Rare Earth Element Biosorption onto Bacterial Surfaces in Engineered and Soil Systems
Bacterial surfaces can be used in engineered systems through the implementation of bioreactors as unconventional means of extracting critical metals. This dissertation discusses a genetically modified gram-negative bacterium, dLBTx8 induced Escherichia coli, which possesses lanthanide binding tag peptides that chelate and preferentially adsorb rare earth elements over other co-existing metals. A thermodynamic surface complexation model was first built to compare the cell surface terbium binding mechanisms of wild type and dLBTx8 engineered E. coli (Ch. 2). This surface complexation model was then expanded upon to include adsorption of yttrium and thirteen lanthanide elements (Ch. 3). The engineered dLBTx8 E. coli cells were immobilized in a non-sorbing polyethylene glycol diacrylate (PEGDA) polymer and packed into fixed-bed columns to demonstrate proof-of-principle bioreactor designs. A one-dimensional reactive transport model incorporating advective inter-bead and diffusive intra-bead transport of lanthanides was developed to account for the complex competitive adsorption processes taking place within the dLBTx8 E. coli packed fixed-bed column. After careful calibration and testing of the reactive transport model, predictions were made to optimize separation of valuable rare earths, exemplified by europium vs. lanthanum separation. This dissertation demonstrates the power of surface complexation and reactive transport modeling to allow for future engineering designs that can improve the efficacy of rare earth element bio-extraction and separation from geothermal fluid and mining leachate feedstocks.Bacterial surfaces can also play an important role in soils, surface water, and subsurface geologic systems, where the cell surface-based ligands can adsorb various metals. Due to the reactive nature of their cell surfaces, bacteria can influence fate, transport, and mobility of naturally occurring and anthropogenically introduced metals in soils. While many scientists have investigated the adsorption of divalent metals, such as lead and copper, onto bacterial surfaces found in soils and sediments, a knowledge gap currently exists in understanding how trivalent lanthanides impact these dynamic subsurface transport processes. In general, soil bacterial surfaces have been shown to have particularly strong electrostatic interactions with rare earths, implying that these trivalent metals can act as important controls over the transport of divalent metals. In this dissertation, a gram-positive soil bacterium, Arthrobacter nicotianae, has been shown to adsorb lanthanides through multiple different surface complexation mechanisms, including phosphodiester, phosphoryl, carboxyl, and carbonyl-based amide binding (Ch. 4). These results, supported through attenuated total reflectance Fourier transform infrared spectroscopy measurements (ATR-FTIR), are qualitatively corroborated by thermodynamic modeling of the bacterial cell wall. Notably, the modeling results suggest the relatively high affinities imposed by each of these site types, particularly through phosphate and amide monodentate adsorption. This work contributes to the currently existing knowledge gap whereby soil transport of divalent metals may significantly be affected by the presence of even low concentrations of trivalent rare earth elements.