沸石和改性沸石对孔雀绿(MG)和磺化若丹明(LR)的吸附特性

Adsorption of malachite green(MG) and lissamine rhodamine(LR) on natural and modified zeolite

以斜发沸石为原料制取壳聚糖改性沸石,并对改性沸石进行扫描电镜(SEM)和孔分布表征.改性沸石的表面附着壳聚糖并形成覆盖性多孔状结构,改性前后沸石BET比表面积变化不大,但改性沸石的总孔容积和平均孔径分别增大1.62倍和1.71倍.相对而言,初始浓度变化对天然沸石去除率的影响较小,而改性沸石所受影响较大.沸石对MG的吸附速率明显低于LR,吸附容量明显大于后者,这可能是范德华力和静电斥力综合作用的结果.p H值在2.0—9.0范围内MG的吸附随p H呈上升趋势,p H值超过9.0后又略有下降,LR在p H 4.0—11.0范围内呈下降趋势.反应温度从20℃上升至60℃,天然沸石对LR和MG的吸附容量有所提高.相对于准一级和准二级方程,孔内扩散方程可以更好地描述MG、LR在沸石和改性沸石上的吸附动力学,说明吸附反应主要发生在较大的微孔中,孔内扩散为影响吸附反应速率的主要因素.Langmuir方程和Freundlich方程均能较好描述吸附等温过程,两种沸石均易于与MG和LR发生吸附反应,但改性沸石吸附能力明显优于前者.热力学分析表明,MG和LR在沸石和改性沸石上的吸附过程是吸热反应,吸附反应是自发的过程,且属于物理吸附为主的反应类型.

Modified zeolite was synthesized using natural zeolite （clinoptilolite） and chitosan. The SEM and nitrogen adsorption experiments were carried out to characterize the surface structure. Results indicated that zeolite surface was covered with chitosan and porous structure was formed. There was no significant difference of the BET specific area before and after modification. But the total pore volume and mean pore size were increased by 1.62 and 1.71 times respectively. The initial concentration had more effect on the modified zeolite than natural zeolite. Adsorption of MG onto zeolites was slower than that of LR, and the adsorption capacities of MG were much higher than that of LR. The adsorption was probably due to the combined effect of Vander Waals＇ force and electrostatic repulsion. The adsorption of MG increased with pH in the range of 2.0 and 9.0, but dropped appreciably when pH was above 9.0. The adsorption of LR decreased when pH increased between 4.0 and 11.0. The adsorption capacities of MG and LR onto natural zeolite and modified zeolite increased with temperature from 20 ℃ to 60 ℃. The adsorption kinetics conformed to pore-diffusion reaction much more closely than first-order or second-order reaction, which meant the adsorption took place in rather large pores and diffusion in pores was the key step to determine adsorption velocity. Langmuir equation and Freundlich equation could describe adsorption isotherm satisfactorily. Natural and modified zeolite could easily absorb MG and LR when judged by 1/n value. Results of thermodynamics analysis indicated that adsorptions of MG and LR on zeolites were endothermic reactions. The adsorption reactions were spontaneous and mainly belong to physical adsorptions.