Unraveling Uranium Adsorption Mechanisms on Amidoxime-Modified Multiwalled Carbon Nanotubes Using Statistical Physics

Effective management of uranium levels in soil and water via removal or minimization is imperative for safeguarding both environmental integrity and public health. This study successfully implemented four advanced statistical physics-based adsorption models (monoenergetic monolayer, bienergetic monolayer, trienergetic monolayer and bienergetic bilayer) to accurately describe experimental uranium(VI) uptake onto amidoxime (AO) modified multiwalled carbon nanotubes (AO-MWCNTs) at 298-318 K range. The optimal model, selected through rigorous statistical analysis (R², χ_red², RSS and HYBRID), was deployed to derive essential thermodynamic parameters including entropy, Gibbs free energy and internal energy alongside stereographic metrics. The variations in these relevant factors are carefully inspected in relation to the temperature of adsorption isotherms. Our conducted error quantification procedure identified the single-energy monolayer framework as the most satisfactory and realistic representation of the adsorption data. Moreover, performed numerical analysis demonstrated that the number of uranium bounded by the docking cavity fluctuates in the range of [1.22-0.76] across the temperature range. As the system's thermal energy increased, the number of accessible sites per unit mass of the adsorbent was observed to rise and reached 126,788 for T = 318 K. Finally, thermodynamic analysis revealed that the retention process is endothermic while the negative values of Gibbs free energy confirmed the spontaneous nature of the adsorption process indicating its thermodynamic feasibility. Entropy shows two regimes around a maximum: for C < C₁/₂ (≈12 mg/L), disorder increases sharply; beyond the peak, entropy declines, indicating emerging configurational order. The major outcomes of our investigation provide crucial insights and are expected to significantly contribute to minimizing the environmental and health risks associated with uranium contamination.