Maximizing Lithium Adsorption and Selectivity on Manganese-Based Ion Sieves: Effects of Thermal Treatment, Acid Content, and Operating Conditions

Abstract

Lithium has been proven to be a critical metal for energy transition due to its application as high-grade energy storage. Its extraction has long been limited to conventional sources (i.e., mines and salt lakes), which use solar ponds and chemical treatment methods. However, to respond to the ever-increasing exponential demand for Li⁺, unconventional resources such as subsurface brines from geothermal and oilfields have lately been considered, with technological extraction means being the main challenge. The present research focuses on maximizing lithium adsorption and selectivity on manganese-based ion sieves by optimizing key factors such as the thermal treatment condition for powder calcination, acid content, and batch experiment operating conditions. Thus, it aims to enhance the efficiency of the ion-sieve synthesis process while minimizing energy and reagent consumption, addressing both the performance and scalability challenges of existing methods. It was observed that the lithium excess spinel cubic structure (i.e., Li_1.6Mn_1.6O₄) ion sieve was optimum for lithium recovery when LiMnO₂ was heat treated at 400–450 °C for 4 h at a ramping rate of 10 °C/min. The precursor was then treated with various acids to remove the template Li⁺ from the structure without compromising it, whereby HCl-treated powder registered the highest desorption (95%), CH₃COOH the lowest Mn²⁺ dissolution (9%), and H₃PO₄ the highest adsorption (29.5 mg/g). Hence, CH₃COOH was the best delithiation medium when the material recyclability was the main focus, while HCl serves well to enhance the final recovery efficiency of lithium ions from the sieve structure. The adsorption of the optimum Li_1.6Mn_1.6O₄ spinel cubic structure treated with 0.5 M HCl acid solution was described as a Langmuir monolayer model with an equilibrium retention capacity of 34.25 mg/g and dynamic pseudo-second-order chemisorption. The powder selectivity performance, Li⁺ ≫ Mg²⁺ > Fe²⁺ > Na⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ > K⁺ was primarily a function of the structure’s memory effect, with a secondary dependence on size and charge. When applied to synthetic lithium-rich Oilfield brine from Buchan (U.K.), Leduc (Canada), and Somerset, the extraction performance was recorded to be 20, 23, and 27 mg/g, respectively, at an S/L ratio of 1 g/L. The effects of operating conditions were also evaluated, with adsorption increasing with pH and brine temperature while decreasing moderately with stirring rate, Mg, and Na/Li ratio.