Preparation Of Graphene-based Composites And Their Catalytic Properties By Non-thermal Plasma

Parabens are widely used as antiseptic and bactericides in food and daily chemical fields.Due to their extensive use,they have been detected in surface water,groundwater,drinking water,sewage,air,dust and soil,especially in the water environment.They have caused serious threats to the environmental safety of water bodies.Graphene composites became a hot spot in the field of materials in recent years,especially in the catalytic degradation of organic pollutants.Compared with traditional catalytic materials,they have the advantages of high efficiency and stable performance.Dielectric barrier discharge?DBD?is one of the most common ways to generate non-thermal plasma?NTP?.There are a large number of different kinds of active particles in DBD induced NTP?DBD-NTP?.Compared with other chemical methods,the resulting active particles induced by DBD-NTP are more diverse,more active,and more susceptible to reaction with the organics.Compared with the conventional advanced oxidation processes,the non-thermal plasma has significant advantages such as high efficiency,simple operation,and wide application range.In this paper,methylparaben?MeP?was used as the target contaminant,and the degradation mechanisms of aquesous MeP by ZnO-rGO,ZnFe₂O₄-rGO and g-C₃N₄-rGO combined with DBD-NTP were studied.At the same time,the structure and photoelectric properties of three graphene-based composite catalytic materials were characterized.Finally,the factors affecting the degradation of MeP during the reaction and the degradation pathways of MeP were also revealed.Firstly,the mechanism of MeP degradation in aqueous solution by DBD-NTP combined with zinc oxide-reduced graphene oxide?ZnO-rGO?nanosheets was investigated.The ZnO-rGO nanosheets were synthesized by hydrothermal method and characterized by X-ray diffraction?XRD?,nitrogen adsorption-desorption isotherm,scanning electron microscope?SEM?,transmission electron microscope?TEM?,UV-Vis absorption spectroscopy?UV-Vis?,photocurrent test and X-ray photoelectron spectroscopy?XPS?.The effects of various factors?discharge power,initial concentration of MeP,initial pH value and air flow rate?on the degradation of MeP were evaluated and the changes of solution pH,conductivity and TOC during the degradation process were discussed.The results showed that when the discharge power was 20 W,the airflow rate was 20 L/h,the ZnO-rGO nanosheets dosage was 0.015 g/L,the initial concentration of MeP was 20 mg/L,and the initial pH value was 7.0,the degradation efficiency of MeP achieved 99%at 15 min.The degradation efficiency of MeP in DBD-induced NTP synergistic ZnO-rGO nanosheet treatment solution was about 45%higher than that of DBD-induced NTP treatment of MeP alone.In addition,the role of some reactive species during the degradation process of MeP was also investigated.The degradation process of MeP was well fitted by the pseudo-first-order kinetics.Good stability of the synthetic ZnO-rGO was observed.The degradation intermediates of MeP were determined by gas chromatography-mass spectrometer?GC-MS?and the degradation pathways were proposed.Under the attack of reactive species,conjugated structure destruction,hydroxylation,carboxylation and ring-opening reaction occurred during MeP degradation,resulting in the formation of organic acids and alcohols;furthermore,some of the degradation intermediates were mineralized to H₂O and CO₂.After that,two graphene-based composite materials ZnFe₂O₄-rGO and g-C₃N₄-rGO were prepared.The structure and photoelectric properties of the two catalysts were characterized by XRD,nitrogen adsorption-desorption isotherm,SEM,TEM,UV-Vis,photocurrent test and XPS.The degradation efficiency of MeP by DBD-NTP synergy with ZnFe₂O₄-rGO and g-C₃N₄-rGO was approximately 25%and24%higher than that of DBD-NTP,respectively.The effects of discharge power,initial concentration of MeP,initial pH and air flow on the degradation efficiency of MeP were investigated.The degradation of MeP by DBD-NTP synergy with ZnFe₂O₄-rGO and g-C₃N₄-rGO follows the first order kinetics.The changes of O₃ and H₂O₂ content in the degradation process were studied,which proved that·OH,H₂O₂ and O₃ played an important role in the reaction.The pH of the solution gradually decreased and the conductivity gradually increased during the degradation of MeP by DBD-NTP in combination with ZnFe₂O₄-rGO and g-C₃N₄-rGO.The degradation intermediates of MeP were determined by GC-MS,and the degradation pathway of MeP was presumed based on the detected degradation intermediates,which mainly caused conjugate structure destruction,demethylation,hydroxylation,carboxylation and ring-opening reaction.As described above,some of the degradation products are decomposed into H₂O and CO₂.