Lithium extraction from geothermal brines: Adsorption-desorption studies on 226Ra (228Ra) and 210Pb on lithium-titanium-oxides (LTO)

Extracting lithium from geothermal brines contributes to meet the growing demand for lithium -ion batteries and other energy applications. Deep geothermal brines of the Upper Rhine Graben (URG) contain up to 200 mg L-1 of lithium and a wide range of dissolved elements, including Th-U series radionuclides. This study presents data from a radiochemical monitoring conducted as part of pilot tests for lithium extraction at the Bruchsal geothermal site under site -specific thermodynamic conditions (T = 60 C-degrees; p = 18 bar). The results show an uptake of the long-lived radionuclides Ra-226 (228Ra) and 210Pb by the granulated lithium -titanium -oxide (LTO) sorbent employed. Increasing pH values favor the adsorption of the naturally occurring radionuclide, due to increasing negative charge of the LTO surface. The radioactive uptake of radium was found to be nearly in adsorption equilibrium state after 12 extraction cycles performed. In contrast, 210Pb is permanently removed from the Bruchsal brine by more than 96 % reflecting its high capability to approach the adsorbent surface during adsorption compared to the competing cations. Radioactive load of both radium and lead isotopes on the LTO adsorbent could be reduced using 0.5 M HCl as desorption agent. In this context, desorption percentages ranging from 50 to 100 % per extraction cycle were obtained.