Reduction of water-soluble colloidal manganese dioxide by thiourea: a kinetic and mechanistic study

Kinetic data for the colloidal MnO2 - thiourea redox system are reported for the first time. The reduction of water-soluble colloidal MnO2 by thiourea (sulfur containing reductant) in aqueous perchloric acid medium has shown that it proceeds in two stages, i.e., a fast stage followed by a relatively slow second stage. The log ( absorbance) versus time plot deviates from linearity. The kinetics of both the stages was investigated spectrophotometrically. The first-order kinetics with respect to [ thiourea] at low concentration shifts to zero-order at higher concentration. The reaction rate increases with [HClO4] and the kinetics reveals complex order dependence in [HClO4]. Addition of P2O74- and F- in the form of Na4P2O7 and NaF, respectively, has inhibitory effect on the reaction rate. The reaction proceeds through the fast adsorption of thiourea on the surface of the colloidal MnO2. A mechanism involving the protonated thiourea as the reactive reductant species is proposed. The observed results are discussed in terms of Michaelis - Menten/Langmuir- Hinshelwood model. From the observed kinetic data, colloidal MnO2 - thiourea adsorption constant (K-ad1) and rate constant (k(1)) were calculated to be 1.25 x 10(10) mol(-1) dm(3) and 3.1 x 10(-4) s(-1), respectively. The variation of temperature does not have any effect on the reaction rate.