二价／单价阳离子混合体系的纳滤过程

Nanofiltration of mixed salt solution with monovalent and divalent cations

采用DK膜对(Mg2+、K+、Cl-)、(Ca2+、Na+、Cl-)三元离子混合体系进行分离实验,通过道南细孔-介电(DSPM-DE)模型对其分离性能进行评价。结果表明:料液中二价阳离子与单价阳离子浓度比越大,分离因子(SF)越小,越有利于分离;操作压力的增加亦能提高DK膜的分离能力,即SF随压差或渗透流率增加而减小,最后趋于稳定,存在一个极限SF。模型分析发现膜荷电密度(Xd)及孔内介电常数(εp)不仅与离子浓度有关,还与离子类型有关,且Xd及εp均随二价阳离子含量增加而单调减小;而SF随|Xd|、εp的减小而减小,证实了Xd和εp的大小直接决定二价阳离子与单价阳离子分离效果。

Mixed salt solutions containing three species of ions（Mg^2＋、K^＋、C1^-） and （Ca^2＋、Na^＋、C1^-） were filtrated with a commercially available DK nanofiltration membrane. The Donnan-steric pore model and dielectric exclusion （DSPM-DE） model was implemented to evaluate the separation performance. The results revealed that the higher the ratio of divalent cations to monvalent cations was, the better the facilitated separation of them was. The separation performance was also improved when working pressure or permeate flux was increased, and there was a limiting separation factor （Sr）. Model analysis showed that membrane charge density （Xa） and dielectric constant （εp）within the pores depended on ion concentration and type （value and sign）. ｜Xd｜ and εp decreased monotonously with increasing divalent cation concentration. Separation efficiency of divalent and monvalent cations directly depended on Xd and εp ,which was confirmed by the fact that Sr decreased with the decreasing ｜Xd｜ and εp.