Study On The Structure Design,preparation And Structure-activity Relationship Of Cellulose-based Adsorption Materials With Multiple Adsorption Sites

Heavy metal pollution in water directly threatens human health and the growth of animals and plants.Cellulose-based heavy metal adsorption materials have attracted much attention because of their advantages such as high adsorption capacity,high removal efficiency and non secondary pollution.Recently,how to improve the adsorption rate and removal rate of cellulose-based adsorption materials for low concentration heavy metal ions,so as to make them reach the discharge standard of heavy metal ions in drinking water has become a research problem.Based on the molecular structure,this paper taked this difficulty as the research targe to design and prepare the cellulose-based absorbents containing N adsorption sites,the cellulose-based adsorption materials including N,O,S adsorption sites and the hyperbranched cellulose-based amphoteric adsorbent containing N,O adsorption sites,and also studied the adsorption performance of heavy metal ions Cr（Ⅵ）,Cu（Ⅱ）and Pb（Ⅱ）on the adsorbents.The mechanism of cellulose-based adsorbents adsorbed heavy metal ions were explored combining the qualitative and quantitative characterization of technology.（1）Biomass-based adsorption materials with high amino density were prepared by one-step rapid cross-linking method.Solid amine adsorption materials with amino different structures（small molecule tetraethylene pentaamine（TEPA）,branched polyethyleneimine（BPEI）,amino terminated hyperbranched polyamine（HBP-NH₂））and different matrixes（bagasse fiber（BF）,pulp fiber（PF）,microcrystalline cellulose（MCC））were prepared by adjusting the structure of amine and matrix type.The experimental results showed that biomass-based adsorbents with different sizes and structures could be prepared by one-step rapid cross-linking method.The series of biomass-based solid amine adsorbents had high amino density（>10.63 mmol/g）,and could remove high concentration（100.00 mg/L）and low concentration（1.00 mg/L）of Cr（Ⅵ）to meet the content standard of Cr（Ⅵ）in drinking water（<0.05 mg/L,GB 5749-2006）.Moreover,the adsorption rate of Cr（Ⅵ）could be promoted when the biomass matrix was used as the material skeleton.When the hyperbranched structure was introduced into the material to prepare cellulose-based hyperbranched adsorption material,the nano cavity of the material further improved the adsorption rate of Cr（Ⅵ）,and the Cr（Ⅵ）with low concentration（1.00 mg/L）could be completely removed by adsorption within the detectable range after 5 minutes.The adsorption mechanism of Cr（Ⅵ）by biomass-based solid amine adsorption material were electrostatic action,chelation reaction and redox reaction,which realized the efficient removal of Cr（Ⅵ）by the material.At the same time,based on the heterogeneous reaction of biomass fiber,a one-step rapid cross-linking method was used to prepare the biomass-based adsorbents with high amino density.This method was generally suitable for different cellulose matrix and different structures of amine functional reagents,which overcomes the disadvantages of traditional grafting method which has many preparation steps and low conversion of reactants.（2）In order to achieve the simultaneous removal of anionic and cationic heavy metal ions in low concentration by cellulose based adsorption materials,MCC/TEPAA with high amino density was prepared by one-step crosslinking method,from the point of view of collaborative introduction of multiple adsorption sites（N,O,S）.Then bis（carboxymethyl）trithiocarbonate（BCTTC）with O and S sites was grafted on the surface of the adsorbent.And cellulose-based adsorption material with multiple adsorption sites（MCC/TEPAA-BCTTC）was prepared by condensation reaction.MCC/TEPAA-BCTTC was rich in functional groups,including amino group（8.86 mmol/g）,sulfur group（2.06mmol/g）and carboxyl group（5.74 mmol/g）,which provided a large number of active adsorption sites for its adsorption of heavy metal ions Cr（Ⅵ）,Cu（Ⅱ）,Pb（Ⅱ）.The experimental results showed that under the condition of low concentration,MCC/TEPAA-BCTTC could remove Cr（Ⅵ）（1.00 mg/L）in 3 minutes to the discharge standard of Cr（Ⅵ）（GB 8978-1996）,and could remove Cu（Ⅱ）（3.00mg/L,in 1 minute）and Pb（Ⅱ）（1.00 mg/L,in 180 minutes）to the content standard of Cu（Ⅱ）（<2.00 mg/L）and Pb（Ⅱ）（<0.01 mg/L）in drinking water（GB 5749-2006）.The adsorption of anionic Cr（Ⅵ）in water by cellulose-based adsorption materials with multiple adsorption sites mainly depended on the electrostatic action,chelation and redox reaction of N atom adsorption sites,while the adsorption of cationic Cu（Ⅱ）and Pb（Ⅱ）mainly depended on the chelation of S and O adsorption sites.The N,O and S atom adsorption sites of the material could remove the anionic and cationic heavy metal ions simultaneously and efficiently through the cooperation mechanism.（3）In order to improve the adsorption rate of cellulose-based adsorption materials for low concentration heavy metal ions,hyperbranched polyamines and hyperbranched polycarboxylic acids with N and O adsorption sites were introduced into MCC to prepare hyperbranched cellulose based amphoteric adsorption materials with high density of functional groups.The simultaneous removal effect and the adsorption mechanism of low concentration anionic and cationic heavy metal ions were studied.The experimental results showed that the hyperbranched cellulose-based solid amine adsorbent（MCC/HBPA）was prepared by one-step rapid cross-linking method,in which MCC was the matrix and hyperbranched polyamine with amino terminal（HBP-NH₂）（branched degree was 0.887）was the functional reagent.And then the hyperbranched cellulose-based amphoteric adsorbent（MCC/HBPA-COOH）was prepared by condensation reaction with carboxyl terminated hyperbranched polycarboxylic acid（HBP-COOH,branched degree was 0.793）)。The adsorption material has a high density of functional groups（19.92 mmol/g）,in which the density of amino group and carboxyl group were 9.14 mmol/g and 10.78 mmol/g,respectively.The results of adsorption experiment showed that under the condition of low concentration,MCC/HBPA-COOH could remove Cr（Ⅵ）（1.00 mg/L）in 3 minutes to the content standard of Cr（Ⅵ）in drinking water（<0.05 mg/L,GB 5749-2006）,and the removal rate to reach to 100%within the detectable range after adsorption for 10minutes.Besides,MCC/HBPA-COOH could remove Cu（Ⅱ）（3.00 mg/L）in 1 min to the content standard of Cu（Ⅱ）in drinking water（<2.00 mg/L,GB 5749-2006）.Therefore,the hyperbranched three-dimensional structure effectively promoted the rapid adsorption of Cr（Ⅵ）and Cu（Ⅱ）,and realized the rapid and efficient removal of Cu（Ⅱ）and Cr（Ⅵ）.In this paper,we could control the preparation of cellulose-based solid amine adsorption materials with N adsorption sites,cellulose based adsorption materials with N,O and S adsorption sites,and hyperbranched cellulose based amphoteric adsorption materials with N and O adsorption sites through the regulation of adsorption sites of cellulose based adsorption materials,so as to achieve the selective removal of Cr（Ⅵ）or the simultaneous removal of Cr（Ⅵ）,Cu（Ⅱ）and Pb（Ⅱ）.The results could provide theoretical basis and technical support for the structural design and preparation of high efficiency cellulose-based adsorption materials and the removal of heavy metal ions from polluted water.