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

The effects of pH and dissolved oxygen (DO) on the reduction of Cr(VI) by d issolved Fe(II) were investigated for aqueous solutions having relatively l ow buffering capacities. All solutions were maintained at a constant ionic strength (generally 0.05 M) and temperature (23 +/- 2 degreesC). For the ma jority of the experiments conducted, initial concentrations of Fe(II) and C r(VI) were 50 and 20 muM, respectively, representing a deficient amount of Fe(II) (i.e. nonstoichiometric conditions). Experiments conducted in the ab sence and presence of DO were performed in an anaerobic chamber and in vess els open to the atmosphere, respectively. Specific initial pH values were o btained by adjusting the pH of Cr(VI) and Fe(II) stock solutions prior to t heir 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) reductio n rates for our systems increased with increasing pH (pH ranges of 3.5-6 an d 3.5-7.2 for oxic and anoxic experiments, respectively). Because of our po orly 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 r esulted in faster rates of Cr(VI) reduction than when the pH was adjusted p rior to mixing the stock solutions together; this observation is likely due to the presence of microenvironments in the reactors for which local, shor t-term pH values greatly exceed the equilibrium value (i.e, mixing is slowe r than the reduction reaction in these high pH microenvironments). The mola r ratios of Fe(II) oxidized to Cr(VI) reduced were close to the expected st oichiometric value of 3 for the majority of our experimental systems, which shows that DO will not cause a serious interference in most applications u sing Fe(II) to reduce Cr(VI). (C) 2001 Elsevier Science Ltd. All rights res erved.