MnO₂-decorated biochar composites of coconut shell and rice husk: An efficient lithium ions adsorption-desorption performance in aqueous media

Lithium (Li⁺) is used in various applications involving pharmaceuticals, textile dyes, and batteries. Therefore, the demand for environmentally friendly and effective materials for Li⁺ uptake and recovery continues to increase. Herein, rice husk (RH) and coconut shell (CS) biomasses were used to fabricate honeycomb-networked biochar (BC) precursors via slow pyrolysis. RHBC- and CSBC-based MnO₂ composites were synthesized by depositing MnO₂ in various ratios onto RHBC and CSBC by varying the KMnO₄ concentration (2%, 3%, and 4%), followed by simple ultrasonication and heat-treatment methodologies. The structural and physicochemical properties of all of the fabricated composites were analyzed using several different instrumental methods. The batch adsorption experiments were performed for comparative Li⁺-adsorption studies of RHBC-Mnx and CSBC-Mnx composites by optimizing several parameters (pH, adsorbent dose, Li⁺ initial concentration, and contact time). The comparative adsorption analysis revealed that the RHBC-Mnx composites exhibited stronger Li⁺-adsorption ability than the CSBC-Mnx composites and that increasing the MnO₂ deposition to 3% in both cases led to maximum Li⁺ adsorption capacities (62.85 mg g⁻¹ and 57.8 mg g⁻¹), respectively. The kinetic studies show that Li⁺ adsorption proceeds through the pseudo-second-order mechanism. Li⁺ recovery was successfully carried out using HCl (eluting agent), thereby demonstrating the benefits of synthesized composites at the industrial scale. The current work indicates that the fabricated RHBC-Mnx and CSBC-Mnx composites may have potential for use as economical composites in eco-friendly applications such as Li⁺ adsorption and recovery from aqueous media.