Green electrochemical process for metronidazole degradation at BDD anode in aqueous solutions via direct and indirect oxidation

A comparative review on the mitigation of metronidazole residues in aqueous media using various physico-chemical technologies

In the last few decades, pharmaceuticals have emerged as a new class of serious environmental pollutants. The presence of these emerging contaminants even in minimal amounts (micro- to nanograms) has side effects, and they can cause chronic toxicity to health and the environment. Furthermore, the presence of pharmaceutical contaminants in water resources leads to significant antibiotic resistance in bacteria. Hence, the removal of antibiotics from water resources is essential. Thus far, a wide range of methods, including adsorption, photodegradation, oxidation, photolysis, micro-/nanofiltration, and reverse osmosis, has been used to remove pharmaceutical contaminants from water systems. In this article, research related to the processes for the removal of metronidazole antibiotics from water and wastewater, including adsorption (carbon nanotubes (CNTs), magnetic nanocomposites, magnetic molecularly imprinted polymer (MMIP), and metal-organic frameworks), filtration, advanced oxidation processes (photocatalytic process, electrochemical advanced oxidation processes, sonolysis and sonocatalysis) and aqueous two-phase systems (ATPSs), was reviewed. Results reveal that advanced oxidation processes, especially photocatalytic and sonolysis processes, have high potential in removing MNZ (more than 90%).

Simultaneous electrochemical removal of three microcystin congeners and sulfamethoxazole in natural water

Microcystins (MCs), frequently detected in freshwater ecosystems, have raised significant human health and ecological concerns. New approaches are being developed to control and remove MCs. In this study, we examined factors influencing the efficacy of electrochemical oxidation as a means of control. Anode material (Pt/Ti, Ta2O5-IrO2/Ti, SnO2-SbO2/Ti, boron-doped diamond (BDD/Si), anode surface area ratios and solution volumes, initial pollutant concentrations, and the co-existing antibiotic sulfamethoxazole (SMX) were investigated. MCs and SMX were dissolved in filtered Taihu Lake water to simulate the natural aquatic environment. The results showed that non-active anodes, lower initial concentration of MC, larger surface area ratio of cathode to anode, and smaller ratio of reaction solution volume to anode surface area could promote the degradation target pollutants. Under optimal conditions in this study, the degradation rates of MC-LR, MC-YR, MC-RR, and SMX each reached more than 90% within 6 h, and the removal efficiency of MC-YR was the highest among three congeners. The effect of SMX on the degradation of MC congeners depended mainly on their concentration differences, such that when the initial concentration of SMX was one to two orders of magnitude lower than microcystin, the presence of SMX would promote the degradation of MCs. In contrast, when the initial concentration of SMX was higher than that of microcystin by approximately an order of magnitude, sulfamethoxazole would inhibit the degradation of MCs by between 4.6% and 24.5%. Ultra-high-performance liquid chromatography tandem mass spectrometry analysis revealed that the three MC congeners were electrochemically degraded through aromatic ring oxidation, alkene oxidation, and bond cleavage on the ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid) side chain. Notably, the removal of MCs was accompanied by a decline in the hardness of the reaction water. This study provided insights into electrochemical degradation of microcystins and antibiotics in natural water, offering suggestions for its practical application.

Electrochemical oxidation of tetrahydrofurfuryl acohol on boron-doped diamond anode: Influence of current density and electrolyte solution

Tetrahydrofurfuryl alcohol (THFA), a widely applied raw materials, intermediate and solvent in the fields of agricultural, industry (especially in nuclear industry), is a potentially hazardous and non-biodegradable pollutant in wastewater. In this study, the electrochemical degradation pathways of THFA by a boron-doped diamond (BDD) anode with different current density (jappl = 20, 40 and 60 mA cm-2) and electrolyte solution (KNO3, KCl and K2SO4) was carefully investigated. The results exhibit that high chemical oxygen demand (COD) removal and mineralization rates were achieved by rapid non-selective oxidation in electrolyte solutions mediated by hydroxyl radicals (∙OH) and active chlorine (sulfate) under constant current electrolysis. In-depth data analysis using the high performance liquid chromatography and liquid chromatography/mass spectroscopy, the underlying removal pathways of THFA in KNO3, KCl and K2SO4 electrolyte solutions are proposed according to the effect of different mineralization mechanisms.

Bimetal-oxide (Fe/Co) modified bagasse-waste carbon coated on lead oxide-battery electrode for metronidazole removal

Current study covers the preparation and application of a commercial modified lead oxide battery electrode (LBE) in electrochemical oxidation (ECO) of metronidazole (MNZ) in an aqueous phase. Modified electrode is prepared by doping of bimetal-oxide (Fe and Zn) nanoparticles (NPs) & single metal-oxide (Fe/Zn) on bagasse-waste carbon (bwc) which is further coated on LBE. The modified LBE electrode surface was examined for metal-oxide NPs through X-ray diffraction analysis (XRD). Different electrodes are prepared by varying combinations of two metal-oxide based on molar ratio and tested for electrochemical characterization and MNZ removal test. Based on large oxygen evolution potential in a linear sweep volumetry (LSV) analysis and high MNZ removal rate, the best electrode has been represented as Fe1:Co2-bwc/LBE which contains Fe & Co molar ratio of 1:2. Moreover, equilibrium attained at faster rate in degradation process of MNZ, where pseudo first order kinetics of 2.29 × 10-2 min-1 was obtained under optimized condition of (MNZ:100 mg/L, pH:7, CD: 30 mA/cm2 and electrolyte: 0.05 M Na2SO4). Maximum MNZ removal, total organic carbon removal (TOC), mineralization current efficiency (MCE) & energy consumption (EC) of 98.7%, 85.3%, 62.2% & 96.143 kW h/kg-TOC removed are found in 180 min of treatment time for Fe1:Co2-bwc/LBE electrode. Accelerated service life test confirms that the stability of modified electrode is enhanced by 1.5 times compared to pristine LBE. Repeatability test confirms that modified LBE (Fe1:Co2-bwc/LBE) can be utilized up to 3 times.

An optimized electro-fenton pretreatment for the degradation and mineralization of a mixture of ofloxacin, norfloxacin, and ciprofloxacin

The electro-Fenton process (EFP) is a powerful advanced oxidation process beneficial to treating recalcitrant contaminants, and there has been a continuing interest in combining this technology to enhance the efficiency of conventional wastewater treatment processes. In this work, an optimized EFP process is performed as pretreatment for the degradation and mineralization of three blank fluoroquinolones (FQs) drugs: ofloxacin (OFL), norfloxacin (NOR), and ciprofloxacin (CIP). The optimization of the experiment was carried out using a Box-Behnken experimental design. Faster and complete degradation of the drugs mixture was achieved in 90 min with 61.12 ± 2.0% of mineralization in 180 min, under the optimized conditions: j = 244.0 mA cm-2, [Fe2+] = 0.31 mM, and [FQs] = 87.0 mg L-1. Furthermore, a low toxicity effluent was obtained in 90 min of the experiment, according to bioassay toxicity with Vibrio fischeri. Five short-chain carboxylic acids, including oxalic, maleic, oxamic, formic, and fumaric acids, were detected and quantified, in addition to F- and NO3- inorganic ions. The inhibition of the reactive oxygen species with scavenger proof was also evaluated in this paper.

Degradation of levofloxacin from antibiotic wastewater by pulse electrochemical oxidation with BDD electrode

Antibiotic-containing wastewater is a typical biochemical refractory organic wastewater and general treatment methods cannot effectively and quickly degrade the antibiotic molecules. In this study, a novel boron-doped diamond (BDD) pulse electrochemical oxidation (PEO) technology was proposed for the efficient removal of levofloxacin (LFXN) from wastewater. The effects of current density (j), initial pH (pH0), frequency (f), electrolyte types and initial concentration (c0(LFXN)) on the degradation of LFXN were systematically investigated. The degradation kinetics under four different processes have also been studied. The possible degradation mechanism of LFXN was proposed by Density functional theory calculation and analysis of degradation intermediates. The results showed that under the optimal parameters, the COD removal efficiency (η(COD)) was 94.4% and the energy consumption (EEC) was 81.43 kWh·m-3 at t = 120 min. The degradation of LFXN at pH = 2.8/c(H2O2) followed pseudo-first-order kinetics. The apparent rate constant was 1.33 × 10-2 min-1, which was much higher than other processes. The degradation rate of LFXN was as follows: pH = 2.8/c(H2O2) > pH = 2.8 > pH = 7/c(H2O2) > pH = 7. Ten aromatic intermediates were formed during the degradation of LFXN, which were further degraded to F-, NH4+, NO3-, CO2 and H2O. This study provides a promising approach for efficiently treating LFXN antibiotic wastewater by pulsed electrochemical oxidation with a BDD electrode without adding H2O2.

Copper Nanowire Networks: An Effective Electrochemical Peroxymonosulfate Activator toward Nitrogenous Pollutant Abatement

Herein, we developed an electrochemical filtration system for effective and selective abatement of nitrogenous organic pollutants via peroxymonosulfate (PMS) activation. Highly conductive and porous copper nanowire (CuNW) networks were constructed to serve simultaneously as catalyst, electrode, and filtration media. In one demonstration of the CuNW network's capability, a single pass through a CuNW filter (τ < 2 s) degraded 94.8% of sulfamethoxazole (SMX) at an applied potential of -0.4 V vs SHE. The exposed {111} crystal plane of CuNW triggered atomic hydrogen (H*) generation on sites, which contributed to effective PMS reduction. Meanwhile, with the involvement of SMX, a Cu-N bond was formed by the interactions between the -NH₂ group of SMX and the Cu sites of CuNW, accompanied by the redox cycling of Cu²⁺/Cu⁺, which was facilitated by the applied potential. The different charges of the active Cu sites made it easier to withdraw electrons and promote PMS oxidation. Theoretical calculations and experimental results were combined to suggest a mechanism for pollution abatement with CuNW networks. The results showed that system efficacy for the degradation of a wide array of nitrogenous pollutants was robust across a broad range of solution pH and complex aqueous matrices. The flow-through operation of the CuNW filter outperformed conventional batch electrochemistry due to convection-enhanced mass transport. This study provides a new strategy for environmental remediation by integrating state-of-the-art material science, advanced oxidation processes, and microfiltration technology.

Preparation of Porous Ti/RuO2-IrO2@Pt, Ti/RuO2-TiO2@Pt and Ti/Y2O3-RuO2-TiO2@Pt Anodes for Efficient Electrocatalytic Decomposition of Tetracycline

Electrocatalytic oxidation (ECO) has attracted attention because of its high efficiency and environmental friendliness in water treatment. The preparation of anodes with high catalytic activity and long service lifetimes is a core part of electrocatalytic oxidation technology. Here, porous Ti/RuO₂-IrO₂@Pt, Ti/RuO₂-TiO₂@Pt, and Ti/Y₂O₃-RuO₂-TiO₂@Pt anodes were fabricated by means of modified micro-emulsion and vacuum impregnation methods with high porosity titanium plates as substrates. The scanning electron microscopy (SEM) images showed that RuO₂-IrO₂@Pt, RuO₂-TiO₂@Pt, and Y₂O₃-RuO₂-TiO₂@Pt nanoparticles were coated on the inner surface of the as-prepared anodes to form the active layer. Electrochemical analysis revealed that the high porosity substrate could result in a large electrochemically active area, and a long service life (60 h at 2 A cm^-2 current density, 1 mol L^-1 H₂SO₄ as the electrolyte, and 40 °C). The degradation experiments conducted on tetracycline hydrochloride (TC) showed that the porous Ti/Y₂O₃-RuO₂-TiO₂@Pt had the highest degradation efficiency for tetracycline, reaching 100% removal in 10 min with the lowest energy consumption of 167 kWh kg^-1 TOC. The reaction was consistent with the pseudo-primary kinetics results with a k value of 0.5480 mol L^-1 s^-1, which was 16 times higher than that of the commercial Ti/RuO₂-IrO₂ electrode. The fluorospectrophotometry studies verified that the degradation and mineralization of tetracycline were mainly ascribed to the •OH generated in the electrocatalytic oxidation process. This study thus presents a series of alternative anodes for future industrial wastewater treatment.

Wastewater treatment by anodic oxidation in electrochemical advanced oxidation process: Advance in mechanism, direct and indirect oxidation detection methods

Electrochemical Advanced Oxidation Process (EAOP) has been applied to the degradation of refractory pollutants in wastewater due to its strong oxidation capacity, high degradation efficiency, simple operation, and mild reaction. Among electrochemical processes, anodic oxidation (AO) is the most widely used and its mechanism is mainly divided into direct oxidation and indirect oxidation. Direct oxidation means that pollutants are oxidized at the anode by direct electron transfer. Indirect oxidation refers to the generation of active species during the electrolytic reaction, which acts on pollutants. The mechanism of AO process is controlled by many factors, including electrode type, electrocatalyst material, wastewater composition, pH, applied current and voltage levels. It is very important to explore the reaction mechanism of electrochemical treatment, which determines the efficiency of the reaction, the products of the reaction, and the extent of reaction. This paper firstly reviews the current research progress on the mechanism of AO process, and summarizes in detail the different mechanisms caused by influencing factors under common AO process. Then, strategies and methods to distinguish direct oxidation and indirect oxidation mechanisms are reviewed, such as intermediate product analysis, electrochemical test analysis, active species detection, theoretical calculation, and the limitations of these methods are analyzed. Finally some suggestions are put forward for the study of the mechanism of electrochemical advanced oxidation.

Ti/PbO2 Electrode Efficiency in Catalytic Chloramphenicol Degradation and Its Effect on Antibiotic Resistance Genes

Livestock farming has led to the rapid accumulation of antibiotic resistance genes in the environment. Chloramphenicol (CAP) was chosen as a model compound to investigate its degradation during electrochemical treatment. Ti/PbO₂ electrodes were prepared using electrodeposition. The prepared Ti/PbO₂-La electrodes had a denser surface and a more complete PbO₂ crystal structure. Ti/PbO₂-Co electrodes exhibited improved electrochemical catalytic activity and lifetime in practice. The impact of different conditions on the effectiveness of CAP electrochemical degradation was investigated, and the most favorable conditions were identified (current density: I = 15.0 mA/cm, electrolyte concentration: c = 0.125 mol/L, solution pH = 5). Most importantly, we investigated the effects of the different stages of treatment with CAP solutions on the abundance of resistance genes in natural river substrates (intI1, cmlA, cmle3, and cata2). When CAP was completely degraded (100% TOC removal), no effect on resistance gene abundance was observed in the river substrate; incomplete CAP degradation significantly increased the absolute abundance of resistance genes. This suggests that when treating solutions with antibiotics, they must be completely degraded (100% TOC removal) before discharge into the environment to reduce secondary pollution. This study provides insights into the deep treatment of wastewater containing antibiotics and assesses the environmental impact of the resulting treated wastewater.

Efficient eradication of antibiotic and dye by C-dots@zeolite nanocomposites: Performance evaluation, and degraded products analysis

Metronidazole (MET), a recalcitrant antibiotic from the nitro-imidazole family and commercially used Rhodamine B (RhB) dye, contributes a huge to water pollution, which needs to eliminate, preferably by photocatalytic degradation technique. The Cdots@zeolite (CDZ) nanocomposites with different weight ratios (1:1, 1:3, 1:5, 5:1, 1:7) were synthesized hydrothermally to degrade MET and RhB molecules. The CDZ composites were characterized by XRD, BET, EDS, and XPS technique which verifies the crystalline nature, incorporation of C-dots into zeolite frameworks with high surface area (∼187 m²/g). The morphology, d-spacing and lattice planes were analyzed by SEM images, HR-TEM and SAED analysis. The maximum degradation (∼79%) was achieved at an optimum catalyst dose of 0.2 g/L and pH 4 for MET and that of RhB was ∼90% at a catalyst dose of 0.4 g/L. The PZC (point of zero charge) value for CDZ composite was about pH 3.4, which justifies the maximum removal of MET at pH 4. The obtained rate constants 'k' were found to be 0.0081, 0.0041, and 0.0101 min^-1 in sun, UV, and visible light sources, respectively. The real industrial wastewater sample has been treated to give ∼68% of COD and ∼62% TOC removal. Moreover, the intermediates of plausible degradation pathways were identified by the m/z values obtained from GC-MS analysis.

Electro-oxidation of Ni (II)-citrate complexes at BDD electrode and simultaneous recovery of metallic nickel by electrodeposition

The simultaneous electro-oxidation of Ni (II)-citrate and electrodeposition recovery of nickel metal were attempted in a combined electro-oxidation-electrodeposition reactor with a boron-doped diamond (BDD) anode and a polished titanium cathode. Effects of initial nickel citrate concentration, current density, initial pH, electrode spacing, electrolyte type, and initial electrolyte dosage on electrochemical performance were examined. The efficiencies of Ni (II)-citrate removal and nickel metal recovery were determined to be 100% and over 72%, respectively, under the optimized conditions (10 mA/cm², pH 4.09, 80 mmol/L Na₂SO₄, initial Ni (II)-citrate concentration of 75 mg/L, electrode spacing of 1 cm, and 180 min of electrolysis). Energy consumption increased with increased current density, and the energy consumption was 0.032 kWh/L at a current density of 10 mA/cm² (pH 6.58). The deposits at the cathode were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These characterization results indicated that the purity of metallic nickel in cathodic deposition was over 95%. The electrochemical system exhibited a prospective approach to oxidize metal complexes and recover metallic nickel.

Fluidized ZnO@BCFPs Particle Electrodes for Efficient Degradation and Detoxification of Metronidazole in 3D Electro-Peroxone Process

A novel material of self-shaped ZnO-embedded biomass carbon foam pellets (ZnO@BCFPs) was successfully synthesized and used as fluidized particle electrodes in three-dimensional (3D) electro-peroxone systems for metronidazole degradation. Compared with 3D and 2D + O₃ systems, the energy consumption was greatly reduced and the removal efficiencies of metronidazole were improved in the 3D + O₃ system. The degradation rate constants increased from 0.0369 min^-1 and 0.0337 min^-1 to 0.0553 min^-1, respectively. The removal efficiencies of metronidazole and total organic carbon reached 100% and 50.5% within 60 min under optimal conditions. It indicated that adding ZnO@BCFPs particle electrodes was beneficial to simultaneous adsorption and degradation of metronidazole due to improving mass transfer of metronidazole and forming numerous tiny electrolytic cells. In addition, the process of metronidazole degradation in 3D electro-peroxone systems involved hydroxyethyl cleavage, hydroxylation, nitro-reduction, N-denitrification and ring-opening. The active species of ·OH and ·O₂^- played an important role. Furthermore, the acute toxicity LD₅₀ and the bioconcentration factor of intermediate products decreased with the increasing reaction time.

Simultaneous electrochemical removal of Microcystis aeruginosa and sulfamethoxazole and its ecologic impacts on Vallisneria spiralis

In this study, the simultaneous removal effects of electrochemical oxidation with boron-doped diamond anodes at different current densities were tested on Microcystis aeruginosa and sulfamethoxazole. Flow cytometry and non-invasive micro-test technology were applied to study the physiological states of M. aeruginosa and Vallisneria spiralis leaf cells. As the current density increased, the degradation effect of electrochemical oxidation on sulfamethoxazole and microcystin-LR increased and exceeded 60% within 6 h. In addition, population density of M. aeruginosa, fluorescence response of chlorophyll a, and cytoplasmic membrane integrity decreased, whereas the proportion of cells with excessive accumulation of intracellular reactive oxygen species (ROS) increased. The effect of electrochemical oxidation on the cell population of M. aeruginosa continued after the power was turned off. The physiological state of V. spiralis leaf cells was not severely affected at 10 mA/cm² for 24 h. Higher current intensity and longer electrolysis time would induce apoptosis or necrosis. In order to achieve a higher target pollutant removal effect and simultaneously avoid damage to the lake ecosystem, the current intensity of the electrochemical oxidation device should not exceed 10 mA/cm², and a single electrolysis treatment should range from 6 h to 24 h.

Iron Phosphide Precatalyst for Electrocatalytic Degradation of Rhodamine B Dye and Removal of Escherichia coli from Simulated Wastewater

Electrocatalysis using low-cost materials is a promising, economical strategy for remediation of water contaminated with organic chemicals and microorganisms. Here, we report the use of iron phosphide (Fe2P) precatalyst for electrocatalytic water oxidation; degradation of a representative aromatic hydrocarbon, the dye rhodamine B (RhB); and inactivation of Escherichia coli (E. coli) bacteria. It was found that during anodic oxidation, the Fe2P phase was converted to iron phosphate phase (Fe2P-iron phosphate). This is the first report that Fe2P precatalyst can efficiently catalyze electrooxidation of an organic molecule and inactivate microorganisms in aqueous media. Using a thin film of Fe2P precatalyst, we achieved 98% RhB degradation efficiency and 100% E. coli inactivation under an applied bias of 2.0 V vs. reversible hydrogen electrode in the presence of in situ generated reactive chlorine species. Recycling test revealed that Fe2P precatalyst exhibits excellent activity and reproducibility during degradation of RhB. High-performance liquid chromatography with UV-Vis detection further confirmed the electrocatalytic (EC) degradation of the dye. Finally, in tests using Lepidium sativum L., EC-treated RhB solutions showed significantly diminished phytotoxicity when compared to untreated RhB. These findings suggest that Fe2P-iron phosphate electrocatalyst could be an effective water remediation agent.

Comparative electrochemical oxidation of the secondary effluent of petrochemical wastewater with electro-Fenton and anodic oxidation with supporting electrolytes

Electro-Fenton (EF) oxidation has high oxidation abilities and is widely used in the treatment of biorefractory and chemically refractory organic wastewater. However, it generates a large amount of iron sludge, which limits large-scale application. In this work, the comparative study of EF oxidation and anodic oxidation (AO) of the secondary effluent of petrochemical wastewater using boron doped diamond anode is carried out. In EF oxidation, the effects of Fe²⁺ concentration, pH value, and current density are investigated. The optimal conditions consist of the following: Fe²⁺ concentration of 1.5 mmol·L^-1, pH of 4, and current density of 10 mA·cm^-2. In AO process, the effect of adding SO42-, Cl^-, NO3-, PO43-, and CO32- is investigated; the optimal conditions can be obtained by adding a Na₂SO₄ solution (0.075 mol·L^-1). When compared with AO, although EF oxidation has a higher treatment efficiency, its energy consumption is higher, and the generated effluent (with 155 g of iron sludge·m^-3) dramatically increases the post-treatment cost, thereby limiting its large-scale application. For AO with Na₂SO₄ solution (0.075 mol·L^-1) and a COD removal efficiency of 70%, the corresponding treatment time is 1.34 h and the energy consumption is 2.44 kWh·m^-3.

Degradation of antibiotics, organic matters and ammonia during secondary wastewater treatment using boron-doped diamond electro-oxidation combined with ceramic ultrafiltration

In this study, a BDD electrolytic oxidation-ceramic membrane ultrafiltration (EO-CM) system for the removals of antibiotics, organic matters and ammonia in wastewater was evaluated. Sulfamethazine (SMZ) was degraded following a pseudo first-order kinetics. The removal rate of SMZ improved with the increase of electro-oxidation time (0-60 min) and current density (5-30 mA/cm²). During the BDD electro-oxidation process, H₂O₂ and hydroxyl radicals (•OH) were generated which were detected by N, N-diethyl-p-phenylenediamine (DPD) method and electron paramagnetic resonance spectroscopy (EPR), respectively. Chemical oxygen demand (COD) was able to be removed by EO and CM processes, in which proteins and humic acids were regarded as the main removed components measured using excitation-emission matrix (EEM) technique. Moreover, BDD electro-oxidation pretreatment could make the CM process maintain a high water flux and significantly control the membrane fouling and relieve transmembrane pollution. In addition, the removal of ammonia was enhanced with the increase of chloride ions (Cl^-) in wastewater during EO process due to the generation of active chlorine (i.e., ClO^-, HClO, or Cl₂) from the oxidation of Cl^-. Chloramine and nitrogen were produced in the oxidation of ammonia by active chlorine. Overall, the results of this study suggest that BDD EO-CM system is a promising process for removing antibiotics, organic matters and ammonia.

Heterogeneous Photocatalysis of Metronidazole in Aquatic Samples

Metronidazole (MET) is a commonly detected contaminant in the environment. The compound is classified as poorly biodegradable and highly soluble in water. Heterogeneous photocatalysis is the most promoted water purification method due to the possibility of using sunlight and small amounts of a catalyst needed for the process. The aim of this study was to select conditions for photocatalytic removal of metronidazole from aquatic samples. The effect of catalyst type, mass, and irradiance intensity on the efficiency of metronidazole removal was determined. For this purpose, TiO₂, ZnO, ZrO₂, WO₃, PbS, and their mixtures in a mass ratio of 1:1 were used. In this study, the transformation products formed were identified, and the mineralization degree of compound was determined. The efficiency of metronidazole removal depending on the type of catalyst was in the range of 50-95%. The highest MET conversion (95%) combined with a high degree of mineralization (70.3%) was obtained by using a mixture of 12.5 g TiO₂-P25 + PbS (1:1; v/v) and running the process for 60 min at an irradiance of 1000 W m^-2. Four MET degradation products were identified by untargeted analysis, formed by the rearrangement of the metronidazole and the C-C bond breaking.

Co/La modified Ti/PbO2 anodes for chloramphenicol degradation: Catalytic performance and reaction mechanism

Chloramphenicol (CAP) is widely used in daily life, and its abuse hurts human health, so a suitable method is needed to solve the problem. In this study, the Ti/PbO₂ electrodes prepared by the electroplating method were characterized. The CAP degradation effect and mechanism were investigated. It was shown that the electrode surface had a dense plating with a characteristic peak of β-PbO₂ as the active component. The electrode had an oxygen precipitation potential of 1.695 V and a corrosion potential of 0.553 V, and a long service life (505.4 d). The degradation of CAP at Ti/PbO₂ electrode followed a first-order kinetic reaction. The optimal degradation conditions (current density of 12.97 mA cm^-2, electrolyte concentration of 50 mM, and solution pH of 6.38) were obtained by the response surface curve method. The degradation rate of CAP was 99.0% at 60 min. The results showed that the reactive groups leading to CAP degradation were mainly ·OH and SO₄^2-, and only a tiny portion of CAP was directly oxidized on the electrode surface. The addition of Cl^- favored the degradation of CAP, but reduced the mineralization rate. LC-MS analysis showed that ·OH mainly attacked the asymmetric centers (C1, C2) of weakly bound hydrogen atoms, resulting in underwent addition and substitution reactions. CAP was converted into two substances with m/z = 306 and m/z = 165. Finally, inorganic substances such as CO₂ and H₂O were generated. This study provided a new idea for preparing Ti/PbO₂ electrode with high performance and the safe and efficient degradation of CAP.

Reusable Ag@TiO2-Based Photocatalytic Nanocomposite Membranes for Solar Degradation of Contaminants of Emerging Concern

Two significant limitations of using TiO2 nanoparticles for water treatment applications are reduced photocatalytic activity under visible radiation and difficulty recovering the particles after use. In this study, round-shaped Ag@TiO2 nanocomposites with a ≈21 nm diameter and a bandgap energy of 2.8 eV were synthesised by a deposition-precipitation method. These nanocomposites were immobilised into a porous poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) matrix and well-distributed within the pores. The photocatalytic activity of Ag@TiO2/PVDF-HFP against metronidazole (MNZ) under solar radiation was evaluated. Further, an adaptive neuro-fuzzy inference system (ANFIS) was applied to predict the effect of four independent variables, including initial pollutant concentration, pH, light irradiation intensity, and reaction time, on the photocatalytic performance of the composite membrane on MNZ degradation. The 10% Ag@TiO2/PVDF-HFP composite membrane showed a maximum removal efficiency of 100% after 5 h under solar radiation. After three use cycles, this efficiency remained practically constant, demonstrating the membranes' reusability and suitability for water remediation applications.

Synergetic metronidazole removal from aqueous solutions using combination of electro-persulfate process with magnetic Fe3O4@AC nanocomposites: nonlinear fitting of isotherms and kinetic models

The removal of metronidazole (MNZ) from aqueous solutions by the electro-persulfate (EC–PS) process was performed in combination with magnetic Fe3O4@activated carbon (AC) nanocomposite. In the first step, the Fe3O4@AC nanocomposites were synthesized and characterized using energy-dispersive X-ray spectroscopy (XRD), vibrating-sample magnetometer (VSM) and field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), mapping, and Fourier-transform infrared spectroscopy (FTIR) analysis. The effect of Fe3O4@AC, PS and EC processes were studied separately and in combination and finally, the appropriate process for MNZ removal was selected. The effect of key parameters on the EC–Fe3O4@AC–PS process including pH, Fe3O4@AC dosage, initial MNZ concentration, and PS concentration were investigated. Based on the results obtained, the Fe3O4@AC had a good structure. The MNZ removal in EC, PS, Fe3O4@AC, EC–Fe3O4@AC, EC–PS, EC–Fe3O4@AC–NaCl, EC–Fe3O4@AC–PS, and EC–Fe3O4@AC–PS–NaCl processes were 0, 0, 59.68, 62, 68.94, 67.71, 87.23 and 88%, respectively. Due to the low effect of NaCl insertion on the EC–Fe3O4@AC–PS process, it was not added into the reactor and optimum conditions for the EC–Fe3O4@AC–PS process were determined. Under ideal conditions, including MNZ = 40 mg/L, Fe3O4@AC dose = 1 g/L, pH = 3, PS concentration = 1.68 mM, current density (CD) = 0.6 mA/cm2 and time = 80 min, the MNZ removal was 92%. Kinetic study showed that the pseudo-second-order model was compatible with the obtained results. In the isotherm studies, the Langmuir model was the most consistent for the data of the present study, and the Q max for Fe3O4@AC dose from 0.25 to 1 g/L was 332 to 125 mg/g, respectively.

Kinetics of the Organic Compounds and Ammonium Nitrogen Electrochemical Oxidation in Landfill Leachates at Boron-Doped Diamond Anodes

Electrochemical oxidation (EO) of organic compounds and ammonium in the complex matrix of landfill leachates (LLs) was investigated using three different boron-doped diamond electrodes produced on silicon substrate (BDD/Si)(levels of boron doping [B]/[C] = 500, 10,000, and 15,000 ppm—0.5 k; 10 k, and 15 k, respectively) during 8-h tests. The LLs were collected from an old landfill in the Pomerania region (Northern Poland) and were characterized by a high concentration of N-NH₄⁺ (2069 ± 103 mg·L⁻¹), chemical oxygen demand (COD) (3608 ± 123 mg·L⁻¹), high salinity (2690 ± 70 mg Cl⁻·L⁻¹, 1353 ± 70 mg SO₄²⁻·L⁻¹), and poor biodegradability. The experiments revealed that electrochemical oxidation of LLs using BDD 0.5 k and current density (j) = 100 mA·cm⁻² was the most effective amongst those tested (C_8h/C₀: COD = 0.09 ± 0.14 mg·L⁻¹, N-NH₄⁺ = 0.39 ± 0.05 mg·L⁻¹). COD removal fits the model of pseudo-first-order reactions and N-NH₄⁺ removal in most cases follows second-order kinetics. The double increase in biodegradability index—to 0.22 ± 0.05 (BDD 0.5 k, j = 50 mA·cm⁻²) shows the potential application of EO prior biological treatment. Despite EO still being an energy consuming process, optimum conditions (COD removal > 70%) might be achieved after 4 h of treatment with an energy consumption of 200 kW·m⁻³ (BDD 0.5 k, j = 100 mA·cm⁻²).

Anodic oxidation of ciprofloxacin using different graphite felt anodes: Kinetics and degradation pathways

Ciprofloxacin (CIP) is ubiquitous in the environment which poses a certain threat to human and ecology. In this investigation, the physical and electrochemical properties of graphite felt (GF) anodes which affected the anodic oxidation (AO) performance, and the CIP removal effect of GF were evaluated. The GFs were used as anodes for detection of ·OH with coumarin (COU) as molecule probe and removal of CIP in a 150 mL electrolytic cell with Pt cathode (AO-GF/Pt system). The results showed that hydrophilic GF (B-GF) owned higher sp³/sp² and more oxygen-containing and nitrogen-containing functional groups than the hydrophobic GF (A-GF). Moreover, B-GF possessed higher oxygen evolution potential (1.12 V), more active sites and stronger ·OH generation capacity. Above mentioned caused that B-GF exhibited more superior properties for CIP removal. The best efficiencies (96.95%, 99.83%) were obtained in the AO-B-GF/Pt system at 6.25 mAcm^-2 after 10 min (k₁, 0.356 min^-1) and 60 min (k₂, 0.224 min^-1), respectively. Furthermore, nine degradation pathways of CIP in AO-B-GF/Pt system were summarized as the cleavage of the piperazine ring, cyclopropyl group, quinolone ring and F atom by ·OH. It provides new insights into the removal and degradation pathways of CIP with GF in AO system.

Electrochemical oxidative degradation of X-6G dye by boron-doped diamond anodes: Effect of operating parameters

Boron-doped diamond (BDD) is an excellent electrode material. As the anode in an electrochemical degradation tank, BDD has been receiving widespread attention for the treatment of azo dye wastewater. In this study, electrochemical oxidation (EO) was applied to electrolyze reactive brilliant yellow X-6G (X-6G) using BDD as the anode and Pt as the cathode. To balance the degradative effects and power consumption in the electrolysis process, the effects of a series of operating parameters, including current density, supporting electrolyte, initial pH, reaction temperature and initial dye concentration, were systematically studied. The oxidative process was analyzed by color removal rate, and the degree of mineralization was evaluated by TOC. The optimal experimental parameters were finally determined: 100 mA cm^-2, 0.05 M Na₂SO₄ electrolyte, pH 3.03, 60 °C, and an initial X-6G concentration of 100 mg L^-1. As a result, color completely disappeared after 0.75 h of electrolysis, and TOC was removed by 72.8% after 2 h of electrolysis. In conclusion, the EO of a BDD electrode as an anode can be a potent treatment method for X-6G synthetic wastewater.

Differentiation Between Metronidazole Residues Disposal by Using Adsorption and Photodegradation Processes Onto MgO Nanoparticles

Background

Metronidazole (MNZ) is an antibiotic form that is considered as a dangerous environmental pollutant due to its widespread use as growth promoters in livestock and aquaculture operations along with its therapeutic application for humans.

Purpose

The objective of this work was to conduct a comparative study between the efficiency of the adsorption and photocatalytic degradation of MNZ in an aqueous solution by using magnesium oxide nanoparticles (MgO NP) under different effects, as well as evaluate the performance, reusability and cost study.

Materials and Methods

Several instruments such as XRD, EDX, SEM, and TEM were used to characterize the chemical composition and morphological properties of the synthesized MgO NP, while the GC-MS analysis was used to monitor the degradation pathway of MNZ particles within 180 min. The simple photo-batch reactor was used to investigate the degradation of MNZ under the effect of UV radiation, initial concentration of MNZ, pH, catalyst loading, inorganic salts addition, time, and temperature.

Results

The degradation efficiency is mainly divided into two steps: 35.7% for maximum adsorption and 57.5% for photodegradation. Adsorption isotherm models confirmed that the process nature is chemisorption and appropriate Langmuir model, as well as to be a nonspontaneous and endothermic reaction according to the thermodynamic study. Adsorption constant during dark condition is smaller than typical adsorption equilibrium constant derived from the Langmuir-Hinshelwood kinetic model through photodegradation of MNZ that follows pseudo-first-order kinetics. Toxicity rates were reduced considerably after the photodegradation process to 88.21%, 79.84%, and 67.32% and 57.45%, 51.98%, and 43.87% by heamolytic and brine shrimp assays, respectively, for initial MNZ concentrations (20, 60, and 100 mg/L).

Conclusion

We significantly recommend using MgO NP as a promising catalyst in the photodegradation applications for other organic pollutants in visible light.

Comparison of Ag and AgI-Modified ZnO as Heterogeneous Photocatalysts for Simulated Sunlight Driven Photodegradation of Metronidazole

Ag and AgI-modified ZnO composites (Ag/ZnO and AgI/ZnO) were synthesized in facile ways. The photocatalysts were used for the photodegradation of metronidazole (MNZ) under the irradiation of simulated sunlight. The results of experiments showed that both Ag/ZnO and AgI/ZnO had a specific molar ratio to reach the best performance. Ag/ZnO performed better in the photodegradation of MNZ than AgI/ZnO under the same conditions. The reaction rate constant of AgI/ZnO was less affected by the variation of initial concentration of MNZ or pH values. The main reactive oxygen species of the photocatalytic process are OH, O2− and h+, but the free radicals which play the most critical part differ in Ag/ZnO and AgI/ZnO. Several intermediates were revealed by LC–MS/MS analysis. The stability of the photocatalysts was evaluated by a series of repeated MNZ photodegradation experiments. The results showed that AgI/ZnO had better stability than Ag/ZnO.

Advanced electrochemical treatment of real biotreated petrochemical wastewater by boron doped diamond anode: performance, kinetics, and degradation mechanism

Petrochemical wastewater is difficult to process because of various types of pollutants with high toxicity. With the improvement in the national discharge standard, traditional biochemical treatment methods may not meet the standards and further advanced treatment techniques would be required. In this study, electrochemical oxidation with boron doped diamond (BDD) anode as post-treatment was carried out for the treatment of real biotreated petrochemical wastewater. The effects of current density, pH value, agitation rate, and anode materials on chemical oxygen demand (COD) removal and current efficiency were studied. The results revealed the appropriate conditions to be a current density of 10 mA·cm^-2, a pH value of 3, and an agitation rate of 400 rpm. Moreover, as compared with the graphite electrode, the BDD electrode had a higher oxidation efficiency and COD removal efficiency. Furthermore, GC-MS was used to analyze the final degradation products, in which ammonium chloride, formic acid, acetic acid, and malonic acid were detected. Finally, the energy consumption was estimated to be 6.24 kWh·m^-3 with a final COD of 30.2 mg·L^-1 at a current density of 10 mA·cm^-2 without the addition of extra substances. This study provides an alternative for the upgrading of petrochemical wastewater treatment plants.

Removal of 5-fluorouracil by solar-driven photoelectrocatalytic oxidation using Ti/TiO2(NT) photoelectrodes

The efficient and safe degradation of drugs present in wastewater requires the design of a new material possessing high activity for that process. In addition to other methods, photoelectrocatalysis (PEC) merges the strengths of both photocatalytic and electrochemical methods, and the efficiency could be enhanced by the type of photoelectrode material. To address this challenge, three Ti/TiO₂ nanotube-based photoelectrodes, differing in their tube morphology, were prepared by anodic oxidation and employed for the degradation of the 5-fluorouracil (5-FU) drug by the PEC process. The highest efficiency for 5-fluorouracil (5-FU) degradation by PEC was observed for the photoelectrode with a 1.7 μm length, 65 nm diameter and 8 nm wall thickness of TiO₂ nanotubes, which were prepared by Ti foil anodization at 30 V. The effects of applied potential, irradiation intensity, initial pH and 5-FU concentration on PEC were investigated. Furthermore, our findings showed that the mechanism of photoelectrocatalysis in the presence of TiO₂ nanotubes is based on ∙OH and h⁺ activity. To determine the 5-FU degradation pathway, the organic byproducts were identified by LC-MS analysis. Furthermore, the ecotoxicity evaluated during PEC dropped with decreasing 5-FU concentration.

Effect of electrolytes on the simultaneous electrochemical oxidation of sulfamethoxazole, propranolol and carbamazepine: behaviors, by-products and acute toxicity

In this work, the effect of supporting electrolytes on the simultaneous electrochemical oxidation of the pharmaceuticals sulfamethoxazole (SMX), propranolol (PRO), and carbamazepine (CBZ) in aqueous solutions has been studied. Based on the identified by-products, the degradation mechanisms were proposed and the acute toxicity was evaluated for each electrolyte. Assays were carried out in batch mode in a 2 L undivided reactor using a niobium coated with boron-doped diamond (Nb/BDD) mesh anode and Ti cathode at 2.5 A in presence of different supporting electrolytes (Na₂SO₄, NaCl, or NaBr) at the same concentration of 7 mM. The degradation rates were higher in the assays with NaCl and NaBr. Reaction by-products were identified by gas chromatography-mass spectrometry. Indirect oxidation by electrogenerated reactive halogen species (RHS) was the main mechanism when halide ions were used as electrolytes. Ten by-products were detected using Na₂SO₄ as electrolyte, while 19 (12 non-halogenated and 7 halogenated) and 20 (10 non-halogenated and 10 halogenated) using NaCl and NaBr respectively. The proposed degradation pathways involve transformation (hydroxylation, deamination, desulfonation, and halogenation) and bond rupture to produce less molecular weight compounds and their further transformation until total degradation. Chlorinated and brominated by-products confirm halogenation reactions. The electrogenerated RHS presented a significant inhibition effect on Vibrio fischeri; nevertheless, acute toxicity was not presented using Na₂SO₄ as electrolyte and a pharmaceutical concentration of 5 μg/L. In this view, the role of the supporting electrolyte in electrochemical oxidation process is crucial since it strongly influence degradation rate, by-products, and acute toxicity.

Pre-treatment of soft drink wastewater with a calcium-modified zeolite to improve electrooxidation of organic matter

Wastewater from soft drink manufacturing, having a high organic load (chemical oxygen demand (COD) = 4,500 mg L^-1) and high alkalinity (2,653.7 mg L^-1 CaCO₃; pH 12), was pretreated with a calcium-modified zeolite to reduce the alkalinity and improve the electrooxidation of organic matter. The natural zeolite clinoptilolite was modified in various ways with Ca(OH)₂ and CaCl₂. The CaCl₂-modified zeolite (ZSACaCl-72h) was more effective for the treatment of soft drink wastewater than the congener modified with Ca(OH)₂, where the former reduced the alkalinity by 86% after 8 h. Electrooxidation of soft drink wastewater without zeolite pre-treatment was carried out with boron-doped diamond (BDD) electrodes under the optimal conditions (current intensity: 3 A; sample pH: 12), with 98% and 94.05% reduction of the COD and total organic carbon (TOC), respectively, after 14 h of treatment. Soft drink wastewater pretreated with calcium-modified clinoptilolite was also electrooxidized using the BDD system. The results showed that the pre-treatment was extremely convenient, reducing the treatment time to 6 h compared to the electrooxidation of wastewater. At a current intensity of 3 A, the treatment time was 8 h, with 100% reduction of colour and COD and 97.5% reduction of TOC.

Carbon quantum dots as a fluorophore for “inner filter effect” detection of metronidazole in pharmaceutical preparations

With houttuynia cordata as carbon source, photoluminescent carbon quantum dots (CDs) were obtained via a one-step hydrothermal procedure. The absorption band of metronidazole (MNZ, maximum absorption wavelength at 319 nm) can well overlap with the excitation bands of CDs (maximum excitation wavelength at 320 nm). A fluorescent approach has been developed for detection of MNZ based on the inner filter effect (IFE), in which as-prepared CDs act as an IFE fluorophore and the MNZ as an IFE absorber. We have investigated the mechanism of quenching the fluorescence of CDs and found that the IFE leads to an exponential decay in fluorescence intensity of CDs with increasing concentration of MNZ, but showed a good linear relationship (R² = 0.9930) between ln(F₀/F) with the concentration of MNZ in the range of 3.3 × 10⁻⁶ to 2.4 × 10⁻⁴ mol L⁻¹. Due to the absence of surface modification of the CDs or establishing any covalent linking between the absorber (MNZ) and the fluorophore (CDs), the developed method is simple, rapid, low-cost and less time-consuming. Meanwhile, it possesses a higher sensitivity, wider linear range, and satisfactory selectivity and has potential application for detection of MNZ in pharmaceutical preparations.

CDs were prepared using Houttuynia cordata via hydrothermal process, the absorption band of MNZ can well overlap the excitation bands of CDs, a simple, rapid approach for detection of MNZ was established on the basis of IFE.

Electrochemical filtration process for simultaneous removal of refractory organic and particulate contaminants from wastewater effluents

Versatile electrochemical reactions are effective for removing a wide range of water contaminants. This study focuses on the development and testing of bifunctional electrocatalytic filter anodes as reactive and separating media for the simultaneous removal of refractory dissolved organic and particulate contaminants from real wastewater effluents. The results show that the TiO₂ particle interlayers formed between the Ti fiber support and the top composite metal oxide catalyst layers assist in reducing filter pores to an effective size range that enables removal of most particulates within the wastewater. The double-sheet design, which is a sandwich-structured module with an internal void space for permeate, prevents filter fouling, and transmembrane pressure can be maintained at a very low level of <5 kPa during batch and continuous flow reactor operations. Substantive and simultaneous removal of dissolved organics (e.g., chromophores, fluorophores, 1,4-dioxane, chemical oxygen demand, and total organic carbon) and particulate matter (i.e., turbidity) are achieved, although removal rates and efficacies differ depending on the electric current density applied. Decolorization and particulate rejection occur swiftly and immediately, but 1,4-dioxane degradation is relatively slow and quite time-dependent. Possible 1,4-dioxane degradation pathways during electrocatalysis are also proposed. Electrochemical filtration technology shows considerable promise for use in the next generation of advanced wastewater treatment solutions.

Cytostatic drug removal using electrochemical oxidation with BDD electrode: Degradation pathway and toxicity

In the presented study, electrochemical oxidation of five anticancer drugs (5-fluorouracil (5-FU), ifosfamide (IF), cyclophosphamide (CF), methotrexate (MTX), imatinib (IMB)) using boron doped diamond (BDD) electrode was investigated. In the first step the operating parameters of electrolysis were optimized. Studies have demonstrated a significant influence of applying current density, temperature, pH of solution and initial concentration of 5-FU on the process efficiency. A comparison of the decomposition rate of all the tested drugs showed a decrease in the pseudo-first order rate constants in the following order: k(IMB) > k(MTX) > k(CF) ≈ k(IF) > k(5-FU). Mineralization current efficiency (MCE) was determined for all the drugs based on the removal amount of total organic carbon (TOC) and their values decreased in the same order as values of drug degradation rate k. Based on the identified degradation products, electrochemical oxidation pathways of the decomposed drugs were proposed. In the case of CF, IF and 5-FU the degradation process occurred mainly through ketonization, hydroxylation and dehalogenation, while MTX and IMB were decomposed by attack of hydroxyl radicals on benzyl position in parent compounds. An important part of the research was the evaluation of eco-toxicity of electrochemically treated drug solutions against Lemna minor. Toxicity of initial 5-FU and MTX solutions towards L. minor were observed but after electrochemical treatment its toxicity decreased. The opposite trend was observed for CF and IF. In this case no significant toxicity was observed for the initial solutions of these drugs, while after electrochemical treatment an increase in growth inhibition of L. minor was found.

Ultrasonic power improvement of flumequine degradation effectiveness in aqueous solution via direct and indirect action of mechanical acoustic wave

The current research work aimed to describe the roles of ultrasonic power under sono-Fenton process in the degradation of flumequine (FLU) in water. For this purpose, the effects of some parameters including temperature, ferrous ion concentration, chemical oxidant concentration (S₂O₈^2- and Cl^-) and the initial pH value of the reaction kinetics were investigated. Results showed that the degradation of FLU antibiotic was accelerated by ultrasonic irradiation and the presence of an inorganic oxidant. The sono-generation of active species such as hydroxyl radicals (HO and HOO) and sulfate radicals (SO₄^-) as strong oxidizing agents improved the FLU degradation. In fact, the peroxydisulfate anion (S₂O₈^2-) has been identified as among parameters that enhanced the degradation process. Under optimal conditions, 98% of the flumequine removal was carried out within 80 min at 60 °C.

Metronidazole removal by means of a combined system coupling an electro-Fenton process and a conventional biological treatment: By-products monitoring and performance enhancement

In order to mineralize Metronidazole (MTZ), a process coupling an electro-Fenton pretreatment and a biological degradation was implemented. A mono-compartment batch reactor containing a carbon-felt cathode and a platinum anode was employed to carry out the electro-Fenton pretreatment of MTZ. A total degradation of MTZ (100 mg L^-1) was observed at 0.07 mA.cm^-2 after only 20 min of electrolysis. Yet, after 1 and 2 h of electrolysis, the mineralization level remained low (16.2% and 32% respectively), guaranteeing a significant residual organic content for further biological treatment. LCMS/MS was used to determine the intermediates by-products and hence to propose a plausible degradation pathway. An increase from 0 to 0.44 and 0.6 for 1 and 2 h of electrolysis was observed for the BOD₅/COD ratio. Thus, from 1 h of electro-Fenton pretreatment, the electrolysis by-products were considered biodegradable. A biological treatment of the electrolysis by-products after 1 and 2 h was then realized. The mineralization yields reached very close values, about 84% for 1 and 2 h of electrolysis after 504 h of biological treatment, namely close to 89% for the overall process, showing the pertinence of the proposed coupled process.