Effect of oxidation degree of GO nanosheets on microstructure and performance of polysulfone-GO mixed matrix membranes

Graphene oxide with increasing degree of oxidation (GO-x) were incorporated in polysulfone (PSF) mixed-matrix ultrafiltration membranes. Synthesized GO-x were characterized for average particle size by AFM and DLS method; oxygen functionalities and nanostructure properties by XPS, FT-IR, XRD, Raman analysis, TEM, and elemental analysis; surface charge by zeta potential. Increasing oxidation degree, increased the proportion of carbonyl functional groups, exfoliation, lowered zeta potential and particle size in GO. GO-x with higher oxidation degree showed significantly improved dispersibility in N-methyl-2-pyrrolidone and polysulfone matrix. Blending hydrophilic GO-x, enhanced phase-separation kinetics improving porosity, tensile strength, permeability, protein rejection, and fouling resistance of membranes. With the increase in the oxidation degree of the matrix, contact angle and surface charge on the membranes decreased. 2%-GO-7/P membrane showed permeate flux of 352 kg m−2 h−1 bar−1 while, 1%-GO-7/P membranes acquired permanent negative charge and displayed permeability of 250 kg m−2 h−1 bar−1 with BSA rejection of about 97%, compared to control membrane permeability of 102 kg m−2 h−1 bar−1 with BSA rejection of about 91%. Thus, the membrane flux-rejection trade-off is overcome. 1%-GO-7/P membrane displayed exceptional flux recovery of over 99% on simple DI water flushing. The superior performance of GO-x/P membranes is attributed to faster diffusion rate of water across GO matrix over that through the polymer solution, which increased the number of thermodynamic instabilities giving higher rate of phase inversion resulting in narrow pore size distribution and higher pore density, demonstrating simple approach for enhancing performance of UF membranes.