Article pubs.acs.org/est
Decreased Salinity and Actinide Mobility: Colloid-Facilitated Transport or pH Change? Brian Haliena,† Hangping Zheng,† Nathan Melson,† Daniel I. Kaplan,‡ and Mark O. Barnett*,† †
Department of Civil Engineering, Auburn University, Auburn, Alabama 36849, United States Savannah River National Laboratory, Aiken, South Carolina 29803, United States
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S Supporting Information *
ABSTRACT: Colloids have been implicated in influencing the transport of actinides and other adsorbed contaminants in the subsurface, significantly increasing their mobility. Such colloid-facilitated transport can be induced by changes in groundwater chemistry that occur, for example, when high ionic strength contaminant plumes are displaced by infiltrating rainwater. We studied the transport and mobility of Th(IV), as an analogue for Pu(IV) and other tetravalent actinides [An(IV)], in saturated columns packed with a natural heterogeneous subsurface sandy sediment. As expected, decreases in ionic strength both promoted the mobilization of natural colloids and enhanced the transport of previously adsorbed Th(IV). However, colloid-facilitated transport played only a minor role in enhancing the transport of Th(IV). Instead, the enhanced transport of Th(IV) was primarily due to the pH-dependent desorption of Th(IV) caused by the change in ionic strength. In contrast, the adsorption of Th(IV) had a marked impact on the surface charge of the sandy sediment, significantly affecting the mobility of the colloids. In the absence of Th(IV), changes in ionic strength were ineffective at releasing colloids while in the presence of Th(IV), decreases in ionic strength liberated significant concentrations of colloids. Therefore, under the conditions of our experiments which mimicked acidic, high ionic strength groundwater contaminant plumes, Th(IV) had a much greater effect on colloid transport than colloids had on Th(IV) transport.
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facilitated transport has been implicated.6 Colloid-borne actinides can migrate as fast or faster (due to size exclusion effects) than groundwater,6 and the potential for colloidfacilitated transport remains one of the largest uncertainties related to the geological disposal of nuclear waste.1 In groundwater, mobile colloids may be formed due to (1) the release of fine particles from the porous media by chemical, physical, and biological disturbances or (2) the in situ precipitation of supersaturated mineral phases.8 Ryan et al.9 reported that the most common change in groundwater chemistry is ionic strength, which can be decreased by the infiltration of dilute freshwater from natural rain events, irrigation, or artificial groundwater recharge. During these events, groundwater aquifers containing solutions of higher ionic strength are displaced by solutions of lower salinity, potentially inducing the detachment of colloidal fines from sediments. This effect is particularly important at waste sites, where actinides have sometimes been introduced into the subsurface in very high ionic strength solutions (>5 M).10 Over time, these hypersaline solutions can be replaced by infiltrating
INTRODUCTION Actinides have been and are being produced as a result of nuclear weapons and nuclear power production.1 Improper storage, handling, and disposal, as well as nuclear weapons testing, have released large quantities of actinides (and other contaminants) into the subsurface, and some of the worst contamination is found at U.S. Department of Energy (DOE) sites.2 In addition, future permanent geological repositories for spent nuclear fuel will potentially introduce even more actinides into the subsurface.3 Actinides are toxic, radioactive, and longlived, and the study of subsurface actinide transport is extremely important in assessing the existing and future risk of nuclear technologies.4 Although actinides can exist in two or more oxidation states simultaneously,5 Th is always tetravalent in natural waters, Pu is often predominantly tetravalent, even under oxidizing conditions, and both U and Np may be tetravalent under reducing conditions.4,6 Tetravalent actinides [An(IV)] tend to be strongly surface reactive and have very low solubility (2000 μm), 97% sand (2000−53 μm), 2.9% silt (53−2 μm), and 0.2% clay (
