Phosphonoformic acid, oxalic acid, glyoxylic acid, and 10 additional organic compounds that are structurally related to them have been reacted with synthetic MnO2 (birnessite), consisting of 22% MnIII and 78% MnIV, and synthetic MnOOH (manganite), consisting solely of MnIII. Significant concentrations of dissolved MnIII were detected in reactions of phosphonoformic acid with MnOOH, indicating that ligand-assisted dissolution took place. Reaction of phosphonoformic acid with MnO2, and reaction of all other organic reactants with either MnOOH or MnO2, yielded only MnII, indicating that reductive dissolution was predominant. As far as reductive dissolution reactions are concerned, MnO2 yields a range of reactivity that is nearly 20-times greater than that of MnOOH. Oxidation converts phosphonoformic acid into orthophosphate ion, glyoxylic acid into formic acid, pyruvic acid into acetic acid, and 2,3-butanedione into acetic acid. When differences in surface area loading are accounted for, oxalic acid, pyruvic acid, and 2,3-butanedione yield virtually the same dissolution rates for the two (hydr)oxides. At pH 5.0, glyoxylic acid reacts 14-times faster with MnO2 than with MnOOH. MnO2 reacts more slowly than MnOOH by a factor of 1/16th with oxamic acid, 1/20th with lactic acid, and 1/33rd with dimethyl oxalate. Adsorptive, complexant, and reductant properties of the 13 organic reactants are believed responsible for the observed reactivity differences.
ASJC Scopus subject areas
- Geochemistry and Petrology