Tikalon Blog by Dev Gualtieri

Rare Earths from Bacteria

May 29, 2023

Many plants concentrate trace elements. One example is locoweed, the common name for plants of the species Astragalus, which concentrate selenium up to several percent of their dry weight.[1] Nickel is a valued metal (Nickel is about a hundred times more expensive than iron), and Pycnandra acuminata, a species of plant in the family, Sapotaceae, concentrates nickel from the nickel-rich soil of New Caledonia. The dry weight of its sap contains up to 25% nickel citrate, and the sap has a turquoise-green color as a consequence.

Astragalus lentiginosus (spotted locoweed) and Pycnandra acuminata

Left, Astragalus lentiginosus, also known as the "spotted locoweed." Right, Pycnandra acuminata

Brazil nuts, the seeds of Bertholletia excelsa in the family Lecythidaceae, concentrate selenium up to 83 micrograms per gram (shell included).[2] Toxic effects of selenium start at about 700 micrograms; so, don't eat too many Brazil nuts at one time.

(Left, Wikimedia Commons image by Stan Shebs. Right, a Wikimedia Commons image by Benoit Henry. Click for larger image.)

Bacteria are known to concentrate toxic heavy metals, and that's inspired genetic engineers in their attempts to maximize this trait. While the main purpose is for bioremediation, such bioaccumulation can be a means of extracting valuable heavy metals for reuse, rather than disposal.[3] In particular, the sulfate-reducing bacteria, Desulfovibrio desulfuricans, will reduce U⁶⁺ to U⁴⁺ to provide an easy means of recovering uranium from contaminated waters and waste streams.[4] Also interesting are rare earth concentrating bacteria, as discussed later in this article.

A good test of an inorganic chemist is whether he or she can name all of these uncommon elements while looking at just their symbols on the periodic table. The most technologically useful rare earths are yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium , and lutetium (Lu). (Created using Inkscape. Click for larger image.)

Rare earth elements are used in many industrial and consumer products. These include catalysts, the major use, the magnets used in electric motors for drones and automobile windows, laser crystals, and phosphors. The estimated value of United States imports of rare-earth compounds and metals was $160 million in 2021, a significant increase from $109 million in 2020.[5] China's 2021 production in tons of rare earth oxide equivalents was 168,000, followed by the United States at 43,000, Myanmar (Burma) at 26,000, and Australia at 22,000.[5]

China controls the world market in rare earth metals, and it has been limiting exports of these elements. As a consequence, other countries have been searching for new sources of these metals. Just this year, reserves of one million metric tons were discovered in Kiruna, Sweden. Rare earth deposits have been discovered in seafloor mud in the Pacific Ocean.[6-7] As I wrote in a previous article (Rare Earths from Coal Waste, April 18, 2022), coal ash, a byproduct of coal combustion, contains rare earth elements.[8-9] Acid mine drainage contains sulfuric acid that dissolves rare earths from surrounding rocks. An estimated 6,000 metric tons of rare earths are created annually in acid mine drainage sites.[10-11]

A team of German scientists from the Technical University of Munich (Garching, Germany), and the University of Applied Sciences Kaiserslautern (Pirmasens, Germany) have found that some exotic photosynthetic cyanobacteria efficiently absorb rare earths from wastewater, and the rare earths can be extracted from their biomass.[12-14] Their findings have been published in an open access paper in the journal, Frontiers in Bioengineering and Biotechnology.[12]

One fundamental problem in rare earth extraction from ore is that they are chemically similar. As a consequence, their extraction requires environmentally toxic chemicals and a lot of energy.[12] Rare earth recovery from industrial wastewater, and acid mine drainage, as mentioned above, is becoming an encouraging alternative.[12] Biosorption of metal ions by photosynthetic microorganisms, a metabolically passive and rapid process that involves the reversible binding of ions from aqueous solutions to biomass is a viable alternative method for both bioremediation and metal recovery.[12,14] The present study builds from 2021 research of heavy metal removal by cyanobacteria.[13]

Says study's first author, Michael Paper, a scientist at the Technical University of Munich,

"We found that biomass derived from cyanobacteria has excellent adsorption characteristics due to their high concentration of negatively charged sugar moieties, which carry carbonyl and carboxyl groups. These negatively charged components attract positively charged metal ions such as REEs, and support their attachment to the biomass."[14]

Seven terrestrial and five aquatic cyanobacterial strains were investigated for their absorption of rare earths from aqueous solution.[13-14] They are found in habitats with harsh environmental conditions, such as the arid soils in Namibian deserts, rock crevices in South Africa, natron lakes in Chad, and polluted streams in Switzerland.[13-14] Most of these had never been checked for their bioremediation potential.[13-14]

If it's green, it must be biology - Professor Thomas Brück taking samples at the cyanobacteria photobioreactor for extraction of rare earth elements.

There's a laboratory adage as follows: If it's green, it's biology; if it stinks, it's chemistry; and, if it doesn't work, it's physics. (TUM image by Andreas Heddergott, released under the Creative Commons Attribution License. Click for larger image.)

The rare earth elements, cerium, neodymium, terbium, and lanthanum, were used to screen the adsorption capacity of the bacteria, and a not previously investigated new species of Nostoc, Nostoc sp. 20.02, showed the highest adsorption capacity with 84.2-91.5 mg/g.[12] This is about 10% of the bacterial dry mass. Others showed adsorption capacities of 47.2 to 83.5 mg/g.[12] Absorption was rapid at the mildly acidic pH of 5-6, but decreased at more acidic pH.[12,14] Most of the cerium in solution was captured within five minutes.[13] Says study author, Thomas Brück,

"The cyanobacteria described here can adsorb amounts of REEs corresponding to up to 10% of their dry matter. Biosorption thus presents an economically and ecologically optimized process for the circular recovery and reuse of rare earth metals from diluted industrial wastewater from the mining, electronic, and chemical-catalyst producing sectors."[14]

It was observed that rare earth biosorption was possible even at low metal concentration, and the process was more efficient when there were no competing cations, such as zinc, lead, nickel, or aluminum.[13-14] Overall, the application of cyanobacteria in bioremediation and recovery of rare earth elements is a promising method for the development of an industrial process, but needs to be to be further optimized and adjusted.[12] In a subsequent project, the research team plans to conduct more experiments on a larger scale with a view to industrial application of the results.[13]

Rare Earths from Bacteria

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