Rejection of pharmaceuticals by graphene oxide membranes: Role of crosslinker and rejection mechanism

Graphene oxide (GO)-based membranes were fabricated by first immobilizing GO nanosheet using polydopamine (PDA) on the polyvinylidene fluoride (PVDF) substrate and then tuning the interlaminar spacing and properties by the different crosslinkers (i.e., ethylenediamine (EDA) or beta-cyclodextrin functionalized EDA (beta-CD-EDA)) to investigate rejection performance and mechanism of three pharmaceuticals. The d-spacings of EDA crosslinked membrane (PDA-GO/EDA) and beta-CD-EDA crosslinked membrane (PDA-GO/beta-CD-EDA) were determined to be 1.13 and 0.97 nm, respectively, which were much higher than that of PDA coated GO membrane (PDA-GO) of 0.78 nm. The moderate and even relatively low rejection ratio of the pharmaceuticals demonstrated that molecular sieving didn't contribute significantly to the pharmaceutical rejection as expected. In addition to molecular sieving, the electrostatic and chemical interactions from the active sites GO might play the significant and noticeable role on pharmaceutical rejection. When adsorption was not much higher in some scenarios (i.e., carbamazepine at all pH, sulfadiazine @ pH = 11 and propranolol @ pH = 3), it was plausible that conformation of GO nanosheets dramatically changed, which resulted in decrease of d-spacing and thus much higher rejection ratio. In contrast, when adsorption was much higher in some scenarios (sulfadiazine @ pH = 3, 7 and propranolol @ pH = 7, 11), the solubility in the membrane matrix increased and therefore the rejection ratio decreased. The disagreement demonstrated that more attention should be paid on the interactions between pharmaceutical and GO membranes, although the stability of the membrane was enhanced by crosslinking.