Effect of electrode rinse solutions on the electrodialysis of concentrated salts

[Display omitted] •The selection of the preferred electrode rinse for treating 3% NaCl is described.•The preferred electrode rinse increased the initial ion flux by 37–56%.•NaOH added to Na2SO4 increased the conductivity of the rinse solution.•1 g/l NaOH added to the electrode rinse decreased the initiation voltage by 0.42 V.•A flux model including reaction potentials predicts the lower initiation voltage. The data presented indicate that modest attention to the chemistry of the electrode rinse solution can yield as much as 56% improvement in process efficiency. When operating electrodialysis (ED) with salt concentrations greater than 10,000 mg/l (as NaCl) the amperage can be relatively high even at low voltages. To maximize the overall rate of ion transport from diluate to concentrate, there is a need to minimize resistances in the electrode cells since these can represent as much as 30% of the total resistances in the entire process and in this particular ED unit, represented resistance roughly equivalent to that within the membrane stack. This work describes the performance of a 10-cell pair, 200 cm2 (0.02 m2) per membrane, pilot ED unit operated in batch mode at 5 V potential. The feedstock to the stack was varied from 0.5% to 6% NaCl. Results from Volt-amp profiles (2.0–15 V) were used as the rationale for choosing the preferred electrode rinse solution, 90 g/l (kg/m3) disodium sulfate at pH 12.5. The recommended solution of 30 g/l (kg/m3) disodium sulfate at neutral pH was tested against stronger solutions (60, 90, and 120 g/l (kg/m3) disodium sulfate) at neutral pH and with 1 g/l (kg/m3) sodium hydroxide added to yield solutions at pH 12.5. There were stark differences in the performance of the ED unit between the solutions at pH 7 and those at pH 12.5. The slopes of the Volt-amp profiles improved indicating less resistance to ion flow at pH 12.5. Most of the difference can be attributed to the greater conductivity of sodium hydroxide compared to disodium sulfate, where even a modest addition of 1 g/l (kg/m3) sodium hydroxide increased the conductivity of the solution between 8 and 20% depending on the concentration of disodium sulfate. Most notably, the initiation voltage was 2.39 ± 0.08 V with solutions at pH 7 and 1.99 V ± 0.04 V when the rinse solutions were around pH 12.5, a difference of around 0.42 V. Since the full batch runs were performed at 5.0 V, this 0.42 V differential represented a loss of 8% in process efficiency. A series of tests s