Magnesium isotope fractionation associated with biotic and abiotic weathering -and- Developing a scalable method for rare earth element extraction from non-traditional feedstocks using engineered Escherichia coli

Abstract

This dissertation is divided into two sections, with the first discussing magnesium (Mg) isotope behavior during biotic and abiotic rock weathering, and the second describing the application of engineered microbes for the selective recovery of rare earth elements (REEs) from non-traditional feedstocks. Magnesium isotopes exhibit observable mass dependent fractionation during a variety of rock weathering processes, permitting them to serve as a tracer of chemical and biological weathering. First, Bacillus subtilis endospore-mediated forsterite dissolution experiments demonstrating the effects of cell surface reactivity on Mg isotope fractionation are discussed. The endospore surfaces preferentially adsorbed 24Mg from the forsterite dissolution products, with calculated adsorbed Mg isotope compositions reaching δ26Mg = approx. -0.51‰ compared to the aqueous phase at approx. -0.37‰. These results demonstrate the effect of cell surface reactivity on Mg isotope fractionation in isolation, separate from other biological processes such as metabolism and organic acid production. Second, the Mg isotope compositions of granite and granodiorite weathering profiles were examined to investigate Mg isotope behavior during the weathering of felsic rocks. Mg isotope fractionation in these profiles was primarily controlled by primary biotite weathering and secondary illite formation and weathering. Finally, North Cascade Volcanic Arc basalts and andesites were analyzed for Mg isotopes to assess the extent and mechanism of crustal contribution to this magmatic system. The δ26Mg of these samples vary from within the range of ocean island basalts (the lightest being -0.33 ± 0.07‰) to heavier compositions (as heavy as -0.15 ± 0.06‰). The heavy Mg isotope compositions are best explained by the addition of crustal materials during assimilation and fractional crystallization. The results show that Mg isotopes may be a valuable tracer of crustal input into a magma, supplementing more traditional methods. Next, we investigate biosorption as a potential means of recovering REEs from non-traditional feedstocks. We examined how REE adsorption by engineered E. coli is controlled by various geochemical factors relevant to natural geofluids, including total dissolved solids (TDS), temperature, pH, and the presence of specific competing metals. REE biosorption is largely unaffected by high TDS concentrations, although high concentrations of some metals (e.g., U and Al) and decreasing pH below 5-6 were found to limit REE recovery. REE extraction efficiency and selectivity increase with temperature up to ~70°C, which is best explained by the thermodynamic properties of metal complexation on the bacterial surfaces. Together, these data demonstrate the potential utility of biosorption for selective REE recovery from geothermal fluids; however, the cells alone are generally not suitable for industrial-scale extraction operations. In the second portion of this section, we immobilize the engineered cells by encapsulating them in polyethylene glycol diacrylate (PEGDA) microparticles for use in fixed-bed columns. We demonstrate that optimal REE recovery (~2.6 mg Nd/g dry sorbent) occurs at an influent flow rate of ≤1 ml/min, pH of 6, and maximum REE concentration of ~21 mM. The microparticles exhibit minimal loss in performance over >9 adsorption/desorption cycles. Furthermore, they have a strong preference for heavy REEs, particularly Eu, Sm, Yb, and Lu, which may permit the separation of individual rare earth metals. The results of this study represent a major step towards making biosorption a viable industrial scale REE extraction technology.