Interfacial interaction energy and pore blockage analysis of protein fouling in ultrafiltration-like forward osmosis

Protein fouling on forward osmosis (FO) membranes with reverse osmosis (RO)-like selective layers is well understood, yet the fouling mechanism in ultrafiltration (UF)-like FO membranes remains largely unexplored. UF-like FO membranes, characterized by uniform pore sizes with a less distinct difference between the top and bottom surfaces compared to RO-like FO membranes, are hypothesized to exhibit similar fouling behavior regardless of membrane orientations. This study investigates the fouling dynamics in UF-like FO membranes under both active layer-facing feed solution (AL-FS) and active layer-facing draw solution (AL-DS) orientations. Optimal UF-like FO membranes were fabricated by polymer solution formulation and subjected to 6-hour bovine serum albumin (BSA) filtration. Fouling mechanisms were analysed using the pore blockage-cake filtration model and surface interactions were assessed through extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. The membrane with 12 wt% PES (UF-12) demonstrated the highest water flux (34.23 LMH) and the lowest solute flux (17.32 gMH) under 1 wt% poly(sodium styrene sulfonate) (PSS) draw solution. Flux decline (∼60 %) was similar in both orientations, although AL-DS declined faster initially. Pore blockage analysis indicated a transition from pore blockage to cake formation in AL-DS mode and intermediate blocking in AL-FS mode. Surface interaction analysis revealed that van der Waals forces significantly overcame the charge repulsion between co-anionic BSA and the negatively charged membrane surface. BSA agglomeration, driven by hydrophilic attraction, was confirmed by a high ${\gamma }^{\mathrm{LW}}$ (43.90 mJ/m²) and positive ${\Delta G}^{\mathrm{Coh}}$ (39.59mJ/m²). These findings provide insights into fouling mechanisms in UF-like FO membranes, facilitating the development of antifouling membranes for water treatment applications.