Hydrogen peroxide decomposition on manganese oxide (pyrolusite): Kinetics, intermediates, and mechanism
Introduction
There has been increasing interest in finding the physical/chemical characteristics of reaction intermediates from hydrogen peroxide decomposition, because of the high reactivity, which is highly effective for the removal of contaminants (Kwan and Voelker, 2003, Seol and Javandel, 2008). One of the most well-known intermediates, the hydroxyl radical, is generated from hydrogen peroxide decomposition on iron. Known as the Fenton reaction, it has been characterized, and the reaction has been modified (Peyton et al., 1995, Lin and Gurol, 1998, Laat and Gallard, 1999, Lipczynska-Kochany and Kochany, 2008). Moreover, it has been shown that the generation of various reaction intermediates (hydroxyl radical, hydroperoxyl radical, superoxide anion, hydroperoxide anion, oxide radical ion, etc.) is possible (Buxton et al., 1988, Gonzalez and Martire, 1997).
Besides the hydroxyl radical, hydroperoxide and superoxide anion have been investigated for their identification (Pignatello, 1992, Smith et al., 2004). It has been suggested that hydroperoxide anion and superoxide anion including a hydroxyl radical provide a treatment matrix that desorbes, oxidizes, and reduces contaminants (Watts et al., 1999). In particular, hydroperoxide anion and superoxide anion have been proposed as candidates to improve the desorption of organic compounds from soils (Watts et al., 1999). Recently, superoxide anion generated from a modified Fenton system was proven to be a responsible species for carbon tetrachloride transformation (Smith et al., 2004). However, there has been no evidence for the physical role of hydroperoxide/superoxide anion for improving the desorption of contaminants.
In addition to hydrogen peroxide decomposition on iron to generate reactive reagents, hydrogen peroxide decomposition on manganese oxides has been studied to investigate the catalytic activity of manganese oxides (Zhou et al., 1998, Baldi et al., 1998), and the decomposition of hydrogen peroxide on the metals (i.e. both iron and manganese) coated filter showed that hydrogen peroxide decomposition rates mostly depend on manganese concentrations (Miller and Valentine, 1995). Moreover, it has been suggested that hydroperoxide/superoxide anions could be generated from the decomposition of hydrogen peroxide on manganese oxide including metals embedded manganese oxide catalysts (Hasan et al., 1999). However, the detection of hydroperoxide/superoxide anion produced from the reaction of hydrogen peroxide on manganese oxide (pyrolusite) has yet to be reported.
The objective of this study was to find supporting results to evaluate the role of hydroperoxide/superoxide anion for contaminant desorption. To accomplish this objective, hydrogen peroxide decomposition on manganese oxide (pyrolusite) was chosen instead of iron, because it can minimize the effect of the hydroxyl radical (strong oxidant). Hydrogen peroxide decomposition on manganese oxide (pyrolusite) was studied kinetically, and the mechanisms of this decomposition are suggested as being comprised of both the production of oxygen and the involvement of intermediates (hydroperoxide/superoxide anion). To date, the literature has shown little evidence of their existence.
Section snippets
Materials
Double deionized water (DDW) was used with a deionization system (Milli-Q Model Gradient A 10). Manganese oxide (MnO2) from near Pullman, WA was used below the size of 0.088 mm, and was identified to be pyrolusite by XRD analysis (Cu K-alpha1 X-ray radiation, 40 kV beam voltage, and 100 mA beam current). The specific surface area of MnO2 measured by the BET method using nitrogen was 6.063 m2 g−1.
Hydrogen peroxide (35%, Junsei Chemical Co.) was used. Titanium sulfate (Junsei Co.), hydroxylamine
Decomposition of hydrogen peroxide
A study on kinetic decomposition of hydrogen peroxide on manganese oxide at the initial pH 7.0 ± 0.1 was conducted in a batch reactor. The decomposition of hydrogen peroxide can be expressed with the observed first-order rate constants (kobs). The observed first-order rate constants are calculated by non-linear regression using the least-square method. Fig. 1 shows the effect of initial hydrogen peroxide (29.4–441 mM) concentration on the decomposition of hydrogen peroxide in the presence of 7.5 mM
Conclusion
The kinetics of decomposition of hydrogen peroxide on manganese oxide (pyrolusite) at initial pH 7 can be represented by a pseudo first-order rate model. The ratio of [H2O2]/[MnO2] needs to be considered as an important parameter to describe [H2O2] a decomposition rate on manganese oxide. The observed increasing first-order rate constant (kobs) and constant pseudo first-order rate constant () with certain ranges of [H2O2]/[MnO2] (from 11.8 to 39.2) could imply the existence of [H2O2]
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