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Matocha, C. J., D. L. Sparks, J. E. Amonette, and R. K. Kukkadapu. 2001. Kinetics and mechanisms of birnessite reduction by catechol. Soil Sci. Soc. Am. J. 65:58-66.
Kinetics and Mechanism of Birnessite Reduction by Catechol
Christopher J. Matocha,* Donald L. Sparks, James E. Amonette, and Ravi K. Kukkadapu
C.J. Matocha, Dept. of Agronomy, University of Kentucky, N-122 Ag. Sci. Ctr-North, Lexington, KY 40546-0091; D.L. Sparks, Department of Plant and Soil Sciences, University of Delaware, 152 Townsend Hall, Newark, DE 19717; J.E. Amonette and R.K. Kukkadapu, William R.Wiley Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory, Richland, WA, 99352. *Corresponding author (firstname.lastname@example.org).
ABSTRACT 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 (δ -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 d[Mn(II)]/dt = k[CAT]1.0 [SA]1.0 where k=4 (±0.5) (10 -3 L m -2 s -1 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 (Ea) 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.