As civilization marches on, humanity struggles with a lack of suitably clean water and energy. These two are among the most important challenges we face, because solving them could mitigate other issues such as food scarcity, poverty, and poor health. Water and energy, however, are interconnected in a relationship known as the water-energy nexus; it takes copious amounts of energy to purify water on a large scale and a considerable amount of fresh water to produce energy. Roughly 2.5 percent of all water on Earth is fresh, and even less is easily accessible, which further exacerbates water scarcity.
To sustain humanity, we must harness non-traditional sources of water, which have contaminants, using efficient separation systems. President John F. Kennedy highlighted this issue as early as the 1960s. He said, “If we could ever competitively, at a cheap rate, get fresh water from salt water, that would be in the long-range interests of humanity which would really dwarf any other scientific accomplishments.”
Thanks in part to JFK’s efforts, major breakthroughs in desalination (removing salt from saline water) occurred in the 1960s and 1970s with the advent and commercialization of high-performance polymeric membranes. Membranes, with varying degrees of porosity, are ubiquitous in water treatment plants because they efficiently separate or “filter” out salt, bacteria, and other major contaminants. For example, a membrane-based process called reverse osmosis can remove salt from sea water with less energy than competing technologies.
Despite such breakthroughs, there is still much progress needed to address the water-energy nexus. In particular, desalination membranes have not changed considerably in the last 40 years. There is a critical need for systems that can effectively purify varying levels of contaminated water while being energy efficient. Additionally, achieving highly specific molecular selectivity is a frontier area, as it is challenging to filter out contaminants that have similar sizes and chemical interactions.
The U.S. Department of Energy has made a strong commitment to advancing this field by funding three new Energy Frontier Research Centers (EFRCs) to address these challenges: Advanced Materials for Energy-Water Systems (AMEWS), Center for Enhanced Nanofluidic Transport (CENT), and Center for Materials for Water and Energy Systems (M-WET). These EFRCs are composed of strong multi-disciplinary teams. Each center has started up their research programs in the last several months and aims to fill the basic science gaps necessary to spur a paradigm shift in energy-efficient water purification from impaired water.
Advanced Materials for Energy-Water Systems (AMEWS): Investigating where water meets solid
Scientists at AMEWS have targeted water-solid interfaces as the focus of their studies. “The interface is integral to nearly all the scientific challenges underlying water treatment,” said Seth B. Darling, AMEWS Director, “because the bulk of the action happens there regardless of the system being employed.” One issue at the heart of this work is fouling, which occurs when an undesired material builds up at a solid surface and hinders operation. In water treatment, this means a membrane or similar device gets clogged up with “gunk,” much like a coffee filter or drain gets clogged, and the throughput of water slows until no water passes through. Researchers at AMEWS are seeking deeper insight into the core processes to help ultimately develop materials where fouling can be mitigated.
AMEWS aims to answer the following pressing fundamental questions at the water-solid interface: What interactions cause specific contaminants to adsorb, or stick, to a given interface and can this be exploited for selective adsorption? How do chemical catalysts interact with solutes at an interface, and can this be applied to actively remove pollutants or degrade foulants? How do molecules and ions transport near interfaces inside pores, where the interface is now wrapped around the pore? How can highly selective transport be achieved?
The key goal of their fundamental studies is to advance the science needed to engineer next-generation materials. For example, their world-leading expertise in specialized techniques allows them to manipulate an interface in a highly controlled and tunable manner. Using these techniques along with the knowledge gained from their studies, AMEWS could design interfaces that reduce fouling propensity and are highly selective.
Darling notes that to attack an issue as large as water treatment requires a synergistic approach. “We need to put all the key ingredients together—synthesis to create new materials, advanced characterization to probe their emergent properties, and multiscale models to interpret the data and allow for informed design. An EFRC brings all of these people together and allows for research to be done at a level that a single investigator could never approach.” The AMEWS researchers are excited about their opportunity to advance the field, and Darling hopes that the center’s work with early career scientists contributes to the emergence of new leaders in water treatment.
Center for Enhanced Nanofluidic Transport (CENT): Learning how fluids move in small pores
Researchers at CENT are investigating fluid flow in very small nanopores. Mark Reed, who leads CENT’s effort on molecular selectivity, noted, “As nanofluidic channels become smaller, we observe strange behavior in a number of areas. One example is the highly selective transport observed, which would be very useful for the filtration systems applied in water purification.” CENT’s team is specifically studying single digit nanopores (SDNs), which have pore diameters less than 10 nanometers, or, in other words, about ten thousand times thinner than a strand of human hair.
Zuzanna Siwy, who leads CENT’s thrust on ion correlation and solvation, added, “Single digit nanopores are very promising for creating next-generation separation materials because they exhibit remarkable properties, like ionic selectivity comparable to biological ion channels.” Biological ion channels can be up to one thousand times more selective than traditional membranes. The main issue? “The physics of transport in these systems are poorly understood,” said Siwy. CENT researchers want to improve this understanding by answering fundamental questions, including the following: Can the mechanism for flow in very small pores be elucidated and described quantitatively and predictively? How do ions and fluids structure themselves in SDNs? What is the basis for the remarkable ionic and molecular selectivity in SDNs?
CENT is not focused on specific materials, but rather, fundamental understanding that can ultimately predict transport within any nanopore system. Aleksandr Noy, who leads CENT’s research on emergent confinement effects, said, “We want to focus on understanding the physics of transport in confined spaces, such as those encountered in small pores, and envision that our knowledge could then be applied to many materials ultimately changing the way we deal with water.” One of CENT’s main objectives is to lay the scientific foundation regarding transport in SDNs, allowing for transformative molecular separation technologies to be rationally designed.
When reflecting on being part of an EFRC, Reed said, “We learn a lot since we work with the best people in the world, but the greatest part is the emergent discoveries and new ideas we generate that can truly advance the field, which are only possible due to the collaboration and interaction an EFRC affords.”
Center for Materials for Water and Energy Systems (M-WET): Developing design rules for next-generation membranes
M-WET’s team aims to discover and understand the fundamental science necessary to create novel membrane materials for water treatment. Existing membranes exhibit an inherent tradeoff between permeability (throughput) and selectivity (separation efficiency), experience extensive fouling, struggle to discriminate between ions with the same charge (for example, Na+ and Li+), and labor to remove neutral contaminants (for example, boron).
Consequently, current membranes are unsuitable for treating highly impaired water sources, such as those generated by energy-related activities (for example, produced water in the oil industry). Scientists at M-WET also view this as an exciting untapped opportunity because this water can contain high concentrations of valuable resources, such as lithium. However, to access such opportunities, Benny D. Freeman, director of M-WET, said that “Novel sets of materials with high permeability, high selectivity, and fouling-resistant properties that are unattainable today must be constructed to purify water to varied degrees and recover valuable resources using less energy.”
To reach this objective, scientists at M-WET are investigating basic science questions, including the following: How do solutes interact with surfaces having varied levels of hydration, and can hydration be tuned to control adsorption? What are the molecular factors governing selectivity and can membrane chemistries be designed to capture and release target molecules? What pore structures yield excellent permeability and selectivity, and can these structures be designed rationally for membranes?
Freeman explains that M-WET is uniquely equipped to answer such questions. “It is rare to have this team working on a unified front—water chemistry experts, who understand the contaminants in waste water; membrane scientists, who measure transport through membranes; polymer chemists, who can create novel structures; and modeling experts, who develop the theory behind the observed behavior and predictive tools.”
He is also passionate about M-WET being an investment for the future. “It is a special honor and privilege to have the unprecedented opportunity to train the next generation of leaders in the membrane science area aimed at solving one of humanity’s greatest challenges—energy-efficient access to abundant supplies of fit-for-purpose water.”
Outlook for water treatment
The opportunity these new EFRCs present both early career scientists, like myself, and principal investigators to explore the unknown and write the next chapter of fundamental science for water treatment is outstanding and inspiring. The basic science the teams at these EFRCs discover and develop will well-equip researchers to attack the water-energy nexus for years to come. Each EFRC has a diverse team of experts, giving them the resources to make the disruptive impact needed to advance the field and help humanity in the process.
