Question

Some engineers recommend addition of the strong oxidant potassium permanganate (KMnO4) for remediating aquifers contaminated with chlorinated solvents such as trichloroethylene (TCE). At many of these sites, trivalent chromium (Cr(III)) is present, which is the less toxic form of Cr. Addition of KMnO4 might oxidize Cr(III) to its more toxic, hexavalent form (Cr(VI)). Based on the data below, answer the following questions. HCrO4 + 4H+ + 3e = Cr(OH)3(s) + H2O Mn04 + 8H+ + 5e = Mn2+ + 4H20 E° = 1.20 V E° = 1.49 V (a) Under standard conditions, can MnO4 oxidize Cr(III)? Explain. (b) If Cr(III) can be oxidized, this reaction will produce Cr(VI) as HCrO4 , which will be dissolved in the groundwater. Is there a potential adverse effect of using KMnO4 as an oxidant in this case? Explain.

Answer 1

The standard potential change of a reaction can be calculated as the difference between the standard potential of the reduction half-reaction and the standard potential of the oxidation half-reaction. In this case, if we want to oxidize Cr(OH)₃, this has to be the oxidation reaction and the permanganate has to be reduction reaction, so we have:

$\Delta E^{o}=E_{red}-E_{ox}=1.49V-1.20V=0.29V$

The relation between this change in potential and the change in Gibbs free energy, which determines if the process is spontaneous, is given by:

$\Delta G^{o}=-nF\Delta E^{o}$

Which means that if ΔE° is positive, ΔG° is negative and the reaction is spontaneous. This means that yes, Cr(OH)₃ can be oxidized with permanganate.

b) If HCrO₄^- is formed, we will be obtaining the most toxic form of Cr. If this filters into groundwaters, the health hazard is enormous, since we will be adding a toxic species to water that can be used for cropping, consumption, etc. So yes, there is a potential adverse effect when using permanganate in aquifers which might contain Cr(III).