Development of Advanced Graphene-based Composites for Water Purification

Abstract

Presence of a broad range of co-existing water pollutants including heavy metals, organic dyes, organic solvents and oils remains an unresolved environmental challenge. Majority of the existing water purification technologies are expensive, energy-intensive and ineffective, particularly for the removal of ultra- low concentration of heavy metals. This calls for the development of novel sorbent materials and new effective water purification technologies. This thesis consolidates the development of multifunctional graphene-based composites to achieve highly efficient removal of a broad range of water pollutants. Three key aspects are addressed in this thesis: First, mastery of tailoring the multifunctional surface chemistry properties of graphene-based composites is underpinned through the simultaneous binary-heteroatom doping and reduction of graphene oxide (GO), followed by a thiol-ene click reaction using amino-terminated thiol molecules to create multifunctional graphene-based materials with tunable surface chemistry. Second, based on four functionalization strategies (hydrothermal, chemical reduction, thermal and photoinitiated thiol-ene click), multifunctional groups are covalently attached to the GO to address the limitations of commercial adsorbent, activated carbon (AC). Precursors enriched with oxygen- (alginate), nitrogen- (hydrazine and mixed polyamine blend), sulfur- (pentaerythritol tetrakis-mercaptopropionate) as well as combined nitrogen- and sulfur- groups (cysteamine) are used to demonstrate these chemical functionalization concepts for higher mechanical strength, stability, better dispersion in water and enhanced sorption of diverse classes of water pollutants. Magnetic iron oxide nanoparticles are also incorporated to the graphene-based composites for easy separation after the sorption process besides improving the sorption capacity of water pollutants. The developed modification strategies are versatile, low cost, scalable, energy-efficient and eco-friendly compared to highly-energy-intensive activation process of AC. Third, sorption performance of the developed graphene-based composites is studied in terms of the sorption isotherms, kinetics, selectivity, regenerability and real sample analysis. Both hydrothermally- and chemically- reduced polyamine-modified rGO composites follow the Freundlich isotherm models with a maximum sorption capacity of 63.80 mg/g and 59.90 mg/g, respectively, achieved towards Hg²⁺ ions. Langmuir isotherm model is used to describe the interaction of graphene-based composites with their respective contaminants studied and maximum sorption capacity attained as the following: cysteamine-modified partially rGO (169.00 mg/g for Hg²⁺ ions), multithiol functionalized graphene bio-sponge (101.01 mg/g for Pb²⁺ ions and 102.99 mg/g for Cd²⁺ ions) and multifunctional graphene biopolymer foam (789.70 mg/g for methylene blue, 107.00 mg/g for Hg2+ ions and 73.50 mg/g for Cu²⁺ ions). Meanwhile, all the graphene-based composites developed are well-fitted using the pseudo-second order kinetic models. Positive evaluation outcomes suggest that all the developed graphene-based composites outperformed AC and some sorbents reported in the literature in single and simultaneous pollutant removal studies using milli-Q, river and sea water. Different innovative functionalization approaches are demonstrated herein to engineer graphene-based composites with unique multifunctional properties, which promise realistic water purification technology solutions for efficient uptake of diverse classes of water pollutants. Research studies completed in this thesis integrate the fundamental understanding and application of knowledge on the surface, structural, chemical and thermal characteristics of functionalized graphene-based composites to address the most concerning water pollution challenges using cutting-edge and scalable water purification technologies.