| The utilization of wasted bagasse is a hot research topic.Bagasse is the main by-product during sugar production process.Lignin,an important component of bagasse,contains many reactive functional groups,such as phenolic hydroxyl groups,alcoholic hydroxyl groups,and carboxyl groups.Bagasse lignin(BL)has been widely applied in the heavy metal treatment.However,the removal efficiency of heavy metals by the BL is limited and it needs to be improved.The mechanisms of selective removal ability of heavy metals by the BL are rarely reported.Therefore,the main aims of this study are to investigate efficient extraction method to extract BL,and find an appropriate oriented-modification method on the BL.Advanced analytical techniques,such as nuclear magnetic resonance spectroscopy(NMR),fourier transform infrared spectroscopy(FT-IR),and scanning electron microscopy(SEM)were applied to characterize the modified BL.The mechanisms of selective adsorption of heavy metals on the BL were illustrated by density functional theory(DFT)and adsorption models.The results of this study have evidenced the potential utilization of bagasse in the heavy metal pollution treatment,which are shown below:1.After ultrasonic-assisted treatment,the surface of bagasse formed a network structure,which was beneficial for the solvent penetration.This improved the extraction rate of bagasse during the alkali extraction.The extraction rate of ultrasound-assisted bagasse was up to 60.60%,which was 1.42 times more than the unassisted bagasse.BL was sequentially extracted with diethyl ether/methanol to obtain diethyl ether purified alkali bagasse lignin(BLI),methanol purified alkali bagasse lignin(BL2),and insoluble alkali bagasse lignin(BL3).BL2 occupied the highest mass fraction of 57.0%,and its structure exhibited loose porous characteristic.Besides,BL2 had a reduction of aryl ether,while an increase for the radio of G model in structural unit and a decrease for that of S and H models.The total hydroxyl and aliphatic hydroxyl contents were 3.7997 mmol/g and 0.9559 mmol/g,respectively,which were 1.55 times and 1.59 times more than that of BL2.Phenolation amination modification transformed BL2 to amination phenolation modified BL2(A-P-BL2).Phenolation resulted in a reduction in the aliphatic hydroxyl group by 0.5992 mmol/g,and an increase of 0.6083 mmol/g in the phenolic hydroxyl group.This indicates that the phenolic hydroxyl group was introduced in the lignin structure,as a result of the substitution reaction between the phenol para-carbon atom and the positive carbon.This positive carbon was formed by the water molecule removal after the combination of the ortho proton and the alpha carbon hydroxyl group on the lignin fatty chain.The amination modification enhanced the introduced nitrogen content to 9.44%and formed a strong absorption peak of the C-N bond which was showed in the FT-IR spectra.This was due tothe combination of the N atom in the tertiary amine cation and the ortho carbon atom in the phenolic hydroxyl group which located in the benzene ring of phenolation modified BL2(P-BL2)3.The results of batch experiments showed that the adsorption of Cu2+,Pb2+ and Cd2+ by A-P-BL2 was greatly improved,and the equilibrium adsorption capacity could reach 48.54,48.82 and 36.43 mg/g?respectively.The adsorption of Cu2+ and Pb2+ by A-P-BL2 was in accordance with Langmuir isotherm,which represents the uniform single-layer adsorption while the adsorption of Cd2+ can be described by the Freundlich isotherm,which indicates heterogeneous multi-layer adsorption.The adsorption of those three metal ions by A-P-BL2 was a spontaneous exothermic entropy increase process,which conformed to the pseudo-second-order kinetic model and was controlled by processes such as membrane diffusion and intraparticle diffusion4.The results of fixed bed experiments showed that preferential adsorption order of A-P-BL2 was:Cu2+>Pb2+>Cd2+,which also implied the high adsorption selectivity for Cu2+The DFT results illustrated that this preferential adsorption ability on Cu2+ were due to the following reasons:primary amine type complex binding force,frontal orbital energy difference(?EHOMO-LUMO)and Laplace key level(LBO)of Cu2+ were the largest,its natural bond orbital(NBO)value was the smallest,and its central ion charge was also the highest.In summary,by applying the oriented modification and selective adsorption mechanisms illustration landed a theoretical basis for the regulation and engineering application of bagasse lignin in heavy metal removal treatment. |
PDF Full Text Request