Metal ion–humic substance interaction A thermodynamic study

Abstract

Peat and aquatic humic and fulvic acids have been isolated and characterized from a site at Whitray Fell in the area of the Forest of Bowland, UK. The substances cover a range of molecular masses from approximately 2800 (aquatic fulvic acid) to 40500 (peat humic acid). The enthalpies of interaction of these humic substances with a range of metal ions, including Cu²⁺, Cd²⁺, La³⁺ and Al³⁺, have been measured by microcalorimetry in aqueous solutions of ionic strengths ranging from ca. 50 mmol dm⁻³ to 5 mol dm⁻³ at 25 °C. Apart from the interactions of La³⁺ ions at low ionic strength, the enthalpies of interaction were 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 sphere surrounded by a diffuse electrical double layer, the thermodynamic parameters for the discharge of the double layer were estimated. The enthalpy change on discharge of the humic-associated double layer is endothermic 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 is exothermic. These observations suggest that the discharge of the electrical 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.