Pb (II) Biosorption by Ramalina conduplicans Biomass: Adsorption Behavior and Binding Mechanisms

This study presents a rigorous technical evaluation of the sequestration of Lead (Pb (II)), a representative potentially toxic element (PTE), using the biomass of the lichen Ramalina conduplicans Vain. Surface characterization conducted via SEM-EDX, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) confirmed that the lichen biomass possesses a heterogeneous, porous, and amorphous structure. The surface is rich in carboxyl, hydroxyl, and amino functional groups, which undergo distinct morphological transformations following Pb (II) adsorption. Batch experiments achieved a maximum removal efficiency of 99.15% at pH 5 and a dosage of 83.33 g L^-1. Kinetic modeling via nonlinear regression identified the pseudo-second-order model (R² = 0.965) as the superior fit, while nonlinear Boyd and Weber-Morris models confirmed film diffusion as the primary rate-limiting step. Equilibrium data were evaluated using five nonlinear equilibrium models, with the Hill model providing the most accurate description (R² = 0.936). The Hill coefficient (n_H = 1.983) reveals positive cooperativity, indicating that the initial Pb (II) binding enhances the affinity of subsequent binding sites. While the Langmuir model (R² = 0.894) estimated a maximum adsorption capacity (q_max) of 3.889 mg g^-1, the superior fits of Hill, Redlich-Peterson, and Temkin models confirm a complex chemisorption mechanism occurring on a heterogeneous surface. Thermodynamic analysis further categorized the process as both spontaneous and endothermic. In conclusion, R. conduplicans biomass functions as a sustainable and cost-effective biosorbent for Pb (II) removal. While further optimization is required for industrial scaling, its cooperative binding mechanism and high removal efficiency highlight its potential as a green component in integrated PTE remediation strategies.