Study On The Modification Of Carbon Nitride Material And Its Photocatalytic Reduction Of Hexavalent Chromium

Hexavalent chromium ion is one of the common heavy metal ions in wastewater,which has strong oxidizing properties and 500 times more toxic than Cr(III).The long-term existence of hexavalent chromium in the environment seriously affects the ecological environment and human health.Photocatalytic technology is an environmentally friendly technology that is not only simple to operate and free of secondary pollution,but also capable of reducing carcinogenic hexavalent chromium to non-toxic trivalent chromium under visible light.Graphite carbon nitride is a non-metallic organic semiconductor material,which is not only low in cost and easy to prepare,but also has visible light response and good chemical stability,and is widely used in the field of photocatalysis.However,it has problems such as large forbidden band width,fast carrier recombination,and various modification strategies,which needed to improve its photocatalytic activity.In this article,a series of graphite-phase carbon nitride-based photocatalytic materials were prepared through several common modification methods,and hexavalent chromium was used as the main pollutant to investigate its photocatalytic performance and mechanism.The research content is as follows:(1)Firstly,carbon nitride nanosheets were synthesized by thermal polycondensation method,and then cyano groups were introduced on the surface of carbon nitride nanosheets through simple alkali treatment,and the cyano groups were converted by amidoximation of hydroxylamine hydrochloride.It is an amidoxime group,which greatly enhances the adsorption performance and photocatalytic reduction performance of carbon nitride to hexavalent chromium.By means of characterization,the introduction of the cyano group and the successful conversion with the amidoxime group were successfully proved.The sample with the amidoxime group showed excellent ability to adsorb Cr(VI).It successfully adsorbed 70% of Cr(VI)in 1 h,and successfully removed 100% of Cr(VI)in 3 h,and the photocatalytic reaction rate is 2.6 times than that of the cyano-modified sample and about 13.2times than that of the original carbon nitride nanosheet sample.(2)Because the energy band structure of carbon nitride and tungsten trioxide are very matched,an S-type heterojunction photocatalyst composed of carbon nitride nanosheets and tungsten oxide nanosquare sheets were successfully prepared.The shape of the synthesized tungsten trioxide is neat and uniform,and the load on the carbon nitride nanosheets is very uniform.In order to improve the separation efficiency of carrier pairs and make full use of the reducing or oxidizing capabilities of different active species,an experiment was designed to synergistically catalyze hexavalent chromium and tetracycline hydrochloride.And the photocatalytic experiment proved that in a system where Cr(VI)and tetracycline hydrochloride coexisted,the synthesized S-type heterojunction photocatalyst removed 80% of Cr(VI)in 3 h,which was more than that of a single pollutant system,and the reaction rate is 5 times than that of a single pollutant system.(3)The carbon nitride quantum dot solution was successfully prepared by the shear etching effect of the mixed acid solution of concentrated nitric acid and concentrated sulfuric acid on the carbon nitride layered structure,combined with the hydrothermal method,and the carbon nitride quantum dots were loaded on the titanium dioxide nano-flowers to form closely bonded binary heterojunction.Due to its wide band gap,the main photoresponse region of titanium dioxide photocatalyst is in ultraviolet light,and there is no absorption in the visible light region,which greatly affects the practical application in life scenes.However,after combining with carbon nitride quantum dots,the absorption of visible light is significantly enhanced,and this 0D-3D binding mode not only increases the specific surface area of the material,but also provides more active sites,which greatly enhances the light the ability to catalytically reduce hexavalent chromium.After 3 hours of photocatalysis,100%of Cr(VI)was successfully removed,while the titanium dioxide nanoflowers had almost no visible light activity,and the carbon nitride nanosheet alone reduced only 23% of Cr(VI).