Research On Characteristics And Mechanism Of Chromium(?) Adsorption Onto Modified Agroforestry Biocharcoal

Due to excessive discharges of chromium containing waste residues and wastewaters from electroplating,metal processing,leather making and various other types of industries in China,water pollution has become a highly concerned issue because of its harmful effect that can cause serious damage to ecological environment and human health.Biological adsorption using biomass materials as adsorbents is an effective method to remove pollutants from wastewater.However,the application of bio-adsorbents made of agricultural and forestry residues is limited by their shortcomings such as low density and difficulties in both granule formation and particle-water separation.In recent years,more and more attention has been paid to the further optimization of physicochemical properties and surface structures of biomass materials with purpose to improve their adsorption properties.In the present study,modification treatments using rice bran and rape straw as raw materials and ferric chloride hexahydrate as modifier were carried out to produce modified corn bran biochar(Fe/MCB)and rape straw biochar(Fe/RSB).The adsorption capacity of the modified adsorbents to remove Cr(?)ions from aqueous solution was examined.The balance test was used to compare the adsorption advantages of using corn bran biochar(MCB)and rape straw biochar(RSB)as adsorbents.Equilibrium adsorption experiments were conducted to determine the optimal pyrolysis-carbonization conditions for preparing the modified biochar adsorbents.Batch adsorption tests were designed to investigate the effects of adsorbent dose,initial ion concentration,solution pH,reaction time and temperature on Cr(?)ion adsorption.Scanning electron microscopy and Fourier transform infrared spectroscopy technologies were applied for characterization of the adsorbent surface physicochemical properties.of For a better understanding of the Cr(?)adsorption mechanism,linear fitting analyses of the obtained experimental data were further conducted using isothermal adsorption model,dynamic boundary adsorption model,kinetics model and thermodynamic model.The main results are as follows:1)The optimal pyrolysis-carbonization conditions for preparation of the modified corn bran biochar Fe/MCB and modified rape straw biochar Fe/RSB were carbonization at 550? for 4h and carbonization at 650? for 2h,respectively.The applied modification processes solved the low density problems of the raw materials and enhanced the adsorption capacity of the tested adsorbents.Sigificant differences were observed among the adsorption capacities of Fe/MCB,Fe/RSB,corn bran biochar MCB,and rape straw biochar RSB after modification.The adsorption capacity of the tested adsorbents was following the order:Fe/MCB>Fe/RSB>RSB>MCB,and their adsorption capacity qm values were esatimated as 12.01,9.27,4.40 and 4.22 mg/g,respectively.2)SEM and FTIR analysis showed that the surface morphology of the adsorbent,as well as the number and type of functional groups,had changed significantly.The characteristic peak of Fe-O stretching vibration was observed at 571 cm-1 in the infrared spectrum,indicating that the iron oxide ?-Fe2O3 was successfully embedded into Fe/MCB and Fe/RSB.3)The highest equilibrium Cr(?)adsorption density value was obtained at pH=2 and the adsorption density value decreased with increasing pH in the tested range.The protonation effect of the surface functional groups of Fe/MCB and Fe/RSB was stronger under acidic conditions than that under alkaline conditions,giving thus higher Cr(?)adsorption values at lower solution pH levels.4)Experimental data fitness analyses using Langmuir,Freundlich,D-R and Temkin adsorption isotherm models showed that Langmuir isotherm was the best model for description of the entire adsorption process,suggesting that the process of Cr(?)ion removal from aqueous solution onto Fe/MCB and Fe/RSB was similar to single molecule layer adsorption combined with non-uniform surface adsorption.The main controlling steps of the adsorption process were intra particle diffusion and chemical reaction between Cr(?)ions and active radicals possessed on Fe/MCB and Fe/RSB surfaces.5)The R2 values of pseudo-second-order equation were 0.9995 and 0.9999,respectively,The fitted qe value was close to the measured value,indicating that the adsorption behavior of Cr(?)ions onto Fe/MCB and Fe/RSB can be satisfactorily described by the pseudo-second-order equation.Particle diffusion equation fitted curve did not go through the origin,indicating that intragranular diffusion is not the only rate-control step.6)The standard enthalpy change ?H0 was greater than zero,indicating that the adsorption reaction process was endothermic and high temperature was thus favorable to adsorption of Cr(?)ions onto Fe/MCB and Fe/RSB.The ?H0 values of the adsorption reactions obtained under different temperature conditions were found between 4?10 kJ·mol-1,showing the main effect of temperature on Van der Waals' force in the adsorption reaction processes.The standard enthalpy change ?S0 was higher than zero,indicating that the reaction between Cr(VI)ions and the tested adsorbents caused increase in disorder of the adsorption system and thus the reaction was a process of entropy increase.The observed negative values of the standard change in the Gibbs free energy ? G0 confirmed that the absorption reaction processes were spontaneous.