Facile One-step Synthesis Of Magnetically Modified Biochar With Enhanced Removal Capacity For Hexavalent Chromium From Aqueous Solution

As an adsorbent,biochar has the advantages of wide source of raw materials,low cost,large specific surface area,and abundant pore structure.Meanwhile,it has the defects that the material is not easy to recover and poor adsorption behavior for specific pollutant.In this study,waste bamboo was used as a biomass.Ferric chloride and diethylenetriamine(DETA)were used as modifiers to prepare magnetic biochar(MBC),which could be easily recoverable,and magnetically modified biochar(NMBC),which had magnetism and better adsorption behavior.These two adsorbents were compared to adsorb hexavalent chromium in solution.This research explored the optimal preparation conditions of MBC,as well as NMBC with different DETA adding.The adsorption properties of MBC and NMBC for hexavalent chromium under different conditions were also studied using batch experiment.The results showed that the optimal preparation conditions of magnetic biochar were as follows:the pyrolysis temperature was 600?,the pyrolysis time was 1 h,the FeCl3 impregnation ratio was 1.5:1.The initial pH had significant effect on the removal of hexavalent chromium.The removal efficiency of hexavalent chromium under acidic conditions was better than than alkaline conditions.Compared with MBC,NMBC had a greater advantage of adsorbing hexavalent chromium in acidic solutions.The effect of ionic strength on MBC adsorption was insignificant.The optimal dosage was 1 g/L,and the adsorption efficiency was almost the same after multiple recycling.The difference was that the ionic strength had a greater impact on 4NMBC.And after multiple recycling for 4NMBC,the removal efficiency of hexavalent chromium was significantly reduced.The adsorption process of hexavalent chromium by MBC and NMBC was consistent with the Tempkin adsorption isotherm model,in which the maximum adsorption of hexavalent chromium by MBC was 19 mg/g.With the increase of DETA adding,the maximum adsorption of hexavalent chromium by NMBC increased firstly and then decreased.4NMBC could reach the highest maximum adsorption,48 mg/g.The adsorption of hexavalent chromium by NMBC conformed to pseudo second-order kinetic model,which implied the adsorption process was chemisorption based on the formation of chemical bonds.Moreover,the adsorption process of hexavalent chromium by MBC and 4NMBC was an endothermic reaction,and the degree of chaos in solid/liquid interface increased during the adsorption process.The increase in temperature favored spontaneous adsorption of hexavalent chromium by MBC and NMBC.X-ray diffraction(XRD),elemental analysis(EA),scanning electron microscopy(SEM),hysteresis curve,Zeta potential,Fourier transform infrared spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XPS)were used to analyze the physicochemical characteristics of MBC and NMBC.The results showed that both MBC and NMBC contain Fe3O4 crystals,but the addition of DETA affected the degree of Fe3O4 crystallinity in the material,as well as the magnetism.But 4NMBC had enough magnetism to achieve solid-liquid separation by magnets.With the increase of DETA adding,the N content in NMBC gradually increased,and the functional groups in biochar were improved,such as hydroxyl(-OH),amino(-NH),carbon-hydrogen(C-H)and carbon-oxygen double bond(C=O).In addition,the mechanism of MBC and 4NMBC adsorbing hexavalent chromium was studied.The results showed that the adsorption of hexavalent chromium by MBC and 4NMBC included hydrogen bonding and electrostatic attraction besides the adsorption of pores.The difference was that the adsorption of hexavalent chromium by MBC contains oxidation-reduction reaction where hexavalent chromium was reduced to trivalent chromium.On the contrary,the adsorption process of hexavalent chromium by 4NMBC was mainly chemisorption to generate chemical bonds.