Differential thermal analysis of metal-fulvic acid salts and complexes

Abstract

Salts and complexes of fulvic acid, a water-soluble soil humic fraction, with 14 different mono-, di and tri-valent metal ions, were prepared and characterized by differential thermal analysis (DTA). The major exotherm for untreated fulvic acid, signalizing the decomposition of the fulvic acid “nucleus”, occurred at 450° C. For salts of fulvic acid with monovalent ions, major exotherms occurred between 430° and 450° C, but for most polyvalent metal-fulvic acid complexes the major exotherms appeared at significantly lower temperatures. Ferric iron was especially effective in lowering the temperature of the major fulvic acid exotherm. This effect was not catalytic but due to the formation of a chemical complex between iron and fulvic acid. The thermal stabilities of the salts and complexes were related to the nature of the metal-fulvic acid bonding. For metal-fulvic acid salts the main exotherm temperatures tended to decrease as the size of the metal ion increased. For transition metal-fulvic acid complexes the major exotherm temperatures were inversely proportional to the ionization potential of the metal ion. The DTA method permits differentiation between metal chemically complexed by fulvic acid and metal physically mixed with fulvic acid. At the present time this cannot be done by any other method in such a relatively simple manner. Also, the DTA method may serve as a “fingerprint” for the rapid identification of metal-fulvic acid complexes.