METAL ION-HUMIC SUBSTANCE INTERACTION - A THERMODYNAMIC STUDY

Feat and aquatic humic and fulvic acids have been isolated and charact erized from a site at Whitray Fell in the area of the Forest of Bowlan d, UK. The substances cover a range of molecular masses from approxima tely 2800 (aquatic fulvic acid) to 40500 (peat humic acid). The enthal pies of interaction of these humic substances with a range of metal io ns, including Cu2+, Cd2+, La3+ and Al3+, have been measured by microca lorimetry in aqueous solutions of ionic strengths ranging from ca. 50 mmol dm(-3) to 5 mol dm(-3) at 25 degrees C. Apart from the interactio ns of La3+ ions at low ionic strength, the enthalpies of interaction w ere endothermic, in marked contrast to what would be expected for the complexing of metal ions by chemical groupings known to be present in humic substances such as carboxylate, hydroxy and phenolic groups. By representing the humic molecules by a solvent and ion penetrable spher e surrounded by a diffuse electrical double layer, the thermodynamic p arameters for the discharge of the double layer were estimated. The en thalpy change on discharge of the humic-associated double layer is end othermic and ionic strength dependent and of the same magnitude as the experimental enthalpies of ion interaction with the humic substances. In contrast the enthalpy change on discharge of the humic molecules i s exothermic. These observations suggest that the discharge of the ele ctrical double layers makes a significant contribution to the enthalpy changes on ion interaction with humic substances. However, the major contribution to the Gibbs energy change on discharge of the electrical double layer arises from a large increase in entropy.