Innovative reverse osmosis membrane manufacturing with green and nanocomposite carbon quantum dots for desalination purposes

Abstract

The accelerating rate of climate change in tandem with the progression of industrialization has resulted in substantial losses in water supply, with a consequent increase in the global population facing water scarcity. In addition to those already lacking access to potable water, a significant proportion of the global population is facing a decline in available freshwater resources. Membrane processes represent a class of innovative separation technologies employed to ensure the provision of water from high salinity waters or partially treated waters.The reverse osmosis process involves the utilisation of semi-permeable polymeric membranes under pressure as a driving force. Reverse osmosis membranes are typically composed of a three-layer structure. The first layer, designated as non-woven, possesses a thickness of 90 µm and is covered with a polymeric structure. The most commonly employed polymers in this layer include polysulfone, polyethersulfone, and polyetherimide. The second layer is covered with a thin film coating, resulting in the formation of a polyamide layer characterised by high surface selectivity. It is inevitable that certain problems will occur during the operation of the reverse osmosis process. The primary issue is fouling, which is the accumulation of organic, inorganic, and biological contaminants on the surface and pores of the membranes, resulting in a reduction in separation efficiency. Various cleaning procedures may be used to address this issue, though it should be noted that these methods result in additional water and chemical material consumption. Secondly, the polyamide coating on the top layer of reverse osmosis membranes is not resistant to active chlorine. When the active chlorine in the water due to disinfection comes into contact with the membrane, it adheres to the binding ends of various functional groups in the polyamide layer, causing damage to the polyamide layer and decreasing the selectivity performance of the membrane. Thirdly, boric acid, a by-product of desalination and a constituent of seawater (5 mg/L), is not retained by the membrane due to its non-ionic nature at neutral pH (2.573 Å). This is in close proximity to the hydrogen bonded water molecules (2.7 Å). Consequently, reverse osmosis (RO) membranes generally exhibit low boron removal percentages (below 90%) and are incapable of reducing the boron content below 0.5 parts per million (ppm). The integration of innovative nanomaterials within the membrane structure has been identified as a significant advancement in addressing the persistent challenges associated with reverse osmosis membrane performance, as evidenced by a substantial body of research.Quantum dots, defined as artificial semiconductor crystals with a nanometer scale dimension (typically ranging from 1-10 nanometers), exhibit quantum mechanical effects due to their minute size. The properties of these dots, particularly their capacity for light absorption and emission, exhibit a high degree of variability depending on their dimensions. Due to their diverse functional groups, their capacity for facile synthesis based on carbon, their high dispersion ability, their widespread biodegradability, and their integration with membrane technologies, quantum dots have emerged as a subject of considerable research interest. The main aim of this thesis is to enhance the performance of reverse osmosis membranes and extend their lifespan by imparting resistance to fouling and chlorination.The scope of the thesis encompasses the optimization of the manufacturing parameters of reverse osmosis membranes, including the non-woven layer, the support layer, the polymer concentration, and the thin film coating formulation.Subsequently, the efficacy of innovative membranes produced by integrating two distinct quantum dots into the thin film coating layer is examined.