C. Clark, A. Thompson, C. Griggs
U.S. Army Corps of Engineers Engineering Research and Development Center,
Keywords: REE, Biosorption, Scale
Summary:Rare earth elements (REEs) are crucial components in clean energy, electronics, and national defense applications, but these elements typically do not form natural ore deposits, making their recovery difficult. This combination of critical need and scarce supply has led to the exploration of alternative REE sources. One technique uses REE-selective Bacillus subtilis spores due to their unique adsorptive and desorptive capabilities. Additionally, these spores can be genetically modified through the addition of REE-specific proteins (e.g., Lan-M) on active surface sites. Spores are also comparatively advantageous to other recovery methods due to their potential for regeneration. Large-scale REE recovery has been investigated herein using wild-type and genetically modified bacillus subtilis spores in a flow-through crossflow membrane filtration system. The crossflow membrane filtration system allows for the continuous separation (concentration) of REE-adsorbed spores from excess solution. Experiments have been conducted with groundwater along with standard Dysprosium (Dy) salt solutions to study the effects of adsorption rates, the efficiency of dewatering, and the total REE recovery percentages. Interferences in the groundwater, such as iron and aluminum, have proven to limit the adsorptive capabilities of both wild- type and Lan-M spores, while the standard solutions produce valuable results. The standard Dy solutions demonstrate recovery of 85% Dy using the wild type spores, while the Lan-M spores recovered 47% Dy; however, within the first hour of adsorption, Lan-M spores adsorbed 30% more Dy compared to wild type spores – indicating differences in adsorption kinetics. Therefore, while Lan-M spores have a higher adsorption percentage, desorption of Dy from Lan-M was limited, indicated by the lower recovery rate. The crossflow system has the capability to continuously filter 1.5 liters of solution across 2 hours but can be scaled up with the addition of stronger pumps, increased surface area membranes, and increased tubing size. The scalability and efficiency of this recovery process provide a valuable extraction of REEs from solutions.