Study On Photocatalytic Removal Of Aqueous Hexavalent Chromium,Nitrite And Bromate By Polyaniline/W₁₈O₄₉ Composite

With the development of modern agriculture and industry,excess oxoanions including Cr（VI）,nitrite and bromate are widely present in industrial wastewater,natural water and drinking water,and have been demonstrated to be highly toxic,mutagenic and carcinogenic to living organisms.To minimize risks of their pollution,ion exchange,adsorption,reverse osmosis and electrodialysis are often used.However,instead of destroying the oxoanions,removing and concentrating them from raw water is not a permanent solution.In this sense,photocatalysis appears to be the most promising technology,which can convert the aqueous oxoanions to harmless ionic salts,etc.Based on the excellent photocatalytic properties of W₁₈O₄₉ and polyaniline,we designed a series of polyaniline/W₁₈O₄₉9 composites as an efficient photocatalyst for conversion of aqueous Cr（VI）,nitrite and bromate,respectively.Polyaniline coated W₁₈O₄₉(PANI/W₁₈O₄₉)nanocomposite was prepared by in situ oxidative polymerisation method for Cr（VI）reduction.We herein focused on enhancing the stability and the photocatalytic performance of W₁₈O₄₉.The prepared PANI/W₁₈O₄₉ was thoroughly characterized by FTIR,TEM,XRD,BET,UV-vis DRS and PL.The PANI molecules presented a great effect on the light harvesting and photo-charge transfer of the W₁₈O₄₉,and the optimum percentage of was found to be 10 wt%.As for treating Cr（VI）,the effect of important water quality parameters（such as pH,coexisting ions,humic acid,dissolved oxygen,temperature and small molecular weight organic acids）on photocatalytic performance was investigated under the visible light irradiation（λ>420 nm）.Small molecular weight organic acids were shown to exert a dramatic accelerating influence on Cr（VI）reduction in the system.The obtained 10%-PANI/W₁₈O₄₉ can completely catalytically reduce 1 mM Cr（VI）in the presence of tartaric acid（1:3）within 50 min.Meanwhile,it can be recycled at ten times without any loss of photocatalytic efficiency,indicating the high stability of the as-prepared photocatalyst.Polyaniline-decorated three dimensional W₁₈O₄₉ composite(PANI@W₁₈O₄₉)was prepared through a solvent volatilization method.The noble metal-free photocatalyst possesses excellent photocatalytic activity for aqueous nitrite removal with high N₂ selectivity.The prepared sample was thoroughly identified via XRD,Raman,FTIR,SEM,TEM,UV-vis DRS and PL.The catalytic results demonstrated that over 80%N₂ selectivity（initial concentration1.0 mM）was achieved through the PANI@W₁₈O₄₉ without sacrificial agent under 300W Xe lamp irradiation for 60 min.Such advantages were attributed to the built-in junction between n-type W₁₈O₄₉ and p-type PANI,offering suitable redox levels of electron-hole pairs for NO₂^-reaction.The modification of PANI also benefited the light harvesting ability and activated carriers migration,the calculated rate constant of PANI@W₁₈O₄₉ is about four times as high as that of W₁₈O₄₉.Reduced graphene oxide supported polyaniline/W₁₈O₄₉ nanobelt-flower(RGO/PANI@W₁₈O₄₉)ternary composite was designed through a solvent volatilization method for photocatalytic reduction of bromate.The prepared sample was characterized,and the influence of aqueous pH,ions and dissolved oxygen on the bromate reduction was explored.As expected,the introduction of RGO and PANI on the W₁₈O₄₉9 exhibited great synergistic effects on the separation of photogenerated carriers.The calculated reduction rate constant of RGO/PANI@W₁₈O₄₉was about six times as high as that of pure W₁₈O₄₉.Specially,the prepared photocatalyst possessed strong optical absorption in a broad range of 250-2500 nm,and thus displaying excellent catalytic performance in utilization of all solar spectrum energy.Moreover,the RGO/PANI@W₁₈O₄₉ exhibited stable photocatalytic activity in cycling test.Overall,the current study demonstrate the potential of the new obtained photocatalysts in efficient utilization of solar energy for treating Cr（VI）,nitrite and bromate.