The Chemical Transformations of Chromium in Natural Waters – A Model Study

A hybrid chemical kinetic and equilibrium model has been developed to quantitatively assess the dynamic oxidation-reduction (redox) transformations of chromium in natural waters. Simulations are conducted to examine the effects of 11 chemical and environmental parameters on the redox cycle between trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium. Based on the model results, it is found that natural water serves as a reductive environment for chromium under typical conditions, since the common oxidants of Cr(III) (e.g., hydroxyl radical and manganese) do not convert chromium to Cr(VI) at significant rates because of their extremely low levels or slow oxidation kinetics. At low pH (<5.0), sulfite [S(IV)] is the primary reductant for Cr(VI). At higher pH (>6.0), ferrous iron [Fe(II)] becomes the predominant reductant. However, at pH greater than 8.0 and in the presence of dissolved oxygen (DO), Cr(VI) reduction by Fe(II) is greatly suppressed due to the rapid oxidation of Fe(II) by DO. Other reactive species such as hydroperoxyl radical (HO2•/O2•-) and hydrogen peroxide (H2O2) present in sunlit waters can indirectly affect the chromium redox cycle through their reactions with iron and S(IV). pH appears to be an important parameter, as it affects both chemical speciation and reaction kinetics. Chelating agents in natural waters should not significantly affect chromium redox transformations due to their usually low concentrations and generally weak interactions with Fe(II).