Layer-by-layer assembly of inorganic nanosheets and polyelectrolytes for reverse osmosis composite membranes

In this study, we introduce a layer-by-layer (LbL) assembly method to prepare a reverse osmosis (RO) desalination membrane that consists of a hybrid combination of [polyelectrolyte (PE)/montmorillonite (MTM)]n layers. First, adopting poly(allylamine hydrochloride) (PAH) for a PE layer, an RO test showed that the permeate flux of water through (PAH/ MTM)n multilayer-coated membranes decreased from -25.5 to - 8.3 L·m^_2·h^_1 with the increased bilayer number from n = 9 to n = 18. At the same time, the corresponding ion rejection with respect to NaCl is increased from -30 to - 81%. Despite the increased ion rejection performance, (PAH/MTM)n membranes exhibit a poor chlorine resistance, as frequently observed in commercial polyamide-based RO membranes. In our previous study, it was noted that the RO membranes, prepared just from the PE multilayers (i.e., [PAH/poly(acrylic acid) (PAA)]n layers), showed a marked chlorine tolerance, but concomitantly very low permeate flux (-4-5 L·m^_2·h^_1). Considering the significant drawback in each case (poor chlorine tolerance for (PAH/MTM)n layers and low permeate flux for (PAH/PAA)n layers), we proposed to combine the layer constituents primarily by inserting PAH/PAA layers between two adjacent PAH/MTM layers. Indeed, the flux is maintained at -7.5±0.5 L·m^_2·h^_1, comparable to commercial membranes, while the salt rejection ability is as high as - 75±2.5% and the stability against the chlorine attack is well preserved with - 74±5.0% ion rejection after the NaOCl treatment.