Study On The Quick Removal Of Nitrobenzene And Cr(?) With Micro-nano Hierarchical Zero-valent Iron

With the rapid development of China's industry, the discharge of all kinds of industrial wastewater has caused a great threat to the environment. The pollutants in industrial wastewater are complex and toxic, therefore, the development of the methods to remove pollutants in industrial wastewater rapidly is a important significance. Due to its high activity, strong reduction, and friendly environment, nano-zero-valent iron treatment technology has been widely concerned. Howerer, owing to its easy agglomeration, spontaneous combustion, transportation inconvenience and so on, leading to its application is greatly limited. If we can make full use of the advantages of nano-zero-valent iron and overcome its disadvantages, which will greatly broaden the application of the zero-valent iron in the removal of pollutants. Therefore, we prepare micro-nano hierarchical zero-valent iron (referred to as mnZVI) which is assembled by iron nanoparticles with liquid phase reduction method. Powder X-ray diffraction, scanning electron microscopy, high resolution electron microscopy and X-ray photoelectron energy spectrum characterizations show that mnZVI is been formed by self-assembly of iron nanoparticles. In addition, mnZVI has spherical amorphous structure, micro-nano hierarchical structure, and high porosity. What's more, mnZVI possesses the high activity of nano-zero-valent iron and stability of micro-zero-valent iron. Therefore, we use mnZVI to remove some oxidizing pollutants in wastewater, and study its influence factors and mechanism:1. Study on the reduction and removal of nitrobenzene with mnZVI. In the absence of stirring and adjusting pH and the solution for neutral conditions,0.112 g mnZVI reduces 20 mL 20 mg-L-1 nitrobenzene to aniline in 9 s completely. The average removal rate of nitrobenzene reaches 2.22 mg·L-1·s-1, more than 1000 times faster than recently reported in the literature. Series of comparative experiments show that the rapid removal of nitrobenzene lies in the following reasons:(1) mnZVI posesses amorphous structure, large surface and high activity. (2) The Tafel curves show that the corrosion potential of the mnZVI is much lower and much easier to give the electrons to the others. (3) ESR characterization shows that the reaction of mnZVI and H2O can produce a large number of hydrogen free radicals instantaneously. The intermediates of reduction of nitrobenzene with mnZVI is detected by high perfotmance liquid phase chromagraphy. It is speculated that the mechanism of reduction of nitrobenzene as follows:mnZVI can give electronics quickly to H3O+in the solution easily and quickly, then produce large amounts of hydrogen radicals. Hydrogen radicals have strong reduction ability and can rapidly reduce nitrobenzene to aniline.2. The research on the removal of Cr(?) with mnZVI.0.02 g mnZVI can remove 50 mL 20 mg·L-1 potassium dichromate solution in 40 min completely (without adjusting the solution pH, pH= 6.25, stirring speed of 750 rpm).The rate of removal is line well with the first order kinetic equation and much faster compared to the reports in the literatures. The mechanism of removal of Cr(?) is reduction and co-precipitation, and the removal rate increases as the pH of the solution decreases, if regarding the removal process of Cr(?) as 100% adsorption, the adsorption process accords well with Elovich first order dynamics adsorption. In the whloe removal process, Cr(?) in the solution is not detected, which indicates that the removal of Cr(?) contains a redox and a co-precipitation process. The used mnZVI is analysed by XPS, the results shows that the proportion of Cr(?)/Cr(?) on the mnZVI surface increases with the etching time increases, which indicates that the precipitation on the surface of mnZVI hinders its internal electronics transferring to outwards. When the Fe(?) of system is complexed completely, the removal rate of Cr(?) from 100%reduce to 70.9%in 40 min. Therefore, in the process of removal of Cr(?), Fe(0) in the mnZVI plays a major role, while Fe(?) plays a secondary role. Compared mnZVI to its heat treatment material (crystallized mnZVI) in the removal efficiency of Cr(?) and the composition of the two material, finding the amorphous structure of mnZVI is the main reason that the Cr(?) is rapidly removed.