| In recent years,the heavy metal Cr(VI)has caused serious harm to human beings and environment,causing widespread public concern.The adsorption method has become an important technology for Cr(VI)removal because of its low cost,simple operation and good adsorption capacity.In this study,by loading graphene oxide with agricultural waste rice husk,which was used as a raw material,a novel adsorbent named graphene oxide-rice husk biochar(GO-RHB)was synthesized.To decipher the Cr(VI)removal efficiency and reaction mechanism by GO-RHB,the microscopic characterization of materials,the macroscopic influence parameters,and the fitting of various models were studied.To study the effect of Cr(VI)removal by GO-RHB,the adsorption performances,such as GO content,initial pH,adsorbent dosage,initial Cr(VI)concentration,reaction time,temperature,ionic strength and recycling times were investigated.The experiment results showed that the Cr(VI)adsorption capacity by GO-RHB composite material was higher than that of single RHB material,and the optimal loading content of GO was 5%.The pH was a very important parameter during the reaction process,GO-RHB had higher Cr(VI)adsorption capacity under acidic than under alkaline conditions,and the optimum pH value and zero potential of the reaction were both at pH of 2.The optimum adsorbent dosage was 8 g·L-1.The Cr(VI)adsorption capacity was increasing with the increment of Cr(VI)initial concentration and reaction time,and the adsorption equilibrium was reached at 240 min.The Cr(VI)adsorption capacity would be effected when the ionic strength in the solution was high.The Cr(VI)removal efficiency was up to 85.1%after the GO-RHB composite was recycled for 6 times.The adsor:ption kinetics of Cr(?)on GO-RHB followed the pseudo-second-order model,indicating that the adsorption process was mainly chemical adsorption.Furthermore,the Webber-Morris internal diffusion model and the Boyd model fitting revealed that the main controlling step of the diffusion rate was film diffusion.By simulating the adsorption isotherm model,the results showed that the adsor:ption process conformed to the Langmuir model,indicating that the reaction process was monolayer adsorption.The maximum adsorption capacity of Cr(VI)was 48.8 mg·g-1 at 45?.By calculating the relevant parameters of thermodynamics,the results of?H°>0,?S°>0,?G°<0 were obtained,and the value of AG°increased negatively with the temperature increasing,indicating that Cr(VI)removal by GO-RHB was a spontaneous endothermic reaction with entropy increasing.The physicochemical properties of the GO-RHB composite was analyzed by SEM-EDS,FTIR,XPS,BET and Zeta potential.The results showed that the surface roughness of GO-RHB composite increased after GO loading onto RHB,and the specific surface area of GO-RHB material was about 7 times of RHB specific surface area;The FTIR characterization results illustrated that the content of oxygen-containing functional groups such as hydroxyl groups and carboxyl groups on the GO-RHB composite was much higher than that of the RHB material.From the elements content analysis of XPS,the C/O of GO-RHB and RHB were 2.81 and 4.23 respectively,indicating that the content of O element in GO-RHB composites increased a lot;The two orbital peaks of Cr2p3/2 and Cr2pi/2 appeared in the Cr2p spectrum of XPS,showing the presence of Cr(VI)and Cr(III).It can be seen from the spectra of C1s and O1s after adsorption that the binding energy of C=O and C-O increased at the same time,and a new O-Cr peak appeared,indicating that Cr(VI)reacted with the oxygen-containing functional group on the surface of GO-RHB during the reaction process.Based on the results of comprehensive macroscopic experiments,the microscopic characterization of materials and the fitting of various models,it was concluded that the adsorption and removal process of Cr(VI)by GO-RHB composites mainly includedfour parts:(1)pore adsorption;(2)electrostatic action;(3)reduction;(4)complexation. |
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