Preparation Of Amino-Modified Biochar Magnetic Materials And Their Adsorption Properties For Congo Red And Cr(Ⅵ)

Water pollution is a global environmental problem.A large amount of wastewater is discharged into the water environment every year,resulting in serious destruction of clean water resources.Azo dyes and heavy metal ions,two common pollutants in wastewater,are highly toxic and non-biodegradable,which pose a great threat to the ecological environment and human health.How to effectively remove dyes and heavy metal ions from wastewater is an urgent problem to be solved at present.In this work,two new biochar-based adsorbents were synthesized from bamboo and poplar sawdust wastes,and were used to effectively remove Congo red dye（CR）and hexavalent chromium（Cr（Ⅵ））.The main research contents and conclusions are as follows:1.Porous bamboo biochar was firstly prepared from waste bamboo by pyrolysis and NaOH activation,and then Co Fe₂O₄ nanoparticles and amino functional group were introduced to synthesize polyethyleneimine（PEI）modified bamboo pyrolytic carbon magnetic material（MABC-Px）.The characterization results show that MABC-Px is a porous rod-like structure with high thermal stability and saturation magnetization,and can be rapidly separated from solution.After PEI modification,the N content of the material is obviously increased,but the specific surface area and pore volume are reduced.The adsorption performance of MABC-P50 for CR was investigated,and the results showed that the maximum adsorption capacity of MABC-P50 at 298 K was 377.61 mg·g^-1,which was 1.8 times higher than that before modification.The CR adsorption capacity increased significantly with the decrease of p H,and salt ion promoted the adsorption.The adsorption kinetics of CR with low and high concentration were more consistent with pseudo-second-order and Elovich models,respectively.The CR adsorption isotherm conforms to Freundlich model,and the adsorption is a spontaneous,endothermic and entropy-increasing process.The results of mixed dye adsorption experiments indicate that the removal ability of MABC-P50 for MB is greatly enhanced in Congo red/Methylene blue（CR/MB）binary system due to the synergistic adsorption,which is the electrostatic attraction andπ-πsuperposition between CR and MB.The adsorption mechanism of CR on MABC-P50is mainly electrostatic attraction and hydrogen bonding.MABC-P50 can almost completely remove CR dye from simulated textile wastewater.In addition,the adsorbent was regenerated by soaking the material in NaOH/ethanol eluent（NaOH concentration was 0.1 M,NaOH:ethanol=1:1）for 2 h.After 10 times of reuse,the CR removal efficiency can still reach 93.1%.2.Poplar sawdust hydrochar was firstly prepared from waste poplar sawdust by hydrothermal carbonization and KOH pretreatment,and then Fe₃O₄ nanoparticles and amino functional group were introduced to synthesize PEI modified poplar sawdust hydrochar magnetic material（MHTCP）.The characterization results showed that MHTCP was superparamagnetic and could be completely separated from the solution within 1 min.After modification,the N content and zeta potential of the material increased significantly,and PEI formed an amide covalent bond with carboxyl groups of hydrochar.The adsorption performance of MHTCP-5 for CR was investigated,and the results indicated that the maximum adsorption amount of MHTCP-5 at 303 K was930.41 mg·g^-1,which was 3.6 times higher than that of hydrochar before modification.Both salt ions and cationic surfactants can greatly enhance the CR adsorption,and MHTCP-5’s removal ability of CR in complex water system is obviously improved than that in deionized water.The CR adsorption process conforms to Elovich and Freundlich models,and is a spontaneous,endothermic and entropy-increasing process.In addition,MHTCP-5 has higher adsorption capacity for CR than other pollutants,and can completely remove CR from mixed systems.The adsorption mechanism of CR is mainly electrostatic attraction and hydrogen bonding.The column adsorption process of CR is consistent with the Thomas model,and the equilibrium adsorption capacity（q₀）is 782.3 mg·g^-1,which is higher than most reported adsorbents.3.MHTCP was also applied to remove Cr（Ⅵ）from wastewater,the results showed that the maximum adsorption capacity of MHTCP-5 at 303 K was 281.64mg·g^-1,which was 1.03 times higher than that before modification.The Cr（Ⅵ）adsorption amount firstly increased and then reduced with improving p H,and reached the maximum value when p H=2.Cationic and microplastics have no obvious effect on Cr（Ⅵ）adsorption,but humic acid（HA）and cationic surfactant have some promoting effect.The removal process of Cr（Ⅵ）conforms to Elovich and Freundlich models,and is a spontaneous,endothermic and entropy-increasing process.MHTCP-5 has higher Cr（Ⅵ）removal ability than other heavy metals,and shows high selectivity for Cr（Ⅵ）from mixed systems,with adsorption separation factor（SF）ranging from 9.16 to 15.81.In addition,MHTCP-5 exhibits extremely fast adsorption rate for low concentration Cr（Ⅵ）,which can reduce the Cr（Ⅵ）concentration in solution below the drinking water standard(0.05 mg·L^-1)within 5 min,and below 0.004 mg·L^-1 within 10 min.After 17adsorption cycles,the residual concentration of Cr（Ⅵ）is still lower than 0.05 mg·L^-1.The removal process of Cr（Ⅵ）involves adsorption and reduction.Cr（Ⅵ）is first adsorbed to MHTCP-5 by electrostatic attraction and hydrogen bonding,then part of Cr（Ⅵ）is reduced to Cr（III）,and Cr（III）is fixed by coordination.MHTCP-5 shows great practical application potential,when the dosage of adsorbent is 2 g·L^-1,the Cr（Ⅵ）concentration in electroplating wastewater can be reduced to less than 0.004 mg·L^-1.