Metal—fulvate complexes

A number of processes affect the solubility characteristic of metal-hu-mate and metal-fulvate complexes in soils, as well as in natural waters. A major factor is the extent to which the complex is saturated with metal ions. Other factors affecting solubility include pH, adsorption of the complex to mineral matter (e.g., clay), and biodegradation. Under proper pH conditions, trivalent cations, and to some extent divalent cations, are effective in precipitating humic substances from very dilute solutions monovalent cations are generally effective only at relatively high particle concentrations. 

TABLE 11.2. Conditional Association Constants (log K) for the Formation of Metal-Fulvate Complexes... 

ESR examination of nonchemically isolated fulvic acids showed that Mn2+ was the primary paramagnetic species observable (60, 61). Most likely, the soluble-colloidal fraction we identified in the speciation studies consisted primarily of such complexes. Because the ESR spectral characteristics of Mn in fulvic acid complexes are quite similar to Mn(H20)62+, Alberts et al. (62) suggested that the metal-fulvate interaction was weak. Stronger interaction would be expected to lead to changes in peak shape. This view leaves unexplained the ability of the complexes to survive the isolation procedure s long ultrafiltration steps, because weak interactions are usually associated with reversible complexation. 

The data for the speciation of dissolved metals are given Table I. Major fractions of dissolved Cu, Cd, Zn, Ni and Mn were present as fulvate complexes or free ions. The data reported here are consistent with the earlier studies (27) for the Upper Three Runs Creek, and the Bever Dam Creek. Small fractions of these metals were predicted to be... 

Ion-exchange method and assumed 1 1 metal/fulvate mole ratio in the complex. 

Potentiometric titration and assumed 1 1 metal/fulvate mole ratio in the complex. "Potentiometric and acid-base conductimetric titrations and assumed 1 1 metal/fulvate mole ratio in the complex. 

Materials such as humates, fulvates, and melanins are related to, or contain, peptide or protein molecules or moieties. Kinetic patterns for their interactions with metal ions may be complicated by the probability that more than one complex species will be involved. Thus it has been demonstrated that Ni2+-fulvate solutions contain at least four kinetically distinct species, i.e. Ni2+aq and three complexes, as indicated in reactions of such solutions with for example par (514). Moreover in a metal-exchange reaction the metal ions M and M may complex at some... 

Mixed ligand complexes may also exert a significant influence on metal speciation and concentration in natural waters. Fulvate-phosphate complexes of Cu", Pb", Cd" and Zn" are more stable than the metal-fulvic acid complexes. Incorporation of these species into models in order to estimate free metal ion concentrations gave estimates which were in excellent agreement with concentrations determined experimentally using ion-specific electrodes. However, mixed ligand complex formation with humic and fulvic acids has not received widespread attention, despite these results and their potential importance. 

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