Electrocatalytic reduction of low-concentration thiamphenicol and florfenicol in wastewater with multi-walled carbon nanotubes modified electrode

B-Modified Pd Cathodes for the Efficient Detoxification of Halogenated Antibiotics: Enhancing C-F Bond Breakage beyond Hydrodefluorination

Halogenated antibiotics pose a great threat to aqueous environments because of their persistent biotoxicity from carbon-halogen bonds. Electrochemical reduction (ER) is an efficient technology for dehalogenation, but it still suffers from limited efficiencies in breaking C-F bonds. Herein, we present a strategy to enhance C-F cleavage and promote detoxification by loading benchmark palladium cathodes onto boron-doped carbon. This improves the florfenicol (FLO) degradation rate constant and defluorination efficiency by 1.24 and 1.05 times, respectively, and improves the defluorination of various fluorinated compounds. The cathode with optimal B content shows superior mass activity for FLO degradation (1.11 mmol g-1 Pd min-1), which is 5.9 times that of commercial Pd/C and is among the best-reported cathodes. Notably, the exclusive formation of the direct defluorination product (i.e., FLO-F) on Pd/B-C implies a higher intrinsic C-F cleavage ability endowed by B doping. As revealed by experiments and theoretical calculations, boron modification enhances palladium binding and induces stronger strain effects and higher electron density for surface palladium atoms, which boosts H* generation and reduces the energy barrier for C-F cleavage. This study provides an effective cathode design strategy to enhance C-F activation, which may broadly benefit the destruction and detoxification of fluorinated organics that are limited by sluggish C-F cleavage kinetics.

Performance and degradation pathway of florfenicol antibiotic by nitrogen-doped biochar supported zero-valent iron and zero-valent copper: A combined experimental and DFT study

Fluorinated compounds are a class of organic substances resistant to degradation. Although zero-valent iron (Fe0) has a promising reducing capability, it still fails to degrade fluorine-containing antibiotics (i.e., florfenicol) efficiently. In this study, we applied a simple one-pot pyrolytic approach to synthesize nitrogen-doped biochar supported Fe0 and zero-valent copper (Cu0) composite (Fe/Cu@NBC) and investigated its performance on florfenicol removal. The results clearly showed that approximately 91.4% of florfenicol in the deionized water was removed by Fe/Cu@NBC within 8 h. As the reaction time was extended to 15 d, the total degradation rate of florfenicol reached 96.6%, in which the defluorination and dechlorination rates were 73.2% and 82.1%, respectively. Both experimental results and density functional theory calculation suggested that ∙OH and ·O2- triggered β-fluorine elimination, resulting in defluorination prior to dechlorination. This new finding was distinct from previous viewpoints that defluorination was more difficult to occur than dechlorination. Fe/Cu@NBC also had a favorable performance for removal of florfenicol in surface water. This study provides a new insight into the degradation mechanism and pathway of florfenicol removal in the Fe/Cu@NBC system, which can be a promising alternative for remediation of fluorinated organic compounds in the environment.

Oxalated zero valent iron enables highly efficient heterogeneous Fenton reaction by self-adapting pH and accelerating proton cycle

Heterogeneous Fenton reactions of zero-valent iron (ZVI) requires the sufficient release of Fe(II) to catalyze the H₂O₂ decomposition. However, the rate-limiting step of proton transfer through the passivation layer of ZVI restricted the Fe(II) release via Fe⁰ core corrosion. Herein we modified the shell of ZVI with highly proton-conductive FeC₂O₄·2H₂O by ball-milling (OA-ZVI^bm), and demonstrated its high heterogeneous Fenton performance of thiamphenicol (TAP) removal, with 500 times enhancement of the rate constant. More importantly, the OA-ZVI^bm/H₂O₂ showed little attenuation of the Fenton activity during 13 successive cycles, and was applicable across a wide pH range of 3.5-9.5. Interestingly, the OA-ZVI^bm/H₂O₂ reaction showed pH self-adapting ability, which initially reduced and then sustained the solution pH in the range of 3.5-5.2. The abundant intrinsic surface Fe(II) of OA-ZVI^bm (45.54% vs. 27.52% in ZVI^bm, according to Fe 2p XPS profiles) was oxidized by H₂O₂ and hydrolyzed to generate protons, and the FeC₂O₄·2H₂O shell favored the fast transfer of protons to inner Fe⁰, therefore, the consumption-regeneration cycle of protons were accelerated to drove the production of Fe(II) for Fenton reactions, demonstrated by the more prominent H₂ evolution and nearly 100% H₂O₂ decomposition by OA-ZVI^bm. Furthermore, the FeC₂O₄·2H₂O shell was stable and slightly decreased from 1.9% to 1.7% after the Fenton reaction. This study clarified the significance of proton transfer on the reactivity of ZVI, and provided an efficient strategy to achieve the highly efficient and robust heterogeneous Fenton reaction of ZVI for pollution control.

Evaluation of the effect of biofilm formation on the reductive transformation of triclosan in cathode-modified electrolytic systems

The performance of electrochemical reduction is often enhanced by electrode modification techniques. However, there is a risk of microbial colonization on the electrode surface to form biofilms in the treatment of actual wastewater with modified electrodes. In this work, the effects of biofilm formation on modified electrodes with reduced graphene oxide (rGO), platinum/carbon (Pt/C), and carbon nanotube (CNT) were investigated in triclosan (TCS) degradation. With biofilm formation, the TCS degradation efficiencies of carbon cloth (CC), rGO@CC, Pt/C@CC, and CNT@CC decayed to 54.53 %, 59.77 %, 69.19 %, and 53.97 %, respectively, compared to the raw electrodes. Confocal laser scanning microscopy and microbial community analysis revealed that the difference in biofilm thickness and activity were the major influencing factors on the discrepant TCS degradation rather than the microbial community structure. The electrochemical performance tests showed that the biofilm formation increased the ohmic resistance by an order of magnitude in rGO@CC, Pt/C@CC, and CNT@CC, and the charge transfer resistance was increased by 2.45, 3.78, and 7.75 times, respectively. The dechlorination and hydrolysis governed the TCS degradation pathway in all electrolysis systems, and the toxicity of electrochemical reductive products was significantly decreased according to the Toxicity Estimation Software Tool analysis. This study presented a systematic assessment of the biofilm formation on modified electrodes in TCS reduction, and the undisputed experimental outcomes were obtained to enrich the knowledge of implementing modified electrodes for practical applications.

Electrochemical Reduction of Halogenated Alkanes and Alkenes Using Activated Carbon-Based Cathodes

Granular activated carbon (GAC) is used to sorb a broad range of halogenated contaminant classes, but spent GAC disposal is costly. Taking advantage of GAC's conductivity, this study evaluated the conversion of the GAC to cathodes for electrochemical reductive dehalogenation of 15 halogenated alkanes and alkenes exhibiting a diversity of structures (type of halogen, number of halogens, functional groups) and including contaminants of practical importance (e.g., trichloroethylene). Alkane degradation rates increased with the number of halogens and in the order: chlorine < bromine < iodine. Quantitative structure-activity relationships (QSARs) correlating experimental first-order degradation rate constants for alkanes with molecular descriptors associated with an outer-sphere one-electron transfer calculated using density functional theory indicated that correlations with molecular descriptors improved in the order: aqueous phase reduction potentials (E^0,aq) < energy of the substrate's lowest unoccupied molecular orbital (E_LUMO) < Marcus theory activation free energies (ΔG^‡) ∼ gas-phase standard reduction free energies (ΔG^0,gas). Chlorinated alkene degradation rates increased with decreasing number of chlorines, and QSAR correlations were opposite those of alkanes, indicating a different reaction mechanism. Degradation timescales ranged from 1 min to 3 h with halides as predominant products. These results suggest that the electrochemical reduction of halogenated alkanes and alkenes can be used to regenerate spent GAC.

In-situ chemical attenuation of pharmaceutically active compounds using CaO2: Influencing factors, mechanistic modeling, and cooperative inactivation of water-borne microbial pathogens

Water polluted by pharmaceutically active compounds (PhACs) and water-borne pathogens urgently need to develop eco-friendly and advanced water treatment techniques. This paper evaluates the potential of using calcium peroxide (CaO₂), a safe and biocompatible oxidant both PhACs (thiamphenicol, florfenicol, carbamazepine, phenobarbital, and primidone) and pathogens (Escherichia coli, Staphylococcus aureus) in water. This paper evaluates the potential of using calcium peroxide (CaO₂) as a safe and biocompatible oxidant to remove both PhACs (thiamphenicol, florfenicol, carbamazepine, phenobarbital, and primidone) and pathogens (Escherichia coli, Staphylococcus aureus) in water. The increased CaO₂ dosage increased efficiencies of PhACs attenuation and pathogens inactivation, and both exhibited pseudo-first-order degradation kinetics (R² > 0.90). PhACs attenuation were mainly via oxidization (H₂O₂, •OH/O^•-, and O₂^•-) and alkaline hydrolysis (OH^-) from CaO₂. Moreover, concentrations of these reactive species and their contributions to PhACs attenuation were quantified, and mechanistic model was established and validated. Besides, possible transformation pathways of target PhACs except primidone were proposed. As for pathogen indicators, the suitable inactivation dosage of CaO₂ was 0.1 g L^-1. The oxidability (18-64%) and alkalinity (82-36%) generated from CaO₂ played vital roles in pathogen inactivation. In addition, CaO₂ at 0.01-0.1 g L^-1 can be applied in remediation of SW contaminated by PhACs and pathogenic bacteria, which can degrade target PhACs with efficiencies of 21-100% under 0.01 g L^-1 CaO₂, and inactivate 100% of test bacteria under 0.1 g L^-1 CaO₂. In short, capability of CaO₂ to remove target PhACs and microbial pathogens reveals its potential to be used as a representative technology for the advanced treatment of waters contaminated by organic compounds and microbial pathogens.

Removal of antibiotic thiamphenicol by bacterium Aeromonas hydrophila HS01

Thiamphenicol (TAP) is an amphenicol antibiotic, which has a broad-spectrum inhibitory effect on both gram-positive and gram-negative bacteria. Since it is widely used in animals and aquaculture, its residues in environment may bring potential risk for human health and ecosystems. While TAP can be removed through conventional physical or chemical methods, its bioremediation using microorganisms is less studied. Here, we report the removal of TAP by a bacterial strain, Aeromonas hydrophila HS01, which can remove more than 90.0% of TAP in a living cell-dependent manner. Our results indicated that its removal efficiency can be greatly affected by the growth condition. Proteomics studies revealed a number of differentially expressed proteins of HS01 in the presence of TAP, which may play critical roles in the transportation and degradation of TAP. All these results indicate bacterial strain A. hydrophila HS01 is a new microbial resource for efficiently removing TAP, and may shed new insights in developing bioremediation approaches for TAP pollution.

Effective electrocatalytic elimination of chloramphenicol: mechanism, degradation pathway, and toxicity assessment

The residual antibiotics in different environmental media pose a serious threat to human health and the ecosystem. The high-efficient elimination of antibiotics is one of the foremost works. In this study, chloramphenicol (CAP) was eliminated efficiently by electrocatalytic advanced oxidation process with carbon nanotubes/agarose/indium tin oxide (CNTs/AG/ITO) electrode. The influences of different experimental parameters on the degradation efficiency were systematically studied. Under the optimal conditions (4 V potential, 10 wt% CNTs dosage, and pH = 10), the maximum degradation efficiency of CAP (20 mg L-1) achieved 88% within 180 min. Besides, the electrocatalytic degradation pathway and mechanism for CAP were also investigated, •O2- played a major role in the process of electrocatalytic degradation. Based on the QSAR (quantitative structure-activity relationship) model, the toxicities of CAP and identified intermediates were analyzed. Compared with the parent compound, the maximal chronic toxicity of intermediate ((E)-3-(4-nitrophenyl)prop-1-ene-1,3-diol) for daphnid increased 197-fold. Besides, the hybrid toxicity of the degradation system was further confirmed via disk agar biocidal tests with Escherichia coli ATCC25922, which changed slightly during the degradation process. Based on the above results, it is worth noting that the degradation pathway and toxicity assessment should be paid more attention to the treatment of antibiotic wastewater.

Augmentation of chloramphenicol degradation by Geobacter-based biocatalysis and electric field

Electroactive microorganisms and electrochemical technologies have been separately used for environmental remediation such as antibiotics removal, yet the efficiency of coupling these two methods for chlorinated antibiotics removal is poorly known. Here we tested the synergy of Geobacter sulfurreducens PCA, an electroactive bacteria, and an electrical field, on chloramphenicol removal. Removal is increased two-fold by increasing the temperature from 30°C to 37°C. The cyclic voltammograms and chronoamperometry tests demonstrated that G. sulfurreducens PCA catalyzed chloramphenicol chemical reduction with electrode as excusive electron donor. A critical voltage, -0.6 to -0.5 V vs. Ag/AgCl, was discovered for chloramphenicol degradation with an increase of removal rate about 2.62-folds, from 31.06% to 81.41%. Combined removal with both G. sulfurreducens PCA and an electrical field increased the apparent rate constant and reached 82.77% removal at -0.5 V. Specially, the combined removal at -0.5 V even presented more robust removal efficiency compared to -0.6 V (78.64%) without G. sulfurreducens PCA. Mass spectrometry of degradation products indicates the reduction of nitro into amine groups, and dechlorination into less toxic compounds. Overall, combined biocatalysis and an electrical field is a promising method to remove antibiotics from polluted environments.

Electrocatalytic Dechlorination of Dichloromethane in Water Using a Heterogenized Molecular Copper Complex

The remediation of organohalides from water is a challenging process in environment protection and water treatment. Herein, we report a molecular copper(I) complex with two triazole units, CuT2, in a heterogeneous aqueous system that is capable of dechlorinating dichloromethane (CH₂Cl₂) to afford hydrocarbons (methane, ethane, and ethylene). The catalytic performance is evaluated in water and presented high Faradaic efficiency (average 70% CH₄) across a range of potentials (-1.1 to -1.6 V vs Ag/AgCl) and high activity (maximum -25.1 mA/cm² at -1.6 V vs Ag/AgCl) with a turnover number of 2.0 × 10⁷. The CuT2 catalyst also showed excellent stability for 14 h of constant exposure to CH₂Cl₂ and 10 h of CH₂Cl₂ exposure cycling. The control compound, a copper-free triazole unit (T1), was also investigated under the same condition and showed inferior catalytic activity, indicating the importance of the copper center. Plausible catalytic mechanisms are proposed for the formation of C₁ and C₂ products via radical intermediates. Computational studies provided additional insight into the reaction mechanism and the selectivity toward the CH₄ formation. The findings in this study demonstrate that complex CuT2 is an efficient and stable catalyst for the dehalogenation of CH₂Cl₂ and could potentially be used for the exploration of the removal of halogenated species from aqueous systems.

Facile Construction of a Copper-Containing Covalent Bond for Peroxymonosulfate Activation: Efficient Redox Behavior of Copper Species via Electron Transfer Regulation

Heterogeneous catalysis can be enhanced through the construction of effective atom connection for rapid electron transport on the catalyst surface. Hence, this study proposed a new strategy for electron transfer regulation to facilitate redox cycle of Cu(II)/Cu(I). The objective was achieved by successful construction of copper-containing covalent bond through the in situ growth of porous g-C₃N₄ with oxygen dopants and nitrogen defects (O-CN_D) on CuAl_xO_y substrate (CuAl@O-CN_D). On the basis of X-ray absorption fine structure (XAFS) and other characterization results, the facilitated redox behavior of copper species by electron transfer regulation was ascribed to the formation of a C-O-Cu bond on the porous-rich superficial of the catalyst; these covalent C-O-Cu bonds shortened the migration distance of electrons between Cu(II) and Cu(I) via Cu(I)-O-C-O-Cu(II) bridge. The construction of copper-containing covalent bonds in the catalyst resulted in efficient PMS activation for a rapid redox cycle of Cu(II)/Cu(I), triggering a series of reactions involving the continuous production of three highly active species (SO₄^·-, ^·OH and ¹O₂). The rapid diffusion and transportation of the generated active species from porous structures directly attack typical pharmaceutically active compounds (PhACs), achieving superior catalytic performance. This study provides a new routine to construct a C-O-Cu bond for PMS activation by regulating the electron transfer to accelerate the redox behavior of copper species for environmental remediation.

In-situ dosage of Fe2+ catalyst using natural pyrite for thiamphenicol mineralization by photoelectro-Fenton process

The degradation of the antibiotic thiamphenicol has been studied by photoelectro-Fenton (PEF) process with UVA light using pyrite particles as catalyst source. Pyrite is a sulfide mineral that naturally acidifies the reaction medium and releases Fe²⁺, thus promoting the effective generation of OH from Fenton's reaction. The assays were made in an IrO₂/air-diffusion cell, which yielded similar results to a boron-doped diamond (BDD)/air-diffusion one at a lower cost. In dark conditions, electro-Fenton (EF) process showed an analogous ability for drug removal, but mineralization was much poorer because of the large persistence of highly stable by-products. Their photolysis explained the higher performance of PEF. Conventional homogeneous PEF directly using dissolved Fe²⁺ exhibited a lower mineralization power. This suggests the occurrence of heterogeneous Fenton's reaction over the pyrite surface. The effect of current density and drug content on pyrite-catalyzed PEF performance was examined. The drug heteroatoms were gradually converted into SO₄^2-, Cl^- and NO₃^- ions. Nine aromatic derivatives and two dichloroaliphatic amines were identified by GC-MS, and five short-chain carboxylic acids were detected by ion-exclusion HPLC. A reaction route for thiamphenicol mineralization by PEF process with continuous H₂O₂ and Fe²⁺ supply on site is proposed.

Insights into degradation pathways and toxicity changes during electro-catalytic degradation of tetracycline hydrochloride

The removal of antibiotics has attracted much attention due to their extremely high adverse impacts on the environment. However, the potential risks of degradation intermediates are seldom reported. In this work, the influence of different factors on the electro-catalytic degradation efficiency of tetracycline hydrochloride (TCH) by the prepared carbon nanotubes/agarose/indium tin oxide (CNTs/AG/ITO) electrode was investigated. Under optimal conditions (10 wt% CNTs dosage, pH = 7), the maximum degradation efficiency for TCH (10 mg L^-1) reached up to 96% within 30 min treatment with 4 V potential. Superoxide anions (•O₂^-) played an important role in the electro-catalytic degradation. Totally 10 degradation intermediates were identified using HPLC-MS/MS, and the degradation pathway was proposed. Toxicities of the parent antibiotic and the identified intermediates were calculated using the ECOSAR (Ecological Structure Activity Relationship) program in EPISuite, and results showed that more toxic intermediates were generated. The maximal chronic toxicity for green algae of the intermediate increased 1439.92 times. Furthermore, antimicrobial activity was further verified by disk agar biocidal tests with Escherichia coli ATCC25922 and higher biotoxicity intermediates compared with parent compounds were confirmed to be formed. Therefore, more attention should be paid on the potential risk of degradation intermediates in the treatment of wastewater containing antibiotics.

Heterogeneous activation of peroxymonosulfate by a biochar-supported Co3O4 composite for efficient degradation of chloramphenicols

Herein, a new peroxymonosulfate (PMS) activation system was established using a biochar (BC)-supported Co₃O₄ composite (Co₃O₄-BC) as a catalyst to enhance chloramphenicols degradation. The effects of the amount of Co₃O₄ load on the BC, Co₃O₄-BC amount, PMS dose and solution pH on the degradation of chloramphenicol (CAP) were investigated. The results showed that the BC support could well disperse Co₃O₄ particles. The degradation of CAP (30 mg/L) was enhanced in the Co₃O₄-BC/PMS system with the apparent degradation rate constant increased to 5.1, 19.4 and 7.2 times of that in the Co₃O₄/PMS, BC/PMS and PMS-alone control systems, respectively. Nearly complete removal of CAP was achieved in the Co₃O₄-BC/PMS system under the optimum conditions of 10 wt% Co₃O₄ loading on BC, 0.2 g/L Co₃O₄-BC, 10 mM PMS and pH 7 within 10 min. The Co₃O₄/BC composites had a synergistic effect on the catalytic activity possibly because the conducting BC promoted electron transfer between the Co species and HSO₅^- and thus accelerated the Co³⁺/Co²⁺redox cycle. Additionally, over 85.0 ± 1.5% of CAP was still removed in the 10th run. Although both SO₄^- and OH were identified as the main active species, SO₄^- played a dominant role in CAP degradation. In addition, two other chloramphenicols, i.e., florfenicol (FF) and thiamphenicol (TAP), were also effectively degraded with percentages of 86.4 ± 1.3% and 71.8 ± 1.0%, respectively. This study provides a promising catalyst Co₃O₄-BC to activate PMS for efficient and persistent antibiotics degradation.

Highly efficient electro-generation of H2O2by a nitrogen porous carbon modified carbonaceous cathode during the oxygen reduction reaction

In this work, active carbon fibers (ACFs) were modified with nitrogen-doped porous carbon (NPC) and carbon nanotubes (CNTs) to prepare a novel modified electrode as a cathode.

Biodegradation and metabolic fate of thiamphenicol via Chlorella sp. UTEX1602 and L38

Thiamphenicol (TAP) is a typical medicament in animal husbandry and aquaculture for treating diverse infections. In this work, thiamphenicol biodegradation performance via microalgae was tested. The cultivation results showed that TAP could be biodegraded via the target algae. Chlorella sp. L38 presented strong adaptive ability to high concentration TAP. Biodegradation, biosorption and bioaccumulation were the dominant metabolic fates. Biodegradation contributed around 97% of the total removal efficiency at the TAP concentration of 46.2 mg·L^-1. The removal of TAP by Chlorella L38 and UTEX1602 agreed with the kinetic range of zero-order reaction, and the shortest half-lives were 3.2 d and 5.0 d. Based on the identification of metabolites, the metabolic pathway of TAP by microalgae was proposed, including chlorination, chlorine substitution, dehydration and hydroxylation. Therefore, biological treatment via microalgae has the potential for TAP purification.

Recent progress on carbon nanomaterials for the electrochemical detection and removal of environmental pollutants

Rapid global industrialization and explosive population growth have resulted in an increase in the discharge of harmful and toxic compounds. These toxic inorganic gases, volatile organic compounds, heavy metals, personal care products, endocrine-disrupting chemicals, dyes, and pharmaceuticals are destroying the balance in the Earth and increasing environmental toxicity at an alarming rate. Thus, their detection, adsorption and removal are of great significance. Various carbon nanomaterials including carbon nanotubes, graphene, mesoporous carbon, carbon dots, and boron-doped diamond have been extensively utilized and further proven to be ideal candidates for resolving environmental problems, emerging as adsorbents, electrochemical sensors and electrodes. Herein, we review the recent advances, progress and achievements in the design and properties of carbon nanomaterials and their applications for the electrochemical detection and removal of environmental pollutants.

Recent electrochemical methods in electrochemical degradation of halogenated organics: a review

Halogenated organics are widely used in modern industry, agriculture, and medicine, and their large-scale emissions have led to soil and water pollution. Electrochemical methods are attractive and promising techniques for wastewater treatment and have been developed for degradation of halogenated organic pollutants under mild conditions. Electrochemical techniques are classified according to main reaction pathways: (i) electrochemical reduction, in which cleavage of C-X (X = F, Cl, Br, I) bonds to release halide ions and produce non-halogenated and non-toxic organics and (ii) electrochemical oxidation, in which halogenated organics are degraded by electrogenerated oxidants. The electrode material is crucial to the degradation efficiency of an electrochemical process. Much research has therefore been devoted to developing appropriate electrode materials for practical applications. This paper reviews recent developments in electrode materials for electrochemical degradation of halogenated organics. And at the end of this paper, the characteristics of new combination methods, such as photocatalysis, nanofiltration, and the use of biochemical method, are discussed.

Biodegradability and mechanism of florfenicol via Chlorella sp. UTEX1602 and L38: Experimental study

In this work, florfenicol removal via two kinds Chlorella sp. (UTEX1602 and L38) was investigated. The experimental results indicated that FF could be removed by biodegradation associated with microalgae growth. Compared to Chlorella sp. UTEX1602, L38 had a good self-adjustment capacity at the condition of high initial FF concentration. The biodegradation of FF followed the first order kinetic model with half-lives ranged from 3.53 to 7.63 days at different initial concentration. The removal efficiency of FF could achieve 97% when the FF concentration was set at 46 mg·L^-1. While the FF concentration in the medium increased to 159 mg·L^-1, more than 74% FF could still be purified via Chlorella sp. L38. Therefore, Chlorella sp. L38 could be promising alternative algae to be used for FF removal from different water sources.

Ultrahigh yield of hydrogen peroxide and effective diclofenac degradation on a graphite felt cathode loaded with CNTs and carbon black: an electro-generation mechanism and a degradation pathway

A new graphite felt cathode loaded with carbon nanotubes and carbon black was developed.