Adsorption of Rb + Ions from Geothermal Brine Using δ-MnO2 Obtained by Hydrothermal Synthesis

Separation of rubidium from geothermal brine was investigated by birnessite-type MnO2 (δ-MnO2). In this process, birnessite-type sodium manganese dioxide (Na-Birnessite) used as a precursor was synthesized via coprecipitation and hydrothermal routes. Acid treatment with 1 M HCl mediates the removal of interlayer sodium ions from Na-birnessite to form hexagonal birnessite (H-Birnessite). Adsorption experiments were conducted using both geothermal brine and synthetic Rb+ solutions. The results revealed that δ-MnO2 exhibits remarkable Rb+ uptake, achieving an adsorption capacity of 8.4 mg·g–1 and a removal efficiency of 92.7% under optimum conditions (pH 7, 30 min, 10 ppm, 1 g/L, and 55 °C in the presence of coexisting ions). The kinetic data obeyed both the pseudo-first-order and pseudo-second-order models, indicating mixed-kinetic control, while the loading data showed slightly better correlation with the Langmuir isotherm model than with the Freundlich or Temkin models. The thermodynamic results verified that this process is endothermic, with the adsorption reaction largely favoring product formation and being entropy-driven. The distribution coefficients of metal ions followed the order: Rb+ > Ca2+ > K+ > Mg2+ > Li+, demonstrating the selectivity of δ-MnO2 toward Rb+ ions. δ-MnO2 shows strong potential as an efficient and selective sorbent for rubidium recovery from hypersaline geothermal brines.