Kinetics and mechanism of birnessite reduction by catechol

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 (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.