A bacterial culture can do the work of separating rare-earth elements, or lanthanides, used widely in advanced batteries and magnets (Environ. Sci. Technol. Lett. 2016, DOI: 10.1021/acs.estlett.6b00064). The newly developed method is an initial step in efforts to create alternatives to the time-consuming and less-than-green solvent extractions now used in industry.
Rare-earth elements have their dance cards full these days. Kilograms of lanthanum go into every hybrid car battery, and neodymium magnets power motors in applications ranging from wind turbines to tablets. Rare earths are the darlings of green energy and handheld gadgetry, but demand is outstripping supply. They aren’t actually hard to find—rather, rare earths are hard to isolate. Found together in the same ores, the lanthanides—neighbors on the periodic table—are so physically and chemically similar that they must be painstakingly separated from each other before they can dance on their own.
The industrial process requires iterative extractions in organic solvents to take advantage of slight basicity differences between lanthanides, says William D. Bonificio of Harvard University. He says he once toured a lanthanide mine with row upon row of organic solvent vats arranged in extraction lines as long as a football field. The by-products from these extractions, when not dealt with responsibly, poison land and people near processing plants, particularly in China, where about 95% of rare-earth refining is done. Global dependence on Chinese rare-earth production has also led to supply crises, so new separation methods could help other countries compete better in the rare-earth market.
These challenges led Bonificio and Harvard materials engineer David R. Clarke to search for a greener separation technology. Since heavy-metal remediation often uses bacteria to pluck metals out of complex mixtures, they decided to see if bacteria could distinguish between metals in mixtures of chemically similar lanthanides.
They grew a culture of a strong metal adsorber—a Roseobacter strain from a California saltwater slough—placed the bacteria on a filter, and then pumped a mixture of 14 lanthanides past the bacteria. All of the lanthanides stuck to the bacteria, likely because the positively charged lanthanides bind to phosphate, carboxyl, and other groups on the cell membrane, Bonificio says. The team then guessed that each lanthanide’s adsorption strength might vary slightly depending on the pH, so they passed washes of decreasing pH past the filter, from pH 6.0 down to 1.5. As they hoped, the lanthanides slid off gradually, the lightest ones at higher pH washes, and the heaviest lanthanides not until pH 2 or lower.
In another approach, the team tried to pluck out just one or two target lanthanides from the group. Bonificio and Clarke soaked bacteria in an initial low-pH wash before exposing the cells to the lanthanide mixture. This prevented the lightest lanthanides from sticking at all, allowing only the heaviest ones to stick. They stripped those heavy lanthanides off with an even lower pH wash and ran that solution one more time past a fresh batch of soaked bacteria. They ended up with a solution highly concentrated in only the heaviest two lanthanides, lutetium (30 wt %) and ytterbium (18 wt %), better than a traditional solvent extraction achieves in just two stages. A similar process concentrated the midweight lanthanides.
Bonificio and Clarke have filed a patent on the technique and have now separated rare earths with six other types of metal-adsorbing bacteria.
Pierre Le Cloirec, director of École Nationale Supérieure de Chimie de Rennes, says the separation the bacteria achieved was interesting, but he cautions that this is a small laboratory experiment, and industrial applications are a long way off.