Study On Preparation Of Adsorbent For Heavy Metalions By Modificating Bagasse Pith And Its Mechanism

Nowadays, heavy metal ions pollution is a serious problem in China. It was investigated that1/5of farmland were polluted by Cd, As, Pb and Cr, et. al. Some heavy metal moved into the food chain, which would lead the disease, disability and deformity, or even death of animals or human beings. There are several methods for heavy metal ions removal, such as membrane separation, chemical precipitation, ion-exchange and so on. However, adsorption is the most convenient and effective method. Especially, the biomass absorbents, as environment friendly and renewable resources, have great potential.Sugar cane is one of the main sugar crops,60%of sucrose output in China was produced from sugar cane in Guangxi. Thus, bagasse is the most abundant raw material for pulping and papermaking industry in Guangxi. However,30%-35%(w/w) of bagasse is pith, which is the main waste with high-volume from papermaking and sugaring industry. Therefore, the reasonable and effective utilization of pith, improving its added value, would have more development.The study is on the preparation of absorbent by chemical modifing bagasse pith and its adsorption property for heavy metal ions removal from aqueous solution. The mechanism of the reaction was discussed based on the main components of pith, cellulose and lignin. The adsorption dynamics models were also used to description the heavy metal adsorption process. The main contents and results were shown as followings.The bagasse pith, after being pretreated by alkali and periodate, was reacted with epoxy chloropropane, and then with diethylenetriamine and formaldehyde. The products were used to remove the heavy metal ions from aqueous sigle metal solution. Fourier transform infrared spectroscopy (FTIR) and scanning electronic microscope (SEM) were used to characterize the modified pith. The optimal conditions were30℃, epoxy chloropropane20ml, reaction time3.5h for epoxyl modification and30℃, formaldehyde7ml, diethylenetriamine5ml, reaction time3h for aminated modification. FTIR spectra showed that the stretching peaks of C=N、 C-N、N-H exist in the modified bagasse pith, and some curled fragments adhered on the surface of modified pith charactorized by SEM. The adsorption capbility of metal ions Cu(II) and Pb(Ⅱ) was62.75mg/g and46.32mg/g for modified pith, respectively, compared to8.25mg/g and17.85mg/g for pith, at the pH6.5, ion concentration150mg/L and contact60min.To study the actions of bagasse pith in the processes of modification and adsorption, the main components, cellulose and lignin, were seperated. They were modified by the method for pith modification, and their adsorption properties were investigated, by FTIR, X-ray and SEM were the main analysis approaches for their characterization. As the results shown, the cellulose in pith played an important role in modification of pith under the similar optimal coditions. However, the lignin in the pith may play a supporting role, as the differences existed in the optimal factors for modification, though the epoxy group, amido group and carbonyl group were introduced. The study on their heavy metal adsorption capbility suggested that the cellulose and lignin made some contribution to adsorption of Pb(II) and Cu(II) for modified pith.Furthermore, based on the generation of aldehyde generation and cellulose degradation in periodate oxidizing cellulose, the selectly oxidation kinetics model was constructed, and a variable factor R (the ratio of aldehyde content to the degradation of cellulose fiber) was introduced to evalue the oxidation levels. The kinetics model was as [DGic]=C0(C1-exp(-a,K,t)-a3K3t for oxidation and [L]=(a2K2+asK3)t for degradation. In the equations, K1, K2and K3were the parameters. The equation for evaluation index R was as R=[DGlc]/[L]=MbmDGlc/MamL=Mb/Ma·F/L·(1-L).The experimental data were fitted by the models, and the results shown that there was a better correlation between the dynamic model and the experimental data. Thus, the oxidation degree of bagasse fiber oxidized by periodate can be quantitative evaluated based on this model. The interpretation of results suggested that the yield and aldehyde content of the2,3-dialdehyde cellulose fiber could be controled by the reaction time and temperature at the same concentration level of periodate. In order to study the modification mechanism of pith, the quantized structures of cellulose unit and lignin unit were calculate by quantum chemistry software based on DFT B3LYP/6-31G, and the nuclear magnetic resonance (13C-NMR) of possible structures were discussed. The results showed that the hydroxyl groups on C2, C3and C6of cellulose unit were the sites with high reaction activity. Lignin could be attacked by nucleophile, and the syringyl lignin unit is easier than guajacyl lignin unit. The charges on the hydroxy-ortho carbon of hydroxy-phenyl lignin unit and guajacyl lignin unit were between-0.15eV and-0.16eV, which meant weak chemical bonding between hydrogens and carbons. Therefore, the active hydrogen would be easly removed under the protonation of formaldehyde.The adsorption dynamics models and control models were also used to describe the heavy metal adsorption process, and FTIR spectroscopy was used to study the active sites for heavy-metal adsorption. The results showed that the Pb(Ⅱ) and Cu(Ⅱ) ions adsorption can be well described by both Pseudo-first-order model and Pseudo-second-order model. Webber model could describe the adsoption process by two stages, and diffusion was found to be the rate-limiting step atinitial stage, but the approach equilibrium point the diffusion model could not fitted well.Aimed to discusse the active site, the FTIR spectroscopy of lignin and cellulose before and after adsorption. The results suggested that the sites for adsorption were related to hydroxyl and amido groups.