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.

The low boric acid ionization percentage into the diluate boundary layer constitute a serious shortage during desalination of boron-rich waters. By analyzing the nature of this problem we concluded that concentrate pH should be kept as high as possible to assure reliable boron transport since back-diffusing hydroxyl ions are suspected to increase the ED transferable boron form content, namely borate, present into the diluate boundary layer. To prove the above a set of experiments at varying concentrate pH levels was conducted in laboratory. The boron-rich water examined was preliminarily desalinated industrial landfill leachate from Tarnowskie Góry, Poland, chemicals storage. Our experiments proved a drastic enhancement in boron transport with concentrate pH increase. The process indices, such as boron flux and boron electric current efficiency were found to depend on the concentrate pH, since the hydroxyl ion present into the concentrate diffuses against the electric field into the diluate and improves the boric acid ionization percentage. Moreover, more than 100% boron electric current efficiency observed at pH of 12.0 suggests that electrodialysis is enhanced by other, than electromigration process, more particularly by ion exchange (Donnan dialysis through anion exchange membrane). Thus the hydroxyl exchanges with the borate present in the diluate boundary layer. The estimated cost of both, preliminary desalination and boron removal by the electrolysis at concentrate pH of 12 equals to $0.80/m3. Its low value proves that ED at elevated concentrate pH may compete with IE presently used to treat Tarnowskie Gory landfill leachate.