Role of manganese dioxide in the oxidation of aqueous sulphur(IV) in oxic and anoxic suspensions

Abstract

The kinetics of direct oxidation of Sulphur(IV) by MnO₂ has been studied in stirred suspensions in nitrogen atmosphere and that of autoxidation reaction in stirred suspensions in equilibrium with atmospheric oxygen. Sulphate has been detected to be the only oxidation product of sulphur(IV) in both the cases. In the <i style="mso-bidi-font-style:normal">p</i>H range 4.6-5.8, the kinetics of direct oxidation of sulphur (IV) in anoxic suspensions agreed with the rate law (I). On the other hand at <i style="mso-bidi-font-style:normal">p</i>H 7.9 for the same reaction a much simpler rate law (II) is obeyed. <BR><BR> <i>-</i>d[S(IV)]<i>/dt </i>= {(A[H⁺]^-1+B)[MnO₂]₀ [S(IV)]₀}/ { 1 +C[S(IV)]₀[H⁺]^-1} ... (I)<br> <i>-</i>d[S(IV)]<i>/dt </i>= {<i style="mso-bidi-font-style:normal">k</i>₃<i style="mso-bidi-font-style:normal">K</i>₃[MnO₂]₀ [S(IV)]₀}/{1 +<i style="mso-bidi-font-style: normal">K</i>₃[S(IV)]₀} ... (II) <BR> At 30°C, the values of empirical constants are: A = 4.8×10^-10 mol g^-1 s^-1; B = 2.8×10^-5 L

g^-1s^-1; C = 1.1 × 10^-3; <i>k</i>₃<i> </i>= 9.3 ×10^-7 s^-1and <i>K</i>₃<i> </i>= 34. For MnO₂ catalysed autoxidation in oxic suspensions the rate data pertaining to <i style="mso-bidi-font-style:normal">p</i>H range 4.6-5.8 fitted Eq. (III) and those <BR>

<i>-</i>d[S(IV)]<i>/dt </i>=<i style="mso-bidi-font-style:normal"> k</i>₃[MnO₂]₀ [S(IV)]₀[H⁺]^-0.34 ... (III) <BR> <i>-</i><span style="font-size:12.0pt;font-family:" times="" new="" roman","serif";mso-bidi-font-style:="" italic"="">d[S(IV)]<i>/dt </i>=<i style="mso-bidi-font-style:normal"> k</i>₈[MnO₂]₀ [S(IV)]₀[H⁺]^-0.4 ... (IV) <br><br>

in the <i style="mso-bidi-font-style:normal">p</i>H range 6.5-7.0 fitted Eq. (IV). At 30°C, <i>k</i>₅<i> </i>and k₈ have values of 1.4 × 10^-4 L^0.66 g^-1 mol^-0.34 s^-1 and 3.2 L^1.4<i> </i>g^-1 mol^-0.4s^-1. The rate of autoxidation is independent of oxygen partial pressure. The atmospheric conversion rates at 30°C have been calculated.