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SOIL CHEMISTRY

Kinetics of Nonenzymatic Decomposition of Hydrogen Peroxide by Torrifluvents

^a Dep. of Soils and Agricultural Chemistry, Alexandria Univ., Alexandria, Egypt
^b Dep. of Soils and Waters, Tanta Univ.,Egypt

^* Corresponding author (aelprince{at}gmail.com).

Peroxide application has been proposed as a method to control soil aeration. The objectives of this study were to formulate the rate law, identify the inorganic catalyst, identify preferred active centers, and hypothesize a mechanism for the H₂O₂ decomposition reaction in Torrifluvents. The rate of decomposition of H₂O₂(aq) by Torrifluvents (10 locations in the Nile Delta) and synthetic Mn oxides was determined by measuring with a gas burette the volume of O₂(g) given off. Heat pretreatment showed that the catalytic activity of the air-dried soil is essentially nonenzymatic on the time scale of the reaction. Pretreatment of the soil samples by NH₂OH–HCl (pH 2), designed for selectively dissolving MnO₂(s), completely deactivated their catalytic capacity. The pseudo-first-order rate coefficient, k*, for Torrifluvents was expressed by the equation: k* = k_o[MnO₂(s)](10^–pH/4), where k_o is a constant. Contrary to cryptomelane and pyrolusite, values of k* for birnessite satisfied the above equation, indicating that pedogenic birnessite was the active catalyst in Torrifluvents. Diethylenetriamine pentaacetic acid (DTPA) was found to be a more effective inhibitor than ethylenediamine tetraacetic acid (EDTA). A high value of the activation energy (E) was connected with a high value of the pre-exponential factor (A) of the Arrhenius equation. It was suggested that the birnessite surface has two kinds of active centers: Mn^III/Mn^II and Mn^IV/Mn^III. While the former is less numerous (lnA = 37 ± 4) with low activation energy (E = 75 ± 4 kJ mol^–1), the latter is numerous (lnA = 62 ± 3) and of high activation energy (E = 144 ± 17 kJ mol^–1). The Habes and Weiss mechanism explained the experimental results obtained.