Reduction and Simultaneous Removal of 99Tc and Cr by Fe(OH)2(s) Mineral Transformation

Technetium (Tc) remains a priority remediation concern due to persistent challenges, including mobilization due to rapid reoxidation of immobilized Tc, and competing comingled contaminants, e.g., Cr­(VI), that inhibit Tc­(VII) reduction and incorporation into stable mineral phases. Here Fe­(OH)2(s) is investigated as a comprehensive solution for overcoming these challenges, by serving as both the reductant, (Fe­(II)), and the immobilization agent to form Tc-incorporated magnetite (Fe3O4). Trace metal analysis suggests removal of Tc­(VII) and Cr­(VI) from solution occurs simultaneously; however, complete removal and reduction of Cr­(VI) is achieved earlier than the removal/reduction of comingled Tc­(VII). Bulk oxidation state analysis of the final magnetite solid phase by XANES shows that the majority of Tc is Tc­(IV), which is corroborated by XPS measurements. Furthermore, EXAFS results show successful, albeit partial, Tc­(IV) incorporation into magnetite octahedral sites. Cr XPS analysis indicates reduction to Cr­(III) and the formation of a Cr-incorporated spinel, Cr2O3, and Cr­(OH)3 phases. Spinel (modeled as Fe3O4), goethite (α-FeOOH), and feroxyhyte (δ-FeOOH) are detected in all reacted final solid phase samples analyzed by XRD. Incorporation of Tc­(IV) has little effect on the spinel lattice structure. Reaction of Fe­(OH)2(s) in the presence of Cr­(III) results in the formation of a spinel phase that is a solid solution between magnetite (Fe3O4) and chromite (FeCr2O4).