Preparation of GO@DCN nanocomposite modified PVDF membranes for improved membrane performance and exploration of formation mechanism

Abstract

Two-dimensional nanocomposite membranes (NCMs) have attracted increased attention, but the effects of nanomaterials on membranes and phase inversion mechanisms are still unclear. In this study, graphene oxide (GO) was combined with nitrogen defect-modified g-C3N4 (DCN) nanosheets by grinding to obtain GO@DCN, with subsequent doping into polyvinylidene fluoride (PVDF) by non-solvent induced phase separation (NIPS) to prepare a series of hydrophilic NCMs. The thermodynamic ternary phase diagram of NCMs is reported for the first time. Compared with pure PVDF, the phase inversion of NCMs was accelerated with the addition of DCN, GO and GO@DCN nanomaterials, which lead to sequential increment in porosity, providing channels for water molecules. Besides, the Density Function Theory(DFT) calculations indicated a strong interaction between PVDF and GO@DCN, which also contributes to the rapid phase inversion of the PVDF/GO@DCN. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory demonstrates that the attraction between PVDF/GO@DCN2.0 and Bovine Serum Albumin (BSA) is halved compared to PVDF and BSA, indicating superior antifouling ability. At the same time, the negative zeta potential of NCMs rose, which enhanced the repulsion between NCMs and the negatively charged BSA contaminants in water. Thus, in contrast to PVDF, the permeate flux of PVDF/GO@DCN2.0 increased from 161.6 Lm−2h−1 to 387.8 Lm−2h−1 and the rejection of BSA from 80.1% to 91.6%, respectively. The flux recovery ratio (FRR) of PVDF/GO@DCN2.0 improved 30.4%, and irreversible fouling decreased 29.6%. After five cleaning cycles, the NCM still maintained excellent stability.