PNNL

Abstract

Two studies in the early 1980s described the leaching behavior of americium (Am) disposed as part of acidic high-salt processing wastes from the Hanford Site’s Plutonium Finishing Plant to nearby ground sediments. These batch leach experiments showed that the Am concentrations followed a linear log [Am] versus pH relationship with a slope of –1. Column leach experiments in the second study, however, did not follow this relationship and only ~30% of the americium desorbed even after extensive column leaching. Here, the 1980s research is re-examined along with previously unpublished information and, in light of recent published work, a plausible mechanism is proposed to explain these phenomena. Amorphous silica in the contaminated sediments is postulated to be the substrate responsible for both the exchangeable Am available for leaching and the retained low-leachable Am made evident in the column leach experiments. The exchangeable Am3+ in the contaminated sediment leach experiments behaves with pH dependence similar to that observed for uptake onto amorphous silica of sodium (Na+), calcium (Ca2+), barium (Ba2+), cadmium (Cd2+), uranyl (UO22+), ferric (Fe3+), chromic (Cr3+), cupric (Cu2+), plumbous (Pb2+), uranium(IV) (U4+), plutonium(IV) (Pu4+), zirconium (Zr4+), analogue lanthanide (gadolinium, Gd3+, europium, Eu3+, and lutetium, Lu3+) and curium (Cm3+) ions as well as other studies with Am3+. Correspondingly, the residual low-leachable Am3+ revealed in the column leach experiments is attributed to incorporation of Am3+ within amorphous silica by dynamic Am3+ sorption and silica precipitation processes.