Study On The System Of Photocatalytic Reduction Of Uranium(?)

Due to its high energy density and low carbon dioxide emissions,nuclear energy had been attracted widespread attention.Uranium,as the major constituents of nuclear fuel,is usually released into the natural environment from uranium mining,milling,nuclear experiment and so on.The removal of uranium pollutants became an urgent and meaningful issue in view of the threats to ecosystem and human health induced by uranium's chemotoxicity and radiotoxicity.A variety of technologies such as evaporation,solvent extraction,adsorption and chemical precipitation had been used to fight against the uranium pollutants.Although some of these technologies had achieved encouraging results,some technical defects had limited their application.For example,the evaporation was hindered by a high energy costs and a large amount of organic reagents were consumed by the solvent extraction.The adsorption was limited the saturation of adsorption materials and the variety of surrounding environment,uranium pollution is likely to be returned to the aqueous system,leading to secondary pollution,while chemical precipitation cannot be applied to dilute solutions.The uranium species in natural environment mainly consist of highly mobile hexavalent uranium(VI)and relatively immobile tetravalent uranium(IV),thus reduction of soluble uranium(VI)into sparingly soluble uranium(IV)was believed to be a plausible approach to fight against the uranium pollutants.Chemical reduction,photocatalytic reduction and microbial reduction have been explored for uranium(?)reduction,among which,photocatalytic reduction as a simple,green and efficient method has been highly emphasized.However,the technology of photocatalytic reduction of uranium(?)is still in its infancy.It is an imperative task to develop a simple and applicable system for photocatalytic reduction of uranium(?).The system for photocatalytic reduction of uranium(?)are designed rationally.We realize the transformation from photocatalytic reduction of uranium(?)under full spectrum to photocatalytic reduction of uranium(?)under visible light and then achieved uranium(?)photoreduction under visible light without any sacrificial agents.The specific contents are as follows:1.We conduct a first study of uranium(?)photoreduction under full spectrum by the g-C3N4/TiO2 composites constructed by ultrasonic dispersion strategy.Due to the matched energy level alignment between g-C3N4 and TiO2,the type ? heterojunction is formed after their combination.The separation of photogenerated charge carriers are enhanced by the ? heterojunction,thus gC3N4/TiO2 composites exhibit higher photocatalytic performance for uranium(?)reduction than both of pure g-C3N4 and TiO2.The reaction rate constant of the optimal g-C3N4/TiO2 composites is 3.9 times than pure TiO2.In addition,the photocatalytic removal of uranium(?)activity by g-C3N4/TiO2 composites is affected by solution pH,uranium(?)concentration,light source,hole scavenger and metal ions.2.We report a first study of visible-light-driven photocatalytic reduction of uranium(?)using visible light responsive ZnFe2O4.It is a simple,efficient and applicable photocatalytic system for uranium(?)photoreduction.The ZnFe2O4 catalysts with different morphologies,i.e.nanoparticles,rods and microspheres,are successfully obtained by tailoring reaction conditions.The photoreduction of uranium(?)under visible light is achieved over these ZnFe2O4 samples.The photocatalytic activity for these ZnFe2O4 catalysts follows the order of rods>microspheres>nanoparticles.With the initial uranium(?)concentration at 50 ppm,98%uranium(?)reduction efficiency is achieve by ZnFe2O4 rods during 60 min visible light irradiation with small catalyst dosage of 0.2 g/L.The effects of catalyst dosage,hole scavenger(CH3OH)dosage and solution pH on the photocatalytic reactions are investigated in detail using ZnFe2O4 rods.Furthermore,XPS,Mott-Schottky test and photocurrent test are used to explore the mechanisms of the uranium(?)photoreduction.The results show that uranium(?)is successful photoreduced to uranium(IV),and the uranium(?)photoreduction is clearly promoted with the aid of methanol.In addition,ZnFe2O4 rods own good stability,recyclability and magnetic separability.All these features make ZnFe2O4 a promising photocatalyst for radioactive environmental remediation.3.We conduct a first study of uranium(?)photoreduction by the visiblelight-driven 2D/1D Ti3C2/CdS nanorods composites constructed by electrostatic assembly strategy.Under visible light,Ti3C2/CdS achieve efficient uranium(?)photoreduction without any sacrificial agents.1D CdS nanorods are randomly dispersed on the surface of 2D Ti3C2 nanosheets,forming a 2D/1D composite structure.The optimal Ti3C2/CdS composites possess enhanced specific surface area,improved visible light response and accelerated electron-hole separation,thus exhibit higher photocatalytic performance for uranium(?)reduction than pure CdS.In addition,the Ti3C2/CdS composites also own the enhanced photostability than CdS.Ti3C2 in the composites acts as a Janus co-catalyst for the photoreduction of uranium(?),in view of the fact that it can simultaneously enhance the photocatalytic performance and the stability of CdS.Under typical weak alkaline conditions,the optimal Ti3C2/CdS catalysts achieve a high uranium extraction capacity of 500 mg/g during 40 min visible light irradiation.Meanwhile,the Ti3C2/CdS composites keep high performance regardless of acidic,neutral and alkali conditions,which make them simple,economic,and applicable photocatalyst for uranium(?)removal from environment samples.In this work,we achieve uranium(?)photoreduction under full spectrum with g-C3N4/TiO2,firstly.Then the system which ZnFe2O4 acts as the catalyst and methanol acts as the hole scavenger for uranium(?)photoreduction under visible light is obtained.At last,using Ti3C2/CdS composites,we achieve efficient uranium(?)photoreduction without any sacrificial agents under visible light.This work offers new clues for radioactive environmental remediation.