Divalent Cation Removal by Donnan Dialysis for Improved Reverse Electrodialysis

Area of Science:

• Membrane science and technology
• Sustainable energy production
• Water treatment

Background:

• Divalent cations in feedwater reduce the efficiency of membrane processes like reverse electrodialysis (RED).
• RED generates power from salinity gradients, but divalent cations increase resistance and decrease voltage.
• Removing divalent cations from freshwater is key to improving RED power density.

Purpose of the Study:

• To investigate the removal of divalent cations from freshwater using Donnan dialysis (DD) with seawater as the draw solution.
• To evaluate the impact of DD pretreatment on the performance of a subsequent RED process.

Main Methods:

• Donnan dialysis (DD) process utilizing a cation exchange membrane to remove divalent cations.
• Membrane stack operation for DD with short residence times.
• Integration of DD-pretreated freshwater into a reverse electrodialysis (RED) system.

Main Results:

• A 76% reduction in divalent cation content was achieved in natural freshwater within seconds using DD.
• The permselectivity of the cation exchange membrane was crucial for minimizing salt leakage.
• Pretreatment with DD resulted in improved gross and net power densities of RED by 9.0% and 6.3%, respectively.

Conclusions:

• Donnan dialysis is an effective, chemical- and electrode-free method for removing divalent cations from freshwater.
• DD pretreatment enhances RED performance by reducing freshwater resistance through the exchange of divalent for monovalent cations.
• This integrated membrane system offers a promising approach for sustainable energy generation from salinity gradients.