Lead(II); Adsorption enthalpy; Natural coal; Calorimetry

Immobilization of lead(II) ions from aqueous solutions on natural coal was investigated to compare calculated and measured adsorption enthalpies. For the purpose, adsorption isotherms were measured at temperatures 30, 60 and 80°C. Adsorption enthalpy ?H was evaluated from temperature dependence of the equilibrium constant of adsorption using the van´t Hoff equation. Thus, the value of ?H = 27 kJ mol-1 was obtained manifesting endothermic effect of lead(II) immobilisation on the coal. However, based on the flow and immersion calorimetric measurements, the exothermic character of lead(II) adsorption on the studied coal was proven with a value of about -7 kJ mol-1. The reason for the evident discrepancy is not clear.