Kinetics and statistical physics modeling of heavy metal ions adsorption onto functionalized pyrite composite: Experimental and modeling

Abstract

Heavy metal ions such as Cu²⁺, Cd²⁺, Ni²⁺, and Zn²⁺ pose serious threats to water quality and public health due to their toxicity and persistence. In this study, a functionalized pyrite composite (FeS₂ core coated with silica) was synthesized and evaluated for its adsorption performance toward these heavy metal ions. Batch adsorption experiments were conducted under varying pH (2−10), contact time (10–60 min), temperature (298–343 K), and initial concentration (100–500 mg/L). Adsorption isotherms were analyzed using Langmuir, Freundlich, Redlich–Peterson, Toth, and other models. The Langmuir model best described the data (R² > 0.999), indicating monolayer adsorption with maximum capacities of 145.71 mg/g for Cd²⁺ and 81.44 mg/g for Cu²⁺. Kinetic analysis showed that the pseudo-second-order (PSO) model best fit the data (R² > 0.998), suggesting chemisorption as the dominant mechanism. Thermodynamic parameters (ΔG°, ΔH°, ΔS°) were calculated from Van't Hoff plots. Negative ΔG° values confirmed spontaneity, while positive ΔH° and ΔS° supported the endothermic and entropy-driven nature of the process. Statistical physics modeling further revealed the number of active sites, the mean adsorption energy (E < 20 kJ/mol), and the adsorption stoichiometry factor (n). These results indicated predominant physisorption with partial chemisorption, particularly for Ni²⁺ and Cu²⁺. The study demonstrates that functionalized pyrite composite is a cost-effective, regenerable, and high-capacity adsorbent for heavy metal ion removal. The integration of isotherm, kinetic, thermodynamic, and statistical physics models offers mechanistic insight and predictive power for practical water treatment applications.