As part of a study exploring the pathways and rates of Cr-VI reduction
under environmentally relevant conditions, this paper examines the ki
netics and mechanism of Cr-VI reduction by mandelic acid in the presen
ce of 1.0 g/L TiO2. The redox reaction is dramatically catalyzed by th
e oxide surface, and benzoylformic acid and benzaldehyde are the major
products of mandelic acid oxidation. Comparisons of adsorption and re
action among mandelic acid, methyl mandelate, and atrolactic acid sugg
est that Cr-VI adsorption is required for the reaction to take place,
while the adsorption of organic reductant may be unnecessary. The reac
tion, to our surprise, is zero order with respect to mandelic acid, bu
t fractional order with respect to the ester methyl mandelate. This ch
ange in reaction order indicates that Cr-VI reduction by mandelic acid
and methyl mandelate occurs via different rate-limiting steps.