The influence of d-mannitol on the effectiveness of boric acid transport during electrodialytic desalination of aqueous solutions

Abstract

Previous studies have shown that electrodialytic (ED) desalination can be effective in separating boric acid from salts, and a model for the transport of boric acid during ED desalination has been proposed.In this study, the influence of d-mannitol on the effectiveness of boric acid transport during ED desalination of aqueous solutions was investigated. The experimental results of periodic desalination were compared with results predicted from a previously proposed model of boric acid mass transport. Both the concentrate and the electrode rinse solution contained lower than predicted amounts of boric acid, especially at high diluate boric acid concentration. This difference was shown to be due to the reaction between monoborate and d-mannitol, which resulted in the formation of mono- and dichelate complexes and therefore significantly reduced the equilibrium concentration of boric acid in the diluate. This phenomenon was observed despite the low pH of the diluate. The equilibrium concentration of boric acid in the diluate was calculated using the model for chemical equilibrium. The results were then analyzed with the model for boric acid transport. A good agreement between the predicted and the measurements results was found. The combined model was therefore shown to be accurate in predicting the effectiveness of boric acid transport during ED desalination of mixtures of boric acid and d-mannitol. The effect of d-mannitol was such that it reduced the equilibrium concentration of boric acid in the diluate. This, in turn, reduced the boric acid flux across the membrane. The extent of the above reduction in boron flux can be predicted based on the combined model provided in this work. No evidence of borate complexes and d-mannitol transport across IEMs was observed.