Adsorption/Passivation Of Cadmium By Fe-Mn Oxide-Biochar

Biochar has a large specific surface area and porosity.It can be made by a wide range of raw materials with low cost and high adsorption performance.Much attention has been paid on biochar because of the increasing of carbon sinks and improving soil quality after its addition to Cd-contaminated soil.At present,biochar has become an effective agricultural soil restoration material.In this paper,the pristine biochar?BC?was pyrolyzed at 600?for2 h under N₂.BC was impregnated with ferric nitrate,and then pyrolyzed again at 600°C for0.5 h under N₂ to obtain Fe-Mn binary oxide-biochar?FMBC?.The physical-chemical characteristics of BC and FMBC were revealed by Scanning electron microscopy?SEM?coupled with energy dispersive X-ray spectroscopy?EDX?,X-ray photoelectron spectroscopy?XPS?,X-ray diffraction?XRD?,Fourier transform infrared?FTIR?spectroscopy and specific surface area?SSA?analyses.Factors that affecting the Cd???adsorption,as well as its adsorption mechanisms by FMBC in aquatic Cd???solution were investigated.The changes of soil p H,Cd speciation,cation exchange capacity,ammonium nitrogen,available potassium and available phosphorus were studied after FMBC addition to the actual Cd-contaminated soil.The main conclusions were as followings:?1?The Cd???adsorption by FMBC confirms the quasi-second-order kinetic model,indicating that the adsorption process is dominated by chemical adsorption;the Cd???adsorption kinetics by both materials were more adaptable to the Langmuir model,and the maximum adsorption capacity is 95.23 mg/g?FMBC?>30.58 mg/g?BC?.?2?SEM-EDS,XRD,FTIR and XPS analysis results show that iron manganese oxide is successfully loaded on the surface of BC,and FMBC has a larger specific surface area?8.8030 m²/g?and pore volume?0.0205 cm³/g?.The forms of Cd???in FMBC are Cd₂Mn₃O₈,Cd₃P₂,Cd P₂ and Cd?OH?₂.The carboxyl group and hydroxyl group decrease,the Mn Ox peak almost disappears,and the hydroxyl group and carboxyl group form complex with Cd???after Cd???adsorbed by FMBC.Manganese oxide forms a precipitate with Cd???.Cd-O group appeared after FMBC adsorbed Cd???,and Cd?OH?₂ and Cd O accounted for63.9 wt%and 36.1 wt%respectively.Ion exchange,precipitation,complexation and cation-?bond interaction are the main mechanisms of FMBC adsorption of cadmium.?3?FMBC was added to the Cd-contaminated soil at a dosage of 1%for the passivation experiment showed that soil p H?from 6.79 to 7.44?and cation exchange capacity?CEC??from 4.75 cmol/kg to 17.20 cmol/kg?were increased significantly.The content of soil organic carbon and the available potassium were also increased.No obvious changes were observed for the available soil phosphorus and ammonium nitrogen.The available soil cadmium decreased 7.24%-42.50%.The soil exchangeable cadmium content for the FMBC treatment decreased significantly,while the Fe-Mn oxide-bound cadmium content and residue cadmium content increased.Compared with the controlled,the soil exchangeable cadmium decreased at different culture time after FMBC addition:90d?57.31%?>60d?32.42%?>30d?18.01%?.