| Heavy metals and organic pollutants caused by human beings have become major threats to the environment and organisms.The methods to remove these pollutants include physical adsorption,chemical precipitation,membrane filtration,photocatalysis and so on.Among these methods,photocatalysis is a promising technology,which can achieve the one step to remove pollutants over light irradiation.Traditional photocatalysts such as titanium dioxide,zinc oxide and cadmium sulfide have some problems such as easy recombination of photogenerated carriers and poor stability.It is urgent to find new and efficient photocatalysts to reduce and degrade heavy metals and organic pollutants.In this research,two new metal-organic frameworks were synthesized based on the principle of crystal engineering.The photocatalytic reduction of hexavalent chromium and the degradation of dye wastewater were studied.At the same time,the photocatalytic efficiency of Cr（Ⅵ）in the presence of dyes was explored,and the reaction mechanism was verified.For the synthesis of metal-organic frameworks’composite,UiO-66,MIL-100（Fe）,g-C3N4 and BiOI have been selected as the original catalysts,and their compounds’photocatalytic activities for the reduction of hexavalent chromium were explored under white or visible light irradiation,and the mechanism was explored.The main works of this paper are as follows:1.Two new 2D metal-organic frameworks,[Cd（bpy）（H2O）L]n（BUC-66）and {Co（bpy）（H2O）L]·H2O}2n（BUC-67）,（H2L=cis-1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid,bpy=4,4’-bipyridine,BUC is taken from the first three initials of the Beijing University of Civil Engineering and Architecture）,have been synthesized under hydrothermal conditions,which were characterized by X-ray single crystal diffractometer, powder X-ray diffraction,fourier transform infrared spectroscopy and thermogravimetric analysis.The results revealed that both BUC-66 and BUC-67 displayed outstanding photocatalytic Cr（Ⅵ）reduction performances with reduction efficiency more than 98% within 30 min under UV light irradiation,much better than that of commercial P25（24%）.Also,BUC-66 and BUC-67 could photocatalytically decompose organic dyes,such as methylene blue,rhodamine B,methyl orange and reactive red X-3B.These two metal-organic frameworks exhibited considerable photocatalytic activities in the Cr（Ⅵ）/organic dyes matrix.Finally,a possible photocatalytical reaction mechanism have been proposed and verified in detail.2.Series direct Z-scheme g-C3N4/UiO-66（BGxUy）heterojunctions constructed from 3D ultra-stable metal-organic framework UiO-66 and 2D nonmetallic polymer semiconductor g-C3N4 sheets were facilely fabricated using ball-milling method,which were characterized by powder X-ray diffraction,fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,N2 adsorption-desorption measurement,thermogravimetric analysis,scanning electron microscopy,transmission electron microscopy,high-resolution transmission electron microscopy,UV-Vis diffuse reflectance spectroscopy, photoluminescence spectroscopy and electrochemical impedance spectroscopy measurement.The photocatalytic activities of Cr（Ⅵ）reduction over the g-C3N4/UiO-66 complex were tested upon the irradiation of white light.The influence factors on photocatalytic performances of BG60U40（the mass ratio of UiO-66 to g-C3N4 was 40:60） like different organics（tartaric acid,citric acid and oxalic acid）,pH（2-8）,real sunlight and different co-existing foreign ions（ions in tap water,lake water,simulated sea water, and simulated leather tanning water）were clarified.The experiment results demonstrated that the optimal BG60U40 exhibited superior Cr（Ⅵ）reduction efficiency than pristine UiO-66 and g-C3N4 due to the declined recombination of photo-induced charge carriers.At the same time,photoluminescence spectroscopy and electrochemical impedance spectroscopy confirmed the improvement of photocatalytic performance.As well,BG60U40 displayed good reusability and stability in the cyclic experiment.The corresponding mechanism of photocatalytic Cr（Ⅵ）reduction was proposed and verified by electrochemical analysis,electron spin resonance measurement,density functional theory calculations and series capture experiments toward active species.3.Series direct Z-scheme heterojunctions were facilely fabricated using ball-milling method,which were prepared by Fe-containing MOFs "MIL-100（Fe）" nanospheres and bismuth-based semiconductor "BiOI" that excited by visible light.And they were characterized by powder X-ray diffraction,fourier transform infrared spectroscopy, thermogravimetric analysis,UV-Vis diffuse reflectance spectroscopy,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy, photoluminescence spectroscopy and electrochemical impedance spectroscopy measurement.The photocatalytic activities of Cr（Ⅵ）reduction was studied under visible light.It was found that when the mass ratio of MIL-100（Fe）to BiOI was 40:160,the reduction efficiency of Cr（Ⅵ）was the highest and the Cr（Ⅵ）has been reduced completely at 80 minutes,which was significantly better than that of pure MIL-100（Fe） （4%）and BiOI（80%）.At the same time,photoluminescence spectroscopy and electrochemical impedance spectroscopy confirmed the improvement of photocatalytic performance.The effects of pH,small-molecular organics and coexisting ions over the photocatalytic activities were investigated.Finally,the active substances in photocatalytic process were captured by electron spin resonance measurement and series capture experiments,and the corresponding mechanism of photocatalytic process was proposed. |
