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Electron Paramagnetic Resonance Stopped-Flow Kinetic Study of Manganese(II) Sorption-Desorption on Birnessite

Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

J. A. Franz and D. M. Camaioni

Pacific Northwest Lab., P.O. Box 999, Richland, WA 99352

*Corresponding author.

ABSTRACT

Many important reactions involving colloidal suspensions are rapid. Here, we introduce the application of a technique capable of rapidly measuring a reactant species (within 20 ms after the reaction initiation) in situ: an electron paramagnetic resonance spectroscopically monitored stopped-flow method (EPR-SF). The utility of this technique is demonstrated by investigating the sorption of Mn²⁺ on -MnO₂. The sorption reaction was complete in <1 s, with >80% of the Mn²⁺ being sorbed within 200 ms. A first-order rate dependence on Mn²⁺ was observed. Measurement of the initial reaction rate allowed the forward (sorption) rate constant to be determined (k_f = 3.74 x 10^–3 s^–1), and the reverse (desorption) rate constant was determined using an integrated reversible first-order rate expression (k_r = 3.08 x 10^–4 s^–1). Using these rate constants, the predicted time dependence of Mn²⁺ sorption was in good agreement with the measured sorption rate. The results indicate that chemical kinetics are being measured that allow determination of precise reaction rates and mechanisms.

Received for publication December 2, 1991.