Interactions between soil and aniline residues increase with temperature. Exothermic values of free energy change point out the higher affinity of the Soignies soil colloids for p-chloroaniline, in comparison to aniline, with increasing temperature. The magnitude of adsorption of these aniline residues was inversely related to their water solubilities.
The temperature coefficient of the thermodynamic equilibrium constant, expressed per unit of organic matter, emphasizes the greater reactivity of Soignies soil organic matter. The net increase in entropy change, calculated for that substrate, indicates higher stability of these amine-soil colloids complexes. Results of the thermodynamic study suggest that diffusion processes of p-chloroaniline within a porous adsorbent, like Soignies soil organic matter, must play a leading part in the overall energetic process of the adsorption reaction. This view is further supported by values of adsorption rate constant derived from the rates of adsorption of p-chloroaniline by the two soil types.