Development of Superpermeable Wood-Based Forward Osmosis Membranes

Novel lignin-based membranes with excellent water permeability and salt rejection characteristics were developed for forward osmosis (FO) processes. The fabricated membranes are composed of three layers: (i) a bottom layer of nanofibrous electrospun sulfonated kraft lignin (SKL) (70 wt %) and poly(vinyl alcohol) (PVA) (30 wt %), (ii) an intermediate layer of PVA-glutaraldehyde (GA) hydrogel, and (iii) a top thin film of a selective polyamide (PA) layer. We coated the SKL-PVA support surface with four different PVA hydrogel concentrations (0.25, 0.5, 1, and 2 wt %). Increasing the concentration of the PVA hydrogel, the thickness of the deposited layer was increased accordingly. After forming the PA layer on top of the PVA hydrogel layer by the interfacial polymerization reaction, lignin-based thin film composite (LTFC-X) membranes were obtained, in which X denotes the PVA hydrogel concentration. According to the cross-sectional TEM images, the thickness of the deposited PA layer dropped from 450 nm on the LTFC-0.25 membrane to less than 60 nm in the LTFC-1 membrane. However, a further increase in the PVA hydrogel concentration enhanced the PA layer thickness to over 250 nm. Among all synthesized membranes, the LTFC-0.5 membrane demonstrated the lowest structural parameter (S) of 191.67 ± 5.88 μm, resulting in a minimal internal concentration polarization in this membrane. The FO performances of membranes were evaluated in two different configurations: the active PA layer facing the feed solution (ALFS) and the PA layer facing the draw solution (ALDS). The LTFC-0.5 membrane provided maximum water flux (Jw) in both ALDS (61.33 ± 2.85 LMH) and ALFS (56.60 ± 2.99 LMH) modes. The LTFC-0.5 membrane also showed the lowest specific salt flux (Js/Jw) of 0.087 ± 0.004 g/L among all fabricated membranes in the ALDS mode. However, the LTFC-1 membrane provided the best performance in the ALFS mode with a minimum Js/Jw value of 0.090 ± 0.011 g/L.