Chemical Factors Influencing Colloid Mobilization and Th(IV) Transport through Saturated Subsurface Sediment
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In recent years, subsurface colloid mobilization has attracted considerable attention due to its suspected role in enhancing the transport of strongly adsorbing contaminants. Such colloid-facilitated contaminant transport can be induced by changes in groundwater solution chemistry, for example decreases in ionic strength and increases in pH. In this study, colloid mobilization and its effect on the transport of Th(IV), an analog for Pu(IV), were investigated after introducing changes in solution chemistry to saturated columns packed with a heterogeneous, Fe-containing subsurface sand collected from the Savannah River Site (SRS). Transport experiments were performed by sorption/mobilization experiments, where Th(IV) was first sorbed to the SRS sediment under conditions minimizing colloid detachment (e.g. high ionic strength) and then mobilized by conditions releasing natural colloids as a result of decreased ionic strength or elevated pH. This method simulated realistic scenarios at DOE waste sites, where groundwater plumes of radioactive waste containing high concentrations of electrolytes have been replaced by infiltrating dilute rainwater from the surface. After Th(IV) was sorbed to the SRS sediment at a pH of 4, decreases in solution salinity promoted the mobilization of natural colloids from the surface of the SRS sediment, along with significantly enhancing the transport of Th(IV). However, contrary to the original hypothesis, colloid-facilitated transport played only a minor role in enhancing the transport of Th(IV). Instead, the enhanced transport of Th(IV) was primarily attributed to a decreased pH front accompanying the change in solution salinity, causing higher concentrations of Th(IV) to desorb from the SRS sediment. Results from this study also showed that the sorption of Th(IV) had a significant impact on the surface charge of the SRS sediment, affecting the release of colloids. In the absence of Th(IV), colloids were released from the SRS sediment only after increases in influent solution pH, while changes in solution salinity at the lower pH range were ineffective at releasing colloids. In contrast, when Th(IV) was sorbed to the SRS sediment, changes in solution salinity at the lower pH range liberated significant concentrations of colloids.