Lithium recovery from geothermal brines: An investigation into radioactive nuclide uptake on lithium‑manganese-oxide (LMO) granules

Abstract

Due to the growing demand for lithium, the exploitation of non-conventional lithium sources such as Li-bearing geothermal brines is currently on the rise. These brines contain up to 200 mg L−1 of lithium along with a diverse range of other dissolved elements, including naturally occurring radionuclides. In this study, data from a radiochemical monitoring process performed during the pilot plant testing of direct lithium extraction (DLE) at a geothermal site in the Upper Rhine Graben (URG) are presented. The tests were conducted under thermodynamic p-T conditions of the geothermal plant (T = 60 °C, p = 18 bar). The results present an uptake of the long-lived radionuclides 226Ra (228Ra) and 210Pb by the granulated lithium‑manganese-oxides (LMO) sorbent employed. Elevated pH values enhance the sorption of nuclides formed in the Th-U decay series, due to the increased negative charge on the LMO surface. The radioactivity uptake on the sorbent was found to be time-dependent. Experiments with a 5-min contact time resulted in relatively low sorption capacities, while extending the sorption time to 30 min significantly increased the accumulation of Ra isotopes and 210Pb on the sorbent surface. During the regeneration phase, the radioactive load on the LMO sorbent could be reduced using 0.5 M HCl. The mass balance analysis of the investigated radionuclides demonstrates desorption percentages ranging from 16% to 100% per extraction cycle.