Preparation Of Carbon-based/Nano-zero-valent Iron Composites And Their Removal Properties Of Cr(Ⅵ) And U(Ⅵ) In Water

This article is based on the preparation of biochar-based/nano-zero-valent iron composites and the removal properties of Cr（Ⅵ）and U（Ⅵ）.Two kinds of adsorbent composites-wheat straw biochar/nano zero-valent iron composite（ABC/NZVI）and red tea powder biochar/nano zero-valent iron（RBC/NZVI）,were prepared successfully.The physical and chemical properties of the composite were measured by means of scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X ray diffraction（XRD）,Fourier infrared spectroscopy（FTIR）,thermogravimetric analysis（TG）,surface area testing（BET）and X line photoelectron spectroscopy（XPS）.Effects of solution pH、adsorption time,adsorption temperature and biochar pyrolysis temperature on the adsorption properties of target ions via subsequent batch adsorption experiments on Cr（Ⅵ）and U（Ⅵ）.Combined with the adsorption model and the microscopic characterization of the material before and after adsorption,the adsorption mechanism of composite materials on Cr（Ⅵ）and U（Ⅵ）was discussed.（1）Preparation of amino-modified biochar/nano-zero-valent iron composites（ABC/NZVI）at different pyrolysis temperatures and their adsorption on Cr（Ⅵ）.The pretreated wheat straw was pyrolyzed at 300℃,400℃and 500℃by high temperature pyrolysis to achieve preparation of biochar precursors.After amino modification from KH-550,NZVI were loaded on the ABC via chemical reduction.ABC/NZVI composites were successfully prepared.The elemental composition,morphology and surface structure of the composites were analyzed with a series of characterization results.XRD diffraction patterns proved that NZVI was loading on biochar successful.Through the comparison between BC and ABC/NZVI in the FTIR spectra,the scissors peaks of 3503 cm^-1and 3437 cm^-1 were found to be amino functional groups.TG curves shows that the final weight loss rate of the ABC/NZVI increases,indicating that the thermal stability of the composite was improved,which may be caused by the loading of silane groups from KH-550 and NZVI.During the batch adsorption experiments on Cr（Ⅵ）,the optimum pH for adsorption is 2 and the increase of reaction temperature is beneficial to the adsorption of Cr（Ⅵ）.The loading of NZVI resulted to increasement of the adsorption capacity of biochar to Cr（Ⅵ）by about 7 times.By comparing the fitting parameters of the two fitting models,it is found that the pseudo-second-order kinetic model could better mimic the removal of Cr（Ⅵ）,indicating that chemisorption occurred at the removal process.By comparing the adsorption properties of three materials at different pyrolysis temperatures,it is found that the adsorption capacity of 300℃pyrolysis temperature is optimal.The removal mechanism of Cr（Ⅵ）is studied with combine from the results of XPS fitting spectra and batch adsorption experiment.On the one hand,it could be concluded that both surface adsorption and electrostatic attraction occurred at the removal of Cr（Ⅵ）,which may be due to the complexation of amino functional groups.On the other hand,the reduction occurred in which NZVI were oxidized to Fe²⁺and Fe³⁺,while Cr（Ⅵ）were reduced to Cr（III）.（2）Preparation of red tea powder biochar/nano-zero-valent iron composite（RBC/NZVI）and its adsorption on Cr（Ⅵ）and U（Ⅵ）.In this study,NZVI was dispersed on the surface of biochar from red tea powder via chemical reduction to prepare RBC/NZVI composites for the removal of Cr（Ⅵ）and U（Ⅵ）.The elemental composition,morphology and surface structure of the composites were analyzed through a series of characterization results.XRD diffraction pattern proved that NZVI was loading on biochar successful.The improvement of final weight loss rate in the comparison of TG curves between RBC and RBC/NZVI,suggested that the thermal stability of the composites were improved.The removal properties of Cr（Ⅵ）and U（Ⅵ）were measured by batch removal experiments,indicated that the most suitable pH for Cr（Ⅵ）and U（Ⅵ）removal were 2 and 6.At the same time,the kinetics performance are fast adsorption and gradually reach the adsorption equilibrium,which the pseudo-second-order kinetic model were utilized to fitting the kinetics curve better.Combined XPS spectrum and batch adsorption experiments,to investigate the removal mechanism of RBC/NZVI on Cr（Ⅵ）and U（Ⅵ）.It turned out that when Cr（Ⅵ）and U（Ⅵ）were removed,not only surface adsorption occurred,but also heavy metal ions were reduced from higher valence to lower valence by NZVI.