Fabrication and characterization of novel forward osmosis membranes based on layer-by-layer assembly

Abstract

In the study of synthesizing LbL FO membranes, the membrane substrate was firstly tailored (high porosity, finger-like pores, thin thickness, and high hydrophilicity) to achieve a small structural parameter of 0.5 mm. Secondly, the polyelectrolyte layers were deposited on the support membrane top surface for a number of times by soaking repeatedly in poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene-sulfonate) (PSS) in an alternative sequence. Increasing the number of polyelectrolyte layers improved the selectivity of the LbL membranes while reducing their water permeability. The more selective membrane 6#LbL (with 6 polyelectrolyte layers) had much lower reverse solute transport compared to 3#LbL and 1#LbL. The FO water flux was found to be strongly affected by both membrane water permeability and reverse solute transportation. Severe solute reverse transport was observed for the active-layer-facing-draw-solution (AL-DS) membrane orientation, which is likely due to the suppression of Donnan exclusion by the high ionic strength of the draw solution. In contrast, the active-layer-facing-feed-solution (AL-FS) orientation showed remarkable FO performance (15, 20, and 28 L/m2.h at 0.1, 0.5, and 1.0 M MgCl2, respectively, for membrane 3#LbL using distilled water as feed solution).