METAL-BINDING ORGANIC MACROMOLECULES IN SOIL: 3. COMPETITION OF Mg(II) AND Zn(II) FOR BINDING SITES IN HUMIC AND FULVIC-TYPE MODEL POLYMERS

Binding of Zn(II) and Mg(II) on several humic and fulvic acidlike model polymers was studied using the dialysis technique of Zunino and Martin (1977b). By interpreting the data in terms of maximum binding ability (MBA) and affinity term (k2), some metal-coordination properties of these humiclike macromolecules are discussed. The following are the most significant results:(1) Polymers with both carboxylic and phenolic OH-containing functional groups show higher coordination power for Zn(II) and Mg(II) than those containing only OH groups.(2) Zn(II) is preferred over Mg(II) in the fulvic and humic acid-type polymers with and without nitrogen and sulfur.(3) NH2 and SH groups favor zinc coordination and enhance the afinity between the polymers and the metal.(4) High zinc-polymer affinities are obtained at very low zinc loading, indicating the presence of specific groups or arrangements of groups for Zn(II).(5) In fulvic acid-type polymers without nitrogen and sulfur, there might be special arrangements of groups highly specific for Mg(II).(6) The specificity for Mg(II) of these fulvic acidlike polymers increases Zn(II) retention.The results are pertinent to heavy metal pollution and plant nutrition processes. The large preference for Zn(II) ions suggests that most naturally occuring cations in soils and waters would not successfully compete with polluting heavy metals for electron donor sites on organic macromolecules. Thus in soils, the heavy metals would tend to accumulate in the organic fraction. It was demonstrated that in the metal-model polymer association, the lower the metal content the higher the affinity or the enrgy linking between the metal and the functional groups, which experimentally supports the cascadelike hypothesis proposed by Zunino and Martin to explain the soil organic matter behavior in transition metal translocation and availability to plants in soil.