Study On Preparation And Performance Of Potassium Ionic Exchange Membranes

Ion-exchange membrane is one of the methods for extracting potassium from seawater. Potassium ionic sieve has the similar structure and function with ion-exchange membrane and it also has special selectivity to K+. Combining potassium ionic sieve and ion-exchange membrane together can overcome the shortcomings of potassium ionic sieve membrane and improve the selectivity to K+ than that of ion-exchange membrane.One kind of ionic sieve with a specific memory function of potassium is prepared by hydrothermal method, and its saturated exchange capacity to potassium in seawater is 56.96 mg/g. The potassium ionic sieve is modified using silane coupling agent KH-560. Effects of reaction conditions such as content of coupling agent and reaction time on the surface modification are discussed. The potassium ionic sieve before and after modification are characterized respectively by XRD, SEM, FT-IR, and thermogravimetric analysis. The results show that KH-560 can graft onto the surface of potassium ionic sieve successfully, and the grafting ratio is 1.25%. The optimal modification conditions of potassium ionic sieve are: the mass ratio of coupling agent to potassium ionic sieve is 2:1 and the coupling reaction time is 7 h.Blending the potassium ionic sieve into PVA matrix, and commercial ion-exchange membrane as the support membrane to prepare hybrid and modified potassium ionic exchange membrane. Membrane performance is tested by bi-ionic potential experiment. This method is used to test membrane potentials of CaCl2, KCl solutions, and then calculate permselectivity coefficient T. According to the T, the conditions of preparing membranes are determined. Effects of dispersant types and contents on membrane surface morphology are discussed. At the same time, effects of reaction conditions such as content of potassium ionic sieve, concentration of PVA, content of crosslinker and reaction time on membrane performance are also discussed. Finally, the optimal conditions of preparing membrane are: the mass ratio of dispersant PAA to potassium ionic sieve is 0.4; content of potassium ionic sieve is 14%; concentration of PVA is 8.96%; content of glutaraldehyde is 0.1%; crosslinking time is 10min.Scanning electron microscopy (SEM) is used for membrane surface and section morphology characterization. The results show that: potassium ionic sieve are well dispersed on the surface of modified membrane, and part of them embedded into the surface pores of commercial membrane. The modified layer can roughly cover the surface and pores of commercial membrane.The results of contact angle experiments show that the hydrophilic performance of the modified membrane is improved apparently, and the contact angle is 78.1°least than commercial membrane. In addition, the relevant characterizations of membranes before and after modification are also conducted and compared. The results show that: ion exchange capacity (IEC) and water content (φw) present decreasing trends for the modified membrane, while membrane resistance (R) after modification slightly rises. This may be because of joining the PVA which does not have ionic exchange groups, reducing the membrane ion-exchange group content. Furthermore, PVA is not conductive, so that the membrane resistance increases.Do electrodialysis experiments on KCl/CaCl₂, KCl/MgCl₂, KCl/NaCl solutions respectively. The results show that: modified membrane have the higher selectivity to K⁺ than commercial membrane. Especially commercial membrane has no selectivity to K⁺ in KCl/NaCl solution. Modified membrane shows priority selectivity to K⁺ in every solution apparently. The largest separation factors of K⁺/Ca²⁺, K⁺/ Mg²⁺, K⁺/Na⁺ are equal to 154.89, 137.94 and 5.00 respectively. Under the same time, the largest differences between commercial membranes are 93.61, 86.84 and 4.24 respectively. These datas show that: compared to commercial cation exchange membrane, modified membrane separation performance to K⁺ improved significantly. The results of the above studies show that combining potassium ionic sieve with commercial cation exchange membrane to prepare potassium ionic exchange membrane is feasible. But its performance has to be further improved in the future.