The Role of Localized Reducing Zones in Cr(VI)aq Removal by the Hanford Sediments Under Hyperalkaline Conditions

High level waste fluids (HLWF) with high pH and ionic strength and rich in Cr(VI) have leaked from underground single shell storage tanks at the Hanford Site, WA. A recent study of the sediments cored beneath the tanks indicated that Cr(VI) migrated faster than 137Cs but was retarded with respect to 99Tc. Our previous experimental work also showed that CrO4-2 transport was retarded under extreme alkaline conditions. Decreased mobility of CrO4-2 was the result of base catalyzed mineral dissolution, subsequent release of Fe(II) from Fe(II)-bearing soil minerals (biotite and chlorite), and, ultimately, Cr(VI)aq reduction by Fe(II)aq to less mobile Cr(III). If CrO4-2 attenuation occurs via reduction, the presence of localized reducing conditions is required in the inherently oxidized environment of the Hanford vadose zone. These O2-depleted zones may be created when Fe(II) is released during dissolution. Our objective, therefore, was to investigate the role of the localized reducing zones created during mineral dissolution, in the attenuation of CrO4-2. Batch experiments with Hanford sediments and simulated HLWF were conducted at 323 K in the presence or absence of oxygen. Sediments were contacted with 0.192 M Cr(VI), 1M NaNO3 and varying concentrations of NaOH and Al solutions, and the changes in the soil solution composition as a function of time were followed. Results showed that while the presence or absence of O2 had no apparent effect on the extent of dissolution (similar trends of the Si and Fe release were observed in the O2-free and O2-rich experiments), CrO4-2 fate was closely related to the presence of O2 in the system. By the end of experiments (42 days), the initial CrO4-2 was totally removed from the aqueous phase in the experiment where base-induced dissolution occurred under O2-free conditions. In this experiment, the rate of CrO4-2 removal was closely related to the extent of sediment dissolution. In contrast to the O2-free experiment, only a limited amount of CrO4-2 was removed from the aqueous phase in the treatments of the experiment where dissolution occurred in the presence of O2. Appreciable CrO4-2 removal was only observed in the Al-free, 4 M NaOH treatment of the O2-rich experiment. The decreasing trend of CrO4-2 concentration with time observed in this treatment was very similar to the trend observed in its O2-free counterpart. Dissolution of the soil minerals probably mobilized substantial quantities of Fe(II) in this treatment. It is likely that a portion of the Fe(II) released into the soil solution consumed the O2 creating localized zones with predominantly anoxic conditions and excess Fe(II)aq where Cr(VI) reduction may have occurred. In order to better understand quantitatively the behavior of Cr(VI) in the Hanford sediments, a modeling strategy was pursued to first model the relatively simpler batch systems without Cr, followed by batch systems in the presence of Cr. The results from these experiments were modeled using the reaction path option of the computer code FLOTRAN. Because of the high ionic strength of the fluids, the Pitzer model was used to calculate activity coefficient corrections. Preliminary results suggest a reasonably good fit to the experimental data can be obtained for these batch experiments. This work will provide the foundation for modeling more complex column experiments and field-scale contaminant plumes involving Cr(VI).