| Visible-light photocatalysis was an environmental friendly and efficient advanced oxidation technology for the removement of the pollutants and the pre-treatment of sample.This technology's advantages were high efficiency,low energy consumption,simple operation,mild reaction conditions,wide application range,and little secondary pollution.Titanium dioxide is the most widely used photocatalyst for its advantages of large specific surface area,non-toxicity,low cost,and long service life.However,the usage of TiO2 was limited by its strong polar,high surface free energy,easily reunitation,and poor dispersing capability.Meanwhile,because of its broad energy gap,the photocatalytic process should be induced by the ultraviolet light.The extreme low surface coverage of organic pollutants on the catalyst TiO2 resulted in the low adsorption and photocatalytic efficiency.Surface modification of nano-TiO2 could increase the light utilization and the surface coverage of organic pollutants,which were crucial for the photocatalytic activity of heterogeneous photocatalysis.Aromatic pollutants(e.g.,acid violet 43,methyl violet,and 4-nitrophenol)were characterized by a high chemical/bio logical oxygen demand,deeply tinct,and strong toxicity,and some of them were USEPA priority pollutants.They were difficult to be degraded by usual technologies,including biotechnology,physical and chemical methods,so,the study of environmental friendly,efficient,economical and convenient method was important.Chemical oxygen demand(COD)was the key index for the assessment of organic pollution degree in aqueous system.The conventional K2Cr2O7 method for COD determination was accurate,reliable,.and reproducible,but it has some drawbacks,such as low detection sensitivity,long time consumption,complicated and tedious operation,using huge amounts of water and electric power,incomplete oxidation and mineralization,undesirable and high consumption of expensive(Ag2SO4),corrosive(concentrated H2SO4)and toxic(Cr and Hg)reagents and secondary pollution.Therefore,developing a rapid,simple,environmental friendly and accurate method for COD detection still remains a challenge in environmental and analytical field.The conclusions of this study were as follows:(1)Using different particle size nano-TiO2(5-10,25,40,60,100nm)to adsorb acid violet 43,the adsorption kinetics and thermodynamics were investigated.Researches of kinetics show that the pseudo-second-order model was found to explain the adsorption kinetics most effectively.It was also found that the membrane diffusion and internal diffusion were the main control factor in the adsorption.Researches of thermodynamics show that the adsorption behavior of nano-TiO2 for acid violet 43 obeys the Langmuir isotherm model.The heat of adsorption ?H>0 means that the sorption process is an endothermic process,and ?H<40 kJ/mol means that the sorption process is mostly physical adsorption.At the same time,Gibbs free energy ?G<0 indicates that the sorption process is a spontaneous process.(2)As a post-synthesis treatment method,surface modification for change of the sizes and crystalline structures of TiO2 nanoparticles is explored for the first time.After surface modification,several new phenomena are explored:(a)Crystalline structure of TiO2 nanoparticles is gradually transformed from anatase to brookite;(b)Sizes of TiO2 nanoparticles become smaller;(c)Surface areas of TiO2 nanoparticles increase;(d)Its UV-vis wavelength response range is expanded from 375 nm to 600 nm;(e)Both adsorption ability and photodegradation activity toward 4-nitrophenol and methyl violet are remarkably enhanced.(3)After surface modification,TiO2 nanoparticles as photocatalysts were employed to develop an effective,rapid,simple,and environmental friendly method for chemical oxygen demand(COD)detection.The COD value was calculated from the changed absorbance of Mn(?).When Ti02 was surface modified by 24.82?g/mg of 5-SA and used under the optimal conditions,the linear range of the calibration curves was expanded as 0.3-400mg/L.This new approach overcame the disadvantages of TiO2 for COD detection techniques,i.e.,the UV irradiation,the low oxidation percentage,and a narrow linear range(2-12mg/L,50-300mg/L).The 5-SA_TiO2-KMnO4 method was applied for the determination of tap waters,Jiu-long river waters,hoggery wastewaters,municipal wastewaters treatment plant influents,and dying wastewaters.The results agreed well with the certified value determined by K2Cr2O7 method.Moreover,the recoveries were reasonable for COD detection,in a range of 95-103%. |
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