Highly crosslinked layer-by-layer polyelectrolyte FO membranes: Understanding effects of salt concentration and deposition time on FO performance

Layer-by-layer (LbL) deposition of polyelectrolytes onto a negative charge membrane has been investigated under a highly crosslinking condition for forward osmosis (FO). The influence of salt concentration and deposition time on LbL FO membranes performance has been investigated in order to optimize the deposition process, followed by the investigation of polyelectrolyte layer's crosslinking. In the crosslinking steps, polycation layers of the three bilayer LbL membrane were firstly crosslinked using glutaraldehyde (GA) as the crosslinker. Compared to a non-crosslinked membrane, the reverse salt fluxes were reduced by 63% and 58% under the PRO (pressure retarded osmosis) testing mode using 0.5 M MgCl2 and 0.5 M NaCl as draw solutions, respectively. Subsequently, the polyanion layer of that membrane was photo-crosslinked under ultraviolent (UV) at the wavelength of 254 nm. The reverse salt flux of the resulting membrane was further reduced by 55% and 53% under the PRO testing mode using 0.5 M MgCl2 and 0.5 M NaCl as draw solutions, respectively. Specially, the LbL membrane with only one bilayer after cross-linking by both GA and 4 h-UV could achieve a water flux of about 11 LMH and a reverse flux of 8 gMH under the PRO test using 0.3 M NaCl as a draw solution. This is the first time, a LbL polyelectrolyte membrane has been successfully demonstrated as a FO membrane with good rejection toward NaCl while none of previously reported LbL polyelectrolyte membranes have achieved based on the best of our knowledge.

► Effects of salt concentration and deposition time on the FO performance of LbL membranes have been investigated.
► PAH and PSS layers were crosslinked using glutaraldehyde and ultraviolent, respectively.
► Crosslinking technologies have been proven to significant enhance the membrane's selectivity to NaCl.
► The crosslinked (PAH/PSS)1 membrane can achieve Jv at 11 LMH and Js at 8 g LM using 0.3 M NaCl as the draw solution.
► The micro-structure of the membranes was examined using a positron annihilation spectroscopy (PAS).