Efficient degradation of 2,4-dichlorophenol in water by sequential electrocatalytic reduction and oxidation with a Pd-MWCNTs/Ni-foam electrode

Our previous study indicated excellent dechlorination efficiency and phenol conversion rate in the electrocatalytic reduction of 2,4-dichlorophenol (2,4-DCP) with a Pd-MWCNTs/Ni-foam electrode; it is deserved to investigate whether this electrode can efficiently degrade phenol in electro-Fenton oxidation (EFO) process and realize the effective mineralization of 2,4-DCP in aqueous solution. In this work, the sequential electrocatalytic reduction and oxidation of 2,4-DCP were studied after examining phenol degradation in the EFO process. The results showed that the removal efficiency of 0.31 mM phenol could reach 96.76% after 90-min degradation with the rate constant of 0.0367 min^-1, and hydroxy radicals (·OH) were the main active species in the EFO process. In the sequential electrocatalytic reduction and oxidation processes, the removal efficiencies of 2,4-DCP, phenol, and total organic carbon (TOC) reached 99.72%, 97.07%, and 61.45%, respectively. The possible degradation mechanism of 2,4-DCP was proposed through monitoring the reaction products, and the stability and reusability of the electrode were also examined. This study suggested that 2,4-DCP in wastewater can be effectively mineralized to realize its efficient degradation through the sequential electrocatalytic reduction and oxidation.

Highly efficient electro-Fenton process on hollow porous carbon spheres enabled by enhanced H2O2 production and Fe2+ regeneration

Electro-Fenton (e-Fenton) is a promising method for wastewater treatment that relies on powerful ·OH generated via the decomposition of electro-generated H₂O₂ catalyzed by Fe²⁺. In this regard, developing a catalyst capable of simultaneously producing H₂O₂ and accelerating Fe²⁺ regeneration is of considerable importance; however, this remains a challenge because of the difficulty in modulating the electronic microenvironment. Herein, a hollow porous carbon sphere catalyst (HPCS) is developed to synchronously enhance H₂O₂ generation and accelerate Fe³⁺/Fe²⁺ cycling by constructing an electron-rich microenvironment via surface curvature regulation. The Fe²⁺ regeneration efficiency reaches 35.5% on HPCS featuring a larger curvature structure (HPCS-TPOS), which is 1.6 times higher than the smaller curvature HPCS-S catalyst (22.8%). Density functional theory reveals that the electron-rich microenvironment on the outer surface of high curvature structure promotes Fe²⁺ regeneration. The H₂O₂ production rate on HPCS-TPOS is 47.2 mmol L^-1 h^-1, exceeding the state-of-the-art e-Fenton catalysts reported. Benefiting from the concurrent high-efficiency of H₂O₂ production and Fe²⁺ regeneration, HPCS-TPOS e-Fenton is demonstrated to be efficient for sulfamethoxazole removal with the kinetic rate of 0.30-0.72 min^-1 at pH 3-7. This work offers new insight into the design of efficient catalysts by rationally regulating curvature structures for wastewater treatment.

Biochar cathode: Reinforcing electro-Fenton pathway against four-electron reduction by controlled carbonization and surface chemistry

Porous biochars have attracted tremendous interests in electrochemical applications. In this study, a family of biochars were prepared from cellulose subject to different carbonization temperatures ranging from 400 to 700 °C, and the biochars were in-situ activated by a molten salt (ZnCl₂) to construct a hierarchically porous architecture. The activated porous biochars (ZnBC) were used as a carbocatalyst for electro-Fenton (EF) oxidation of organic contaminants. Results showed that high-temperature carbonization improved the activity of biochar for four-electron oxygen reduction reaction (ORR) due to the rich carbon defects, while the mild-temperature treatment regulated the species and distribution of oxygen functional groups to increase the production of hydrogen peroxide (H₂O₂) via a selective two-electron ORR pathway. ZnBC-550 was the best cathode material with a high ORR activity without compromise in H₂O₂ selectivity; a high production rate of H₂O₂ (796.1 mg/g/h) was attained at -0.25 V vs RHE at pH of 1. Furthermore, Fe(II) addition induced an electro-Fenton system to attain fast decomposition of various organic pollutants at -0.25 V vs RHE (reversible hydrogen electrode) and pH of 3 with a satisfactory mineralization efficiency toward phenolic pollutants. The EF system maintains its excellent stability for 10 cycles. Hydroxyl radicals were identified as the dominant reactive oxygen species based on in situ electron paramagnetic resonance (EPR) analysis and radical quenching tests. This study gains new insights into electrocatalytic H₂O₂ production over porous biochars and provides a low-cost, robust and high-performance electro-Fenton cathode for wastewater purification.

An upgraded electro-Fenton treatment of wastewater using nanoclay-embedded graphene composite prepared via exfoliation of pencil rods by submerged liquid plasma

In this work, two types of electrochemical electrodes were synthesized using two types (i.e., 4 black (4B) and hard black (HB)) of pencil rods during submerged liquid plasma (SLP) process. At high potential (3 kV) electrons, the SLP process offered an effective exfoliation route for the disorientation of the graphite sp² domain to produce two nanoclay-graphene composite electrodes with a few graphene layers (thickness = 4-9 layers) and high dispersibility (< 19% settlement: 4 h) in polar/non-polar solution (52-53.1% settlement: 4 h). Their performance was then evaluated towards the electro-Fenton (EF) degradation of lindane using a coated Fe₃O₄ plate (as Fenton catalyst). Accordingly, both 4B- and HB-ENcGe electrodes showed high specific capacitance values (473 and 363 F g^-1) at 0.05 A g^-1 and excellent triangular charge-discharge patterns (< 9% and 35% reduction of capacitance, respectively after 1000 cycles (charging rate: 0.2 A g^-1)). At pH 3 and current density of 6.5 mA cm^-2, 4B-ENcGe exhibited superior EF degradation performance (99.4% after 60 min) against 2.5 mg L^-1 lindane (H₂O₂ generation capacity: 2.53 mmol. h^-1, current efficiency: 89.4%, and stability: up to 5th cycles). The complete EF-based mineralization of lindane suggests that these electrodes should offer one-step cost-effective treatment for wastewater contaminants.