Organic and Inorganic 2D Materials for Selective Separation Membranes

Abstract

Laminar membranes (LMs) of aligned flakes of 2D materials are a promising new class of selective separation membranes, showing good water flux and solute rejection compared to existing technologies. However most LMs show poor rejection of monovalent salts without functionalisation, presenting a barrier for the application of LMs in pressing applications such as desalination. This thesis details three ways to control the water flux and salt rejection of LMs. Firstly the material used to make the membrane is evaluated. The water permeance and salt rejection are measured for LMs of different 2D materials produced by sonication; namely graphene, MoS2 and WS2. Despite its hydrophobicity, graphene LMs show a high water permeance of 280 L m-2 h-1 bar-1 in dead end measurement, comparable to MoS2 LMs, owing to its loose membrane structure. LMs of commercial graphene nanoplatelets (GNP) on the other hand show very low water flux, owing to its increased flake size over graphene, while WS2 LMs are fragile and delaminate easily from the PVDF support. The role of exfoliation method for producing the 2D materials in the LM is investigated by comparing LMs of MoS2 produced by microfluidizing and sonication. These LMs show remarkably similar filtration properties despite the microfluidized MoS2 flakes being 10 times larger. Composite LMs made from two different 2D materials are stable and simple to make. If the different 2D materials are well dispersed throughout the LM, the permeance and rejection values are between those of LMs of the constituent components, showing the ability to tune membrane properties by addition of components. A post-fabrication method of membrane modification, the compression of LMs of MoS2 and graphene by external pressures of up to 20 bar, shows improved rejection and lower water permeance, as voids between the flakes are decreased in volume. At an optimum pressure of 6 bar, the NaCl rejection of a graphene LM is increased from 5 to 65%, while water flux is unchanged. The compression does not reverse readily, and stable, high rejection LMs are produced. Finally, the conditions during filtration are used to control flux through LMs. Comparing the measurement of ionic conductance of LMs, under an electrical potential gradient across the membrane, to the salt flux under diffusion indicates both are suitable techniques to measure the permeability. With a potential directly applied to a conductive LMs, electro-responsive behaviour is seen. The water permeance of a graphene LM is increased from 220 to 2100 L h-1 m-2 bar-1 when -1 V potential is applied, while the flux of NaCl decreases by half; and near-identical changes are seen at +1 V. Graphene can be added to LMs of MoS2 to improve its conductivity and enable its use in electroresponsive membranes. Research completed in this thesis shows how the salt rejection and water flux of LMs can be controlled by selection of the starting materials, by a post-fabrication compression step, and by controlling electric potential during filtration.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor) & Mark Bissett (Supervisor)