Kinetics and mechanism of birnessite reduction by catechol

C. J. Matocha, D. L. Sparks, J. E. Amonette, R. K. Kukkadapu

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63 Scopus citations


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]1.0where 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.

Original languageEnglish
Pages (from-to)58-66
Number of pages9
JournalSoil Science Society of America Journal
Issue number1
StatePublished - 2001

ASJC Scopus subject areas

  • Soil Science


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