High-performance functionalized keratin for efficient lithium recovery: Experimental and statistical physics insights

This study investigates the potential of functionalized biosorbents for efficient lithium recovery from aqueous solution. Six biosorbents (keratin, calcium alginate, cellulose, chitosan, pectin, and date seeds) were functionalized with acetic, sulfuric, or phosphoric acid. Phosphoric acid-functionalized keratin (keratin@PA) exhibited the highest lithium adsorption capacity. Characterization techniques, including FTIR, XRD, TGA, SEM, SEM-EDX, and BET analyses, confirmed successful functionalization and revealed structural modifications, enhanced thermal stability, and increased surface porosity. Adsorption kinetics followed a pseudo-second-order model, while equilibrium data best fit the Langmuir isotherm, indicating monolayer adsorption on a homogeneous surface with a maximum adsorption capacity of 189.8 mg/g at an initial lithium concentration of 100 mg/L. Advanced statistical physics modeling, particularly the monolayer model with one energy level (M1), provided insights into the adsorption mechanism, revealing that keratin@PA facilitates multi-molecular adsorption with lithium ions adopting non-parallel orientations. The increase in saturation adsorption capacity with temperature suggests an endothermic process and enhanced adsorption efficiency at elevated temperatures. Regeneration tests validated the reusability of keratin@PA, demonstrating high initial desorption efficiency and stable performance over multiple cycles. Keratin@PA also exhibited exceptional selectivity for Li-ions, demonstrated by high separation factors ranging from 11.3 to 22.0 relative to common competing ions (Mg, K, Na, Ca). This work presents keratin@PA as a potential biosorbent for sustainable lithium recovery applications.