Facile fabrication of covalent organic framework (COF-LZU1) membrane for water recovery from saline oil-in-water emulsion via membrane distillation

Membrane fouling and wetting are the main challenges for membrane distillation (MD) that constrains its application especially in water recovery from saline oil-in-water emulsion. Herein, a novel covalent organic framework (COF) modified membrane was developed via facile in-situ growth of COF-LZU1 (synthesized by p-phenylenediamine and 1,3,5-benzenetricarboxaldehyde) on electrospun polyvinylidene fluoride (PVDF) substrate (average pore size: 0.80 μm, average porosity: 85.5%). This newly developed membrane exhibited high surface roughness (1.22 μm), suitable porosity (57.3%), well-distributed pore size (0.67 μm) and thin thickness (58.0 μm). In addition, the hydrophilic COF-LZU1 layer (water contact angle 79.9°) and hydrophobic PVDF substrate (water contact angle 140.5°) endowed this Janus-like membrane with high fouling and wetting resistance. As a result, relatively higher water flux and milder conductivity increase were shown on COF-LZU1@PVDF-2 membrane during the treatment for saline oil-in-water emulsion with 0.0-4.0 g L⁻¹ n-hexadecane and 50-80°C feed temperature. The anti-fouling and anti-wetting mechanisms were also quantitatively investigated by the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory and liquid entry pressure (LEP) value. The relevant findings may pave the way in high performance COF modified membrane development with high water flux and high permeate quality for robust membrane distillation.