Polyelectrolyte Multilayers for Forward Osmosis, Combining the Right Multilayer and Draw Solution

Polyelectrolyte multilayers (PEMs) are highly promising materials as selective separation layers on the inside of hollow fiber membranes. We investigate the forward osmosis (FO) performance of poly(4-styrene sulfonate) (PSS)/poly(allylamine) (PAH)- and PSS/poly(ethylene imine)-based PEM membranes and apply different draw solutions. The PEM membranes show different separation properties, demonstrated by their retention behavior under nanofiltration conditions against five relevant draw solutions (NaCl, MgCl2, MgSO4, Na2SO4, and trisodium citrate (TSC)). Subsequently, the relation between PEM properties and FO performance was studied for various draw solutions. PSS/PAH membranes showed the lowest reverse salt flux for MgCl2 and MgSO4, while the PSS/PEI membranes showed the lowest reverse salt flux for Na2SO4 and TSC. Combining the right PEM membrane with the right draw solution is thus key to getting low reverse salt fluxes in FO. Cross-linking of the PEMs resulted in an overall lower salt and water flux; however, the reverse salt flux selectivity was not affected.