Alternate pulses of ultrasound and electricity enhanced electrochemical process for p-nitrophenol degradation

Defect-enriched (H2PO4-, Cr3+)-α-Fe2O3/β-In2S3 composites for visible light degradation of 4-nitrophenol

In this work, the high-activity (H₂PO₄^-, Cr³⁺)-α-Fe₂O₃ (PCF) with abundant oxygen vacancies (OVs) and the high specific area was obtained by co-adding H₂PO₄^- and Cr³⁺. Defect-enriched PCF/β-In₂S₃ composites were prepared by low-temperature hydrothermal processes. The prepared composites exhibited improved photocatalytic degradation of 4-nitrophenol under visible light irradiation.The SO bond between PCF and β-In₂S₃ promoted the formation of tight heterojunction composites and increased the OVs concentration. Under the synergistic effect of photo-Fenton, defects, and heterojunction, the PCF/β-In₂S₃ composites effectively promoted the separation of photogenerated carriers and accelerated the production of active substances (•OH, •O₂^-, ¹O₂, and h⁺), leading to the improvement of photocatalytic-Fenton degradation performance. This work provided a new strategy for the preparation of highly efficient photocatalysts.

Recent progress on ultrasound-assisted electrochemical processes: A review on mechanism, reactor strategies, and applications for wastewater treatment

The electrochemical advanced oxidation processes (EAOPs) have received significant attention among the many other water and wastewater treatment technologies. However, achieving a desirable removal effect with a single technique is frequently difficult. Therefore, the integration of ultrasound technique with other processes such as electrocoagulation, electro-Fenton, and electrooxidation is a critical way to achieve effective organic pollutants decomposition from wastewater. This review paper is focused on ultrasound-assisted electrochemical (US/electrochemical) processes, so-called sonoelectrochemical processes of various organic pollutants. Emphasis was given to recently published articles for discussing the results and trends in this research area. The use of ultrasound and integration with electrochemical processes has a synergistic impact owing to the physical and chemical consequences of cavitation, resulting in enhancing the mineralization of organic pollutants. Various types of sonoelectrochemical reactors (batch and continuous) employed in the US/electrochemical processes were reviewed. In addition, the strategies to avoid passivation, enhanced generation of reactive oxygen species, and mixing effect are reviewed. Finally, concluding remarks and future perspectives on this research topic are also explored and recommended.

Tannic acid reinforced electro-Fenton system based on GO-Fe3O4/NF cathode for the efficient catalytic degradation of PNP

In order to overcome the sluggish kinetics of the redox conversion between Fe³⁺ and Fe²⁺ in Fenton process, we established a novel electro-Fenton system based on GO-Fe₃O₄ cathode and tannic acid (TA) for the efficient degradation of p-nitrophenol (PNP). Under the optimal degradation parameters (including the initial PNP concentration of 20 mg L^-1, pH = 5, current density of 30 mA cm^-2 and feeding ratio of PNP: TA = 1:2), the TA reinforced GO-Fe₃O₄ electro-Fenton system exhibited the removal rate of PNP over 90.1 ± 0.2%, the COD removal rate of 69.5 ± 0.84% and satisfactory reusability (with the removal rate of ∼80% after 5 recycles). The excellent degradation performance of the proposed TA reinforced GO-Fe₃O₄ electro-Fenton system was partly attributed to the optimized morphology (with the particle size of Fe₃O₄ reduced to tens of nanometers, pore size decreased by ∼80% and pore volume increased by 24.3 times) and larger specific surface area (increased by 72.7 times) after compositing GO with Fe₃O₄, which exposed more active sites. In return, the electron transfer process, the two-electron oxygen reduction reaction (ORR) and the degradation efficiency were promoted in the cooperation of GO and Fe₃O₄. Moreover, the incorporated TA would form a TA-Fe(III) complex to promote the reduction reaction from Fe³⁺ to Fe²⁺, which strengthened the self-circulation of Fe²⁺ and Fe³⁺ and indirectly enhanced the conversion of H₂O₂ to ROS to decompose PNP into smaller organic fragments or mineralize into CO₂, H₂O, NO₂^- or NO₃^-, etc. Obviously, the incorporation of TA provided a promising strategy to improve the electro-Fenton efficiency and realize the efficient removal of PNP in wastewater.

A novel technique for the reduction of pesticide residues by a combination of low-intensity electrical current and ultrasound applications: A study on lettuce samples

In this study, effects of low-intensity electrical currents (200, 800 and 1400 mA), ultrasound frequencies (24 and 40 kHz) and their combinations were applied at the duration period of 2, 4, 6, 8, and 10 min for the degradation of captan, thiamethoxam and metalaxyl residues in lettuce samples. Residues of the pesticides were determined by gas chromatography with tandem mass spectrometry and electron capture detector. The results indicated that the combination of low-intensity electrical current and ultrasound was found to be effective for the reduction of the pesticides. The most effective combination was obtained to be current of 1400 mA and ultrasound frequency of 24 kHz at 10 min. Under this circumstance, 92.57, 81.99 and 93.09% of captan, thiamethoxam and metalaxyl residues were decreased, respectively. The findings suggest that the combination of low-intensity electrical current and ultrasound applications has an important potential for the degradation of pesticide residues.

Application of ultrasonic technology in postharvested fruits and vegetables storage: A review

It has been an important research topic and a serious applicable issue to extend storage time of fruits and vegetables using advanced scientific and effective technology. Among various approaches, ultrasound has been regarded as one of the most pollution-free and effective technical means to significantly improve the preservation of fruits and vegetables. This paper summarizes the application of ultrasonic technology in fruits and vegetables storage in recent years, including removal of pesticide residues and cleaning, sterilization, enzyme inactivation, effect on physico-chemical indexes. Additionally, we also discussed limitations and negative effects of ultrasonic treatment on fruits and vegetables such as damages to tissues and cells. Furthermore, a proper application of ultrasonic technology has been proven to effectively extend the storage period of postharvest fruits and vegetables and maintain the quality. Moreover, the combination of ultrasound and other conventional preservation technologies can further improve the preservation in a coordinate manner and even have a broader application prospect.

Fabrication of a double-layer membrane cathode based on modified carbon nanotubes for the sequential electro-Fenton oxidation of p-nitrophenol

To improve the electrocatalytic efficiency of the cathode and provide a wider pH range in the electro-Fenton process, N-doped multi-walled carbon nanotubes (NCNTs) and ferrous ion complexed with carboxylated carbon nanotubes (CNT-COOFe²⁺) were used to fabricate the diffusion layer and catalyst layer of a membrane cathode, respectively. The morphology, structure, and composition of CNT-COOFe²⁺ were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The oxygen reduction performance of NCNT was evaluated using cyclic voltammetry (CV) and the rotating disk electrode technique (RDE). In addition, a potential application of the cathode in sequential electro-Fenton degradation of p-nitrophenol (p-NP) was investigated. The results revealed that iron was successfully doped on the carboxylated carbon nanotubes in ionic complexation form and the content of iron atoms in CNT-COOFe²⁺ was 2.65%. Furthermore, the defects on the tube walls provided more reactive sites for the electro-Fenton process. A combination of CV and RDE data indicated that NCNT had better electrocatalytic H₂O₂ generation activity with a more positive onset potential and higher cathodic peak current response than CNT. A p-NP removal rate of 96.04% was achieved within 120 min, and a mineralization efficiency of 80.26% was obtained at 180 min in the sequential electro-Fenton process at a cathodic potential of - 0.7 V vs SCE and neutral pH. The activity of the used cathode was restored simply through electro-reduction at - 1.0 V vs SCE, and a p-NP removal rate of more than 70% was obtained at 60 min after six regeneration cycles.

Simple Environmentally-Friendly Reduction of 4-Nitrophenol

The low molecular-mass organic compound 4-nitrophenol is involved in many chemical processes and is commonly present in soils and in surface and ground waters, thereby causing severe environmental impact and health risk. Several methods have been proposed for its transformation (bio and chemical degradation). However, these strategies not only produce equally or more toxic aromatic species but also require harsh operating conditions and/or time-consuming treatments. In this context, we report a comprehensive and systematic study of the electrochemical reduction of 4-nitrophenol as a viable alternative. We have explored the electrochemical reduction of this pollutant over different metallic and carbonaceous substrata. Specifically, we have focused on the use of gold and silver working electrodes since they combine a high electrocatalytic activity for 4-nitrophenol reduction and a low electrocatalytic capacity for hydrogen evolution. The influence of the pH, temperature, and applied potential have also been considered as crucial parameters in the overall optimization of the process. While acidic media and high temperatures favor the clean reduction of 4-nitrophenol to 4-aminophenol, the simultaneous hydrogen evolution is pernicious for this purpose. Herein, a simple and effective electrochemical method for the transformation of 4-nitrophenol into 4-aminophenol is proposed with virtually no undesired by-products.

Degradation of p-Nitrophenol using magnetic Fe0/Fe3O4/Coke composite as a heterogeneous Fenton-like catalyst

A Coke supported Fe₃O₄ and Fe⁰ composite (Fe⁰/Fe₃O₄/Coke) was prepared for the first time with the aim of evaluating its ability to be used as heterogeneous catalyst for the Fenton degradation of p-Nitrophenol (p-NP). A four factor Box-Behnken design (BBD) coupled with response surface methodology (RSM) was applied to evaluate the effects of several operating parameters, namely Fe⁰/Fe₃O₄/Coke dosage, reaction temperature, initial pH and H₂O₂ concentration, on the removal efficiency of p-NP. A significant quadratic model (p-value<0.0001, R²=0.9952) was derived using analysis of variance (ANOVA). Optimum conditions were determined to be 1.3g/L catalyst, 32°C, pH3.1 and 11.3mM H₂O₂. 100% of p-NP (100mg/L) conversion and 81% of COD removal were achieved after 120min of reaction time, respectively, under the optimum conditions, which agreed well with the modeling prediction. The recyclability of Fe⁰/Fe₃O₄/Coke was also investigated after three successive runs, in which p-NP degradation performances showed a slight difference with the first oxidation cycle with an acceptable iron leaching. Moreover, according to the main intermediate products identified by gas chromatography-mass spectrometry (GC-MS), a possible pathway of p-NP degradation was proposed based on hydrogen radicals ([H]) or hydroxyl radicals (•OH) mechanism.