Building Nanostructures Toward Improved RO Membrane Performance

Author: Nasser Alotaibi

  • Thesis download: available for open access on 6 Feb 2020.

Alotaibi, Nasser, 2017 Building Nanostructures Toward Improved RO Membrane Performance, Flinders University, School of Chemical and Physical Sciences

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Abstract

In this thesis, single wall carbon nanotubes (SWNTs) were embedded into the polyamide layer of RO membranes. Functionalisation of SWNTs was performed to improve their distribution in the polyamide layer. To study the functionalisation reactions of SWNTs, model reactions were undertaken. Anthracene-9-carboxylic acid was the model substrate of the SWNTs because of the similarity of its aromatic properties. The produced amine compounds from the model substrate reactions were explored for formation of supramolecular structures. Benzene-1,3,5-tricarboxylic acid chloride (TMC) was used to synthesise benzene-1,3,5-tricarboxamide (BTA) derivatives as supramolecular motifs. Four BTA molecules, with varingin aliphatic chain lengths, were synthesised and characterised. Supramolecular structures were formed through the self-assembly process in a chloroform solution. Due to the chain length differences between the molecules, different self-assembled shapes were observed. Thus, this self-assembly behaviour as a function of time was investigated for the shortest and longest aliphatic chain molecules. This self-assembly was investigated by monitoring the change in fluorescence, FT-IR and SEM as a function of time. It was found that the long-chain molecule showed faster self-assembly and with greater overlap between the aromatic rings, due to the flexibility of the long-chain molecule. Moreover, the molecule with the long chain showed higher fluorescence and longer fibre formation on a substrate. After investigating the self-assembly behaviour and the influence of chain length on the model substrate, the functionalisation reaction progressed to the SWNTs in order to embed them into the polyamide layer of the RO membrane. The influence of chain length on the distribution of SWNTs into the polyamide layer was investigated as well. Two molecules with different aliphatic chain lengths were synthesised and separately grafted to SWNTs. Raman and FT-IR showed successful grafting of SWNTs with the synthesised molecules. Synthesis of the polyamide layer was performed by the interfacial polymerisation method. TMC (0.1 wt%) in the organic phase was allowed to react with mphenylenediamine (MPD) (2 wt%) in the aqueous phase on the top surface of the polysulfone (PSf) layer, forming a thin polyamide film. To modify the polyamide layer, the two grafted SWNTs were dispersed separately in the organic phase prior to the polymerisation. Two modified polyamide layers, with a variety of mass loadings of grafted SWNTs, were synthesised. Characterisation of the modified polymers was obtained by Raman, FT-IR, AFM and SEM. Membrane water flux and salt rejection were determined using 2000 ppm sodium chloride as the feed solution. The modified membranes showed an improvement in water flux, but there was a decrease in salt rejection. For example, an improvement in flux from 24 to 32 Lm-2 h-1 for SWNTs-free membrane and modified membrane with 5 wt% SWNTs respectively was observed, while for the same membranes the salt rejection decreased from 73% to 67%. Further investigation of the chain-length influence was obtained by the model reaction. Anthracene-9-carboxylic acid was used to mimic the SWNTs and synthesise the model reaction molecules. The model membranes were synthesised using the model molecules as a mimic for grafted SWNTs. The flux and salt rejection of the model membranes were determined under the same conditions as the modified membranes.

Keywords: RO membrane , polyamide membrane , CNTs, SWNT, BTA
Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Chemical and Physical Sciences
Supervisor: Prof. Joe Shapter