Preparation Of Magnesium Compound Modified Corncob Biochar And Its Pb(?) Adsorption Performance

With the development of social economy and the growth of industrial scale,heavy metal lead pollution in water has become a colossal menace.At present,using biochar,which is a cheap,green and recyclable material with excellent physical and chemical properties,as adsorbent to adsorb heavy metal is regarded as one of the most suitable methods due to its low price,operability and high capacity.Modifying biochar with metallic oxide/ metal hydroxide,especially magnesium oxide/ magnesium hydrate,is the most effective way to improve the adsorbing ability of biochar on heavy metal.This study used corn cob and its residue after industrialization utilization,xylose residue,which were two kinds of cheap and environmental biomass,as raw materials,and modifying origin biochar with magnesium oxide/ magnesium hydrate/ Layered double hydroxides of different morphologies by impregnation-pyrolysis and coprecipitation method,respectively.After preparing series of modified biochar with outstanding adsorption capacity on lead ions,we tested their properties,such as isothermal adsorption properties,adsorption kinetics,antijamming capability and preferential adsorption.With morphology characterization and confirmatory experiment,we analyzed their adsorbing mechanism.The results are as follows:Firstly,using xylose residue as raw material,origin biochar AM-400 was prepared by carbonized at 400?,and then modifying AM-400 with magnesium oxide particles by impregnation-pyrolysis method at 600 ? for different time to product MgO-BC series biochar.Among this series,MgO-BC-2,which were under pyrolysis for 2 h at 600 ?,had the best adsorption effect.Experiments showed that the adsorption of lead ions by MgO-BC-2 was in accordance with the Langmuir model,and the theoretical maximum adsorption capacity was 1921.2 mg/g,while the adsorption fitted the pseudo-second order kinetics.At the same time,its adsorption of lead ions has a strong selectivity.Secondly,using AM-400 as raw material and melamine as nitrogen source,the Ndoped magnesium oxide modifyed biochar MgO-N-BC was pepared by impregnationpyrolysis method at 600 ? for 2h.Adsorbing experiments showed that the adsorption of lead ions by MgO-N-BC was in accordance with the Langmuir model,and the theoretical maximum adsorption capacity was 1455.9 mg/g,while the adsorption fitted the pseudosecond order kinetics.It has a strong selectivity for the adsorption of lead ions,and could maintain the adsorption capacity of 930.5 mg/g under t pH of 3.0.The main adsorption mechanisms of MgO-N-BC on lead ions include ion exchange,formation of hydrargillite precipitation and adsorption of nitrogenous functional groups.Among them,the carbonates needed for the formation of hydrargillite precipitation come from the carbon dioxide in the air,and the precipitation reaction is also the rate-determining step of the whole adsorption process.Thirdly,using corn cob as raw material,origin biochar CM-600 was prepared by carbonized at 600 ? for 2 h,and then modifying CM-600 with Mg-Al layered double hydroxide(LDH)by co-precipitating method to prepare MA-BC series biochar.Among this series,MA-BC-4/1 with magnesium/aluminum ratio of 4:1 has the best adsorption effect.Adsorbing experiments showed that the adsorption of lead ions by MA-BC-4/1 was in accordance with the Langmuir model,and the theoretical maximum adsorption capacity was 467.1 mg/g,while the adsorption fitted the pseudo-second order kinetics.The main adsorption mechanism of MA-BC-4/1 on lead ions includes ion exchange and precipitation of hydrargillite.Among them,the carbonates needed for the formation of hydrargillite precipitation mainly come from the carbonates enriched and stored between LDH layers,so that the adsorption rate of MA-BC-4/1 to lead ions is significantly increased,and the equilibrium can be reached within 90 min.