Chemically Modified Pine Bark as a Sustainable and Low-Cost Biosorbent for Efficient Th(IV) Removal from Water: Characterisation, Kinetic, Isotherm, and Thermodynamic Studies

The adsorption behaviour of thorium (Th(IV)) ions on modified pine bark (MPB) was systematically investigated with respect to pH, adsorbent dosage, temperature, and contact time.Kinetic evaluation using the pseudo-second-order (PSO) model indicated that the equilibrium adsorption capacity (q e ) of MPB, measured at pH 3, achieved a maximum uptake of approximately 90 %, with a q e value of 12.97 mg g -1 at 25 °C.Adsorption isotherm analysis was conducted utilising Langmuir, Freundlich, and Dubinin-Radushkevich models.The Langmuir model showed a better correlation (R 2 ) than the Freundlich model, suggesting monolayer adsorption with a degree of surface heterogeneity.Thermodynamic parameters, including enthalpy (∆H°), entropy (∆S°), and Gibbs free energy (∆G°), were calculated at different temperatures.At 25 °C, the thermodynamic values were ∆H° = 83.10kJ mol -1 , ∆G° = -6.58kJ mol -1 , and ∆S° = 301.18J mol -1 K -1 .These results confirm that adsorption was spontaneous and endothermic, with an increase in entropy.Overall, MPB exhibited significant potential as an effective adsorbent for the removal of Th(IV) ions from aqueous solutions.