Preliminary Study On Preparation And Applications Of Monovalent Ion Selective Cation Exchange Membranes

At present, there are still some requirements expected to be met for cation exchange membrane (CEM) to separate different valent ions from some specific applications, for examples, salt production from seawater, efficient recovery of waste acid in hydrometallurgy, purification of some heavy metal ions,abatement of membrane fouling resulting from the scale formation of bi-valent hydroxide during electrodialysis process, etc. With respect to this, there are almost no domestic reports though a series of studies have been carried out abroad. According to literatures, the surface modification materials are confined to several kinds of conducting polymers such as polypyrrole, polyaniline, polyethyleneimine, etc. which have some disadvantages including complicated preparing method, unfriendship to environment, high cost and so on. Recently, natural and environment-friendly chitosan obtained mainly from the sea has been used tentatively as separation membrane materials in microfiltration, ultrafiltration, nanofiltration, reverse osmosis, pervaporation and gas separation, etc. However, to the best of our knowledge, no one considers chitosan as a kind of surface modification material for ion exchange membrane (IEM). Moreover, it is worth noting that quaternized chitosan in the whole pH embraces (positive) electric charges. Therefore, quaternized chitosan is employed tentatively as surface modification material for commercial ion exchange membrane in order to obtain monovalent ion selective cation exchange membrane in this work.In this paper, the 2-hydroxy-3-trimethyl ammonium propyl chitosan (HAPC) is synthesized and introduced into the surfaces of commercial heterogeneous and homogeneous cation exchange membranes by electrodeposition. After that, the HAPC are cross-linked. So, two types of CEMs with the selective separation performance are obtained. The influences of preparing condition on selective separation performance of modified membrane are investigated, including concentration of modified solution, electrodeposition time and different cross-linking systems with the help of the dynamic electrodialysis experiments of H+/Zn²⁺ binary system. It is found that the selective separation performance of modified membrane prepared in the modified solution with a relatively low concentration is indistinctive, whereas the "bipolar membrane" could be obtained in the modified solution with a relatively high concentration. Apparently, if the electrodeposition time is too short, the modified membrane with selective separation performance couldn't be prepared. The relatively long electrodeposition time could make the structure of the modified membrane loose. The separation effects of modified membrane before and after crosslinking on H+/Zn²⁺ binary system are measured, discovering the selective separation performance of the uncrosslinked modified membranes decrease rapidly, especially in the beginning. Whereas the modified membranes after cross-linking keep a good selective separation effects for a long time, which indicates the cross-linking process can effectively extend the life span of modified membrane. Of course, the molecular structure of cross-linking reagent and bonding types could also affect selective separation performance and life span of the modified membrane.In addition, the relevant characterizations of membranes before and after modification are also conducted and compared. According to the relevant literatures and reports, the preparing conditions, including modified solution concentration: 2g (quaternized chitosan)/L(0.1M NaCl); electrodeposition time:2h; the cross-linking system: epichlorohydrin/acetone=0.45/50, wt%, the cross-linking temperature: 50°C; and the cross-linking time: 26h, are adopted. The SEM and AFM are simultaneously used for membrane morphology characterization. The results indicate that membrane surface after modification appears more tight and smooth. Ion exchange capacity (IEC) and water content ( ? w) present decreasing trends for the modified membrane because the sulfuric acid groups are partly neutralized by quaternized chitosan and many strong hydrophilic groups disappear resulting from the cross-linking process. Membrane conductivity (K) after modification slightly rises which may be attributed to the introduction of the modified layer with positive electric charges. It is observed from the I-V charts of the modified membrane that limit current density( I lim) decreases, the smooth length of the curve becomes longer, and the resistances of Ohmic and Electroconvection regions increase after modification, which indirectly confirm the modified layer could selectively separate different valent ions. The fact that permeability coefficient (D) of the modified membrane also decreases mainly because most membrane pore sizes are covered by the modified layer. The most important selective separation performance is evaluated by the dynamic electrodialysis experiments in three systems, including the H+/Zn²⁺, H+/Al³⁺and H+/Zn²⁺/Al³⁺. The results indicate the selective separation effect between H+ and Zn²⁺, Al³⁺ are all fine due to the obvious differences of the electrostatic repulsion forces between the ions and modified layer. However, the separation effect between Zn²⁺ and Al³⁺ seems to be indistinctive, which may be related to the pore size sieving effect owing to the smaller hydrated ion radius of Al³⁺.Finally, the application prospects of selective separation membrane modified by quaternized chitosan are originally discussed. The relevant practical application systems are simulated, including salt production from brine seawater (Na+,K+,Mg²⁺, Ca²⁺,Sr²⁺ and Ba²⁺), lithium extraction from seawater (Na+, Li+ and Mg²⁺), the removing of ammonium from ammonia nitrogen wastewater (NH4+,K+,Mg²⁺ and Ca²⁺) and the recycle of acid from hydrometallurgy industry (H+,Cu²⁺ and Al³⁺). In this work, selective separation performance is investigated by concentration membrane potential method and bi-counter-ionic membrane potential method, respectively. Results show that the conclusions from the two methods are consistent on the whole. With respect to the field of salt production from brine seawater, the obvious separation effects between Na+ and other ions are observed which can be attributed to the differences of electrostatic repulsion between Na+ and other divalent cations. On the other hand, the separation of Na+ and K+ is related to the pore size sieving effect. Li+ separation effect of the modified membrane is seriously interfered by Na+ during the extraction of lithium from seawater The separation effect between Li+ and Mg²⁺ seems to be obvious. Without question, the electrostatic repulsion plays a main role during the aforementioned process. NH4+ removing rate is almost not observed from the application of disposing and recycling ammonia nitrogen wastewater, which can be understood from the pore size sieving effect caused by bigger hydrated ion radius of NH4+. For the recycling acid from metallurgical industry, the separation effect of H+, Cu²⁺ and Al³⁺ could be observed because electrostatic repulsion forces produced by ions and modified layer can be arranged as follows: Al³⁺>Cu²⁺>H+.In this work, the preparing technique, structure, property and preliminary applications of CEMs with selective separation performance are investigated in detail. The experimental results indicate that it is feasible to use 2-hydroxy-3-trimethyl ammonium propyl chitosan to modify surface of commercial base membrane for preparing cation exchange membrane with selective separation performance between monovalent and multivalent ions.