MECHANISMS RESPONSIBLE FOR RETENTION OF MANGANESE IN THE COLLOIDAL FRACTION OF SOIL

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Abstract
Digestion of Mn-saturated clays in dilute MnCl2 solutions produced results which indicated that Mn retention was due to oxidation rather than to reaction with the clay itself. Heat-treatment of the Mn-saturated clays and subsequent x-ray analysis showed that MnO2 had formed either prior to or during the heat treatment. A thermodynamic examination of Mn oxidation revealed that oxidation could occur at the pH values used in the experiments. Conductometric titration of MnCl2 solutions with NaOH in O and N atmospheres confirmed the thermodynamic theory. In addition, it was shown that manganous hydroxide precipitation is not a prerequisite for manganese oxide formation. To account for the observed retention of Mn in soils in absence of O2 and biological activity, the hypothesis was made that Mn can be retained by soil organic matter in the form of a chelate complex. Evidence was presented to support this hypothesis.