Microscopic evaluation of Li, Cd, and Co removal from aqueous solutions using equisetum and teucrium: statistical...

This study provides a microscopic interpretation of Li⁺, Cd²⁺, and Co²⁺ adsorption onto Equisetum and Teucrium, integrating experimental isotherms with a grand-canonical statistical physics framework. Li⁺ and Cd²⁺ follow Type I monolayer adsorption behavior, whereas Co²⁺ exhibits Type II multilayer behavior. Li⁺ adsorption was best described by an ideal-gas Langmuir monolayer model, with high site densities (W _m = 152.05 and 145.32 mg g^-1) and the highest capacities (Q _m = 147.48 and 139.50 mg g^-1), consistent with the highest configurational entropy peak (S _max = 100-106 J K^-1) and a larger number of accessible adsorption microstates. Cd²⁺ obeys a Langmuir monolayer model with lateral interactions (Q _m ≈ 104 mg g^-1), while Co²⁺ follows a BET-type multilayer model with two adsorption energies (E ₁ = 31.7-29.75 kJ mol^-1; E ₂ = 24.21-22.34 kJ mol^-1) and lower capacities (Q _m = 34.19 and 22.62 mg g^-1). Internal energy profiles and dual-plateau isotherms support the occurrence of Co multilayer adsorption. The n values obtained for all three metals are close to unity, suggesting that each adsorption site on the wild herbs accommodates at most one ion. Energetic analysis suggests partial chemisorption for Li⁺ (E _1/2 = 42.14 and 40.88 kJ mol^-1) and physical adsorption for Cd²⁺ (38.69-38.22 kJ mol^-1) and Co²⁺ (31.7-22.34 kJ mol^-1), consistent with their predominantly exothermic adsorption behavior. Excellent agreement with the models (R ² > 0.99, χ ² < 0.22), together with negative Gibbs free energy values, suggests the spontaneous nature of the adsorption process and highlights Equisetum and Teucrium as efficient biosorbents for Li⁺ and promising adsorbents for Cd²⁺ and Co²⁺.