Current process for separating rare-earth metals is time consuming, expensive, and dangerous. Processing one ton of rare earths can produce 2,000 tons of toxic waste. Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a cleaner method to extract rare earth metals. Their process, described in a paper published in the ACS journal Environmental Science and Technology Letters, uses bacteria filters and solutions with pH no lower than hydrochloric acid.
Bacteria filters have long been used to bioabsorb toxic elements from wastewater or filter metals from mine drainage systems. Recent research has also shown that some rare earths can bioabsorb. David Clarke, the Extended Tarr Family Professor of Materials at SEAS, and his graduate student William Bonificio, wondered if all rare earths could be filtered through bacteria.
They immobilized a bacteria (Roseobacter sp. AzwK-3b) from marine algae on an assay filter and passed a solution of mixed rare earths (lanthanides) through it. The bacteria bioabsorbed all the elements as they passed. Then the team pumped solutions of various pH balances through the filter. With each successive pH wash, different rare earths detached.
The researchers found that lighter lanthanides, such as Europium and Praseodymium, desorbed with higher-pH washes while heavier lanthanides, such as Thulium, Lutetium, and Ytterbium, desorbed with lower pH.
The team also found that if they wanted to separate only the heaviest metals, such as Thulium, which is commonly used in lasers and portable X-rays, they could block the bacteria’s receptors that absorb the lighter rare earths and only use a low-pH solution.
We found that it is possible to concentrate a solution of equal concentrations of each lanthanide to nearly 50 percent of the three heaviest lanthanides in just two passes. This surpasses existing industrial practice.—William Bonificio
This research was supported by the Harvard University Center for the Environment and the Office of Naval Research.
William D. Bonificio et al. (2016) “Rare-Earth Separation Using Bacteria,” Environmental Science & Technology Letters doi: 10.1021/acs.estlett.6b00064