Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide

Here, Cr­(VI) effects on Tc-immobilization by Fe­(OH)2(s) are investigated while assessing Fe­(OH)2(s) as a potential treatment method for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc­(VII) and Cr­(VI) removal is contingent on reduction to Tc­(IV) and Cr­(III). Furthermore, complete removal of both Cr and Tc depends on the amount of Fe­(OH)2(s) present, where complete Cr and Tc removal requires more Fe­(OH)2(s) (∼200 g/L of simulant), than removing Cr alone (∼50 g/L of simulant). XRD analysis suggests that Fe­(OH)2(s) reaction and transformation in the simulant produces mostly goethite (α-FeOOH), where Fe­(OH)2(s) transformation to goethite rather than magnetite is likely due to the simulant chemistry, which includes high levels of nitrite and other constituents. Once reduced, a fraction of Cr­(III) and Tc­(IV) substitute for octahedral Fe­(III) within the goethite crystal lattice as supported by XPS, XANES, and/or EXAFS results. The remaining Cr­(III) forms oxide and/or hydroxide phases, whereas Tc­(IV) not fully incorporated into goethite persists as either adsorbed or partially incorporated Tc­(IV)-oxide species. As such, to fully incorporate Tc­(IV) into the goethite crystal structure, additional Fe­(OH)2(s) (>200 g/L of simulant) may be required.