Raina Maier, a University of Arizona professor of environmental science, has a special connection to World Water Day. Her research in environmental microbiology and water remediation is positioned squarely at the confluence of science, sustainability and societal well-being.

World Water Day, officially designated in 1993 by the United Nations, is observed annually on March 22. The day was intended to inspire action regarding the global water crisis, with the ultimate focus being "water and sanitation for all by 2030."

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This year's "Water for Peace" campaign highlights how water scarcity, pollution and unequal access can exacerbate tensions between communities and nations.

Maier's work focuses on the crucial role of microorganisms in water and soil environments and their application in metal extraction and environmental cleanup. In this Q&A, she talks about the microbially produced surfactants or soaps, their role in recovering critical metals from water streams and the biosurfactants' overall effect on water treatment.

Q: Could you elaborate on the significance of using microbes for critical metal recovery?

A: A critical metal is a metal needed for electrification of society. They are important for making copper wires, batteries and electronics in general. As we transition from petroleum-based energy towards green energy and infrastructure, we need a growing amount of these metals. In collaboration with scientists from Clemson University and Georgia Tech University, we did a first analysis of the natural and waste waters in the U.S. and found that a significant portion of the U.S. demand for the critical rare earth elements could be met by harvesting the elements from these water sources.

That would reduce the need for hard rock mining, which requires large amounts of energy and water to take rock out of the ground, crush and extract metals from it. Harvesting metals directly from natural and waste waters has the potential to save a lot of energy. We are developing a harvesting technology that uses surfactants or soaps that are made by bacteria. In collaboration with University of Arizona chemists, we now can make these bacterial surfactants synthetically and apply them to selectively take the rare earth elements out of wastewater solutions. We are working with several mining companies who are interested in the technology to harvest metals from mining waste streams and want to help us move it along to commercialization.

Q: Can you explain in detail about how these biosurfactants capture metals?

A: I got interested in biosurfactants as a graduate student and continued studying them as a new faculty member at the University of Arizona. One of the discoveries my lab made in the early 1990s was that these surfactants could bind metals, thanks to the fact that the surfactants have complex structures that create a metal-binding pocket. The pocket is just the right size, so that it fits and binds large metals like rare earth elements better than common soil and water ions like calcium or magnesium. We have worked over the years to understand this metal binding and to develop technologies to recover metals from real world solutions.

Q: What type of microbes are they? How does this metal recovery work on a large scale?

A: Our initial work focused on a bacterial surfactant called rhamnolipid, which is produced by Pseudomonas aeruginosa and related species. Rhamnolipids have either one or two fatty acid or lipid tails that don’t like to mix with water, and one or two sugar heads that do like to mix with water. Bacterial rhamnolipids come as complex mixtures of 40 or more different rhamnolipids — the tails might be longer or shorter, or there could be one or two sugars, for instance. So, there is batch-to-batch variability when you produce these microbially. The thought behind making them synthetically is that not only can one make a single rhamnolipid, but one can choose the surfactant you want to make and make it with high purity and in large quantity. So, the ability to make these surfactants synthetically has opened a new door, because we now can choose the structure we want to make. We're working with modelers at the University of Arizona to make rhamnolipid-like surfactants with different size pockets. This research is based on the hypothesis that we can tune the structure of the surfactant pocket to be selective for a particular metal or a rare earth element.

Q: Do these biosurfactants produce a qualitative change in the water streams?

A: Wastewater from mining activities, including acid mine drainage, contains a large variety of metals. It is estimated that acid mine drainage affects and degrades the quality of 10,000 miles of waterways in the United States. We are currently working with mining companies to create treatment platforms that would allow us to sequentially remove all metals from these mining waste streams to produce water that can be reused or returned to the environment. This involves using a variety of approaches and steps to first separate metals from each other and then recover them for reuse or disposal. The bioinspired surfactants are part of the last steps of this technology and are applied to selectively remove metals, like the rare earth elements, that are of value for reuse.

Q: Has critical metal recovery using biosurfactants been implemented already?

A: It has not been implemented in the field yet. But we have groundwater samples from the U.S. Department of Energy that contain uranium and we have several mining company wastewaters that are very complex with a multitude of metals at very different concentrations. We are working to develop strategies on these actual samples. So we moved from very fundamental research for understanding the surfactant-metal interactions, to working with model metal mixture solutions we've created in the lab, to now working with real world solutions. The next step is to build and test a pilot scale facility in the field, which is what we're hoping to do soon.

Q: What other applications do these biosurfactants have pertaining to water?

A: These biosurfactants are truly amazing molecules. We have found that they have application for use in dust suppression. Right now, mining companies suppress dust on their roads and mine tailings piles by watering several times a day. But water is a precious commodity. So, we are testing adding these same surfactants to the water. These surfactants help for a crust formation on the mine tailings surface that reduces the need for such frequent water application.  

Another area of interest is the treatment of groundwater that is contaminated with uranium. We have many such sites on the Navajo Nation in Arizona. Many of these communities don't have access to advanced water treatment systems, so our team envisions building column systems packed with these surfactants that could locally treat groundwater to remove uranium and provide potable water.