Investigation of Stimuli-Responsive Polymer Nanocomposites for Dye Treatment in Aqueous Solution
Date
2019-09-24
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Journal ISSN
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ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
Currently, environmental pollution associated with dyes has become an issue of global concern that is gaining significant attention. Developing inexpensive nanocomposites via combination of functional properties of dissimilar materials is of increasing interest for dye treatment in aqueous solution. Chitosan is a natural, low-cost, biocompatible and biodegradable polymer that is widely researched and applied in advanced water treatment. π-conjugated polyaniline is a notable polymer for adsorption purposes due to the relatively low cost of the monomer precursor, environmental stability, ease of doping/de-doping chemistry and its relatively high nitrogen content. Among conductive polymers polyaniline is one of the most highly studied. Unfortunately, there are some technical limitations associated with each respective polymer; chitosan and polyaniline. Therefore, the application chitosan and PANI in adsorption and catalytic treatment processes of dyes is somewhat limited since single component polymer systems possess limited adsorption sites and low surface area without further synthetic modification.
This thesis investigates various approaches for dye removal from aqueous solution via development of chitosan and polyaniline-based stimuli-responsive nanocomposite materials. These approaches continue to be of great interest since they combine functional properties of dissimilar materials to design multifunctional nanocomposites with enhanced properties. The major aim of this thesis is the development of nanocomposite materials for treatment of dye pollutants from aqueous solution.
In the first section of this thesis, to overcome the weaknesses of chitosan, it was modified to develop pH-responsive polymer brush nano adsorbents for controlled adsorption of methylene blue. By grafting poly anion functional groups to the chitosan backbone, and integrating such a system onto iron oxide nanocrystals, significant enhancement was observed for methylene blue adsorption. This effect was achieved through electrostatic interactions and osmotic pressure, where the regeneration of the nanocomposite was facilitated with stimuli-responsiveness and magnetic separation.
In the second section redox-responsive catalyst systems were developed and utilized towards reduction of 4-nitrophenol with sodium borohydride. Single component precursor (Ag nanoparticles, chitosan and polyaniline) materials are observed to display poor catalytic reduction with 4-nitrophenol reduction. By interfacing a stimuli-responsive polymer with silver nanoparticles, greater catalytic activity was demonstrated for the enhanced reduction of 4-nitrophenol with respect to the single component precursors. Nanocomposites of this type offered a new and efficient approach for conversion of 4-nitrophenol to 4-aminophenol.
Following on the research of methylene blue adsorption, polyaniline/chitosan magnetic nanocomposites were synthesized and used for removal of methylene blue via adsorption in aqueous media. Molecular selective adsorption of methylene blue and methyl orange was measured to bring insight into the adsorption properties of polyaniline nanostructures. In addition, the interaction of polyaniline with NaCl was investigated as a path to further explore dye adsorption onto a polymer in a de-doped state. During this project, it was discovered that polyaniline interacts with salt, which resulted in weakening of the intramolecular hydrogen bonding as confirmed by dynamic light scattering and Raman spectroscopy. The salt-responsive behavior of nanocomposites and their preference to adsorption of a cationic dye over an anionic dye proved that methylene blue likely associates with de-doped polyaniline in a similar fashion as Na+ ions do via cation-dipole interactions.
To better understand the observed experimental results of salt and dye interaction with polyaniline, nitrogen containing self-healing materials were encapsulated inside polyaniline macroparticles (size of ca. 500 nm) and their release upon exposure to salt was monitored. Therefore, a versatile-activation type of delivery systems was developed. The kinetics of release further unraveled the details of methylene blue adsorption and disintegration of polyaniline upon interaction with salt. The results demonstrated that such interactions and disintegration of polyaniline is the result of a combination of various factors (such as osmotic pressure within polymer shell, polymer permeability, polymer flexibility and dynamic motion) that work in a cooperative fashion. In particular, the role of anion hydration according to the Hofmeister series plays a key role on stabilizing the doped polyaniline-M+ complex.
Description
Keywords
Stimuli-responsive, Nanocomposite, organic dyes, adsorption, chitosan
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry