Natural organic matter influences the dissolution and stability of reduced technetium(IV) and uranium(IV)

Reductive precipitation and immobilization of soluble technetium (as pertechnetate, Tc(VII)O4-) and uranium (as uranyl, U(VI)O22+) to sparingly soluble Tc(IV) and U(IV) species have been proposed as one of the promising remediation technologies to immobilize uranium and technetium in situ in the subsurface. However, the dissolution and long-term stability of reduced Tc(IV) and U(IV) species are poorly understood, particularly in the presence of natural and synthetic organic ligands, which are known to form complexes with these metals or radionuclides and thus cause their mobilization. In this study, the kinetics of both ligand-promoted and oxidative dissolution of Tc(IV) and U(IV) solids are determined, and their mobility is evaluated in the presence of natural organic matter (e.g.,humic acid and fulvic acid) and synthetic ethylenediaminetetraacetate (EDTA). We found that EDTA and the humic acid are among the most effective in promoting the ligand-induced dissolution of Tc(IV) and U(IV) by complexation. However, EDTA is found to suppress the oxidative dissoltuion of Tc(IV) and U(IV), whereas the humic acid enhances the oxidative dissolution due to its redox reactive functional properties. Furthermore, the oxidative dissolution is found to be much quicker than the ligand-promoted dissolution by humic substances. Studies of the dissolution and stability of reduced U(IV) in a contaminated sediment column confirms that both the synthetic and natural organic ligands can cause the mobilization of U(IV) although the dissolution rate is relatively slow. Because these organic ligands commonly co-exit at comtaminated sites, our results suggest that their presence can potentially impact the long-term stability and mobility of reduced Tc(IV) or U(IV) and should be considered in designing remediation strategies using the reductive precipitation approach.