The Horizon Europe CRM-geothermal project: Deliverable 3.4 - Report: Proof-of-concept oxalogenesis-oxalotrophy pathway for biorecovery of Li and Sr

In this report we present the data supporting the feasibility of implementing a pathway combining fungal oxalogenesis and bacterial oxalotrophy for the biorecovery of Li from deep geothermal fluids. While we provide data only for Li, we also provide a theoretical background on how Sr could be targeted using the same approach. The biorecovery approach consists of the following steps: 1) Oxalic acid (OA) production by a fungal strain, 2) Fluid filtration with oxalic acid in order to remove impurities such as Ca, and 3) Li immobilization as Li-carbonates via fluid alkalinization by bacterial oxalotrophy.

The data obtained under laboratory condition demonstrate the feasibility of the pathway. A strain of the fungus A. niger was able to produce OA at concentrations that are compatible with the next step in the pathway, i.e. at least 10 g/L. Fluid cleaning with OA in order to remove Ca (and other elements) via oxalate precipitation was possible for brines containing up to 54 g Ca/L which corresponds to the high salinity synthetic brine that the project partners agreed upon as experimental test solution. Finally, while actual Li₂CO₃ precipitation was not experimentally achieved yet, a simple geochemical model indicated that given a Li concentration of at least 400 ppm, Li₂CO₃ precipitation via oxalotrophy is theoretically possible. In addition, a sequential system has been proofed in order to allow the oxalotrophic bacteria to resume growth and alkalinize its medium.

As a result, in order to combine the three steps of the pathway at a pilot scale we propose to set up a sequential system. For this, further experiments will concentrate on optimizing OA production by our best OA producing strain A. niger, increasing Li concentration in the Ca-cleaned fluid via biosorption, and optimizing the stages of the sequential system in order to maximize Ca removal vs Li recovery as Li₂CO₃.