Bacterial Approaches to the Recovery of Scarce Metals

Abstract

Many of the scarcest metals are critical to future energy technologies. However, these metals often have limited supplies, and their current production and recycling methods are complicated and use toxic chemicals. In order to ensure the availability of these metals alternative methods for their recovery need to be explored. This thesis describes biological methods for the recovery of some of these metals, specifically the lanthanides and tellurium. It is one of the first investigations for the biogenic recovery of either of these metals, making it unique in the field. The lanthanides are critical elements in the high performance magnets used in wind turbines, electric vehicles, and other 'green' technologies, but they are difficult to separate from one another because of their chemical similarity. We demonstrate a biogenic method based on lanthanide adsorption to the bacteria Roseobacter sp. AzwK-3b, followed by subsequent desorption as a function of pKa using a semi-continuous flow process. The desorption behavior suggests that the basicity of the individual lanthanides is important in determining their biosorption and desorption behavior. Similar selectivity was also found using phosphatidic acid liposomes. It is possible to concentrate a solution of equal concentrations of each lanthanide to nearly 50% of the two heaviest lanthanides in only two stages of enrichment, surpassing existing industrial practice. This suggests that there is an opportunity to harness the diversity of bacterial surface chemistry and liposome chemistries to fine tune the separation and recovery of these technologically important metals, and to do so in an environmentally benign manner. Tellurium is used in photovoltaic (PV) modules and thermoelectric generators, however it is not abundant in the earth's crust and is difficult to produce. We show that the hydrothermal vent microbe Pseudoalteromonas sp. strain EPR3 can convert tellurium from a wide variety of compounds, industrial sources, and devices into metallic tellurium and a gaseous tellurium species. These include metallic tellurium, tellurite, copper autoclave slime, tellurium dioxide, tellurium-based PV material (cadmium telluride), and tellurium-based thermoelectric material (bismuth telluride). Despite the fact that many of these tellurium compounds are considered insoluble in aqueous solution, they can nonetheless be transformed by EPR3, suggesting the existence of a steady state soluble tellurium concentration during tellurium transformation. Insights from these experiments on the mechanisms of tellurium precipitation and volatilization by bacteria, and their implications on tellurium production and recycling are discussed.