A new analysis by researchers at MIT’s Center for Bits and Atoms (CBA) has found that inactive yeast may be effective as an inexpensive, abundant, and simple material for removing lead contamination from drinking water supplies. The study shows that this approach can be effective and economical, even down to contamination levels of one part per billion. Serious harm to human health is known to occur even at these low levels.
The method is so effective that the team calculated that the yeast waste discharged from a single Boston brewery would be enough to treat the city’s entire water supply. Such a fully sustainable system would not only purify the water, but also divert what would otherwise be a waste stream requiring disposal.
The results are detailed today in the journal Nature Communications Earth and Environment, in a document by Patritsia Statathou, research scientist at MIT; Christos Athanasiou, postdoctoral fellow at Brown University and visiting scholar at MIT; MIT professor Neil Gershenfeld, director of the ABC; and nine others at MIT, Brown, Wellesley College, Nanyang Technological University and National Technical University of Athens.
Lead and other heavy metals in water are a significant global problem that continues to grow due to electronic waste and releases from mining operations. In the United States alone, more than 12,000 miles of waterways are impacted by acidic mine drainage rich in heavy metals, the nation’s leading source of water pollution. And unlike organic pollutants, most of which can eventually be broken down, heavy metals do not biodegrade, but persist indefinitely and bioaccumulate. They are either impossible or very expensive to remove completely by conventional methods such as chemical precipitation or membrane filtration.
Lead is highly toxic, even in minute concentrations, particularly affecting children as they grow. The European Union has reduced its standard for lead allowed in drinking water from 10 parts per billion to 5 parts per billion. In the United States, the Environmental Protection Agency has declared that no level in the water supply is safe. And average levels in surface water bodies globally are 10 times higher than 50 years ago, ranging from 10 parts per billion in Europe to hundreds of parts per billion in South America.
“We don’t just need to minimize the existence of lead; we have to eliminate it in drinking water,” says Stathatou. “And the thing is, conventional treatment processes don’t do that effectively when the initial concentrations they need to remove are low, in the parts-per-billion range and below. Either they fail to fully remove those traces, or to do so they consume a lot of energy and produce toxic by-products.
The solution the MIT team is investigating isn’t new — a process called biosorption, in which an inactive biological material is used to remove heavy metals from water, has been known for a few decades. But the process has only been studied and characterized at much higher concentrations, at more than one part per million. “Our study demonstrates that the process can indeed operate efficiently at much lower concentrations of typical real-world water supplies, and investigates in detail the mechanisms involved in the process,” Athanasiou said.
The team studied the use of a type of yeast widely used in brewing and in industrial processes, called S. cerevisiae, on pure water with added traces of lead. They demonstrated that a single gram of inactive dried yeast cells can remove up to 12 milligrams of lead in aqueous solutions with initial lead concentrations below 1 part per million. They also showed that the process is very quick, taking less than five minutes.
Because the yeast cells used in the process are inactive and desiccated, they require no special care, unlike other processes which rely on living biomass to perform such functions which require nutrients and sunlight to keep the materials active. Additionally, yeast is already abundantly available, as a waste product from beer brewing and various other fermentation-based industrial processes.
Stathatou estimated that to clean up the water supply for a city the size of Boston, which uses about 200 million gallons a day, it would take about 20 tons of yeast a day, or about 7,000 tons a year. In comparison, a single brewery, the Boston Beer Company, generates 20,000 tons per year of surplus yeast that is no longer useful for fermentation.
The researchers also performed a series of tests to determine that yeast cells are responsible for biosorption. Athanasiou says that “Exploring biosorption mechanisms at such difficult concentrations is a difficult problem. We were the first to use a mechanical perspective to unravel biosorption mechanisms, and we discovered that the mechanical properties of yeast cells change significantly after lead absorption. This provides fundamentally new information for the process.
Designing a practical system to treat the water and recover the yeast, which could then be separated from the lead for reuse, is the next step in the team’s research, they explain.
“To expand the process and set it up, you have to embed these cells in a sort of filter, and that’s the work that’s currently underway,” says Stathatou. They are also investigating ways to recover both the cells and the lead. “We need to do more experiments, but it’s possible to recover both,” she says.
The same material can potentially be used to remove other heavy metals, such as cadmium and copper, but this will require further research to quantify the effective rates of these processes, the researchers say.
“This research has revealed a very promising, low-cost, and environmentally friendly solution for lead removal,” says Sivan Zamir, vice president of Xylem Innovation Labs, a water technology research company, who was not associated with this research. “It also deepened our understanding of the biosorption process, paving the way for the development of materials suitable for the removal of other heavy metals.”
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