Sorption of lithium and competing ions from geothermal brine to lithium manganese oxide sorbent and implication for industrial use

One of the biggest challenges for direct lithium extraction (DLE) from brines, such as geothermal or salar fluids, is the selective extraction of Li with only minor co-extraction of other elements. Due to their high selectivity, spinel-structured lithium manganese oxide (LMO) sorbents are promising for application in DLE. We synthesized the LMO Li 1.6 Mn 1.6 O 4 and investigated the sorption of Li and competing elements from a geothermal brine of the Upper Rhine Valley. From the brine isotherm experiments, a maximum Li sorption capacity of 30.6 mg/g is achieved. The Li isotherm is best described through the BET isotherm, indicating that apart from the expected Li + -H + ion exchange, additional sorption processes take place. The sorption selectivity follows the order Mn > Zn > As > Ba > Li > Sr > Ca > K, Mg > Na. A near-complete extraction is achieved for Li, Ba, As, Mn and Zn. Despite their high concentration in the brine (g/L range), only 0.2–4.7 % of Na, Ca and K are extracted. Within both, the alkaline and alkaline earth metals, increasing charge density and thus stronger binding to the hydration sphere correlates with decreased selectivity. The selectivity variation between alkaline earth and alkaline metals is explained by the higher valence of the former. For both groups of the periodic table, the negative correlation between charge density and selectivity and the independence of the sorption capacity from the sorbent to brine ratio indicates electrostatic sorption. The higher selectivity of the LMO for Mn, Zn and As over Li results primarily from chemical binding and complex formation. Regarding an industrial DLE application, the additional sorption processes enable high Li sorption capacities even at low brine pH, potentially avoiding the addition of base to increase Li extraction. The fast sorption kinetics enable the application of the sorbent in high-discharge industrial DLE applications, such as geothermal power plants. The higher selectivity for Ba, As and Zn than Li and their desorption into the stripping solution may be disadvantageous as it may lead to impure products (LiCl solutions) that require further purification or removal prior to the extraction. Since the desorption of these elements is incomplete, this may result in the accumulation at the sorbent, potentially decreasing Li sorption capacity and resulting in expensive disposal of the sorbent as hazardous waste. Despite this, the low selectivity for Na, Ca and K make the sorbent suitable for brines enriched in these elements.