Indirect electroreduction as pretreatment to enhance biodegradability of metronidazole

Improving biodegradability of dissolved organic matter (DOM) in old landfill leachate by electrochemical pretreatment: The effect mechanism of polarity

Enhancing the biodegradability of old landfill leachate is vital for the efficient treatment or resource utilization of municipal solid waste. Electrochemical pretreatment emerges as a promising technology for transformation of refractory dissolved organic matter (DOM). However, the specific impact of polarity on improving biodegradability of DOM remains unclear. In this study, a divided electrolyzer was used to explore the changes in the biodegradability of DOM in old landfill leachate during electrolysis. Meanwhile, the correlation mechanism between BOD5 variation and DOM evolution was explored by spectroscopy and Maldi-TOF-MS analysis. Results shown that different polarities all have positive effect on enhancing the biodegradability of DOM, while the structural changes related with BOD5 are depending on the polarity. In the anode chamber, electrochemical oxidation (EO) generates and eliminates carboxyl groups. Additionally, EO concurrently eliminates humic-like substances, which are challenging for microorganisms to degrade, and protein-like substances, which are easily degradable by microorganisms. This creates a competitive mechanism that coexist the promotion and inhibition for biodegradability. In the cathode chamber, electrochemical reduction (ER) transforms DOM components, accumulating easily useable protein components for microorganisms. Kinetic studies show that EO related BOD5 changes are aptly described by a competition model, considering both generation and removal of bioavailable components. ER related BOD5 changes suit a pseudo-first-order kinetic model. These insights into the transformation of old leachate DOM support the development of methods predicting BOD5 evolution, crucial for future process optimization.

Long-term stable and efficient degradation of ornidazole with minimized by-product formation by a biological sulfidogenic process based on elemental sulfur

Conventional biological treatment processes cannot efficiently and completely degrade nitroimidazole antibiotics, due to the formation of highly antibacterial and carcinogenic nitroreduction by-products. This study investigated the removal of a typical nitroimidazole antibiotic (ornidazole) during wastewater treatment by a biological sulfidogenic process based on elemental sulfur (S0-BSP). Efficient and stable ornidazole degradation and organic carbon mineralization were simultaneously achieved by the S0-BSP in a 798-day bench-scale trial. Over 99.8 % of ornidazole (200‒500 μg/L) was removed with the removal rates of up to 0.59 g/(m3·d). Meanwhile, the efficiencies of organic carbon mineralization and sulfide production were hardly impacted by the dosed ornidazole, and their rates were maintained at 0.15 kg C/(m3·d) and 0.49 kg S/(m3·d), respectively. The genera associated with ornidazole degradation were identified (e.g., Sedimentibacter, Trichococcus, and Longilinea), and their abundances increased significantly. Microbial degradation of ornidazole proceeded by several functional genes, such as dehalogenases, cysteine synthase, and dioxygenases, mainly through dechlorination, denitration, N-heterocyclic ring cleavage, and oxidation. More importantly, the nucleophilic substitution of nitro group mediated by in-situ formed reducing sulfur species (e.g., sulfide, polysulfides, and cysteine hydropolysulfides), instead of nitroreduction, enhanced the complete ornidazole degradation and minimized the formation of carcinogenic and antibacterial nitroreduction by-products. The findings suggest that S0-BSP can be a promising approach to treat wastewater containing multiple contaminants, such as emerging organic pollutants, organic carbon, nitrate, and heavy metals.

Activation of sulfite by micron-scale iron-carbon composite for metronidazole degradation: Theoretical and experimental studies

In recent years, sulfite (S(Ⅳ)), as an alternative to persulfates, has played a crucial role in eliminating antibiotics in wastewater, so there is an urgent need to develop a cheap, environmentally friendly, and effective catalyst. Zero-valent iron (ZVI) has great potential for activated S(Ⅳ) removal of organic pollutants, but its reactivity in water is reduced due to passivation. In this study, a micron-scale iron-carbon composite(mZVI@C-800) prepared via high-temperature calcination was coupled with S(Ⅳ) to degrade metronidazole (MNZ). Under the optimized reaction conditions of mZVI@C-800 dosage of 0.2 g/L and S(Ⅳ) concentration of 0.1 g/L, the MNZ removal rate was up to 81.5 % in acidic and neutral environments. The surface chemical properties of the catalysts were characterized by different analytical techniques, and the corresponding catalytic mechanism was analyzed based on these analytical results. As a result, Fe²⁺ is the main active site, and ^·OH and SO₄^·- were the dominant active species. The increase in efficiency was attributed to the introduction of carbon to enhance the corrosion of mZVI further releasing more Fe²⁺. Additionally proposed were the potential response mechanism, the degradation path, and the toxicity change rule. These results demonstrate that the catalytic breakdown of antibiotics in wastewater treatment can be accelerated by the use of the outstanding catalytic material mZVI@C-800.

Activation of O2 by zero-valent zinc assisted with Cu(II) for organics removal: Performance and mechanism

This study proposes a method to activate O₂ by accelerating the corrosion process for zero-valent zinc (ZVZ) with the assistance of Cu(II), promoting the consecutive production of reactive oxygen species. The mechanisms for reactive oxygen species generation are clarified with metronidazole (MTZ) as the targeted pollutant. The outcome suggests the association of Cu(Ⅱ) and ZVZ presents an apparent cooperative activity, an enhancement of 85% in MTZ removal is attained for the ZVZ/Cu(Ⅱ) system after 10 min compared to that for ZVZ. Analysis of the mechanisms involved indicates that this improvement is due to the addition of Cu(Ⅱ), which can accelerate the corrosion of ZVZ. In addition, quenching experiments and electron paramagnetic resonance (EPR) technology show that superoxide radicals (·O₂^-) result in rapid MTZ degradation. The primary component that is liable for O₂ activation and a certain amount of H₂O₂ generation is verified to be ZVZ. Moreover, Cu(I) is detected in the ZVZ/Cu(Ⅱ) system, which arises from a direct reduction pathway driven by ZVZ and an indirect reduction pathway driven by active hydrogen atoms.

A novel system coupling an electro-Fenton process and an advanced biological process to remove a pharmaceutical compound, metronidazole

The objective of this study was to improve the mineralization of metronidazole, a recalcitrant antibiotic by the development of a new combined process coupling electro-Fenton and a biological process. For biotreatment, various strategies were considered bioaugmentation, bioacclimatation and biostimulation alone or combined. So, the novelty of this strategy is to combine advanced oxidation process with advanced biological process. The conventional biotreatment with activated sludge after 120 h of culture, led to 58.1% mineralization, whereas the pure isolated strains, from activated sludge culture in the presence of metronidazole by-products, identified as Pseudomonas putida (strain A) and Achromobacter sp. (strain B), led to 37.2% and 40.1% respectively. After original acclimation of the isolated strains to electrolysis by-products, the mineralization levels reached 75.6% and 72.9% for strains A and B respectively after 120 h of culture. The results showed that the mineralization of metronidazole by-products was the most important in the case of the combination of autochthonous bioaugmentation and biostimulation, with 96.1% after 120 h of treatment. By coupling the two processes, the global treatment reached therefore a mineralization yield of 97% with a reduction in processing time of 16 days compared to previous conventional biological treatment.

Simultaneous removal of antibiotic and nutrients via Prosopis juliflora activated carbon column: Performance evaluation, effect of operational parameters and breakthrough modeling

In this study, the behavior of mono-component (metronidazole/phosphate/nitrate, MET/PO₄^3-/NO₃^-) and multi-component (MET+PO₄^3-+NO₃^-) adsorption in fixed-bed adsorption column was investigated using Prosopis juliflora activated carbon (PJAC). The influence of column operating parameters such as bed depth (H: 5-15 cm), influent flow rate (Q: 0.5-2 L/h) and adsorbate concentration (C_o: 25-100 mg/L) on breakthrough curves were evaluated. The experimental data was correlated with breakthrough models viz. Thomas, Adams-Bohart, Yoon-Nelson and bed depth service time (BDST) models. The results showed that the Thomas model fitted the experimental data better than other models in predicting the breakthrough characteristics for the removal of MET, PO₄^3- and NO₃^- by PJAC. The maximum adsorption capacity found by Thomas model was 9.70, 8.21 and 5.57 mg/g for MET, PO₄^3- and NO₃^-, respectively. In multi-component systems, antagonistic behavior in sorption of MET, PO₄^3- and NO₃^- was observed and as a result, adsorption capacity was 1.2-1.5 folds lesser than that observed in mono-component system. In conclusion, results of the present study indicate that the PJAC can be successfully employed for the removal of MET, PO₄^3- and NO₃^- using fixed-bed adsorption column; however, the column design for multi-component mixture should be based on rapid breakthrough sorbate.

Pre-electrochemical treatment combined with fixed bed biofilm reactor for pyridine wastewater treatment: From performance to microbial community analysis

To overcome the high biotoxicity and poor biodegradability of pyridine and its derivatives, a pre-electrochemical treatment combined with fixed bed biofilm reactor (EC-FBBR) was designed for multi-component stream including pyridine (Pyr), 3-cyanopyridine (3-CNPyr), and 3-chloropyridine (3-ClPyr). The EC-FBBR system could simultaneously degrade these pollutants with a mineralization efficiency of 90%, especially for the persistent 3-ClPyr. Specifically, the EC could partially degrade all pollutants, and allow them to be completely destructed in FBBR. With EC off, Rhodococcus (35.5%) became the most abundant genus in biofilm, probably due to its high tolerance to 3-ClPyr. With EC on, 3-ClPyr was reduced to an acceptable level, thus Paracoccus (21.1%) outcompeted among interspecies competition with Rhodococcus and became the dominant genus. Paracoccus was considered to participate in the subsequent degradation for the residual 3-ClPyr, and led to the complete destruction for all pollutants. This study proposed promising combination for effective treatment of multi-component pyridine wastewater.

Safe and efficient degradation of metronidazole using highly dispersed β-FeOOH on palygorskite as heterogeneous Fenton-like activator of hydrogen peroxide

In this study, the highly dispersed β-FeOOH on palygorskite (H-β-FeOOH/PAL) was prepared and investigated as Fenton catalyst for H₂O₂ activation towards metronidazole (MTZ). Based on electron spin resonance and probe techniques, hydroxyl radicles (^•OH) as main reactive oxygen species was confirmed, and much more ^•OH were generated over H-β-FeOOH/PAL than T-β-FeOOH/PAL using traditional impregnation method. The enhanced Fe(III) to Fe(II) cycle due to the highly dispersed β-FeOOH leads to the fast degradation of metronidazole over H-β-FeOOH/PAL. 92.8% of MTZ degradation efficiency was achieved within 180 min by adding 17 mmol/L H₂O₂ and 40 mg/L of H-β-FeOOH/PAL at pH 6.0. The bacterial cytotoxicity during MTZ degradation process also decreased with reaction time and achieved zero at 66 min, indicating MTZ was not only efficiently degraded but also safely transformed. Besides, the possible MTZ degradation pathway was further proposed based on the intermediates identified by HPLC-MS.

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.

Reactive oxygen and iron species monitoring to investigate the electro-Fenton performances. Impact of the electrochemical process on the biodegradability of metronidazole and its by-products

In this study, the monitoring of reactive oxygen species and the regeneration of the ferrous ions catalyst were performed during electro-Fenton (EF) process to highlight the influence of operating parameters. The removal of metronidazole (MTZ) was implemented in an electrochemical mono-compartment batch reactor under various ranges of current densities, initial MTZ and ferrous ions concentrations, and pH values. It was found that under 0.07 mA cm^-2, 0.1 mM of ferrous ions and pH = 3, the efficiency of 100 mg L^-1 MTZ degradation and mineralization were 100% within 20 min and 40% within 135 min of electrolysis, respectively. The highest hydrogen peroxide and hydroxyl radical concentrations, 1.4 mM and 2.28 mM respectively, were obtained at 60 min electrolysis at 0.07 mA cm^-2. Improvement of the biodegradability was reached from 60 min of electrolysis with a BOD₅/COD ratio above 0.4, which was reinforced by a respirometric study, that supports the feasibility of coupling electro-Fenton and biological treatment for the metronidazole removal.

Oxidative removal of metronidazole from aqueous solution by thermally activated persulfate process: kinetics and mechanisms

Metronidazole (MNZ) is widely used in clinical applications and animal feed as an antibiotic agent and additive, respectively. Widespread occurrence of MNZ in wastewater treatment and hospital effluents has been reported. In this study, the mechanism of MNZ degradation in aqueous solutions via thermally activated persulfate (TAP) process was established under different conditions. The kinetic model was derived for MNZ degradation and followed pseudo-first-order reaction kinetics and was consistent with the model fitted by experimental data (R ² > 98.8%). The rate constant increased with the initial dosage of persulfate, as well as the temperature, and the yielding apparent activation energy was 23.9 kcal mol^-1. The pH of the solutions did not have significant effect on MNZ degradation. The degradation efficiency of MNZ reached 96.6% within 180 min for an initial MNZ concentration of 100 mg L^-1 under the optional condition of [PS]₀ = 20 mM, T = 60 °C, and unadjusted pH. [Formula: see text] and HO ^· were confirmed using electron paramagnetic resonance (EPR) spectra during TAP process. Radical quenching study revealed that [Formula: see text] was mainly responsible for MNZ degradation at an unadjusted pH. MNZ mineralization evaluation showed that the removal efficiency of total organic carbon (TOC) reached more than 97.2%.

Metronidazole removal in powder-activated carbon and concrete-containing graphene adsorption systems: Estimation of kinetic, equilibrium and thermodynamic parameters and optimization of adsorption by a central composite design

Metronidazole (MNZ) removal by two adsorbents, i.e., concrete-containing graphene (CG) and powder-activated carbon (PAC), was investigated via batch-mode experiments and the outcomes were used to analyze the kinetics, equilibrium and thermodynamics of MNZ adsorption. MNZ sorption on CG and PAC has followed the pseudo-second-order kinetic model, and the thermodynamic parameters revealed that MNZ adsorption was spontaneous on PAC and non-spontaneous on CG. Subsequently, two-parameter isotherm models, i.e., Langmuir, Freundlich, Temkin, Dubinin-Radushkevich and Elovich models, were applied to evaluate the MNZ adsorption capacity. The maximum MNZ adsorption capacities ([Formula: see text]) of PAC and CG were found to be between 25.5-32.8 mg/g and 0.41-0.002 mg/g, respectively. Subsequently, the effects of pH, temperature and adsorbent dosage on MNZ adsorption were evaluated by a central composite design (CCD) approach. The CCD experiments have pointed out the complete removal of MNZ at a much lower PAC dosage by increasing the system temperature (i.e., from 20°C to 40°C). On the other hand, a desorption experiment has shown 3.5% and 1.7% MNZ removal from the surface of PAC and CG, respectively, which was insignificant compared to the sorbed MNZ on the surface by adsorption. The overall findings indicate that PAC and CG with higher graphene content could be useful in MNZ removal from aqueous systems.

Direct and indirect electrochemical reduction prior to a biological treatment for dimetridazole removal

Two different electrochemical reduction processes for the removal of dimetridazole, a nitroimidazole-based antibiotic, were examined in this work. A direct electrochemical reduction was first carried out in a home-made flow cell in acidic medium at potentials chosen to minimize the formation of amino derivatives and then the formation of azo dimer. Analysis of the electrolyzed solution showed a total degradation of dimetridazole and the BOD₅/COD ratio increased from 0.13 to 0.24. An indirect electrochemical reduction in the presence of titanocene dichloride ((C₅H₅)₂TiCl₂), which is used to reduce selectively nitro compounds, was then investigated to favour the formation of amino compounds over hydroxylamines and then to prevent the formation of azo and azoxy dimers. UPLC-MS/MS analyses showed a higher selectivity towards the formation of the amino compound for indirect electrolyses performed at pH 2. To confirm the effectiveness of the electrochemical reduction, a biological treatment involving activated sludge was then carried out after direct and indirect electrolyses at different pH. The enhancement of the biodegradability was clearly shown since mineralization yields of all electrolyzed solutions increased significantly.

A selective electrochemical sensor for caffeic acid and photocatalyst for metronidazole drug pollutant - A dual role by rod-like SrV2O6

In the present study, well-defined one-dimensional (1D) rod-like strontium vanadate (SrV2O6) was prepared by simple hydrothermal method without using any other surfactants/templates. The successful formation of rod-like SrV2O6 was confirmed by various analytical and spectroscopic techniques. Interestingly, for the first time the dual role of as-prepared rod-like SrV2O6 were employed as an electrochemical sensor for the detection of caffeic acid (CA) as well as visible light active photocatalyst for the degradation of metronidazole (MNZ) antibiotic drug. As an electrochemical sensor, the SrV2O6 modified glassy carbon electrode (GCE) demonstrated a superior electrocatalytic activity for the detection of CA by chronoamperometry and cyclic voltammetry (CVs). In addition, the electrochemical sensor exhibited a good current response for CA with excellent selectivity, wide linear response range, lower detection limit and sensitivity of 0.01-207 µM, 4 nM and 2.064 μA μM-1cm-2, respectively. On the other hand, as-synthesized rod-like SrV2O6 showed highly efficient and versatile photocatalytic performances for the degradation of MNZ, which degrades above 98% of MNZ solution under visible light irradiation within 60 min. The obtained results evidenced that the improvement of rod-like SrV2O6 might be a resourceful electrocatalyst and photocatalyst material in the probable applications of environmental and biomedical applications.

Increased photocatalytic activity of NiO and ZnO in photodegradation of a model drug aqueous solution: Effect of coupling, supporting, particles size and calcination temperature

Mechanically ball-mill prepared clinoptilolite nanoparticles (NC) were used for increasing photocatalytic activity of NiO and ZnO as alone and binary systems. The semiconductors were supported onto the zeolite during calcination of Ni(II)-Zn(II)-exchanged NC at different calcinations temperatures. XRD, FTIR, SEM-EDX, X-ray mapping, DRS, TEM and BET techniques were used for characterization of the samples. The calcined catalysts at 400°C for 4h showed the best photocatalytic activity for metronidazole (MNZ) in aqueous solution. The mole ratio of ZnO/NiO affected the photodegradation efficiency because activity of the coupled catalysts depends to the both e/h production and electron scavenging processes. In the used system, NiO acted as e/h production source and ZnO as an electron sink. Red shifts in band gaps of the supported coupled semiconductors was observed whit respect to monocomponent one, confirming formation of nanoparticles of the semiconductors onto the zeolitic bed. The best activities were obtained for the NiO_1.3-ZnO_1.5/NC (NZ-NC) and NiO_0.7-ZnO_4.3/NC (NZ₃-NC) catalysts at pH 3, 1.2gL^-1 of the catalysts and 1gL^-1 of MNZ.

Photocatalytic degradation of Metronidazole with illuminated TiO2 nanoparticles

Metronidazole (MNZ) is a brand of nitroimidazole antibiotic, which is generally used in clinical applications and extensively used for the treatment of infectious diseases caused by anaerobic bacteria and protozoans. The aim of this investigation was to degrade MNZ with illuminated TiO2 nanoparticles at different catalyst dosage, contact time, pH, initial MNZ concentration and lamp intensity. Maximum removal of MNZ was observed at near neutral pH. Removal efficiency was decreased by increasing dosage and initial MNZ concentration. The reaction rate constant (k obs ) was decreased from 0.0513 to 0.0072 min(-1) and the value of electrical energy per order (EEo) was increased from 93.57 to 666.67 (kWh/m(3)) with increasing initial MNZ concentration from 40 to 120 mg/L, respectively. The biodegradability estimated from the BOD5/COD ratio was increased from 0 to 0.098. The photocatalyst demonstrated proper photocatalytic activity even after five successive cycles. Finally, UV/TiO2 is identified as a promising technique for the removal of antibiotic with high efficiency in a relatively short reaction time.