ELECTRON TRANSFER AND COORDINATION REACTIONS BETWEEN MnIII,IVO2(BIRNESSITE), MnIIIOOH(MANGANITE), AND OXYGEN-DONOR ALIPHATIC COMPOUNDS
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Organic compounds possess adsorptive, complexant, and reductant properties that determine pathways and rates of reactions with Mn (hydr)oxides. Mn (hydr)oxides are among the most reactive oxidants found in soils and sediments, and often contain a mixture of MnIII and MnIV. Adsorptive properties are responsible for the formation of surface complexes that are the prerequisite of all reactions at (hydr)oxide surfaces. Complexant properties can lead to the detachment of surface-bound MnIII and the formation of MnIII-organic complexes in solution, a process termed ligand-assisted dissolution. Reductant properties make possible the conversion of MnIV and MnIII to much more soluble MnII, a process termed reductive dissolution. Reaction of nearly three dozen structurally-related oxygen-donor aliphatic compounds with two synthetic phases is investigated: MnO2(birnessite), consisting of 22 % MnIII and 78 % MnIV and MnOOH(manganite), consisting solely of MnIII. Using capillary electrophoresis, we are able to monitor the progress of reductive dissolution by analyzing organic oxidation products and the progress of ligand-assisted dissolution by analyzing dissolved MnIII. Reaction of pyrophosphoric acid, methylenediphosphonic acid and phosphonoacetic acid with either MnOOH or MnO2, reaction of 2-phosphono-1,2,4- butanetricarboxylic acid with MnO2, and reaction of phosphonoformic acid and iminodimethylenephosphonic acid with MnOOH, yield dissolved MnIII to a significant extent, indicating that ligand-assisted dissolution takes place. iii Reaction of phosphonoformic acid with MnO2, and reaction of oxalic acid, glyoxylic acid and ten structurally-related compounds with either MnOOH or MnO2, yield only MnII, indicating that reductive dissolution is 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. Reduction of MnO2 by malonic acid, acetoacetic acid, acetylacetone, and structurally-related compounds has been studied. With acetylacetone and related b- diketones, high reactivity generally corresponds to high enol content, which is contributed by the presence of labile a-H atom(s). With malonic acid and related b- dicarboxylic acids, the presence of a-H atom (s) is required for high reactivity. For acetylacetone, acetoacetic acid, and malonic acid, the effect of pH on rates correlates well with the pH dependence of adsorption. Citric acid is oxidized by MnO2 to 3-ketoglutaric acid and acetoacetic acid, yielding two electron equivalents. Citric acid loss, 3-ketoglutaric acid production, acetoacetic acid production, and dissolved MnII production all yield S-shaped curves indicating autocatalysis. Two parallel processes are responsible: the first, relatively slow process involves the oxidation of free citric acid by surface-bound MnIII,IV, yielding MnII and citric acid oxidation products. The second process, which is subject to strong positive feedback, involves concerted reaction of MnII and citric acid with surface-bound MnIII,IV, yielding citric acid oxidation products and two equivalents of MnII.