METAL-BINDING ORGANIC MACROMOLECULES IN SOIL

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Abstract
An experimental technique was devised to measure the maximum binding ability (MBA) of any macromolecule having electron donor groups. It is a dialysis technique wherein polymer portions are isolated in dialysis bags and equilibrated with metal-ion solutions of increasing concentrations. The excess of free metal ions is removed by repeated changes of distilled water. The device proposed allows one to run 12 or more samples simultaneously. By this procedure the amount of metal ions attached to the macromolecuels as a function of the metal-ion concentration of the equilibrium solution is measured. Plotting the amount of metal bound to 1 mg of the polymer vs. free-metal concentration at equilibrium gives Langmuir isotherm-like curves. To calculate MBA the linear Langmuir plot is applied. The limitation and applicability of the Langmuir isotherm is discussed. The Ca(II), Mg(II), Cu(II) and Zn(II) MBA of microbial humic acid-like polymers synthesized by Stachybotrys atra, Hendersonula toruloidea and Eurotium echinulatum, model phenolic polymers synthesized by autoxidative and enzymatic reactions, extracted soil and peat humic acids, Hansenula holstii polysaccharide and karaya gum were determined. For the majority of the polymers the MBA sequence with the metal ions tested was: Cu > Ca > Mg > Zn. This conforms with Ahrland''s concepts of class A and B electron-acceptors and Pearson''s concepts of hard and soft acids and bases considering that the majority of the active sites on the polymers contain O as donor atom (COOH and phenolic OH) and the minority contain N, S and probably P as electron donors. With all metals the sequence of MBA for all microbial polymers was: E. echinolatum > H. toruloidea > S. atra > H. Holstii polysaccharide. The MBA of the model phenolic polymers were similar to the fungus humic acid-type polymers. The MBA of Cu for all the humic acid-type polymers varied from 0.29-2.26 atom-micrograms per mg of polymer. The microbial ecology of soil may markedly influence the MBA characteristics of the naturally occurring SOM and may strongly influence the metal translocation in soils and the availability of essential elements to biological systems.

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