The influence of concentrate alkalinity on electrodialytic boron transport

Both pressure driven and electro-membrane processes show better boron removal efficiency when carried under alkaline conditions when the boric acid is ionized to a large extent. Even under such conditions the electrodialysis (ED) performance is, however, reported to be poor since low boron current efficiency and low boron fluxes are observed. In the present work the influence of the ED concentrate pH was examined with a great detail. Electrodialysis of boron containing waters was carried out in a laboratory ED unit equipped with four pairs of CMX and AMX Neosepta (Tokuyama Co.). A thin spacer with 0.40 mm membrane to membrane distance was applied. The working area of each membrane was 9 cm² and linear flow velocity was equal to 1.75 cm/s. Every experiment was carried out in batch mode. Previously it was observed that during electrodialysis of low salinity boron-containing waters the hydroxide ion transport is promoted over borate and the decrease in diluate alkalinity result in drastic decrease in boron ionization that cause the boron flux to decrease. Further experiments showed that boron transport might be enhanced by high ED concentrate pH value. This was due to hydroxide back diffusion from concentrate to diluate compartments, essentially taking part in the vicinity of the anion-exchange membrane adjacent boundary layer. Thus, despite the relatively low mean ED diluate pH, hydroxide content in the layer adjacent to the membrane is high enough to ensure boron species dissociation. The minute boron current efficiency found in this work was extremely high 213% while maximum boron flux was 320 μg/cm² h at pH of 12. More than 100% boron electric current efficiency suggests that electrodialysis is enhanced by Donnan dialysis. Thus the hydroxyl exchanges with the borate present in the diluate boundary layer in parallel to borate electromigration. Also the costs of the wastewater treatment in the proposed manner was assessed.