As a runner in Chicago, I spend significant amounts of time running along Lake Michigan. Every winter, the water fountains that line Lake Michigan turn off to prevent the water in the pipes from freezing and bursting the pipes. As the temperatures rise in the spring, these fountains typically come back on for public use. However, over the past couple of years, some of these fountains have remained off as the City of Chicago is concerned about lead exposure.
This situation is becoming all-too-common, unfortunately. In other areas in the United States as a result of aging infrastructure and poor water management, people have been exposed to far more substantial amounts of lead in their drinking water than I have been along the Chicago lakefront. (Think of Flint, Michigan and other areas where the effects have been severe for members of the public.) It is a monumental health challenge globally, with access to clean water affecting more than one-third of the world’s population annually. We need new solutions to ensure clean water for all people.
I am fortunate to be working in a scientific field that has a real potential to help. Metal-organic frameworks (MOFs) with their structural flexibility can be precisely tuned to adsorb impurities from water. These porous adsorbents offer an innovative approach to water quality challenges.
Use of adsorbents for clean water applications is hardly new. People routinely use adsorbent-containing water filters to remove contaminants from water in their houses before drinking water. Various governments similarly use adsorbents at the large scale. For example, the Israeli government has developed large water desalination plants capable of generating 600 million cubic yards of water from plentiful seawater.
But the adsorbent technologies currently used in water purification applications have been around for many decades, and the chemistry has matured to the point where further improvements are unlikely. To improve water purification in the 21st century, a new class of adsorbent materials will be needed.
A new adsorbent to lead the way
Many current water purification technologies currently are based on the widely used adsorbent, activated carbon. Activated carbon contains a wide distribution of pore sizes; imagine crumpling up a piece of paper and the variety of deformation and hole size that would ensue. With this wide distribution of pore sizes, activated carbon provides a certain percentage of pores that are ideal for capturing impurities from water, but also contains a wide selection of pores that are not suitable for the targeted separation.
In contrast, MOFs, crystalline 3-D materials constructed from organic ligands and metal nodes, give a highly tunable, orderly framework. By careful selection of the building blocks utilized in MOF synthesis, it is possible to precisely tune them to modulate chemical stability, thermal stability, and adsorption properties — including so that every pore is ideal for capturing the targeted impurities from a water stream. The ability to optimize the pore size distribution of MOFs results in adsorbent materials with higher capacity than current materials on the market. As such, MOFs have the potential to open up new routes to clean water that haven’t be realized by the existing adsorbent materials.
The field of MOFs has rapidly matured over the past 25 years, with many crucial breakthroughs being realized that will allow for MOFs to be deployed in commercial water-purification applications. For example. initially, MOFs were synthesized with copper and zinc that would readily dissolve in water, limiting their potential in water-based applications. By transitioning to other metal nodes and organic ligands, a series of MOF architectures are now available that are stable in water.
This potential became clear to me in 2014 when Professor Yaghi at Berkeley published a systematic study of water adsorption in a series of water stable MOFs. This study highlighted the potential of MOFs for water adsorption applications. As a result of this work and additional studies, it was shown that it is possible to directly capture water from arid air, an innovation that could be used to capture water in water scarce environments. Going beyond this work, it is also possible to tune MOFs to capture the impurities from water with higher capacities that many conventional adsorbents.
The main barriers we are now facing lies in commercializing this new material technology, challenges we are working every day to overcome at NuMat. The first challenge is the scaling of these materials. Scaling must be achieved while maintaining materials performance and reducing the cost of production. The second challenge is developing the optimal form factor. MOFs are synthesized as powders, but to be used in a wide range of applications they must be formed into granules, extradites, or pellets. Finally, prototype testing must be carried out testing materials under real-life conditions, evaluating conditions such as long-term stability and product efficacy under real-world conditions.
I feel privileged to be working in this field, pushing the boundaries of water purification technology. MOFs have the potential to be used in a wide range of water-based applications, and it is up to us to realize their potential in commercial applications.