Problems in Practical Applications of Bipolar Membranes

Electrodialytic dissociation of water with bipolar membranes is economically very attractive for creating acids and bases. There are, however, several severe problems in practical applications, such as the contamination of the products by salts and low current efficiency at high acid and base concentrations.

Salt contamination of the products is related to the properties of the bipolar membrane. The poor current efficiency is the consequence of the proton and hydroxide ion transport in monopolar membranes, as indicated in Figure 4, which illustrates the conversion of Na2SO4 into H2SO4 and NaOH by electrodialytic water dissociation. Figure 4(A) shows the ion transport in the bipolar membrane. What is desired is a flux of H + and OH " ions from the interphase of the bipolar membrane as the result of the water dissociation. However, in addition there is a flux of Na + and SO4~ ions through the bipolar membrane due to incomplete permselectivity of the anion and cation exchange layers. This leads to a contamination of the base by SO4~ ions and the acid by Na+ ions. Since the permeability of the ion exchange layers to SO^ and Na+ increases with increasing acid and base concentration, the contamination is also increasing with increasing concentration, as shown in Figure 4B. This figure shows the salt contamination in sulfuric acid and sodium hydroxide produced by water dissociation in bipolar membranes from a 1molL~1 Na2SO4 solution as a function of the concentration of the acid and base produced.

The current efficiency in water dissociation with bipolar membranes is mainly affected by the properties of the anion exchange membrane which has very poor retention of the protons, as illustrated in Figure 4C. The transport mechanism of protons is based on a tunnelling mechanism, with the consequence that protons can permeate the anion

Figure 4 Schematic diagram illustrating (A) the contamination of an acid and a base by salt to incomplete permeability of the cation and anion exchange layers of a bipolar membrane; (B) experimentally determined salt contamination as a function of the acid and base concentrations; (C) the decrease in current efficiency during the production of acids and bases due to the poor acid-blocking capability of the anion exchange membrane; (D) experimentally determined current efficiency as a function of the produced acid and base concentration.

Figure 4 Schematic diagram illustrating (A) the contamination of an acid and a base by salt to incomplete permeability of the cation and anion exchange layers of a bipolar membrane; (B) experimentally determined salt contamination as a function of the acid and base concentrations; (C) the decrease in current efficiency during the production of acids and bases due to the poor acid-blocking capability of the anion exchange membrane; (D) experimentally determined current efficiency as a function of the produced acid and base concentration.

exchange membrane rather freely. The same is true for the hydroxide ions which can permeate the cation exchange membrane. The net result of the process is that protons and hydroxide ions generated in the bipolar membrane neutralize each other. The proton and hydroxide fluxes and thus the current efficiency depend on the concentration, as shown in Figure 4D. With increasing acid and base concentration, the current efficiency decreases rapidly.

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