Effect of temperature on kinetics and adsorption profile of endothermic chemisorption process: -Tm(III)-PAN loaded PUF system

The sorption behavior of 3.18 × 10 6mol1^-1 solution of Tm(III) metal ions onto 7.25mg 1^-1 of 1-(2-pyridylazo)-2-naphthol (PAN) loaded polyurethane foam (PUF) has been investigated at different temperatures i.e. 303 K, 313K, and 323 K. The maximum equilibration time of sorption was 30 minutes from pH 7.5 buffer solution at all temperatures. The various rate parameters of adsorption process have been investigated. The diffusional activation energy (ΔE_ads) and activation entropy (ΔS_ads) of the system were found to be 22.1 ± 2.6 kJ mol^-1 and 52.7 ± 6.2 J mol^-1 K^-1, respectively. The thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) were calculated and interpreted. The positive value of ΔH and negative value of ΔG indicate that sorption is endothermic and spontaneous in nature, respectively. The adsorption isotherms such as Freundlich, Langmuir, and Dubinin-Radushkevich isotherm were tested experimentally at different temperatures. The changes in adsorption isotherm constants were discussed. The binding energy constant (b) of Langmuir isotherm increases with temperature. The differential heat of adsorption (ΔH_diff), entropy of adsorption (ΔS_diff) and adsorption free energy (ΔG_ads) at 313 K were determined and found to be 38 ± 2kJ mol^-1, 249 ± 3J mol^-1 K^-1 and -40.1 ± 1.1 kJ mol^-1, respectively. The stability of sorbed complex and mechanism involved in adsorption process has been discussed using different thermodynamic parameters and sorption free energy.