but it is often hampered by the slow mass transfer and short lives of electro-generated radicals. Here, we achieve a four times higher kinetic constant (18.9 min−1) for the oxidation of 4-chlorophenol on the reactive electrochemical membrane by reducing the pore size from 105 to 7 μm, with the predominate mechanism shifting from hydroxyl radical oxidation to direct electron transfer. More interestingly, such an enhancement effect is largely dependent on the molecular structure and its sensitivity to the direct electron transfer process. The spatial distributions of reactant and hydroxyl radicals are visualized via multiphysics simulation, revealing the compressed diffusion layer and restricted hydroxyl radical generation in the microchannels. This study demonstrates that both the reaction kinetics and the electron transfer pathway can be effectively regulated by the spatial confinement effect, which sheds light on the design of cost-effective electrochemical platforms for water purification and chemical synthesis.

Electron transfer route tuning caused by photo-irradiation: Stability enhancement of N-doped carbon materials in Fenton-like reactions

Improving the stability of N-doped carbon materials is still a great challenge in Fenton-like reactions. Herein, a hollow structure with TiO2 nanoparticles supported on the inner surface of N-doped carbon spheres (H-TiO2 @NC) is designed for peroxydisulfate (PDS)-based Fenton-like reactions with photo-irradiation. Impressively, the stability of H-TiO2 @NC is greatly enhanced with photo-irradiation. The degradation rate constant of bisphenol A (BPA) with photo-irradiation is ∼61 times larger than dark condition after four runs. The photovoltage enables more electrons to be transferred from BPA to PDS via active carbons. Less electrons from the carbon framework are expended, thus inhibiting the oxidation and deactivation of H-TiO2 @NC effectively. Additionally, the direct electron transfer (DET) process is improved due to conductivity enhancement, which also weakens the oxidation of carbon skeleton. This work provides a reliable method to enhance the stability of N-doped carbon materials. And giving some new insights into the mechanism of stability enhancement.

Tunable peroxymonosulfate activation by ( −1 1 1) crystal plane exposed δ-MnO2: Oxidant concentration induced intrinsic mechanisms transformation

The dual role of manganese oxides in peroxymonosulfate (PMS) activation for the degradation of organic contaminants makes the mechanism investigation complicated. Herein, a (−111) crystal plane exposed δ-MnO2 (SN-KMO) was synthesized to decipher the debate between direct oxidation process (DOP) and electron transfer process (ETP) mechanisms. Generally, 5 mg/L bisphenol A (BPA) could be efficiently removed in 15 min regardless of in the presence of high (0.5 mM) or low (0.05 mM) PMS concentration. Mechanisms analysis showed that nonradical pathways of DOP and ETP simultaneously contributed to the BPA degradation without the participation of radicals oxidation. Yet, contributions of DOP and ETP at certain PMS dosages differed significantly. Specifically, at high PMS concentration of 0.5 mM, DOP played the dominant role, and the overly protonated manganese active sites could bond with O-O bridged species derived from two PMS molecules to form outer-sphere complexes. Comparatively, at low PMS concentration of 0.05 mM, BPA oxidization was primarily dominated by ETP. The specific ETP was highly dependent on solution pH, and it only existed in the presence of high PMS concentration. The significance of this work lies in synthesizing a (−111) crystal plane exposed δ-MnO2 for efficient BPA degradation with low PMS dosage (0.05 mM) and discovering the unreported outer-sphere complexes for understanding the PMS activation mechanisms by manganese oxides.

Enhancement of anaerobic digestion of ciprofloxacin wastewater by nano zero-valent iron immobilized onto biochar

The commonly used antibiotic ciprofloxacin (CIP) can significantly inhibit and interfere with the anaerobic digestion (AD) performance. This work was developed to explore the effectiveness and feasibility of nano iron-carbon composites to simultaneously enhance methane production and CIP removal during AD under CIP stress. The results demonstrated that when the nano-zero-valent iron (nZVI) content immobilized on biochar (BC) was 33% (nZVI/BC-33), the CIP degradation efficiency reached 87% and the methanogenesis reached 143 mL/g COD, both higher than Control. Reactive oxygen species analysis demonstrated that nZVI/BC-33 could effectively mitigate microorganisms subjected to the dual redox pressure from CIP and nZVI, and reduce a series of oxidative stress reactions. The microbial community depicted that nZVI/BC-33 enriched functional microorganisms related to CIP degradation and methane production and facilitated direct electron transfer processes. Nano iron-carbon composites can effectively alleviate the stress of CIP on AD and enhance methanogenesis.

Electrocatalytic dechlorination of florfenicol using a Pd-loaded on blue TiO2 nanotube arrays cathode

Electrocatalytic hydrodechlorination technology provides an attractive strategy to effectively eliminate the biotoxicity of halogenated organic pollutants (HOPs). Herein, we demonstrated a highly dispersed Pd loaded on blue TiO2 nanotube arrays (Pdx-BTNA) cathode that can achieve efficient dechlorination and detoxification toward florfenicol (FLO). Characterization results indicated that strong interaction between Pd and BTNA was achieved through Pd-O bonds, which facilitated the electron transfer rate. Pdx-BTNA significantly outperformed other cathodes (i.e., BTNA, Pdx-TNA and Pdx-Ti) in the electrocatalytic dechlorination of FLO due to enhanced utilization of active H*. The dechlorination performances of FLO well followed the pseudo-first-order kinetic, and the reaction constant rate on Pdx-BTNA cathode (0.035 min−1) was 17.5, 1.94, 7.0 times higher than that on BTNA, Pdx-TNA and Pdx-Ti cathodes, respectively. Quenching experiments and electrochemical characterization indicated that both direct electron transfer and indirect reduction process mediated by H* were involved in the dechlorination of FLO. The optimal dechlorination efficiency of 98.79% was obtained by Pdx-BTNA cathode within 120 min at 1 mM PdCl2, applied cathode potential of −1.2 V. In addition, Pdx-BTNA exhibited superior stability and durability with almost no Pd leaching (0.0011 mg/L) during five dechlorination cycles. Our research proposed a novel strategy to design low noble Pd-loaded cathode material for efficient dechlorination and detoxification of HOPs.

A double-chamber microbial electrolysis cell improved the anaerobic digestion efficiency and elucidated the underlying bio-electrochemical mechanism

Bioelectrochemical regulation has been proven to enhance the traditional anaerobic digestion (AD) of biowastes. However, most of the research has been conducted using a single-chamber reactor that could not explain the underlying mechanism and/or role of electrodes in CH4 production. Herein, a double-chamber microbial electrolysis cell integrated anaerobic digestion system (MEC-AD) was fed with corn stover (CS) and was exposed to 0.0 V to 1.5 V to elucidate the possible effects of cathode and anode on the AD performance. It was shown that CH4 yield was enhanced by 80.41% at 0.6 V, however, there was a sharp decline in methanogens at 0.9 V, which finally resulted in acidification. Interestingly, electroactive bacteria of Enterococcus were enriched on the electrode at 0.6 V which accelerated the degradation of CS, and thus, enhanced the subsequent mixotrophic methanogenesis dominated by Methanosarcina in the bulk solution of the anode chamber. While, Enterococcus and Methanobacterium were shown to be the dominating microbes in the cathode chamber, which were found syntrophically associated to enhance hydrogenotrophic methanogenesis via the direct interspecies electron transfer process. The synergistic stabilization of both anode and cathode chambers resulted in the final performance improvement in the MEC-AD system. This MEC-AD system provided a deeper understanding for methane enhancement, which will facilitate the scale-up application of the double-chamber systems.

Degradation pathways of atrazine by electrochemical oxidation at different current densities: Identifications from compound-specific isotope analysis and DFT calculation

Current density was the key factor that impacted pollutant degradation by electrochemical oxidation, and reaction contributions at various current densities were non-negligible for the cost-effective treatments of organic pollutants. This research introduced compound specific isotope analysis (CSIA) into atrazine (ATZ) degradation by boron doped diamond (BDD) with current density of 2.5–20 mA/cm2, in order to provide “in-situ” and “fingerprint” analysis of reaction contributions with changed current densities. As results, the increased current density displayed a positive impact on ATZ removal. The ɅC/H values (correlations of Δδ13C and Δδ2H) were 24.58, 9.18 and 8.74 when current densities were 20, 4, and 2.5 mA/cm2, with ·OH contribution of 93.5%, 77.2% and 80.35%, respectively. While DET process favored lower current density with contribution rates up to ∼20%. What's more interesting, though the carbon and hydrogen isotope enrichment factors (εC and εH) were fluctuate, the ɅC/H linearly increased accompanied with applied current densities. Therefore, increasing current density was effective due to the larger ·OH contribution even though side reactions may occur. DFT calculations proved the increase of C–Cl bond length and the delocalization of Cl atom, confirming dechlorination reaction mainly occurred in the direct electron transfer process. While ·OH radical mainly attack the C–N bond on the side chain, which was more benefit to the fast decomposition of ATZ molecule and intermediates. It was forceful to discuss pollutant degradation mechanism by combining CSIA and DFT calculations. Target bond cleavage (i.e., dehalogenation reaction) can be conducted by changing reaction conditions like current density due to the significantly different isotope fractionation and bond cleavage.

Selective Removal of Emerging Organic Contaminants from Water Using Electrogenerated Fe(IV) and Fe(V) under Near-Neutral Conditions.

Fe(IV) and Fe(V) are promising oxidants for the selective removal of emerging organic contaminants (EOCs) from water under near-neutral conditions. The Fe(III)-assisted electrochemical oxidation system with a BDD anode (Fe(III)-EOS-BDD system) has been employed to generate Fe(VI), while the generation and contributions of Fe(IV) and Fe(V) have been largely ignored. Thus, we examined the feasibility and involved mechanisms of the selective degradation of EOCs in the Fe(III)-EOS-BDD system under near-neutral conditions. It was found that Fe(III) application selectively accelerated the electro-oxidation of phenolic and sulfonamide organics and made the oxidation system be resistant to interference from Cl-, HCO3-, and humic acid. Several lines of evidence indicated that EOCs were decomposed via direct electron-transfer process on the BDD anode and by Fe(IV) and Fe(V) but not Fe(VI), besides HO•. Fe(VI) was not generated until the exhaustion of EOCs. Furthermore, the overall contributions of Fe(IV) and Fe(V) to the oxidation of phenolic and sulfonamide organics were over 45%. Our results also revealed that Fe(III) was oxidized primarily by HO• to Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system. This study advances the understanding of the roles of Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system and provides an alternative for utilizing Fe(IV) and Fe(V) under near-neutral conditions.

Insights into the well-dispersed nano-Fe3O4 catalyst supported by N-doped biochar prepared from steel pickling waste liquor for activating peroxydisulfate to degrade tetracycline

The harmless and high-value added utilization of steel pickling waste liquor (SPWL) is a major challenge in the metallurgical advancement. Herein, well-dispersed nano-Fe3O4 (NF) composite with N-doped biochar as a spatial support (NF/0.5N3BC) is synthesized from SPWL and corncob, which has a large specific surface area (159.12 m2/g) and mesoporous structure. Besides, it exhibits excellent degradation efficiency (91.56%) and rate constant (0.042 min−1) of tetracycline (TC, 50 mg/L) by activating peroxydisulfate (PDS). Both free radicals and non-free radicals contribute to the TC degradation, among which the Fe species, defective edges, C = O, graphitic-N, and pyridinic-N are identified as the possible active sites. Particularly, N3BC cooperates with NF to form an internal electronic field to accelerate the Fe(III)/Fe(II) redox cycle and direct electron transfer process. Further, the possible degradation pathways are proposed based on DFT and HPLC-MS analysis, and the toxicity prediction results verify that NF/0.5N3BC + PDS system has an excellent TC degradation/detoxification ability. This study provided a novel strategy for high-value added utilization of SPWL and economically feasible treatment of TC-contaminated wastewater, so as to realizing the goal of “treating waste with waste”.

Electrochemical oxidation of phenol in chloride containing electrolyte using a carbon-coated Ti4O7 anode

Magnéli-phase Ti4O7 is an excellent anode material for electrochemical advanced oxidation process (EAOP). However, the development of Ti4O7 electrodes is always limited by the high cost and high energy consumption in their preparation process. In this study, the carbon-coated Ti4O7 electrode was successfully synthesized by an in-situ pyrolysis method, which exhibited high specific surface area (143.8 m2 g−1) and efficient electron transfer (47.3 Ω). The accelerated service life was measured to be 102.5 h, demonstrating its excellent stability. EAOP performance of the C-coated Ti4O7 electrode was evaluated under various operating conditions. The result showed that a complete degradation of phenol (100 mg L−1) was obtained in 60 min, under the condition of 0.1 M NaCl electrolyte and 20 mA cm−2 current density. Owing to the high oxygen evolution potential (OEP, 2.25 V vs. RHE) of the C-coated Ti4O7 electrode, high TOC removal ratio (68.9%) and low energy consumption (113.8 kWh kg−1TOC) were achieved under the optimal condition. Moreover, the mechanism of phenol degradation was investigated by linear sweep voltammetry (LSV) and free radical scavenging experiments. The results indicated that the degradation of phenol can be mainly attributed to the synergistic effects between adsorbed ·OH (M(·OH)) and Cl−, rather than the direct electron transfer process (DET) on the surface of the C-coated Ti4O7 electrode. This study provides an efficient strategy for the mineralization of phenolic wastewater by the C-coated Ti4O7 electrode.