Production of boric acid by bipolar membrane electrodialysis: Evaluation of commercial and polyelectrolyte multilayers-coated ion exchange membranes

Abstract

Boron is an essential element for many industrial sectors, but the European Union (EU) is entirely dependent on imports for its boron supply. To ensure a sustainable supply of boron for the EU, developing recovery strategies from secondary sources is essential. Thus, boron recovery from seawater reverse osmosis (SWRO) desalination brines using bipolar membrane electrodialysis (BMED) as part of a multi-mineral brine mining process is proposed. BMED experiments were conducted with 35 triplets of commercial membranes (100 cm2), using sodium borate solution (92.5 mM) as feed, at three different pHs (2, 9 and 12), identifying pH 12 as the best option. Moreover, commercial ion exchange membranes from PCA GmbH and Mega a.s. were coated with polyelectrolyte multilayers using layer-by-layer technique. Characterization of the coated membranes included zeta potential, contact angle, ATR-FTIR, transport number and ion exchange capacity analyses. Subsequent BMED experiments with three triplets comparing virgin and modified membranes (100 cm2), using sodium borate solution (92.5 mM) as feed, demonstrated that the coating with 4 bilayers increased the selectivity of membranes. Although the coated RALEX membranes presented the highest selectivity (7.2 for B(OH)4-/Na+ in the acid compartment), uncoated PCA membranes were the optimal choice for H3BO3 and NaOH production due to higher Na and B removal (97 %), higher concentration factors (2.5 for the acid and 4.3 for the base), and higher current efficiency for H3BO3 production (78 %).