Molecular structure of divalent metal ion fulvic acid complexes

A semi-empirical model of Suwannee River fulvic acid based on experimental evidence has been used as a basis for determining the structural characteristics of divalent metal ion binding. Three low-energy configurations of fulvic acid which contain differing orientations of various functional groups are reported. The structures of 1:1 and 2:1 metal ion-fulvic acid complexes have been determined for Cd2+, Pb2+, Mg2+, and Zn2+ bound to pairs of carboxylate groups in the three low energy configurations of fulvic acid. Standard enthalpies of formation are shown to be useful as indicators of metal ion-fulvic acid complex formation. A major characteristic of the phthalate model of Suwannee River fulvic acid is the locational flexibility of the carboxylic acid groups which accommodate ions of differing ionic radii. (C) 1998 Elsevier Science B.V.