Study On The Photocatalytic Reduction Of Cr (Ⅵ) By Selective Adsorption And Photocatalytic Activity Of TiO 2

In recent years, with the development of economy and society, environmental pollution has become an increasingly serious problem. Among the environmental pollution, heavy metal contamination is one of the key of the environmental pollution. Amongthe heavy metal ions pollution, the pollution of Cr（Ⅵ） is particularly acute. Photocatalytic technology is a kind of environmental friendly technology,and it has a broad application prospect in green water treatment, whichhasthe mild reaction conditions, easy to operated, and can directlyuse the sun light. However the research about the photocatalytic technology in waste water treatment on heavy metal contamination develops comparatively slowly, due to the smaller surface area than the traditional adsorbentand the easy photoelectron-hole recombination of the photocatalysts. And the above disadvantages of thephotocatalysts lead to the slow photocatalytic reduction rate of heavy metal ion, especially in thewater wastewhich has low concentration of heavy metal ion, and after the reduction of the heavy metal ion, the product is difficult to stripping from the photocatalyst, which leads to the deactivation of the photocatalyst.The purpose of this paper is to develop an adsorption-photocatalytsis- stripping technology to remove Cr（Ⅵ） in the waste water. Among the semiconducts or photocatalysts, TiO₂ is the most promising photocatalyst because of its advantageous properties of non-toxicity, lowcost, high activity and strong stability in aqueous media. We prepared one kind of the mesoporous single crystal Ti O₂ which has high specific surface area. Also we enhancedthe light utilization rate, reduced the electron-hole recombination, and improved the photocatalytic efficiency of the photocatalyst by the tailoring and modification of the nanostructure（such as mixed or composite surface defect,position control, surface composition modulation, etc.）. According to the characteristics of heavy metal ion wastewater treatment, we attempt to solve the following three key problems:（1） Adsorption of low concentration of Cr（Ⅵ） in the aqueous solution. It will not only quickly remove Cr（Ⅵ） in water, but will also promote the subsequent photocatalytic reduction progress.（2） By changing the surface chemical property of the photocatalyst, Cr（Ⅲ） reduced from Cr（Ⅵ） in photocatalytic reaction canbe desorbed quickly from the photocatalyst surface,avoidingthe photocatalyst poisoning.（3） Explore the mechanism of photocatalyst adsorption and desorption of heavy metal ions, the mechanism ofreduction, the synergistic effect, and discuss the structure-function relationship of the photocatalyst.Based on the above goals, we design several photocatalytic materials as follows.（1） The study of hydroxyl modified mesoporous single crystal TiO₂ preferential adsorption Cr（Ⅵ） and photocatalytic reduction to Cr（Ⅲ）.Using solvothermal method preparedthe mesoporous single crystal TiO₂（MS） with high specific surface area and we successfullymodified hydroxyl group on its surface. XRD, XPS, TEM, SEM, BETetc. were used to characterize these photocatalysts. We tested the adsorptive performance of Cr（Ⅵ） and Cr（Ⅲ） in mixture solution and the photocatalytic reduction of Cr（Ⅵ）. The results show that hydroxyl modified mesoporous single crystal Ti O₂（MS-OH） has high specific surface area（153 m2/g）,meanwhile,MS-OH has a higher performance of selective absorption of Cr（Ⅵ） and photocatalytic reduction of Cr（Ⅵ） than MS.（2） Ti³⁺/TiO₂ preferential adsorption Cr（Ⅵ） and photocatalytic reduction of Cr（Ⅵ） to Cr（Ⅲ）.Using three titanium source one-step method prepared Ti³⁺/TiO₂. Under acid condition, its surface positively charged. Further study of the selective adsorption of hexavalent chromium ion and promote the photocatalysis. Using solvothermal method prepared thenegligiblysmall specific surface area of Ti³⁺/TiO₂, to eliminate the influence of specific surface area before and after the calcinations.which ensures that the adsorption and reduction process only related to its doping content of Ti³⁺. Experimental results demonstrate that the more doping content of Ti³⁺, the more positive charge on its surface, and the higher selective adsorption of Cr（Ⅵ）, which in turn promotes the process of photocatalytic reduction of Cr（Ⅵ）.（3）Single layer graphene/TiO₂ nanoparticles preferential adsorptionCr（Ⅵ） and photocatalytic reduction to Cr（Ⅲ）.Using solvothermal method prepared TiO₂ nanoparticles（nano-TiO₂） and Single layer grapheme/TiO₂ nanoparticles（GO/nano-TiO₂）. Nano-TiO₂ was uniformly loaded on the grapheme surface, which improved the contact site andwas benefit for electron transfer. Proved by the result of the experiment that the presence of graphene effectively improve the selective adsorption of Cr（Ⅵ）, and at the same time improve the photocatalytic reduction rate.