Technetium Behavior in Sulfide and Ferrous Iron Solutions

Abstract

ABSTRACTPertechnetate oxyanion (99TcO4−), a potentially mobile species in leachate from a breached radioactive waste repository, was removed from a brine solution by precipitation with sulfide, iron, and ferrous sulfide at environmental pH's. Maghemite (γ-Fe23 ) and goethite (α-FeOOH) were the dominant minerals in the precipitate obtained from the TcO4−)-ferrous iron reaction. The observation of small particle size and poor crystallinity of the minerals formed in the presence of Tc suggested that the Tc was incorporated into the mineral structure after reduction to a lower valence state. Amorphous ferrous sulfide, an initial phase precipitating in the TcOÝ-ferrous iron-sulfide reaction, was transformed to goethite and hematite (α-Fe2O3 ) on aging. The black precipitate obtained from the TcOÝ-sulfide reaction was poorly crystallized technetium sulfide (Tc2S7 ) which was insoluble in both acid and alkaline solution in the absence of strong oxidants. The results suggested that ferrous- and/or sulfide-bearing groundwaters and minerals in host rocks or backfill barriers could reduce the mobility of Tc through the formation of less-soluble Tc-bearing iron and/or sulfide minerals.