Water Desalination

The increasing global population along with the exploitation of the world's fresh water supply has resulted in a critical shortage of clean drinking water that currently affects over half of the world's population. Since 97% of the world's water is salt water, desalination is a logical choice to ease the water crisis. Although desalination systems can produce large volumes of fresh water, the separation processes are still largely inefficient. New capabilities in micro and nanofabrication have provided new opportunities for increasing the separation efficiency of current desalination processes. At the Device Research Laboratory, we are working on using these novel nanostructures to improve various desalination technologies.
Multifunctional Nanoporous Evaporator for an Efficient Water Desalination System

Typically, photovoltaics (PVs) are used to drive a solar water desalination system (i.e., reverse osmosis, electrodialysis, etc.). However, PVs can convert only about 10-35% of the total solar radiation to electricity, leading to an inefficient overall system. This project team takes an alternative approach, focusing on developing a compact highly efficient solar-driven combined power and desalination system. The overall system is envisioned to be comprised of PVs to generate renewable power and a thermally driven evaporative desalination system.

Specifically, this project focuses on the development of a nanoporous evaporator based water desalination unit, a key component towards realizing the overall system. The evaporator design is targeted to have: 1) high desalination rates, 2) spectral selectivity to maximize the absorption of solar radiation to the evaporator, and 3) anti-fouling to mitigate significant clogging and decreases in efficiency over time. Towards this goal, the project will fabricate and experimentally characterize various membranes with different pore dimensions and porosities in a range of materials including alumina anodiscs as model membrane and polymeric membranes towards a low-cost approach. The project will also explore various solar absorber coatings that can effectively maximize the heating of the membrane with minimal losses. The project will investigate how anti-fouling can be achieved through the changes of surface energetics via modifying the surface design of nano-scale features. High-fidelity models will be developed to facilitate the optimizations. To incorporate the evaporator into the desalination unit, the project will also explore and incorporate condenser designs to efficiently collect the fresh water.

This work is in collaboration with the Masdar Institute of Science and Technology
Capacitive deionization

We are currently investigating is capacitive deionization (CDI) for brackish water desalination. CDI can be a competitive technology for brackish water treatment due to its higher energy efficiencies compared to RO and its more inherent resistance to fouling. However, CDI is still a developing technology where adsorption capacity and salt removal rates into porous, tortuous carbon electrodes is still low. We have designed vertically-aligned carbon nanotube (VA-CNT) electrodes, with minimal tortuosity, to investigate the role of porous geometry on the performance of flow-by CDI devices, specifically examining changes in diffusion resistance, salt adsorption rate and capacity. The porosimetry and capacitance of these electrodes are studied electrochemically in 3-electrode beaker experiments and also in a flow-by CDI prototypical device. We find that in a 1mM NaCl solution, CNT electrodes can adsorb from upto 8 mg salt/g carbon, at rates upto 0.1 mg/g-min. At present, we are investigating methods to increase performance through cell design, electrode porosity, and comparing results with an electric double layer model for macroporous electrodes to inform the design of carbon electrode materials for optimal ion adsorption and throughput in a flow-by CDI device.
Membrane-based reverse osmosis (Past project)

Membrane-based reverse osmosis (RO), which accounts for over 40% of the current worldwide desalination capacity, is limited by the solution-diffusion mode of water transport through a tortuous polymeric active layer. One process we are working on is increasing the water flux of (RO) membranes by utilizing the sub-nanometer porous framework of zeolite-based materials. However, due to the confined nature of these pores, the strong interaction between the solid and liquid can enhance or diminish the transport properties. Surprisingly, MFI zeolites have shown infiltration pressures upwards of 100 MPa, as synthesized. However, by introducing hydrophilic defects this pressure was brought down to 1 kPa, which is well below typical RO operating pressures of ~5 MPa. However, this increased hydrophilicity reduces water diffusivity in the pore, thereby lowering the permeability of the membranes below the estimated transport rate based on diameter alone. These experimental results have been corroborated with novel molecular dynamic simulations to demonstrate that the effect of surface barriers significantly decrease the rate of mass transport through the zeolite crystals. Therefore, in the design of fast-transport membranes for RO, it is important to consider the role of surface barriers on diffusion and the overall device performance.