Graphene oxide for improved water treatment via membrane distillation: membrane synthesis, performance enhancements and environmental impacts

Abstract

This thesis explores the use of graphene oxide (GO) as a membrane coating to improve the performance of water treatment via membrane distillation (MD). Firstly, the limitations and recent advances in membrane distillation were evaluated and compared against conventional membrane desalination (reverse osmosis, RO) and other emerging membrane technologies (forward osmosis and membrane capacitive deionisation). It was found that currently, membrane distillation cannot replace reverse osmosis technologies for the desalination of seawater due to high thermal energy consumption and long-term stability issues (such as fouling and pore-wetting). The environmental impacts of seawater desalination via reverse osmosis and membrane distillation were evaluated. These were compared with scenarios that utilised GO membranes, which have significant studies showing that they can reduce the energy consumption of both technologies through antifouling and high permeability. The results showed that the use of GO membranes could reduce the impacts of RO and MD by 3-7 % and 27-34 % on average, respectively. RO using GO membranes was the most favourable desalination option, with the lowest impacts across most categories when compared to MD. The source of the thermal energy is the most critical factor in reducing the environmental impacts of MD; in fact, the solar thermal energy scenarios had impacts that were 43-93 % lower in nine categories when compared to RO powered by fossil-fuels. The review and the life cycle assessment highlighted opportunities for the application of MD: in the treatment of waters where reverse osmosis cannot be used (fouling wastewater). However, fouling and pore wetting severely affect MD performance during the treatment of these water sources. To combat this, GO coatings were developed on top of commercial polymer supports and polydopamine (PDA) was investigated as an anchor molecule. The membranes were tested against feed water containing 150 ppm of surfactant Triton X-100 and the control membrane (the commercial polymer support) failed from pore wetting after just 15 minutes, whereas the GO layer showed stable operation for 90 h. Next, the method for producing the antiwetting coatings was improved by using a spray coating pyrolysis technique, which was able to deposit GO onto the polymer support. The immobilisation of the GO onto the surface was also improved through this technique and other 2D materials (hBN, MoS2 and WS2) were also explored as surface coatings. The GO membranes showed antifouling properties against a 72 h experiment with feed water containing 150 ppm humic acid and 200 ppm paraffin oil. The permeate flux for the control membrane dropped by 22 %, whereas the GO membrane had a smaller flux reduction of 0.14 % in the same time period. The GO membranes were then tested at pilot-scale, for the first time, and a thermal energy analysis revealed that the GO membranes were 40 % more thermally efficient than the control membranes. Overall, the thesis works to improve the performance of membrane distillation for the improved recovery of water through non-conventional sources.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Adisa Azapagic (Supervisor), Patricia Gorgojo (Supervisor) & Alejandro Gallego Schmid (Supervisor)