Abstract
Desalination is the process of purifying seawater or brackish water by removing salts and other impurities to produce fresh, drinkable water. This technology is increasingly vital in regions facing water scarcity due to population growth, climate change, and limited freshwater resources. A significant by-product of desalination is concentrated reject brine, which presents substantial challenges to marine ecosystems due to its high salinity and potential release of harmful chemicals used in the desalination process. Consequences of releasing reject brine with no proper treatments can lead to ecological imbalances, adversely affecting local marine life and biodiversity. Addressing the environmental impact of brine disposal is critical, and one promising approach is the extraction of valuable minerals from this highly concentrated reject brine. This method not only mitigates environmental harm but also provides economic benefits by recovering high-purity minerals.
This thesis investigates the efficient extraction of magnesium-based minerals from reject brine, the subsequent synthesis of magnesium oxide (MgO)-based composites, and the evaluation of their practical applications. Extracting magnesium-based minerals from desalination reject brine is particularly significant as it provides an alternative to magnesium minerals produced from mining processes, thereby reducing the carbon footprint associated with manufacturing. Furthermore, this approach addresses the adverse effects of carbon dioxide (CO₂) emissions, conserves energy, and mitigates the environmental impact associated with traditional mineral extraction processes.
This study focuses on the following six primary areas:
(1) The development of a database for concentrated reject brine. Using this database, a non-ideal thermodynamic solution model was developed with FactSage commercial software to determine optimal experimental conditions for extracting magnesium and calcium minerals from the brine.
(2) Design and optimization of mineral extraction using the developed solution model to recover high-purity magnesium-based minerals. Magnesium hydroxide (Mg(OH)₂), ammonium magnesium sulfate hexahydrate ((NH₄)₂Mg(SO₄)₂·6H₂O) and magnesium sulfate hydrates (MgSO₄·xH₂O) with purity more than 98+% synthesized by utilizing desalination reject brine. During the extraction process calcium oxalate hydrate (CaC₂O₄·H₂O) was isolated and produced high-purity calcium oxide (CaO). This study is the first to report the production of both CaC₂O₄·H₂O and (NH₄)₂Mg(SO₄)₂·6H₂O directly via a single route using desalination reject brine waste.
(3) This study further investigates the properties of these extracted precursors and explores the potential for producing high-purity reactive MgO as an alternative substitute for Portland cement (PC) blends. It aims to achieve high reactivity and high surface area under commercial-scale manufacturing conditions. In addition to analysing the correlations with calcination variables like temperature and duration, the research examines at how pH conditions affect precipitation and the reactivities of MgO to use in green cement materials.
(4) The applications of MgO are highlighted through the design, development, and synthesis of novel nanomaterials. This study reports, for the first time, the development of innovative MgO-Mg(OH)₂ bi-phase nanocomposites for room temperature CO₂ capture, enabling new CO₂ capture mechanisms. This novel material improves CO₂ capture capacity by approximately 25% compared to standalone MgO. This approach effectively addresses the issue of magnesium carbonate (MgCO₃) formation, which impedes CO₂ adsorption in MgO. The impact of dopant materials on the MgO-Mg(OH)₂ nanocomposites was investigated to explore CO₂ capture efficiency at room temperature. Furthermore, a novel bi-phase MgO-Mg(OH)₂ nanocomposite material was engineered into 2D flower-like structured pillar arrays for application as antibacterial agents, tested against three types of bacteria, achieving 100% elimination after 8 hours.
(5) The potential applications of MgSO₄·xH₂O are explored as a precursor for developing highly efficient N-MgO/g-C₃N₄ nanocomposites. These nanocomposites aim to create low bandgap heterostructures for the treatment of organic wastewater under sunlight, achieving an impressive efficiency of over 90%. In addition, this study tests the theory of using a p-n junction to inhibit the recombination of photo-generated electrons and holes in g-C₃N₄ for the photocatalytic degradation of organic dye waste. This study illustrates the practical applications of low-cost MgSO₄·xH₂O minerals that can be derived from reject brine.
(6) Additionally, the study includes a comprehensive life cycle assessment (LCA) of the production of MgO, CaO, (NH₄)₂Mg(SO₄)₂·6H₂O and MgSO₄ hydrates from reject brine sourced from desalination plants in Singapore. This assessment evaluates the environmental impacts and benefits associated with these processes, highlighting their potential to enhance the sustainability of desalination practices.
This research illustrates the potential for extracting high-quality minerals through eco-friendly methods, contrasting with traditional mineral mining and manufacturing practices. By incorporating reactive MgO derived from reject brine, the aim is to reduce CO₂ emissions linked with numerous industries. Beyond its engineering applications, the research significantly enhances scientific understanding in areas including the thermodynamics of concentrated salt solutions, interactions between nanocomposites and gases, surface, interface polarization, semiconductor band engineering, and photocatalysis. This study aims to explore and enhance Singapore's capability to utilize reject brine effectively, potentially boosting the cost-efficiency and profitability of desalination operations. It tackles brine disposal challenges while offering a sustainable approach to improving the environmental impact of desalination processes, supporting long-term sustainability in diverse industrial applications.