Manganese(II) oxidation kinetics on metal oxide surfaces

Abstract

The rates of oxidation of Mn(II) by oxygen in the presence of α-FeOOH, γ-FeOOH, silica, and δ-Al₂O₃ were studied experimentally. These solids enhanced the rate of Mn(II) oxidation over the initial homogeneous solution rate, the order being γ-FeOOH > α-FeOOH > SiO₂ > δ-Al₂O₃. The kinetic data can be partially accounted for by a rate expression of the form −d[Mn(II)]/dt = k^∗{〉SOH}[Mn²⁺]/[H⁺]²·A·pO₂, where {〉SOH} is the surface concentration of hydroxyl sites and A is the mass concentration of solids in suspension. The reaction on α-FeOOH and γ-FeOOH is strongly temperature dependent (apparent activation energy ∼100 kJ mole⁻¹) and is not affected by normal laboratory light levels. The oxidation rates on α-FeOOH are lower at higher ionic strengths, and are appreciably influenced by individual ions; Mg²⁺, Ca²⁺, silicate, salicylate, phosphate, chloride, and sulfate inhibit the overall oxidation reaction. A surface complex formation model was used to describe the adsorption of Mn²⁺ to the oxide surfaces. In terms of the surface species (>SO)₂Mn, a proposed rate law which partially accounts for observations is −d[Mn(II)]/dt = k″{(〉SO)₂Mn}·A·pO₂. An alternative formulation which accounts for the observed rates postulates the surface species >SOMnOH.