Synthesis And Representation Of G-C₃N₄/FeO_x Complexes For Arsenic Removal By Synchronous Visible Light Catalysis Oxidation And Adsorption

Natural water are arsenite（As（Ⅲ））and arsenate（As（V））.Between them,arsenite exhibits toxicity that is much higher than that of arsenate.Furthermore,the removal of As（Ⅲ）is not efficient in adsorption processes because arsenite usually exists as uncharged forms and has poor affinity for adsorbents.Thus,a common technique for the removal of arsenic usually involves a preoxidation step prior to complete removal.However,the band gaps of most TiO₂-based photocatalysts are generally wide,which results in the scant utilization of visible light.Furthermore,the low adsorption capacity of TiO₂ for arsenic may cause the insufficient removal of arsenic.Recently,g-C₃N₄ has attracted considerable attention owing to its narrow band gap（2.7 e V,ECB = 1.42 V vs NHE）.Also,it can be excited under visible light.However,the photocatalytic activity of pure graphitic carbon nitride is limited by lower efficiency caused by the recombination of photoinduced electron-hole pairs.To overcome these limitations,a lot of photocatalysts with heterojunctions have been synthesized by coupling with other semiconductors including Cd S,TiO₂,Ag2 O,Zn Fe2O4,and Fe₃O₄ for improving the photocatalytic activity.As everyone knows that Fe₃O₄ and α-Fe₂O₃ are two excellent adsorbents for arse nic removal owing to its high adsorption capacity.Thus,Bifunctional mesoporous FeOCN-x and α-FeOCN-x composites were prepared and used for the simultaneous visible-light photocatalytic oxidation and adsorption of arsenic from aqueous solution.The as-prepared composites were characterized.The visible-light catalysis and adsorption ability of two kinds of composites to As（Ⅲ）and As（T）removal performancce were studied in detail,including catalytic oxidation kinetic experiment,the influence of solution p H,visible-light photocatalytic thermodynamics,the effect of coexisting anions to the adsorption of As（Ⅲ）and As（T）,experiments of oxidation mechanism of As（Ⅲ）and recycling experiments.The results of experiments showed that:（1）The XRD indicate that the FeOCN-x is a twophase composite.The HTEM indicates the existence of the heterojunction structure between g-C₃N₄ and Fe₃O₄.The visible-light photocatalytic oxidation kinetic experiment showed thar Fe OCN^-16 had the highest value of the photocatalysis rate constant(0.0066 min^-1),which is about 16.5 and 3.0 times than that of pure Fe₃O₄ and g-C₃N₄,the adsorption behavior of Fe OCN^-16 for As conformed to Langmuir isothermal.the coexisting ions display little effect on the removal of arsenite.When the concentrations of Cl^-,SO₄^2-,NO₃^-,HCO₃^-,and SiO₃^2-are 10 m M,the removal of As（Ⅲ）is 95%,94%,96%,91%,and 89%,respectively.Experiments of oxidation mechanism of As（Ⅲ）indicates that O2·-is the main species for oxidation of As（Ⅲ）in this system.The removals of As（Ⅲ）and As（T）are 94% and 80% respectively,even after five cycles.（2）The XRD indicate that the α-FeOCN-x is a twophase composite.The HTEM indicates the existence of the heterojunction structure between g-C₃N₄ and α-Fe₂O₃.The visible-light photocatalytic oxidation kinetic experiment showed that α-Fe OCN^-12 had the highest value of the photocatalysis rate constant(0.0047 min^-1),which is about 19.2 and 5.4 times than that of pure α-Fe₂O₃ and g-C₃N₄,the adsorption behavior of α-Fe OCN^-12 for As conformed to Langmuir isothermal.When the concentrations of Cl^-,SO₄^2-,NO₃^-,HCO₃^-,and SiO₃^2-are 10 m M,the removal of arsenite is 91%,82%,84%,91%,and 41%,respectively.Experiments of oxidation mechanism of As（Ⅲ）indicates that O2·-is the main species for oxidation of As（Ⅲ）in this system.The removals of As（Ⅲ）and As（T）are 86% and 75% even after five cycles.