In a document published in the journal Nature Communicationscientists explain that such a fully sustainable system would purify water and divert what would otherwise be a waste stream for disposal.
In the study, the team points out that in the United States alone, more than 12,000 miles of waterways are affected by acid mine drainage water rich in heavy metals, the country’s main 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.
“We don’t just need to minimize the existence of lead; we need to eliminate it in drinking water,” researcher Patritsia Statathou said in a press release. “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 they consume a lot of energy and produce toxic by-products.
The role of S.cerevisiae
The solution studied by the MIT team is not 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 looked at 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 quick, taking less than five minutes.
Mechanisms of biosorption
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 pointed out that exploring biosorption mechanisms at such difficult concentrations is a difficult problem.
“We were the first to use a mechanical perspective to unravel the mechanisms of biosorption, and we discovered that the mechanical properties of yeast cells change significantly after lead absorption. This provides fundamentally new information for the process,” she said.
Designing a practical system for treating water and recovering the yeast, which could then be separated from the lead for reuse, is the next step in the team’s research.
“To scale up 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,” Stathatou said.
Scientists are also investigating ways to recover both the cells and the lead.
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.
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