A Clean Way to Harness the Value of Rare Earth Metals

Extracting rare earth metals with bacteria.

The 17 chemically similar elements at the bottom of the periodic table are known as rare earth metals. Due to their similarity, separation of these elements can be very difficult as well as hazardous. The processing of one ton of rare earth metals can produce 2,000 tons of toxic waste.

Now, engineers have developed a clean alternative to conventional separation techniques utilizing bacteria filters and solutions with an acidity no stronger than that of hydrochloric acid.

Bacteria Filters for Separation

The engineering researchers behind this development questioned the capabilities of bacteria filters if applied to all rare earth metals. They began with research which indicated that some rare earth metals are able to bioabsorb.

A solution of lanthanides (mixed rare earth metals) was passed through an assay filter with immobilized bacteria from marine algae. The researchers found the bacteria bioabsorbed all of the passing elements. Those filtered out elements were then drawn from the solution and fixed to the filter surface.

Playing with pH

Next, the researchers experimented with a variety of pH balances and combinations, pumping these pH-varying solutions through the filter. They found that different metals detached at different pH values. Lighter lanthanides, including Europium and Praseodymium, were separated with less acidic washes. Heavier rare earth metals, such as Thulium, Lutetium, and Ytterbium, were filtered out with more acidic solutions.

Rare earth oxides. (Image courtesy of Harvard/John A. Paulson School of Engineering and Applied Sciences.)

Furthermore, they found that the heaviest metals, such as Thulium, could be separated by themselves by blocking the bacteria’s receptors which absorb the lighter rare earth metals. In such cases, only a low-pH solution would need to be used. As a result, it became possible to concentrate a solution of equal concentrations of each type of rare earth metal to almost 50% of the three heaviest types in only two passes.

Not only is this method of separation far superior to current separation techniques for rare earth metals, but it is also much safer for the environment due to the decrease in acidity and the use of biofiltration.

Rare earth metals are found in the vast majority of modern-day technologies, including cell phones, wind turbines and electric cars, but they are only valuable once separated.

This research was conducted by engineers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS).

For further information on this process, read the description of this method in the journal Environmental Science and Technology Letters.