Dozens of exotic bacteria found to passively collect rare earth elements from wastewater

Rare earth elements (REEs) are a group of 17 similar metals, which got their name because they usually occur at low concentrations (between 0.5 and 67 parts per million) within the Earth’s crust. As they are indispensable in modern technology such as light emitting diodes, mobile phones, electromotors, wind turbines, hard disks, cameras, magnets and low energy light bulbs, their demand has steadily increased in recent years , and it is predicted. further increase by 2030.

As a result of their rarity and demand, they are expensive: for example, a kilogram of neodymium oxide currently costs around €200, while the same amount of terbium oxide costs around €3,800. Today, China has a near monopoly on the mining of REEs, although in January 2023 promising new discoveries (more than a million metric tons) were announced in Kiruna, Sweden with great fanfare.

Circular economy

The benefits of moving from a “linear” waste economy to a “circular” economy, where all resources are recycled and reused, are obvious. So could we also recycle REEs more efficiently?

I Frontiers in Bioengineering and Biotechnology, German scientists have shown that the answer is yes: The biomass of some photosynthetic cyanobacteria can efficiently absorb exotic REEs from wastewater; for example those derived from mining, metallurgy, or e-waste recycling. The absorbed REEs can then be washed from the biomass and collected for reuse.

“Here we optimized the REE uptake conditions by the cyanobacterial biomass, and characterized the most important chemical mechanisms for their binding. These cyanobacteria could be used in future eco-friendly processes for the simultaneous recovery and treatment of industrial wastewater REE,” said Dr. Thomas Brück, professor at the Technical University of Munich and last author of the study.

Highly specialized types of cyanobacteria

Biosorption is a metabolically passive process of rapidly and reversibly binding ions from aqueous solutions to biomass. Brück and his colleagues measured the potential for bioabsorption of the REEs lanthanum, cerium, neodymium, and terbium by 12 strains of cyanobacteria in laboratory culture. Most of these strains have never been evaluated for their biotechnological potential before. They were sampled from highly specialized habitats such as arid soils in the Namibian desert, lichen surfaces around the world, natron lakes in Chad, crevices in rocks in South Africa, or polluted streams in Switzerland.

​​​​The authors found that a new uncharacterized species of Nostoc had the highest capacity to biosorb ions of these four REE from aqueous solutions, with efficiencies between 84.2 and 91.5 mg per biomass, and the efficiency was Scytonema hyalinum has the lowest at 15.5 to 21.2 mg per. g. Also effective were Synechococcus elongates, Desmonostoc muscorum, Calothrix brevissima, and a new uncharacterized species of Komarekiella. Biosorption was found to be strongly dependent on acidity: it was highest at pH between five and six, and steadily decreased in more acidic solutions. The process was most effective when there was no “competition” for the bioabsorption surface of the cyanobacteria biomass from positive ions of other metals, non-REE such as zinc, lead, nickel, or aluminum.

The authors used a technique called infrared spectroscopy to determine which chemical functional groups in the biomass were primarily responsible for the biosorption of REEs.

“​​​​We found that biomass derived from cyanobacteria has excellent adsorption characteristics due to its high concentration of negatively charged sugar moieties, which carry carbonyl and carboxyl groups. ,” said first author Michael Paper , a scientist at the Technical University of Munich.

Fast and efficient, with great potential for future applications

The authors conclude that biosorption of REEs by cyanobacteria is possible even at low concentrations of the metals. The process is also fast: for example, most of the cerium in solution was biosorbed within five minutes of starting the reaction.

“The cyanobacteria described here can emit amounts of REE corresponding to up to 10% of their dry matter. Biosorption therefore presents an economically and ecologically optimized process for circular recovery and reuse of rare metals -clay from diluted industrial wastewater from mining, electronic, and the sectors that produce chemical catalysts,” said Brück.

“It is hoped that this system will become economically feasible in the near future, as the demand and market prices for REEs are likely to increase significantly in the coming years,” he said.