Sustainable valorization of biogenic coral limestone waste into calcium aluminate biosorbents for efficient Cr(vi) remediation: characterization, experimental performance, and statistical physics analysis

Abstract

Herein, Hydnophora coral waste was chemically treated and thermally processed to form calcium aluminate ceramic (CAC), which served as an efficient adsorbent for Cr(VI) remediation. The produced CAC biosorbent was characterized using SEM-EDX, XRD, FTIR, and TEM techniques. Adsorption behavior was systematically evaluated through kinetic, equilibrium, thermodynamic, and statistical-physics modeling. Regeneration tests were performed to evaluate stability and reusability over multiple adsorption–desorption cycles. The developed CAC principally included crystalline mayenite (Ca₁₂Al₁₄O₃₃) and Ca₄Al₆O₁₃ phases, confirming successful phase transformation during the fabrication process. Kinetic analysis indicated that the pseudo-first-order model provided the best fit for the adsorption process, while the Liu isotherm most effectively described the equilibrium data. The as-synthesized CAC biosorbent exhibited notable Cr(VI) adsorption capacity (148.65–304.62 mg g⁻¹ across 25–55 °C), reflecting a strong dependence on solution temperature. Thermodynamic analysis confirmed a spontaneous uptake, as ΔG° ranged from −5.064 to −7.528 kJ mol⁻¹ at 298–328 K, and an endothermic process, as ΔH° was +19.601 kJ mol⁻¹. The low ΔE values ranging from 11.45 kJ mol⁻¹ (25 °C) to 13.11 kJ mol⁻¹ (55 °C) indicated that the adsorption system is mainly governed by a physisorption mechanism (ΔE < 15 kJ mol⁻¹). Statistical physics analysis revealed a horizontal multi-docking adsorption configuration, and steric parameters emphasized the significance of the functional group density of CAC in enhancing uptake performance. Additionally, the prepared material exhibited excellent regeneration capability, maintaining high performance over multiple adsorption–desorption cycles. Overall, this work highlights a sustainable method for converting coral limestone waste into high-performance ceramic adsorbents. Additionally, the use of statistical physics theory offers valuable insight into the molecular-level mechanisms controlling Cr(VI) adsorption.