Modification Of Bipolar Membranes On Water Dissociation Performance

Bipolar membrane is a special ion-exchange membrane comprising a anion-exchange layer and a cation-exchange layer. It can dissociate water molecules into protons and hydroxyl ions with the lowest theoretical voltage drop under reverse bias in a direct current electric field. Using this function, bipolar membrane electrodialysis (BMED) has been applied widely in resource recovery, pollution control, and chemical clean processing because of the simple process and low or no waste disposal.Considering the energy consumption and application requirements of bipolar membrane electrodialysis, how to keep a low water dissociation voltage drop and how to make the bipolar membrane endure higher temperature are two projects of significance. Therefore, this research starts with the modification of bipolar membrane's intermediate layer and preparation of hybrid bipolar membranes. Particularly, the main investigations are as follows: (1) the effect of palygorskite-FeCl3 composite on water dissociation and the suppression of Fe loss by palygorskite; (2) the advantage of hybrid bipolar membranes as compared with traditional bipolar membranes.When it comes to the modification of intermediate layer, the bipolar membranes were prepared by using the casting method with FeCl3, palygorskite, palygorskite- FeCl3 composite as the intermediate layer. These membranes were characterized by current-voltage curves and impedance spectra. The water dissociation performance was assessed by comparing the bipolar membrane modified by palygorskite-FeCl3 and the one modified by FeCl3; meanwhile, the evolution of voltage drop across the bipolar membrane was measured. The results indicate: (1) both palygorskite and FeCl3 can act as the catalyst of water dissociation, and concentration and steric effect are the two main factors affecting water dissociation; (2) palygorskite and FeCl3 exhibit a synergetic catalysis on water dissociation, and the optimum concentrations are 5 g dm-3 palygorskite and 0.005 mol dm-3 FeCl3; (3) in view of Fe3+ loss, thickness of intermediate layer, voltage drop, and production of acid/base, the bipolar membrane modified by 5 g dm-3 palygorskite and 0.005 mol dm-3 FeCl3 is more stable.As for hybridization, hybrid bipolar membranes were prepared by casting a cationic layer or hybrid cationic layer on an anionic layer or hybrid layer. The following characterizations include current-voltage curves and voltage drops at different temperatures. The results indicate that (1) most of the hybrid bipolar membranes have stronger catalytic effect on water dissociation than the ones with the same organic ingredients; (2) hybrid bipolar membranes are more heat-stable and their performances are stable at higher temperatures.