Yeast doesn't just cause fermentation; in some cases the microorganisms can even purify water. Even if MIT researchers didn't know exactly how the process worked, the results were clearly visible. Thanks to biosorption, yeast cells bind and absorb heavy metal ions such as lead, even at concentrations below 1 part per million. And according to their calculations, a single brewery in Boston could produce enough waste yeast to purify the entire city's water supply. But when it was first examined in 2021, there was a fatal flaw: Researchers couldn't figure out how to get rid of all that yeast after it bound to the toxic substance.
After three years of further research, the team of chemical, molecular and aerospace engineers say they have a solution: hydrogel shells made from a commonly used, UV-light-sensitive polymer known as polyethylene glycol (PEG).
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“What we decided to do was create these hollow capsules – kind of like a multivitamin pill, but instead of filling them with vitamins, we fill them with yeast cells,” MIT graduate student Devashish Gokhale explained in an announcement at his new co-author of the study in the journal RSC sustainability.
To create their 'multivitamin', the team mixed freeze-dried yeast with water in addition to the PEG polymers. With the addition of UV light, the PEG combined to form approximately half-millimeter, semi-permeable outer capsules around the yeast cells. Even in their new casings, the waste yeast still removed traces of lead from the water as quickly as it did on the barrier.
“These capsules are porous, so the water can go into the capsules and the yeast can bind all that lead, but the yeast itself can't escape into the water,” Gokhale continued.
After confirming that the capsules were strong enough to withstand the fluid forces of tap water, researchers built a proof-of-concept, EPA-grade packed-bed biofilter filled with the new hydrogel yeast beads. Their filter then continuously filtered traces of lead-contaminated water for twelve days, while using less energy than existing systems such as membrane filtration.
Given its relative cost-effectiveness alongside yeast's availability, researchers believe their findings could lead to a sustainable, low-cost alternative to water treatment – something that could bring major benefits to low-income and historically underserved communities dealing with pollution and limited sources of clean water.
In the future, researchers hope to explore methods for recycling and replacing the filter yeast, and develop greener hydrogels from raw materials instead of fossil fuel-derived polymers. They also plan to expand the contaminating capacity of their filters to include microplastics and forever chemicals.
