MEMS-Based Purification and Sensing Technologies for Sustainable Water Management

Abstract

The management of water resources is a pressing global challenge. This research aims to address the issues of water pollution and scarcity by proposing micro-electromechanical systems (MEMS) based solutions for water purification and sensing. In particular, this research presents a new photoelectrocatalytic water purification system, an efficient valveless micropump, and a sensitive heavy metal ion sensor based on MEMS technologies, envisioning for a sensor-based closed-loop control water treatment platform.The photocatalytic degradation of organic matter by titanium dioxide was initially used to remove pollutants from water. A uniform and durable coating of titanium dioxide was successfully applied to various surfaces. These coated surfaces demonstrated high degradation efficiency and long-term stability in degrading organic matter in water. To further improve the photocatalytic degradation efficiency of titanium dioxide, graphene was introduced to form titanium dioxide/graphene nanocomposites. A new photoelectrocatalytic water purification system was developed by combining photocatalysis and electrochemistry. This scheme can be used to simultaneously remove both organic compounds and inorganic heavy metal ions from water. In this system, the working and counter electrodes are switched compared to traditional photoelectrocatalysis systems, with the metal or carbon electrode as the working electrode and the microchannels with photocatalyst serving as the counter electrode. A negative bias potential is applied to the working electrode to reduce the heavy metal ions, and the current flow through the circuit helps transfer the photo-excited electrons from the counter electrode (photocatalyst) to the working electrode. The inorganic heavy metal ions are thus reduced on the working electrodes while the photocatalytic degradation of organic pollutants on the counter electrode is enhanced. Next, a micropump suitable for the water treatment and detection platforms was researched and developed. Valveless fluidic diode micropumps without moving parts are considered suitable for water sensor systems due to their simple structure and rapid mixing. In the design, topology optimization is used to design two-dimensional, fixed-geometry, fluidic diodes of high diodicity, which is the ratio of pressure drops of forward to reverse flows. One of the fluidic diodes, of the Tesla type, shows a diodicity of over five. The numerical simulation was applied to simplify the structures, and the two-dimensional geometry was converted into three-dimensional model for micropumps. Three-dimensional and unsteady numerical analyses of micropump were conducted for pumps with the two diode designs. The experiments conducted with designed diodes showed reliable repeatability and precise control of flow, indicating positive prospects for potential applications. To achieve the envisioned closed-loop control water treatment platform, a highly sensitive low-cost sensor is essential. An electrochemical microfluidic sensor was developed for the high-sensitivity and low-cost detection of heavy metal ions in water. This sensor utilizes a glass carbon/graphene electrode synthesized through layer-by-layer self-assembly and subsequent pyrolysis. The electrode exhibited low overpotential, enabling detection of almost any heavy metal ions in aqueous solutions, and the incorporation of graphene resulted in a high sensitivity. To enhance mass transfer and sensitivity, a valveless micropump was integrated with the electrode, achieving two orders of higher sensitivity than those of measurements with a stationary solution and a detection limit as low as 20 ppt for lead and 100 ppt for cadmium. The developed sensor was employed to simultaneously detect lead and cadmium in water, investigating the mutual influence of heavy metal ions in electrochemistry. A neuron network model was adopted for data analysis. In summary, this study presents a comprehensive approach to addressing the worldwide challenge of sustainable water resource management through the removal of pollutants and the detection of water pollutants. The closed-looped control water treatment platform, based on high-performance sensors, micropumps, and advanced photoelectrocatalysis technologies, provides an efficient and cost-effective solution to water scarcity and pollution. This platform offers promising prospects for the development of effective water management systems, which can contribute to the improvement of global water resources management and enhance the quality of life for people around the world.