MnIII,IV (hydr)oxides are believed to be the principal oxidants of CrIII in the subsurface. In nearly all previous
work on this subject, the CrIII reactant was prepared from inorganic salts (e.g. nitrate, chloride, sulfate). In our present
work, CrIII complexes with the synthetic chelating agents iminodiacetic acid (IDA) and nitrilotriacetic acid (NTA) were
reacted with hydrous manganese oxide (HMO) over a wide pH range to examine rates of reaction and product distribution.
Capillary electrophoresis was used to quantify changes in reactant (CrIII–IDA and CrIII–NTA) and product (CrVI, free IDA
and free NTA) concentrations as a function of time. In addition, a small number of experiments were performed using
solutions prepared from CrIII alum (KCr(SO4)212H2O(s)) as the CrIII reactant. CrIII–IDA and CrIII–NTA were oxidised to
CrVI, but rates were considerably lower than those obtained using inorganic CrIII. Within the timescales of our
experiments, complete conversion of CrIII–NTA occurred at pH >7, but not under moderately acidic conditions, even
when there was a large stoichiometric excess of HMO. MnCl2 addition experiments indicated that the observed reaction
inhibition was attributable to MnII generation during the reaction. Our previous work has shown that citric acid, IDA, NTA
and ethylenediaminetetraacetic acid solubilise CrIII from amorphous Cr(OH)3(s) at appreciable rates. The results of this
study show thatHMO is capable of oxidising the resulting soluble CrIII complexes, providing a viable mechanism for CrIII
oxidation to CrVI over a wide pH range.
Environmental Chemistry 12(1). 33-51