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