By Daeyeon Lee, Professor
University of Pennsylvania
Chemical and Biomolecular Engineering
July 16, 2020
One of the most critical challenges that we face is providing access to clean water for every region and human being. Combinations of different phenomena including climate change, increased human demand, overuse, contamination and pollution continue to exacerbate water scarcity across the world. Given the complexity of factors that contribute to this crisis, multiple approaches must be considered and developed to enable our generation to tackle this existential challenge. One of the approaches that many physical scientists and engineers have been working on involves using various separation (purification) technologies to treat otherwise un-usable water for human use and consumption.
One of the key separation technologies that is currently used and also being extensively investigated relies on membrane separations. In membrane separations, a thin membrane is used to control the rate of transport of different species across the membrane to achieve separations. For example, by enabling water molecules to cross the membrane rapidly, while significantly suppressing (and ideally completely blocking) the crossing of other species such as ions and microorganisms through the membrane, clean water can be produced. One of the key advantages of membrane separations is that it is a relatively energy efficient and less energy intensive technology compared to other separation methods such as distillation. Given how simple and efficient this concept is, it is not surprising that mankind has been using membranes to treat water since ancient times. Despite decades of development in membrane technologies, several challenges and bottlenecks remain, and new innovations will be required to overcome these challenges.
One key issue that often plagues the implementation of membrane technologies for water treatment is fouling. Fouling happens over time, when molecules, debris, cells and materials accumulate on the surface of the membrane, significantly degrading their performance. Membrane fouling leads to higher energy consumption because water has to be “pushed through” additional materials that have collected on the membrane surface. Also, the purity of water can be compromised as undesired species will make their way across the membrane more easily.
One example involves the treatment of produced water from oil and gas exploration and production- through hydraulic fracturing, most commonly known as fracking. During fracking, high-pressure water, along with various chemicals and solid particles, is pumped underground to induce the fracture of rock formation and the release of shale gas. When the high injection pressures are released, water returns to the surface, along with chemicals extracted from the shale. Proper handling of fracking wastewater impacts significant populations, as gas-producing shale formations are staggeringly large. Fracking water needs compete for stressed water supplies as well as generate an enormous quantity of wastewater. A broad range of impurities are present in fracking wastewater, including salts, organic hydrocarbons, metals, solid particles, and radioactive material. This impurity profile and the sheer scale of fracking wastewater production challenge the economics of current separation technologies. Direct discharge of wastewater to surface waters is strictly forbidden by federal law. The current disposal method, now forbidden in several states, is to pump the water into spent wells. This approach poses serious environmental risks, with fears that it will compromise drinking water aquifers. Furthermore, recent increases in seismic activities (e.g., in Oklahoma, Texas etc.) are thought to be induced by this practice.
To avoid the significant risks posed by current disposal methods and to enable water recycle/reuse, reclamation of fracking wastewater is essential. The Department of Energy indicates that energy efficient membrane separation could play a major role, and that the “need is large” for fouling-resistant membranes to treat fracking wastewater. A major challenge, however, is that fracking wastewater contains unusually high concentrations of oily components which are detrimental to many existing membranes. In collaboration with Shu Yang (Penn MSE), we have addressed this issue by developing ultrastable anti-oil fouling coatings that can be readily coated onto existing membranes using scalable methods. One of the most unique advances and features of these coatings is that even if the membrane is somehow fouled with oily components, the nanostructure of the coating will expel the oil and thus revive the oil rejection properties of the membranes. Moreover, the coatings can be easily applied over large areas using simple techniques such as spray coating. Thus, it is not necessary to completely revise how membranes are produced. Simply by adding a coating step, many different types of existing membranes can be modified to resist fouling by oily components.
Figure 1: Nanostructured coating that removes oil from the surface underwater to keep membranes from becoming ineffective due to fouling
Another example of fouling that significantly limits the use of membranes for water treatment is biofouling. Over time, biological materials such as bacteria and viruses will accumulate on the surface of the membrane and create a slimy residue, also known as biofilms. Once developed, these biofilms are extremely difficult to remove simply by adding disinfectants as several studies have shown that bacteria in biofilms are extremely resistant to biocides. Biofilms can also lead to contamination of the filtered water posing a major health threat. In collaboration with Kathleen J. Stebe (Penn CBE) and Martin Haase (a former postdoctoral fellow at Penn and currently at Utrecht University), we have tackled this challenge by developing a new method of producing nanoparticle-decorated porous membranes. Unlike conventional methods of making membranes and subsequently covering the surface of the membranes with nanoparticles, the new approach allows nanoparticles to spontaneously locate themselves on the surface of the pores in the membranes. A high density of nanoparticles at the surface are subsequently modified with molecules that strongly resist the attachment of bacteria, making these membranes highly resistant to biofouling.
Figure 2: Nanoparticle-decorated hollow fiber membrane that resists biofouling
Preventing fouling and thus maintaining the separation properties of membranes over long periods of time under harsh conditions often associated with water treatment, is critical in developing many exciting research innovations in this area. Developments in water treatment and purification will also have a significant impact in enhancing our resilience against natural disasters. Penn researchers in Research and Education in Active Coating Technologies (REACT), an NSF-supported Partnership in International Research and Education program, are collaborating with researchers in France and Korea to further develop technologies that can be readily deployed to enhance the water management of emergency shelters which can be deployed in sites of natural or man-made disasters. Some of the new exciting developments in the field include the possibility of using solar energy to drive water treatment/purification. Solar desalination, an emerging technology, operates completely autonomously off-grid by converting solar energy to create steam which then is separated from brackish water by using porous membranes that let only water vapor through but not liquid water. Successful development of such a technology will depend on close collaborations among researchers of complementing skill sets and expertise. In collaboration with multiple researchers and support from the University Research Foundation, our research group is currently exploring new ways to produce solar desalination membranes that will resist fouling by self-cleaning under solar illumination.
Daeyeon Lee is Professor in Department of Chemical and Biomolecular Engineering at the University of Pennsylvania. He received his BS in Chemical Engineering at Seoul National University and PhD in Chemical Engineering at Massachusetts Institute of Technology. His research focuses on developing deep understanding of the interactions between soft materials near or at interfaces and extending the obtained knowledge to direct the assembly of macroscopic structures that have designed properties and functionality. He has won numerous awards including the 2010 Victor K. LaMer Award, NSF CAREER Award, 2013 3M Nontenured Faculty Award, 2013 AIChE NSEF Young Investigator Award, 2014 Unilever Award for Young Investigator in Colloid and Surface Science and 2017 Soft Matter Lectureship Award.
