β-环糊精/木粉接枝共聚物对Pb^(2+)的吸附动力学和热力学研究

Adsorption of Pb^(2+) from Aqueous Solution by β-Cyclodextrin/Wood Flour Grafted Copolymer:Kinetic and Thermodynamic Studies

以柠檬酸为交联剂,利用酯化交联工艺将功能主体分子β-环糊精接枝到杨木木粉表面制备β-环糊精/木粉(β-CD/WF)接枝共聚物,并用红外光谱仪、热重分析仪和酚酞探针分子技术进行表征。以Pb2+为吸附质,β-环糊精/木粉接枝共聚物为吸附剂,系统探讨接触时间、pH值、Pb2+初始浓度对吸附效果的影响。采用准一级动力学方程、准二级动力学方程、颗粒内扩散模型、Langmuir吸附等温方程、Freundlich吸附等温线方程对试验数据进行拟合。结果表明:β-环糊精不仅被接枝于木粉表面而且可以体现出包合活性;在298 K时,β-环糊精/木粉接枝共聚物对Pb2+吸附平衡的接触时间为30 min,适宜pH值范围为4～8,平衡吸附量随Pb2+的初始质量浓度增加而增加;Pb2+的吸附过程符合准二级动力学方程,即以化学吸附为主,颗粒内扩散不是唯一的吸附速率控制步骤;吸附符合Langmuir吸附等温模型,说明Pb2+的吸附属于单分子层吸附;吸附的吉布斯自由能变为负值,熵变和焓变分别为54.45 J/(mol.K)和13.75 kJ/mol,即该吸附是能自发进行的吸热过程。

β-cyclodextrin/wood flour （β-CD/WF） grafted copolymer was prepared by esterification- crosslinking technology, i.e. , grafting β-CD onto poplar wood flour in the presence of citric acid. The properties of this grafted copolymer were characterized by FF-IR, TGA and the technique of phenolphthalein probe respectively. The adsorption property of this copolymer for Pb^2＋ in a batch adsorption system as a function of contact time, pH and initial Pb^2＋ concentration was investigated in this study. The results showed that β-CD was not only grafted onto the surface of wood flour but also demonstrated an encapsulation activity. The kinetic data were analyzed by the pseudo-first, second-order kinetics and intrapartiele diffusion, and the equilibrium data were analyzed using Langmuir and Freundlich isotherm models. The results showed that adsorption equilibrium was reached within 30 rain at 298 K under the proper pH between 4.0 and 8.0 , and the adsorption capacity increased with the rise of initial Pb^2＋ concentra- tion. It was found that the adsorption kinetics followed the seeond-Order model. This suggested that the adsorption of Pb^2＋ by copolymer was a chemisorption process, and the intraparticle diffusion was not the primary rate-determining step. Besides, Langmuir model fitted the equilibrium data better than the Freundlieh isotherm. It reflected the adsorption of Pb^2＋ on eopolymer was monolayer adsorption. The thermodynamic properties,△G, △H and △S showed that adsorption of Pb^2＋ onto β-CD/WF flour grafted copolymer was endothermic and spontaneous in the temperature range of 293 -β3 K.