Micro-morphology and formation of layer-by-layer membranes and their performance in osmotically driven processes

Abstract

We have fabricated novel multilayer membranes for forward osmosis (FO) processes by layer-by-layer (LbL) assembly of polyelectrolytes and examined the fundamentals of LbL membrane formation and microstructure. The LbL membrane was made of a positively charged poly(allylamine hydrochloride) (PAH) and a negatively charged blend of poly(acrylic acid) (PAA) and poly(sodium 4-styrene sulfonate) (PSS) at a weight ratio of 1:1 on top of hyperbranched polyethyleneimine (PEI) modified Torlon substrates and then crosslinked by glutaraldehyde (GA). The newly developed 3-bilayer membrane has an impressive water flux of 28LMH (Lm−2h−1) and reverse salt flux of 1.97gMH (gm−2h−1) using 0.5M MgCl2 as the draw solution. Positron annihilation spectroscopy (PAS) analyses reveal that PEI densified the upper layer of the substrate while GA crosslinked the selective layer as well as part of the PEI modified substrate. PAS experiments also indicated that growth in membrane thickness was non-linear and became insignificant after around 6-bilayer of deposition. An immobilized osmotic pressure was observed due to the ion adsorption on the LbL layer. FO results show that the pH and salt concentration of polyelectrolytes play important roles on membrane morphology and FO performance. Both water flux and reverse salt flux were strongly influenced by polyelectrolytes’ charge density and the degree of chain coils. Due to strong charge density and chain packing when pH values of PAH and PAA–PSS are 5 and 3–5, respectively, the resultant LbL membranes show both low water flux and low reverse salt flux. The addition of NaCl in polyelectrolyte solutions may facilitate chain coils, rearrange coil size, affect chain packing and create defects depending on NaCl concentration. A relatively high NaCl concentration tends to decompose the coils and weaken the LbL structure, thus increase the water flux but reduce the reverse salt flux.