Reaction of Mn (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds

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% Mn^III and 78% Mn^IV, and synthetic MnOOH (manganite), consisting solely of Mn^III. Significant concentrations of dissolved Mn^III 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 Mn^II, 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.