Sorption of Thorium Onto Subsurface Geomedia

Abstract

Radionuclide contamination has been a significant environmental problem at US Department of Energy (USDOE) sites for many years. In addition, a final disposal solution for radioactive waste, such as deep geological repositories, has also been a topic of investigation in recent years due to the high volume of waste held at surface interim facilities. In this paper, sorption characteristics of Th(IV) onto several geomedia were investigated using batch techniques. Thorium has been used as an analog for other tetravalent actinides such as U(IV) and Pu(IV) since actinides of the same oxidation state exhibit similar chemical behavior. The sorption of Th(IV) has been shown to be strong adsorbate due to its tendency to hydrolyze readily. Multiple batch experiments were performed to evaluate the sorption properties of Th(IV) onto several geomedia in the presence of carbonate. The geomedia used in these studies included two subsurface sediments from the USDOE Savannah River site: a sandy sediment (SRSS) and clayey sediment (SRSC) and two minerals: goethite and kaolinite. These studies showed that sorption of Th(IV) was strong (>95%) for pH 3.5-10 for both SRSS, SRSC, and goethite at a total carbonate concentration of 0.01 M. Th(IV) sorption onto kaolinite was strong for pH 2-5, but sorption decreased from pH 5 to 11. For a total carbonate concentration of 0.1 M, sorption decreased significantly for SRSS from pH 6 to 12 and decreased slightly for SRSC from pH 9 to 11. Precipitation of Th(IV) oxides was also tested in conjunction with these sorption studies. These tests showed that precipitation had a significant role in removal of aqueous Th(IV) from solution at the Th(IV) concentration used. Further investigations indicated the formation of “true” Th(IV) colloids in the absence of geomedia and pseudo-colloids in the presence of geomedia. These studies concluded that as carbonate concentration increases, Th(IV) sorption decreases at near neutral and alkaline pH values. Although sorption of Th(IV) is strong in the same pH range at lower carbonate concentrations, it may be in the form of surface precipitation onto colloids. Thus, the formation of these pseudo-colloids can actually increase the mobility of contaminants.