Order-of-magnitude enhancement in boron removal by membrane-free capacitive deionization

Increasing global water stress motivates a growing interest in seawater desalination by reverse osmosis. Boron is typically weakly removed by reverse osmosis membranes at seawater pH, necessitating expensive post-processes such as caustic agent dosing of the permeate followed by additional filtration steps. It has been previously demonstrated that membraneless capacitive deionization can enable chemical-free boron removal from reverse osmosis permeate, with basic design rules established. However, the level of boron electrosorption per cell charge was limited to similar to 0.5 mu mol/g, a level too low for practical applications. We here explore, both theoretically and experimentally, methods to enhance boron removal by capacitive deionization. We found that reversing the polarity of the applied voltage during the discharge step resulted in an order of magnitude increase in boron electrosorption to nearly 4 mu mol/g with promising energy consumption of 0.2 kW center dot h/m(3). The promise of these results is highlighted when compared with recently-developed boron electrosorption cells requiring bipolar membranes, which demonstrate similar boron removal of 4.35 mu mol/g but with much higher energy consumption of 18.3 kW center dot h/m(3) while incurring significant membrane costs. Overall, we demonstrate for the first time that membrane- and chemical-free electrochemical technologies can remove sufficient boron from RO permeate in a single pass, significantly enhancing its potential to provide energy- and cost-efficient boron removal.

Automated summary

Increasing global water stress has led to a growing interest in seawater desalination by reverse osmosis. Traditional methods of boron removal are costly and require additional filtration steps. This study explores a membrane- and chemical-free electrochemical technology, capacitive deionization, for boron removal. By reversing the polarity of the applied voltage during discharge, boron electrosorption is significantly enhanced, offering a promising energy consumption of 0.2 kW⋅h/m³ and efficient boron removal in a single pass.