Inactivated yeast cells can remove minuscule amounts of lead from water. This discovery was made by Patritsia M. Stathatou, Neil Gershenfeld, Massachusetts Institute of Technology (MIT), Cambridge, USA, Christos E. Athanasiou, MIT and Brown University, Providence, RI, USA, and colleagues. The team has confirmed that baker’s yeast can take up even traces of lead ions from water in the parts-per-billion (ppb) range by passive biosorption, offering an accessible alternative to more complex processes.

 

Lead in Drinking Water

One way in which lead and other heavy metals diffuse into our water supply is when old, lead-containing pipes corrode. In addition, areas with historic or ongoing mining activities often encounter high levels of lead leaching into the environment. And yet, as we know, there should be no lead in drinking water at all, since lead can penetrate into our organs and accumulate in bones and teeth. The European Union has recently set a new target of 5 ppb of lead in drinking water, while the United States Environmental Protection Agency (EPA) demands action if the lead level of 15 ppb is exceeded in more than 10 % of tap water samples, declaring that there is no safe level of lead exposure.

The conventional way to remove lead ions from water is by distillation or reverse osmosis. However, this requires special equipment, and as the remaining lead levels decrease, the process becomes increasingly elaborate and energy-intensive. Searching for more accessible and cost-effective methods, the team investigated whether biological materials can be safely and effectively employed for lead removal. They already knew that yeast cells could bind lead ions in their cell walls, but wanted to ascertain how efficiently they would work as a biosorbent at the relevant concentration levels.

 

Conditions for Biosorption

The team found that biosorption by yeast was efficient and fast, even in the ppb range. However, it was dependent on pH. The team specified the ideal pH value for biosorption to be pH 5, where lyophilized (freeze-dried) yeast cells most efficiently absorbed lead ions from water. At higher pH, lead hydroxides would form precipitates, while lower pH values would be in the range of the isoelectric point of the cell wall components, so they would become protonated and no longer be able to bind the positively charged lead ions.

At optimum pH, however, lead removal to saturation occurred within minutes. The team reported the inactivated yeast cells had a lead uptake capacity of lead uptake of 12 mg of lead per gram of yeast biomass. Adding 5 mg of yeast cells reduced the lead content of solutions in the ppb range by 43 %. The resulting lead-loaded yeast pellet was then separated by centrifugation.

One advantage of yeast cells is provided by their robust cell walls, which remained intact during the lyophilization process. This means that this biological material is inactive, but still intact, and no cell content could leak into the solution from broken cells.

 

Passive Biosorption

The researchers explain that the yeast biosorption process was entirely passive. As changes in the vibrational spectra suggested, lead ions were bound by carboxy and amide functional groups in the cell wall. The team also observed an increased stiffness of the walls after lead binding, suggesting the formation of a thin layer of lead ions sticking the fibrillar structures together.

However, although yeast biosorption worked well for lead, the researchers acknowledged that the affinity for other heavy metals was lower. Other contaminants, such as copper or cadmium, would require different treatment.

Finally, the team addressed the issue of sourcing sufficient yeast biomass to treat entire water supply systems. They argue that the biotechnological industry readily obtains large quantities of this microorganism as a byproduct or even waste. The strain of baker’s or brewer’s yeast (Saccharomyces cerevisiae) that was investigated here is, of course, abundantly produced in the food and drink industry. Not only is sourcing the biomass relatively simple, the researchers also suggest that, in contrast to reverse osmosis membranes, the technology used to produce and use this biological sorbent would be simple.

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• Lead removal at trace concentrations from water by inactive yeast cells,
  Patritsia M. Stathatou, Christos E. Athanasiou, Marios Tsezos, John W. Goss, L. Camron Blackburn, Filippos Tourlomousis, Andreas Mershin, Brian W. Sheldon, Nitin P. Padture, Eric M. Darling, Huajian Gao, Neil Gershenfeld,
  Commun. Earth Environ. 2022.
  https://doi.org/10.1038/s43247-022-00463-0