By Cooper Yerby

Ph.D. Student

University of Pennsylvania

Water scarcity is one of the pre-eminent challenges of the 21st century, affecting roughly 4 billion people globally1. To combat water scarcity, technologies such as desalination will become inevitable to produce drinking water from unconventional sources such as seawater, brackish groundwater, and wastewater. Commonly used desalination processes include distillation, multistage flash processes, reverse osmosis (RO), and electrodialysis. RO is the main desalination method employed at present; however, existing RO infrastructure operates with several challenges, including membrane fouling, high energy consumption, and short durability2. These difficulties contribute to water insecurity domestically and globally; therefore, new fundamental insights for the effective desalination of seawater in RO systems will be required to resolve issues related to water security.

Figure 1: Historic Milestones of Desalination Efficiency and Technological Progress. Desalination efficiency is presented via the Standard Universal Performance Ratio (SUPR). The SUPR is benchmarked at 100 for the desalination efficiency of today’s RO membranes. The SUPR correlates with the kilowatt hours needed to desalinate a square meter of seawater, with an SUPR of 100 tied to 6.5 kilowatt hours per meter squared. Stars denote a time period where a large technological advance in desalination technology occurred. (Shahzad at al., 2019)3

 

Though tremendous progress has been made in the desalination space over the past few decades, as shown in (Figure 1) above, the energetic inputs necessary to drive desalination are still not cost competitive with traditional water procurement and treatment methods. To address current problems and high energetic inputs afflicting RO technologies, my research in the Francisco lab at Penn utilizes computational chemistry to better understand the molecular mechanisms supporting how low-maintenance, low-energy, and long-lived RO processes exist and function in nature.

Using Mangroves as a Template to Study Natural Biological Reverse Osmosis

An example of natural reverse osmosis occurs in mangrove trees, known for their unique ultrafiltration systems that filter more than 99% of salt ions from seawater through the roots4. The water-filtering process in mangrove roots has received considerable attention5,6, however, the fundamental atomic forces behind desalination in mangrove roots and the intermolecular basis behind the creation of negative pressures that drive desalination remain unstudied.

A mangrove’s cell membrane acts as a durable discriminating barrier in which ions are excluded from passive transport4. Water permeates the cell membrane by moving down its chemical potential gradient through nanopores known as aquaporins, with rates of water transport affected by salinity and hydraulic pressure7. This means that selective water transport through mangrove membranes is an energy efficient form of RO2.

It has been theorized that the immense negative internal pressures needed to drive desalination in mangroves can be explained by the cohesion-tension theory7. The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants8. According to the cohesion-tension theory, mangrove trees desalinate salty water using highly negative pressure (or tension) that is generated by evaporative capillary forces in mangrove leaves8. In 2020, experiments in the Elimelech group at Yale showed that a general architecture can be created to induce RO via cohesion-tension principals in synthetic mangrove systems through combining biomimetic root, xylem, and leaf structures created with abiotic materials, as shown in the graphic illustration (Figure 2) below7:

 

Figure 2: Design elements and water flow in the synthetic mangrove. The schematic diagrams show the mangrove tree (left), the synthetic mangrove device (right), and their water transport mechanisms (center insets). In natural mangroves, capillary pressure created by evaporation into substomatal cavities (top left inset) brings about upward water transport through xylem channels (middle left inset) and desalination by aquaporin water channels in root cell membranes, which exclude salt from saline water in the soil (bottom left inset). The highly negative pressure in the root, Proot, overcomes the osmotic pressure of the saline water (πo), which enables water uptake through the root filtration system (i.e., Proot − πroot < Po − πo). (Wang et al., 2020)7


Recreating Biomimetic Reverse Osmosis with Carbon Nanomaterials

In synthetic mangrove systems, carbon nanotubes (CNTs) and graphene nanosheets are materials with the capacity to mimic the chemistry occurring in natural water transport and desalination systems9. CNTs are considered as artificial systems where water and ions are selectively modulated at the CNT entrance. The electroosmotic flow of actively pumped chemicals and net dipolar orientation of water molecules can be controlled through modifying the diameter and design of inner CNT cores. The frictionless water flow within CNTs is 4 to 5 times the rate of water flow seen within aquaporins10. In addition to high water flow rates, CNTs possess other characteristics that make them outstanding candidates for use in water filtration, like high mechanical strength, low density, and high stiffness11. Graphene Nanosheets, consisting of a single layer of carbon atoms in a hexagonal lattice structure, are an ideal material to functionalize to act as a biomimetic membrane material12. Their 2-dimensional shape, flexibility, mechanical strength and, more importantly, tunable surface chemistry give graphene sheets the ability to mimic membrane systems in nature12. By mimicking the physical properties of mangrove systems with physiochemical analogs produced with carbon-based nanomaterials, it may be possible to discern the mechanistic underpinning of salt tolerance in mangroves. Doing so provides new avenues for the development of more precisely engineered biomimetic desalination infrastructure with higher salt rejection and water throughput efficiencies.

Expected Research Outcomes

My work at Penn seeks to provide unparalleled understanding of the unique structural and mechanical properties of RO in mangroves by employing first principles computational physical methodology, known as molecular dynamics (MD) simulation. A detailed mechanistic study of mangrove function, aided by millions of years of evolutionary history, provides an opportunity to learn material design principles that enact a novel ability of low energy, long-lasting, and non-fouling desalination with a high salt rejection rate. Because atomic forces drive desalination, solvent/solute transport through the xylem, and the creation of negative capillary pressures, MD simulations can reveal the mechanics and interplay between biological desalination system processes for a low cost at scales currently immeasurable by existing technologies13,14. Through interrogating the physical mechanisms that drive desalination and water transport in plants, unique mechanical properties in both biological and synthetic systems will be unearthed that have potential to change the way the world thinks about and designs desalination systems.

 

Cooper Yerby is a second-year Ph.D. student from the Department of Earth and Environmental Sciences at the University of Pennsylvania in Dr. Joseph S. Francisco’s lab group. He received a B.S and M.S in Geological Sciences, as well as a minor in Business Finance at the University of Southern California in May of 2019. Cooper is excited to integrate his past knowledge of geobiological systems with first-principles molecular dynamics at Penn to generate new discoveries and research directions in the desalination field. If you are interested in learning more about his research, you can contact him at yerby@sas.upenn.edu.

 

References:

1. M. M., & Hoekstra, A. Y. (2016). Four billion people facing severe water scarcity. Science Advances, 2(2), e1500323

2. Wang, H. (2018). Low-energy desalination. Nature nanotechnology, 13(4), 273-274.

3. Shahzad, M. W., Burhan, M., & Ng, K. C. (2019). A standard primary energy approach for comparing desalination processes. Clean Water, 2(1), 1-7.

4. Kim, K., Seo, E., Chang, S. K., Park, T. J., & Lee, S. J. (2016). Novel water filtration of saline water in the outermost layer of mangrove roots. Scientific Reports, 6(1), 1-9.

5. Kim, K., Kim, H., Lim, J. H., & Lee, S. J. (2016). Development of a desalination membrane bioinspired by mangrove roots for spontaneous filtration of sodium ions. ACS nano, 10(12), 11428-11433.

6. Drennan, P., & Pammenter, N. W. (1982). Physiology of salt excretion in the mangrove Avicennia marina (Forsk.) Vierh. New Phytologist, 91(4), 597-606

7. Wang, Y., Lee, J., Werber, J. R., & Elimelech, M. (2020). Capillary-driven desalination in a synthetic mangrove. Science Advances, 6(8), eaax5253.

8. Angeles, G., Bond, B., Boyer, J.S., Brodribb, T., Brooks, J.R., Burns, M.J., Cavender-Bares, J., Clearwater, M., Cochard, H., Comstock, J. and Davis, S.D., 2004. The cohesion-tension theory. New Phytologist, 163(3), pp.451-452

9. Fornasiero, F., Park, H. G., Holt, J. K., Stadermann, M., Grigoropoulos, C. P., Noy, A., & Bakajin, O. (2008). Ion exclusion by sub-2-nm carbon nanotube pores. Proceedings of the National Academy of Sciences, 105(45), 17250-17255.

10. Majumder, M., Chopra, N., Andrews, R., & Hinds, B. J. (2005). Enhanced flow in carbon nanotubes. Nature, 438(7064), 44-44.

11. Gojny, F. H., Wichmann, M. H. G., Köpke, U., Fiedler, B., & Schulte, K. (2004). Carbon nanotube-reinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content. Composites Science and Technology, 64(15), 2363-2371.

12. Zhang, Y. L., Chen, Q. D., Jin, Z., Kim, E., & Sun, H. B. (2012). Biomimetic graphene films and their properties. Nanoscale, 4(16), 4858-4869.

13. Tsai, S. T., Kuo, E. J., & Tiwary, P. (2020). Learning molecular dynamics with simple language model built upon long short-term memory neural network. Nature Communications, 11(1), 1-11.

14. Hub, J. S., Grubmüller, H., & De Groot, B. L. (2009). Dynamics and energetics of permeation through aquaporins. What do we learn from molecular dynamics simulations?. Aquaporins, 57-76.