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 [1]. At the Device Research Laboratory, we are working on using these novel nanostructures to improve various desalination technologies.

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 [1]. 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 [2] and also in a flow-by CDI prototypical device [3]. 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 [4], 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 [5]. 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 [6]. 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.

  1. T. Humplik, J. Lee, S. C. O’Hern, B. A. Fellman, M. A. Baig, S. F. Hassan, M. A. Atieh, F. Rahman, T. Laoui, R. Karnik, and E. N. Wang, "Nanostructured materials for water desalination," Nanotechnology, vol. 22, no. 29, p. 292001, Jun. 2011. Link.
  2. H.K. Mutha, Y. Lu, I.Y. Stein, H.J. Cho, M.E. Suss, T. Laoui, C.V. Thompson, B.L. Wardle, E.N. Wang, "Porosimetry and packing morphology of vertically-aligned carbon nanotube arrays via impedance spectroscopy," Nanotechnology, 2016
  3. H.K. Mutha, M. Hashempour, B.L. Wardle, C.V. Thompson, E.N. Wang, “The Study of Porous Geometry Design for Capacitive Deionization Devices through Vertically-Aligned Carbon Nanotube Electrodes" The 230th Meeting of the Electrochemical Society, Honolulu, HI Oct 2-7, 2016.
  4. T. Humplik, R. Raj, S.C. Maroo, T. Laoui, E.N. Wang, "Framework water capacity and infiltration pressure of MFI zeolites," Microporous and Mesoporous Materials, 190, p. 84-91, 2014.
  5. T. Humplik, R. Raj, S.C. Maroo, T. Laoui, E.N. Wang, "Effect of Hydrophilic Defects on Water Transport in MFI Zeolites," Langmuir, 30(22), p. 6446-6453, 2014.
  6. M. Fasano, T. Humplik, A. Bevilacqua, M. Tsapatsis, E. Chiavazzo, E.N. Wang, P. Asinari, "Interplay between hydrophilicity and surface barriers on water transport in zeolite membranes," Nature Communications, 7, 2016