Tuning charge density in tethered electrolyte active-layer membranes for enhanced ion-ion selectivity

Abstract

Efforts toward developing membranes for aqueous separations beyond desalination have intensified, in attempts to achieve zero liquid discharge and a circular economy. Treatment of unconventional wastewaters and brines as well as recovery of valuable species require separation of solutes and ions. Recently, tethered electrolyte active-layer membranes (TEAMs) with dense ionizable brush polymers grafted from cellulose ultrafiltration supports were introduced as a robust, highly controllable membrane platform for these aqueous separations. In this study, we investigate crosslinking of single-block TEAMs to increase the effective charge density and coverage of pores by the active layer, and to possibly tap into size-based exclusion mechanisms. We also determine if crosslinking multiblock TEAMs comprising block copolymers of both negative and positive charge can better align blocks, thereby improving ion rejection. Single-block TEAMs with relatively short crosslinkers proved to have the highest divalent co-ion rejection in dilute solutions, at ∼85–95%. NaCl was rejected ∼55 and 80% by crosslinked negatively- and positively-charged TEAMs, respectively. Anion monovalent selectivity, Cl−/SO42−, was as high as ∼25 for negative TEAMs, while the maximum Na+/Ca2+ ratio achieved by positive TEAMs was ∼9.5. This work reinforces the value of ultrathin brush active-layer membranes and TEAMs as important tools to understand fundamental transport through membranes and better control synthesis for targeted selectivity.