Asymmetric Single and Binary Adsorption of Thallium and Antimony Species onto Periwinkle-Shell-Derived Activated Carbon: Equilibrium, Kinetics, Thermodynamics, and Statistical Physics Modeling

Abstract

This study investigates the development of a sustainable adsorbent, periwinkle-shell-derived activated carbon (PSAC), for the remediation of aqueous systems contaminated with thallium and antimony. Structural characterization (BET and FTIR) revealed that PSAC possesses a highly microporous architecture (SBET = 876 m2/g) and a chemically heterogeneous surface. Batch adsorption experiments demonstrated that removal efficiency is highly sensitive to pH, increasing as the solution pH approaches the point of zero charge (pHpzc = 6.8). Kinetic profiles followed the pseudo-second-order model, while intraparticle diffusion analysis confirmed a multi-stage mechanism governed by both surface film diffusion and internal pore transport. Equilibrium data were analyzed using advanced statistical physics formalisms. A monolayer model with lateral interactions provided the most accurate fit for single-component systems, whereas a dual-site model incorporating interaction parameters was statistically superior for the binary system, as confirmed by R2, RMSE, χ2, and AIC testing. Steric parameters revealed that thallium exhibited higher receptor site densityand occupancy compared to Sb(III), which was limited by the steric hindrance of the bulky Sb(III)-tartrate complex. Adsorption energies for thallium were consistently higher than those for antimony, identifying a preferential uptake of thallium in competitive environments. FTIR analysis after adsorption suggested that the adsorption process is mainly governed by weak interactions, potentially including electrostatic attraction and pore filling rather than strong covalent bonding. Thermodynamic analysis derived from the statistical physics parameters confirmed the spontaneity and thermally-activated nature of the process, with configurational entropy profiles revealing greater flexibility for thallium species compared to the sterically constrained antimony-tartrate complex. These results highlight the efficacy of PSAC as a low-cost alternative to commercial carbons and demonstrate the power of statistical physics in decoding the non-ideal behavior of asymmetric binary adsorption systems.