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DIVISION S-2-SOIL CHEMISTRY

Kinetics and Mechanism of Birnessite Reduction by Catechol

^a Dep. of Agronomy, Univ. of Kentucky, N-122 Ag. Sci. Ctr-North, Lexington, KY 40546-0091
^b Dep. of Plant and Soil Sciences, 147 Townsend Hall, Univ. of Delaware, Newark, DE 19717-1303
^c William R.Wiley Environmental Molecular Sciences Lab. (EMSL), Pacific Northwest National Lab., Richland, WA 99352

Corresponding author (cjmato2{at}pop.uky.edu)

The complex interactions of oxidizable organic ligands with soil Mn(III,IV) (hydr)oxide minerals have received little study by in situ spectroscopic techniques. We used a combination of an in situ electron paramagnetic resonance stopped-flow (EPR-SF) spectroscopic technique and stirred-batch studies to measure the reductive dissolution kinetics of birnessite (-MnO₂), a common Mn mineral in soils, by catechol (1,2-dihydroxybenzene). The reaction was rapid, independent of pH, and essentially complete within seconds under conditions of excess catechol at pH 4 to 6. The overall empirical second-order rate equation describing the reductive dissolution rate was where and [CAT] and [SA] are the initial concentrations in molarity and meters square per liter. In the process, catechol was oxidized to the two-electron o-quinone product. The energy of activation (E_a) for the reaction was 59 (±7) kJ mol^-1 and the activation entropy (S) was -78 ± 22 J mol^-1 K^-1, suggesting that the reaction was surface-chemical controlled and occurs by an associative mechanism. Rates of catechol disappearance from solution with simultaneous Mn(II) and o-quinone production were comparable. These data strongly suggest that precursor surface-complex formation is rate-limiting and that electron transfer is rapid. The rapid reductive dissolution of birnessite by catechol has significant implications for C and Mn cycling in soils and the availability of Mn to plants.