adsorption enthalpy; calorimetry; lead(II); phenol; natural coal; activated carbon

Comparison among adsorption enthalpies of thermodynamic analysis and these of direct calorimetric measurements is reported for sorption of lead(II) and phenol on natural coals and activated carbon. Thermodynamic calculations came out from developments of equilibrium adsorption isotherms recorded at 303 K, 333 K and 353 K. Standard adsorption enthalpies ?H0 were calculated from temperature dependences of equilibrium constants of adsorption Ka, with values of Ka being evaluated directly from experimental data and from fitted courses of the Langmuir and Freundlich isotherms. In addition, dependences of isosteric enthalpy on surface coverage were evaluated. Values of the exothermic enthalpies (ranging from -6 kJ mol-1 to -50 kJ mol-1) were obtained, however exclusive endothermic effect for lead(II) adsorption on a sample of natural coal (24 kJ mol-1) was also ascertained. Direct calorimetric measurements using flow and immersion techniques then proved that all the studied adsorptions proceed with exothermic effects from -2 kJ mol-1 to -42 kJ mol-1. It was found that experimental adsorption enthalpies correspond better to mean isosteric enthalpies in the interval 0 ? 1 mol kg-1 of the surface coverages than to the standard adsorption enthalpies ?H0 of the thermodynamic calculations.