Adsorption Of Cr(?) By Magnetic Biochar

Chromium pollution has become an urgent environmental problem in the world.The current treatments of chromium-containing wastewater cover adsorption,ion exchange,precipitation,reverse osmosis methods,etc.Compared with other methods,adsorption methods are simple to operate,environmentally friendly and low cost,therefore they have received widespread attention in recent years.Biochar is a kind of effective adsorbent for removing heavy metal pollutants in waste water due to the present unique surface chemistry,rich pore structure,and low preparation cost.However,the fine particles of biochar powder make it difficult to quickly separate them from aqueous solutions.In this study,waste wheat straw was selected as the raw material for preparing magnetic biochars MBC1,MBC2,MBC3 and MBC4 modified with Fe₃O₄,FeCl₃,FeCl₃/FeSO₄ mixtures and FeSO₄ respectively.MBCs were easily separated from water by magnetic forces.The adsorption performances and mechanism of Cr???in aqueous solution by magnetic biochars were further investigated.XRD,FTIR and XPS characterization analysis results showed that Fe-O groups formed on the surface of magnetic biochars,indicating that magnetic particles were successfully loaded on the surface of the biochars.MBC1 and MBC2 were successfully loaded with Fe₃O₄ and?-Fe₂O₃ respectively,while the two crystalline forms coexisted in MBC3 and MBC4.The resulted magnetic property benefited the separation of biochars from the aqueous solution under the magnetic field.The results of adsorption experiments showed that the magnetic biochars demonstrated the best adsorption performance at pH 2 and the adsorptions approched equilibrium within360-720 min.The adsorption performance for Cr???of magnetic biochars were better than that of unmodified biochar.The saturated adsorption capacity of MBC3reached 34.4 mg·g^-1.A second order mathematical model was established between the adsorption capacity qe and the three independent variables pH,Cr???initial concentration and temperature by response surface tests.Statistical analysis of variance and error showed that the fitted model has a higher confidence level.The model was suitable for the optimization of Cr???adsorption by magnetic biochars.The adsorption of Cr???by magnetic biochars conformed to the quasi-second-order kinetic models and the Langmuir isothermal adsorption models.The adsorption of Cr???on MBCs were single-layer adsorption,limited by the surface adsorption sites.The values of thermodynamic parameters free energy change??G?was negative,enthalpy change??H?was positive,and entropy change??S?was positive,which indicated that the adsorption of Cr???on MBCs were spontaneously endothermic processes.Based on the XPS analysis for the MBCs adsorption before and after,it was revealed that there existed redox reactiones and chemisorption between Cr???and the functional groups including Fe-containing on the MBCs.