Radical attack and mineralization mechanisms on electrochemical oxidation of p-substituted phenols at boron-doped diamond anodes

Encapsulate Co3O4 within ultrathin graphene sheets to enhance peroxymonosulfate activation by tuning surface electronic structures

Heterojunctions composed of cobalt-based materials and carbon materials have been recognized as the efficient catalysts for peroxymonosulfate (PMS) activation to generate reactive oxygen species for the removal of environmental contaminants. However, the role of carbon materials in promoting the heterojunction systems has not been fully understood. This study synthesized a heterojunction material of graphene sheets encapsulating Co3O4 (GCO-500) through the pyrolysis of cobalt MOF and applied it to activate PMS for the removal of lomefloxacin. The results showed a high removal rate of 93.59 % with a degradation rate of k1 = 0.0156 min-1. Co3O4 clusters was encapsulated within ultrathin graphene sheets (<2 nm). DFT calculations revealed that graphene layers improve the electron transfer ability of Co3O4 and increased the d-band center of Co3O4 (-1.61 eV) that promote the adsorption of PMS on GCO-500 (-1.32 eV). In the meanwhile, organic pollutant was enriched in graphene layers with high adsorption energy (-13.08 eV), which greatly enhanced the degradation efficiency of pharmaceuticals. This study provides an effective catalyst for PMS activation and sheds light on the fundamental electronic-level understanding of cobalt-based and carbon heterojunction catalysts in PMS activation.

Adenine phosphate-Cu nanozyme with multienzyme mimicking activity for efficient degrading phenolic compounds and detection of hydrogen peroxide, epinephrine and glutathione

BACKGROUND

With the development of nanotechnology, various nanomaterials with enzyme-like activity (nanozymes) have been reported. Due to their superior properties, nanozymes have shown important application potential in the fields of bioanalysis, disease detection, and environmental remediation. However, only a few nanomaterials with multi-enzyme mimicry activity have been reported. In this study, a novel multienzyme mimic was synthesized through a simple and rapid preparation protocol by coordinating copper ions with N3, N6 (amino), N7, and N9 on adenine phosphate.

RESULTS

The prepared adenine phosphate-Cu complex exhibits significant peroxidase, laccase, and oxidase mimicking activities. The Michaelis-Menten constant (Km) and the maximal velocity (Vmax) values of the peroxidase, laccase, and oxidase mimicking activities of AP-Cu nanozyme are 0.052 mM, 0.14 mM, and 2.49 mM; and 0.552 μM min-1, 6.70 μM min-1, and 2.24 μM min-1, respectively. Then, based on its laccase mimicking activity, the nanozyme was applied in the degradation of phenolic compounds. The calculated kinetic constant for the degradation of 2,4-dichlorophenol is 0.468 min-1 and the degradation efficiency of 2,4-dichlorophenol (0.1 mM) reaches 96.14% at 7 min. Finally, based on the multienzyme mimicking activity of adenine phosphate-Cu nanozyme, simple colorimetric sensing methods with high sensitivity and good selectivity were developed for the detection of hydrogen peroxide, epinephrine, and glutathione in the ranges of 20.0-200.0 μM (R2 = 0.9951), 5.0-100.0 μM (R2 = 0.9970), and 5.0-200.0 μM (R2 = 0.9924) with the limits of quantitation of 20.0 μM, 5.0 μM, and 5.0 μM, respectively.

SIGNIFICANCE

In short, the synthesis of nanozymes with multi-enzyme mimicry activity through coordination between copper ions and small molecule mimicry enzymes provides new ideas for the design and research of multi-enzyme mimics.

Construction of Z-Scheme TiO2/Au/BDD Electrodes for an Enhanced Electrocatalytic Performance

TiO₂/Au/BDD composites with a Z-scheme structure was prepared by orderly depositing gold (Au) and titanium dioxide (TiO₂) on the surface of a boron-doped diamond (BDD) film using sputtering and electrophoretic deposition methods. It was found that the introduction of Au between TiO₂ and the BDD, not only could reduce their contact resistance, to increase the carrier transport efficiency, but also could improve the surface Hall mobility of the BDD electrode. Meanwhile, the designed Z-scheme structure provided a fast channel for the electrons and holes combination, to promote the effective separation of the electrons and holes produced in TiO₂ and the BDD under photoirradiation. The electrochemical characterization elucidated that these modifications of the structure obviously enhanced the electrocatalytic performance of the electrode, which was further verified by the simulated wastewater degradation experiments with reactive brilliant red X-3B. In addition, it was also found that the photoirradiation effectively enhanced the pollution degradation efficiency of the modified electrode, especially for the TiO₂/Au/BDD-30 electrode.

Degradation of 4-chlorophenol through cooperative reductive and oxidative processes in an electrochemical system

Electrochemical treatment can be an effective approach for degrading recalcitrant organic contaminants because its anode/cathode produces powerful oxidizing/reducing conditions. Herein, through the cooperation of the cathodic reductive and anodic oxidative processes, 4-chlorophenol (4-CP) was successfully degraded in an electrochemical system. TiO₂ nanotube arrays (TNTAs)/Sb-SnO₂ and TNTAs/Pd were successfully prepared and served as the anode and cathode electrodes, respectively, to generate oxidative (hydroxyl radical, ·OH) and reductive (chemically adsorbed hydrogen, H_ads) agents. The sequential reduction-oxidation (SRO) process provided a reasonable degradation pathway that accomplished reductive detoxification in the cathode and oxidative mineralization in the anode. The SRO mode achieved dechlorination efficiency (DE) of 86.9 ± 3.9% and TOC removal efficiency of 64.8 ± 4.2% within 3 h and under a current density of 8 mA cm^-2, both of which were significantly higher than those obtained in the sequential oxidation-reduction or the simultaneous redox modes. The increment of current density and reaction time could improve 4-CP degradation performance, but a high current density would decrease the cathode stability and a longer reaction time led to the generation of ClO₄^-. This study has demonstrated that sequential reduction-oxidation can be an effective and tunable process for degrading recalcitrant organic contaminants.

The structure-activity relationship of aromatic compounds in advanced oxidation processes：a review

Advanced oxidation processes (AOPs) are widely used as efficient technologies to treat highly toxic and harmful substances in wastewater. Taking the most representative aromatic compounds (monosubstituted benzenes, substituted phenols and heterocyclic compounds) as examples, this paper firstly introduces their structures and the structural descriptors studied in AOPs before, and the influence of structural differences in AOPs with different reactive oxygen species (ROS) on the degradation rate was discussed in detail. The structure-activity relationship of pollutants has been previously analyzed through quantitative structure-activity relationship (QSAR) model, in which ROS is a very important influencing factor. When electrophilic oxidative species attacks pollutants, aromatic compounds with electron donating groups are more favorable for degradation than aromatic compounds with electron donating groups. While nucleophilic oxidative species comes to the opposite conclusion. The choice of advanced oxidation processes, the synergistic effect of various active oxygen species and the used catalysts will also change the degradation mechanism. This makes the structure-dependent activity relationship uncertain, and different conclusions are obtained under the influence of various experimental factors.

Combined Analytical Study on Chemical Transformations and Detoxification of Model Phenolic Pollutants during Various Advanced Oxidation Treatment Processes

Advanced oxidation processes (AOPs) have been introduced to deal with different types of water pollution. They cause effective chemical destruction of pollutants, yet leading to a mixture of transformation by-products, rather than complete mineralization. Therefore, the aim of our study was to understand complex degradation processes induced by different AOPs from chemical and ecotoxicological point of view. Phenol, 2,4-dichlorophenol, and pentachlorophenol were used as model pollutants since they are still common industrial chemicals and thus encountered in the aquatic environment. A comprehensive study of efficiency of several AOPs was undertaken by using instrumental analyses along with ecotoxicological assessment. Four approaches were compared: ozonation, photocatalytic oxidation with immobilized nitrogen-doped TiO₂ thin films, the sequence of both, as well as electrooxidation on boron-doped diamond (BDD) and mixed metal oxide (MMO) anodes. The monitored parameters were: removal of target phenols, dechlorination, transformation products, and ecotoxicological impact. Therefore, HPLC–DAD, GC–MS, UHPLC–MS/MS, ion chromatography, and 48 h inhibition tests on Daphnia magna were applied. In addition, pH and total organic carbon (TOC) were measured. Results show that ozonation provides by far the most suitable pattern of degradation accompanied by rapid detoxification. In contrast, photocatalysis was found to be slow and mild, marked by the accumulation of aromatic products. Preozonation reinforces the photocatalytic process. Regarding the electrooxidations, BDD is more effective than MMO, while the degradation pattern and transformation products formed depend on supporting electrolyte.

Detection and removal of poly and perfluoroalkyl polluting substances for sustainable environment

PFAs (poly and perfluoroalkyl compounds) are hazardous and bioaccumulative chemicals that do not readily biodegrade or neutralize under normal environmental conditions. They have various industrial, commercial, domestic and defence applications. According to the Organization for Economic Co-operation and Development, there are around 4700 PFAs registered to date. They are present in every stream of life, and they are often emerging and are even difficult to be detected by the standard chemical methods. This review aims to focus on the sources of various PFAs and the toxicities they impose on the environment and especially on humankind. Drinking water, food packaging, industrial areas and commercial household products are the primary PFAs sources. Some of the well-known treatment methods for remediation of PFAs presented in the literature are activated carbon, filtration, reverse osmosis, nano filtration, oxidation processes etc. The crucial stage of handling the PFAs occurs in determining and analysing the type of PFA and its remedy. This paper provides a state-of-the-art review of determination & tools, and techniques for remediation of PFAs in the environment. Improving new treatment methodologies that are economical and sustainable are essential for excluding the PFAs from the environment.

Improving the Treatment Efficiency and Lowering the Operating Costs of Electrochemical Advanced Oxidation Processes

Electrochemical advanced oxidation processes (EAOP®) are promising technologies for the decentralized treatment of water and will be important elements in achieving a circular economy. To overcome the drawback of the high operational expenses of EAOP® systems, two novel reactors based on a next-generation boron-doped diamond (BDD) anode and a stainless steel cathode or a hydrogen-peroxide-generating gas diffusion electrode (GDE) are presented. This reactor design ensures the long-term stability of BDD anodes. The application potential of the novel reactors is evaluated with artificial wastewater containing phenol (COD of 2000 mg L−1); the reactors are compared to each other and to ozone and peroxone systems. The investigations show that the BDD anode can be optimized for a service life of up to 18 years, reducing the costs for EAOP® significantly. The process comparison shows a degradation efficiency for the BDD–GDE system of up to 135% in comparison to the BDD–stainless steel electrode combination, showing only 75%, 14%, and 8% of the energy consumption of the BDD–stainless steel, ozonation, and peroxonation systems, respectively. Treatment efficiencies of nearly 100% are achieved with both novel electrolysis reactors. Due to the current density adaptation and the GDE integration, which result in energy savings as well as the improvements that significantly extend the lifetime of the BDD electrode, less resources and raw materials are consumed for the power generation and electrode manufacturing processes.

Effect of current density and pH on the electrochemically generated active chloro species for the rapid mineralization of p-substituted phenol

The aim of present study is increasing the degradation and mineralization of 4-chlorophenol (4-CP) during electrochemical oxidation with Ti/RuO₂ anodes. Innovatively, the evolution of chlorine-related species and the formations of various inorganic ions were investigated by electrolytic analysis in order to set up whether the formation and consumption of these byproducts associated with either chemical or electrochemical reactions. The effect of operating parameters such as current density, solution pH, treatment time, and electrolyte concentration has been studied. The formation of Cl₂, chlorite (ClO₂^-), and chlorate (ClO₃^-) were detected by adding the known concentration of Cl^- ions at different pH and current densities. Concentration trends of active chloro-species indicate that the degradation of 4-CP and chemical oxygen demand (COD) removal was formed maximum at pH 6 and j of 225.2 Am^-2 in presence of 0.0085 M NaCl. Thus, the 4-CP degradation mainly depends on the radicals and active chlorine formation and a mineralization mechanism was proposed based on intermediates byproducts formation such as catechol, hydroquinone, 1, 4-benzoquinone, and organic acids identify by using the GC-MS and HPLC analysis at the optimum treatment condition. Total organic carbon (TOC) at different pH and current density, mass balance analysis of carbon and inorganic species formation were determined at the optimum treatment conditions of 4-CP. The degradation kinetic of 4-CP was followed the pseudo-first order kinetic model during the each parameters optimization. Specific energy consumption and current efficiency were also used to identify the technical feasibility of the process.

Electrochemical degradation of atrazine by BDD anode: Evidence from compound-specific stable isotope analysis and DFT simulations

Direct charge transfer (DCT) and •OH attack played important roles in contaminant degradation by BDD electrochemical oxidation. Their separate contributions and potential bond-cleavage processes were required but lacking. Here, we carried out promising compound-specific isotope fractionation analysis (CSIA) to explore ¹³C and ²H isotope fractionation of atrazine (ATZ), followed by assessing the reaction pathway by BDD anode. The correlation of ²H and ¹³C fractionation allows to remarkably differentiate DCT process and •OH attack, with Λ values of 18.99 and 53.60, respectively. Radical quenching identified that •OH accounted for 79.0%-88.5% in the whole reaction. While CSIA methods provided biased results, which suggested that ATZ degradation exhibited two stages with •OH contributions of 24.6% and 84.3% respectively, confirming CSIA was more sensitive and provided more possibilities to estimate degradation processes. Combined with Fukui index and intermediate products identification, we deduced that dechlorination-hydroxylation mainly occurred in the first 30 min by DCT reaction. While lateral chain oxidation with C-N broken was the governing route once •OH was largely generated, with the production of DEA (m/z 188), DIA (m/z 174), DEIA (m/z 146) and DEIHA (m/z 128). Our results demonstrated that isotope fractionation can offer "isotopic footprints" for identifying the rate-limiting steps and bond breakage process, and opens new avenues for degradation pathways of contaminants.

Preferential and efficient degradation of phenolic pollutants with cooperative hydrogen-bond interactions in photocatalytic process

Phenolic pollutants as highly toxic and hazardous organics are widely generated from industrial and domestic process. Phenolic pollutants with different hydroxyl position (catechol, resorcinol, hydroquinone, phenol) were preferentially and efficiently oxidized in photocatalytic process (PC) by designing boron-doped TiO₂ (B-TiO₂).The key role for enhancing the photocatalytic activity of B-TiO₂ was the formation of abundant Ti³⁺ species. The formation of Ti³⁺-O weakened the competitive adsorption of H₂O in aqueous solution and favored the formation of cooperative hydrogen bond on the surface of B-TiO₂, leading to enhanced adsorption of phenolic pollutants. The degradation rate constant of B-TiO₂ (k_B-TiO2) was regardless of the corresponding oxidation potential of phenolic pollutants. The k_B-TiO2 for catechol in photocatalytic process was as high as 3.46 min^-1, which was 18.2, 1.6 times higher than that of biodegradation and ozonation methods, respectively. Of note, the preferential removal mechanism of phenolic pollutants was elucidated by in-situ attenuated total reflectance (ATR)-IR and density functional theory calculation (DFT). The results were helpful for developing new preferential oxidation technologies in HO∙-mediated process for selectively removing low concentration but highly toxic pollutants.