Polyelectrolyte multilayers modification of nanofiltration membranes to improve selective separation of mono- and multivalent cations in seawater brine

Abstract

Nowadays, brine produced in the Reverse Osmosis (RO) seawater desalination plants is a significant and largely untapped source of critical minerals and metals, which is usually discharged into the seas as wastewater. As a front face stage in the brine mining process, nanofiltration (NF) membranes can be used to separate brine stream into monovalent and multivalent ion-rich streams. Since high separation factors are required in seawater RO brine, polyelectrolyte multilayers with Layer-by-Layer (LbL) technique were used in this study to modify commercial polyamide NF membranes and improve their performance. Two common polyelectrolytes, poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS) were coated onto two poly-piperazine polyamide NF membranes (NF270 and Desal-5 DL). The effects of the bilayers number and the outer layer charge on membranes’ performance were studied using two crossflow filtration setups with two different membrane sizes to find the membrane with the higher monovalent/multivalent separation factor. Desal-5 DL membrane coated with 5.5 bilayers provided the highest selectivity factors for mono/multivalent cations (> 21), the highest average rejection of multivalent cations group (about 95.7%) for short-term experiments. Another interesting result was the increase in water permeance of the Desal-5 DL membrane after it was coated with 5.5 bLs (indicating more than 20% improvement in pure water permeance). In addition, the stability test with brines at 20 bar showed that this membrane was stable over an extended period of filtration (22 hours) with an even higher selectivity factor of 34 and multivalent cations rejection of 97.7% on average.