The Study On Degradation Of Metronidazole Antibiotic Wastewater By Electro-fenton And Heterogeneous Fenton

The consumption and production of antibiotics in China are the highest in the world.More and more antibiotics and metabolites have been released into our living space,which have caused unpredictable adverse effects on the ecosystem and public health.It is necessary to develop new methods to effectively remove these refractory contaminants from wastewater.Electro-Fenton and heterogeneous Fenton oxidation processes are considered as promising technologies,which are based on the in-situ generation of strong oxidants hydroxyl radical??OH?.The main advantages of these technologies treatment are the following:simple and flexible operation,environmentally friendly and high reaction speed.Nowadays,It is necessary to excogitate an efficient oxygen reduction reaction?ORR?cathode at a wide rang of pH values for hydrogen peroxide production,which is one of the key factors for the industrialization of the electro-Fenton process.The production of H₂O₂ is relatively low due to the limited solubility of oxygen in the electrolyte when conventional electrodes are employed.This above mentioned problem can be overcome by using gas diffusion electrode?GDE?is to generate H₂O₂.The porous structure can eliminate the influence of mass transport of O₂.The development of heterogeneous Fenton catalyst still faces some challenges:low activity under neutral conditions,poor stability and low utilization of hydrogen peroxide.This paper makes some modifications about the traditional electro-Fenton and heterogeneous Fenton oxidation technologies on the aspects of reaction mechanism,catalytic materials and technological process,providing the foundation of industry application.The detailed research contents are as follows:1.To slove the problem of traditional electrochemical Fenton,such as long electrolysis time,high energy consumption,low utilization of hydrogen peroxide and difficult control of reaction conditions,this paper proposes a modified electrochemical Fenton method.The modified electrochemical Fenton consisted of two steps.Firstly soluble Fe²⁺electrochemically produced from sacrificial ironanode?E₀=-0.44 V vs.SHE?was supplied to the solution.Organic compound took place reduction reaction and ferric ions were reduced to ferrous ions during electrolytic process.Secondly hydrogen peroxide was added to the beaker as one of the Fenton reagents to perform the Fenton reaction.The experiment shows that 62.6%of chemical oxygen demand?COD?is removed and the ratio of the five-day biochemical oxygen demand?BOD₅?to COD increases from 0.17 to 0.45 under the optimal experimental conditions?current density of 30 mA/cm2,electrolysis time of 15 min,H₂O₂ dosage of 75 mmol/L,pH 3.00 and stirring time of 2.5 hours?.Meanwhile,the high concentration of metronidazole?1000 mg/L?is dramatically decreased?>95%?.In order to explore the mineralization pathway for metronidazole,the intermediate by-products of the e modified electrochemical Fenton process are investigated by GC/MS,FTIR and HPLC.Good results are achieved in treatment of the real metronidazole wastewater by the proposed method,BOD₅/COD increases from 0.17to 0.35.The total concentration of metronidazole and 2-methyl-5-nitroimidazole in wastewater decreased from 32.32 mg/L to 3.59 mg/L.This method has been successfully applied to a large scale treatment of real metronidazole wastewater as apretreatment process for the biochemical treatment.2.The phosphorus-doped multiwall carbon nanotubes?P-CNTs?catalyst is first used to efficiently generate hydrogen peroxide.It overcomes the problems of low oxygen reduction reaction?ORR?through a two electron,high overpotential,narrow application range of pH and poor stability of cathode materials in the current electro-Fenton technology.These P-CNTs are synthesized by a simple and efficient hydrothermal method,the structure and morphology are characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,Brunauer–Emmett–Teller?BET?surface area,X-ray photoelectron spectroscopy?XPS?,respectively.The electrocatalytic activity of P-CNTs and CNTs for oxygen reduction are determined by cyclic voltammetry?CV?.P-CNTs exhibits a higher catalytic activity for the ORR than CNTs,including a more positive onset potential and a higher current density.The kinetics of the oxygen reduction reaction?ORR?is evaluated by the rotating ring-disk electrode technique.At cathodic potential V=-0.4 V vs SCE,the number of transfer electrons in oxygen reduction is 3.06.The gas diffusion electrode consists of a catalytic layer,a diffusion layer and a Nickel foam substrate.The operational parameters for P-CNTs gas diffusion electrode such as cathodic potential,pH and O₂flow rate are optimized systematically.Under the optimal conditions?Flow rate of0.21 L/min,0.1 mol/L Na₂SO₄,cathodic potential of-0.7 V vs SCE,pH=5?,The accumulation of H₂O₂ reaches to 1291.3 mg/L after 60 min for P-CNTs gas diffusion electrode,while the CNTs gas diffusion electrode produces 415.9 mg/L of H₂O₂.The current efficiency for the formation of H₂O₂ is 88.5%when using P-CNTs gas diffusion electrode,which is higher than 64.7%of CNT gas diffusion electrode in 60min.The concentration of H₂O₂ do not decreased significantly during the reuse of 6times,indicating that P-CNTs gas diffusion electrode has a very remarkable restoration and regeneration performance.In addational P-CNTs gas diffusion cathode exhibits excellent high hydrogen peroxide yield and current efficiency under acidic and neutral conditions,Finally the P-CNTs gas diffusion electrode is experimentally confirmed to be efficient in the degradation of 200 mg/L metronidazole?MNZ?by electro-Fenton?EF?process.Under the conditions that 0.l mol/L Na₂SO₄,0.21 L/min O₂ flow rate,electrolysis time 30 min,Fe²⁺?batch entry?dosages is 0.78 g/L,cathodic potential of-0.7 V vs SCE,the removal rates of metronidazole and COD are 99.9%and 66.3%respectively.3.To slove the problem of heterogeneous Fenton catalysts,such as low catalytic activity in yielding?OH,poor stability and durability of the catalysts,this paper proposes FeOCl/CNTs as heterogeneous Fenton Catalyst.Iron Oxychloride?FeOCl?Nanosheet loaded on carbon nanotubes?CNTs?is first prepared by an impregnation-calcination method,The FeOCl/CNT?56.8 wt%?composite catalyst exhibits typical morphology of CNTs,the FeOCl nanosheets are almost located inside the network of CNTs.It can be found that the FeOCl/CNTs contains many small FeOCl nanosheet?about 200 nm in length and 10 nm in thickness?with good dispersion,Carbon nanotube?CNTs?carriers can be fully and effectively utilized.FeOCl/CNTs are mesoporous materials,The BET surface area and pore volume of FeOCl/CNTs are 53.17 m²/g and 0.224 cm³ g^-1.Metronidazole was used as the target pollutant to measure the activity of FeOCl/CNTs in catalyzing hydrogen peroxide,99.3%MNZ removal efficiency and 68.3%COD removal rate could be achieved after60 min under optimal conditions.The amount of Fe leaching in FeOCl/CNTs is0.00804%.The normalized pseudo first-order kinetic rate constant?kobs?is 0.1877min^-1.No noticeable deactivation of the FeOCl/CNTs catalyst is observed even after 6cycles.The catalytic activity of FeOCl/CNTs is not obviously affected by the solution pH values in the range of pH 2-7.Not just at the FeOCl active sites,H₂O₂ is also being activated and decomposed on the CNTs surfaces to form?OH.Therefore,the high catalytic activity of FeOCl/CNTs is attributed to its multiple catalytic properties.This method is applied in treatment of norfloxacin wastewater.The removal rates of norfloxacin and COD are 97%and 57.0%respectively.The amount of Fe leaching is only 0.00146%.