PVDF membranes containing alkyl and perfluoroalkyl-functionalized graphene nanosheets for improved membrane distillation

Polyvinylidene fluoride (PVDF) membranes containing hydrophobic graphene nanofillers were prepared and tested for membrane distillation applications. The nanofillers were obtained by a two-step process: 1st) chemical grafting of hydrophobic molecules, either octylamine (OA) or perfluoroctylamine (PFOA), to graphene oxide (GO) nanosheets, and 2nd) chemical reduction of functionalized GO (rGO) to remove unreacted oxygen-containing functional groups. This resulted in OA-functionalized reduced GO (OA-rGO) and PFOA-functionalized rGO (PFOA-rGO). The addition of these nanomaterials to PVDF membranes prepared by the phase inversion process led to an increase in the membrane contact angle, and therefore higher hydrophobicity, as well as an increase in the membrane porosity. When comparing both nanofillers, OA-rGO and PFOA-rGO, the latter was more efficient in achieving higher contact angles due to the presence of fluorine atoms, whereas OA-rGO led to a greater enhancement in membrane porosity as compared to PFOA-rGO. MMMs containing 0.7 wt% nanofiller loadings of OA-rGO and PFOA-rGO achieved the highest water fluxes of 9.1 and 8.8 L m- 2 h-1, respectively and salt rejection above 99.9%, which was monitored for at least 162 h of operation for the former. In comparison with pure PVDF (flux of 5 L m- 2 h-1), the addition of OA-rGO and PFOA-rGO nanofillers results in a flux increment of 82% and 76%, respectively.