Listen to this article Rare Earth Elements Collection From Wastewater By A Dozen Exotic Bacteria
Rare earth elements (REEs) are a group of 17 chemically similar metals that have their name because they typically occur at low concentrations (between 0.5 and 67 parts per million) within the Earth’s crust. These rare earth elements are essential for modern technology such as light-emitting diodes, mobile phones, electromotors, wind turbines, hard disks, cameras, magnets, and low-energy lightbulbs. Due to their rarity and demand, they are expensive. In January 2023, there was a great deal of excitement when promising new finds of REEs (more than one million metric tons) were announced in Kiruna, Sweden. It’s important to note that neodymium oxide costs around €200 per kilogram, while terbium oxide costs approximately €3,800 per kilogram. However, China still dominates the mining of REEs.
Circular Economy and REEs
It is evident that a shift from a wasteful “linear” economy to a “circular” economy, where all resources are recycled and reused, can bring numerous benefits. So, can we recycle REEs more efficiently too? In Frontiers in Bioengineering and Biotechnology, German scientists have shown that the answer is yes. The biomass of some exotic photosynthetic cyanobacteria can efficiently absorb REEs from wastewater derived from mining, metallurgy, or the recycling of e-waste. The absorbed REEs can afterwards be washed from the biomass and collected for reuse.
Specialist Strains of Cyanobacteria
Biosorption is a metabolically passive process for the fast, reversible binding of ions from aqueous solutions to biomass. The scientists measured the potential for biosorption of the REEs lanthanum, cerium, neodymium, and terbium by 12 strains of cyanobacteria in laboratory culture. Before this study, most of these strains had not undergone assessment for their biotechnological potential. Researchers collected the strains from specialized habitats, including arid soils in Namibian deserts, lichen surfaces worldwide, natron lakes in Chad, crevices in rocks in South Africa, and polluted brooks in Switzerland.
Cyanobacteria’s Ability to Absorb REEs
The authors found that an uncharacterized new species of Nostoc had the highest capacity for biosorption of ions of these four REEs from aqueous solutions, with efficiencies between 84.2 and 91.5 mg per g biomass, while Scytonema hyalinum had the lowest efficiency at 15.5 to 21.2 mg per g. Also efficient were Synechococcus elongates, Desmonostoc muscorum, Calothrix brevissima, and an uncharacterized new species of Komarekiella. The acidity strongly influenced biosorption: the process had the highest efficiency at a pH range of five to six, and its effectiveness gradually decreased in more acidic solutions. The process was most efficient when there was no “competition” for the biosorption surface on the cyanobacteria biomass from positive ions of other, non-REE metals such as zinc, lead, nickel, or aluminum.
Chemical Mechanisms for Binding REEs
The authors used infrared spectroscopy to determine which functional chemical groups in the biomass were mostly responsible for biosorption of REEs. They 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.
Efficiency and 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, with most certain REEs absorbed within 24 hours. The ability of the cyanobacteria to absorb REEs from wastewater could offer a sustainable, cost-effective alternative to traditional methods of mining, which are often environmentally destructive and involve the use of toxic chemicals
Moreover, this study highlights the potential of using specialist strains of cyanobacteria in the circular economy. It is easy to cultivate and scale up these organisms, which makes them a promising solution for recycling REEs from waste streams. Additionally, using cyanobacteria to recover REEs would reduce the reliance on China, creating a more diverse and secure supply chain.
In conclusion, the discovery of a new species of Nostoc with a high capacity for biosorption of REEs and the potential of specialist strains of cyanobacteria in the circular economy is a significant breakthrough in the sustainable use of REEs. With the demand for REEs set to increase due to their importance in modern technology, it is vital to explore new ways to obtain these valuable metals without damaging the environment. The use of cyanobacteria as a sustainable and cost-effective alternative to traditional mining methods could revolutionize the way we obtain and use REEs.