Effects of pH and dissolved oxygen on the reduction of hexavalent chromium by dissolved ferrous iron in poorly buffered aqueous systems.

The effects of pH and dissolved oxygen (DO) on the reduction of Cr(VI) by dissolved Fe(II) were investigated for aqueous solutions having relatively low buffering capacities. All solutions were maintained at a constant ionic strength (generally 0.05 M) and temperature (23 +/- 2 degrees C). For the majority of the experiments conducted, initial concentrations of Fe(II) and Cr(VI) were 50 and 20 microM, respectively, representing a deficient amount of Fe(II) (i.e. nonstoichiometric conditions). Experiments conducted in the absence and presence of DO were performed in an anaerobic chamber and in vessels open to the atmosphere, respectively. Specific initial pH values were obtained by adjusting the pH of Cr(VI) and Fe(II) stock solutions prior to their mixing or by spiking Cr(VI)-Fe(II) systems with strong base to rapidly increase the pH in situ. Consistent with previous reports, Cr(VI) reduction rates for our systems increased with increasing pH (pH ranges of 3.5-6 and 3.5-7.2 for oxic and anoxic experiments, respectively). Because of our poorly buffered experimental systems, pH values decreased over the course of the reactions which, in turn, caused decreases in the reduction rates with time. Spiking some experimental systems with NaOH to rapidly raise the pH resulted in faster rates of Cr(VI) reduction than when the pH was adjusted prior to mixing the stock solutions together; this observation is likely due to the presence of microenvironments in the reactors for which local, short-term pH values greatly exceed the equilibrium value (i.e. mixing is slower than the reduction reaction in these high pH microenvironments). The molar ratios of Fe(II) oxidized to Cr(VI) reduced were close to the expected stoichiometric value of 3 for the majority of our experimental systems, which shows that DO will not cause a serious interference in most applications using Fe(II) to reduce Cr(VI).