Preparation Of Stabilized Biochar Supported Nano Zero-Valent Iron Composite And Application In Chromium (?) Removal From Water

In recent years,due to its strong reducing ability and high removal efficiency for pollutants,nanoscale zero-valent iron?nZVI?has been widely used in the remediation of organic and inorganic pollutants in groundwater.However,a passivation layer?i.e.oxide layer?readily is formed on the surface of nZVI particles through the reaction of nZVI with non-target substances such as dissolved oxygen,water and/or other solutes in solution.This oxide layer lowers the reducing capacity of nZVI particles by impeding electron transfer from nZVI to the target contaminant.In addition,the freshly prepared nZVI tends to form rapidly larger aggregates because of their magnetic interactions and large surface areas,which also decreases the reactivity of nZVI and thus limits its efficiency of removing contaminants.In this work,nZVI was modified with surface stabilization technology and biochar loading technology,to create a highly stable nZVI composite and apply it to remove chromium?Cr????in water.Here,the synergistic effect of the above two technologies would reduce the aggregation of nZVI and prevent its oxidation.In this paper,peanut shells are selected as biomass raw materials.Biochar?BC?is obtained in a tube furnace by limiting the oxygen and specific temperature.The temperatures were set to 300,500 and 700°C,respectively.Elemental analyzer?OEA?,thermogravimetric analyses?TGA?,scanning electron microscope?SEM?,Fourier transform infrared?FTIR?spectra,and Brunauer-Emmett-Teller?BET?surface area,etc.were used to investigate the influence of carbonization temperature on BC.The obtained BC has porous microstructure with high specific area.With the increase of carbonization temperature,the specific surface area of BC increased from9.062 m²g^-1at 300°C to 61.952 m²g^-1at 700°C;and the average pore diameter has been lowered from 8.958 nm to 3.114 nm.The proportion of micropores continued to increase,reaching 34.27%at 500°C and 40.11%at 700°C.As the carbonization temperature increases,more free radicals are produced on the BC surface.The BC prepared at 500°C and 700°C has high thermal stability.Approximately 22.31%and23.92%weight loss for BC prepared at 500°C and 700°C were observed between room temperature and 800°C in a N₂atmosphere.Conclusively,compared to BC prepared at 300°C?BC300?,BC500 and BC700 have larger pore volume,narrower pore size and higher micropores.The SEM images showed that the nZVI composite?nZVI@BC?supported on biochar?BC500?effectively prevents the agglomeration of nZVI particles,but there was obvious passivation on the surface.After stabilization with sodium citrate?SC-nZVI@BC?and carboxymethyl cellulose?CMC-nZVI@BC?,the passivation phenomenon was effectively improved,and the particles dispersed on BC became regular and smooth.SEM,Vibrating-sample magnetometer?VSM?,and BET characterization showed that the stabilized nZVI@BC has excellent performance.Applied to the removal of Cr???with an initial concentration of 20 mg L^-1,the removal efficiency and removal capacity of Cr???by SC-nZVI@BC reached 100.0%and 199.7 mg g^-1,which was significantly better than that of nZVI@BC(27.2%,54.4mg g^-1)and nZVI(18.7%,37.5 mg g^-1).For the CMC-nZVI@BC system,the decreased magnetization of CMC-nZVI@BC means that the agglomeration between particles is also alleviated.The Cr???removal by nZVI@BC stabilized with CMC reached 80.7%,which is higher than that of nZVI@BC,but lower than that of SC-nZVI@BC under the same conditions.Kinetic studies show that the removal of Cr???by SC-nZVI@BC and CMC-nZVI@BC followed the pseudo second-order kinetics model well.The main influencing factors of Cr???removal in SC-nZVI@BC system were SC dosage,initial p H,mass ratio of C to Fe,and initial Cr???concentration.The appropriate mass ratio of C and Fe is beneficial to the Cr???removal.The Cr???removal increased with the increase of SC molar ratio.The removal capacity of the system for Cr???reached 199.7 mg g^-1at a molar ratio of 1.12 mol%.The acidic environment is conducive to the removal of Cr???,the reaction rate?k₂?is as high as2.121 min^-1?initial p H=5.0?.When the initial Cr???concentration exceeds 20 mg L^-1,the removal capacity of Cr???decreases with the increase of the initial Cr???concentration,to 170.5 mg g^-1at the Cr???concentration of 30 mg L^-1.The calculated maximum removal capacity of Cr???reached 200.0 mg g^-1?initial p H=5.0?,which was higher than that of most Cr???removal by nZVI systems.A mechanism of Cr???remediation by SC-nZVI@BC proposed mainly involves adsorption,reduction and co-precipitation,where SC initiates the depassivation process,dissociating part of the oxides into the solution,thereby further promoting Cr???removal by nZVI@BC.