Graphene Nanocomposite Membranes for Sustainable Water Treatment

Abstract

Membrane distillation (MD) is a promising water treatment technology capable of achieving very high rejections of dissolved non-volatile substances, even with very high feed water concentrations, for a range of high value applications. In this work, functionalised graphene oxide was used to enhance the performance of polyvinylidene fluoride (PVDF) membranes, fabricated via phase inversion and electrospinning. In the first case, the addition of very small (0.5 wt% and 0.3 wt%) of graphene oxide (GO) and 3-(Aminopropyl)triethoxysilane-functionalised graphene oxide (GO-APTS) to PVDF casting solutions resulted membranes with a 52 and 86 % flux enhancement in air gap membrane distillation (AGMD) experiments, respectively, compared to the pure polymer. These membranes were used to treat artificial seawater (35 g L-1 NaCl) and achieved salt rejections of >99.9% throughout testing, showing the potential of the technology for seawater desalination applications. In addition, for the first time, air gap membrane distillation was used to treat synthetic textile dyeing wastewater. This industry’s large consumption of water and generation of significant amounts of waste heat make membrane distillation a potentially suitable water recycling technology. In this work, commercial polytetrafluoroethylene (PTFE) membranes were used to treat simulated textile wastewater containing NaCl, sunset yellow (SY) and rose bengal (RB) dyes and the surfactant, sodium dodecyle sulfate (SDS). Results showed that near-100% removal of salts and dyes was achieved over 20-hour tests with stable fluxes of between 11.7 and 12.6 Lm-2 h-1. When the water contained the surfactant, the flux dropped to ∼3 Lm-2 h-1 after 70 h and the permeate conductivity had risen slightly, indicating the onset of pore wetting and the need for membrane cleaning. In comparison, direct contact membrane distillation (DCMD) tests achieved lower colour and total carbon removal after just 8 h of testing with the surfactant-containing feed solution, suggesting the better suitability of AGMD for this application. Finally, the emerging membrane fabrication technique of electrospinning was successfully utilised to produce high performance PVDF nanofibre membranes for the removal of arsenic via membrane distillation. The performance of these membranes was enhanced by incorporating superhydrophobic graphene, functionalised with polyhedral oligomeric silesquioxane (POSS), into the PVDF nanofibre spinning solutions. The tensile strength of the graphene-enhanced membranes was 4.7 times greater than the pure polymer. In addition, the flux was higher over 24 hours compared to pure PVDF membrane and was stable over 5 days of continuous testing. Arsenic levels were below the detection limit of the mass spectrometer used for analysis (

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Nima Shokri (Supervisor) & Patricia Gorgojo (Supervisor)