In Situ Mobilization of Colloids and Transport of Cesium in Hanford Sediments

Abstract

Radioactive waste, accumulated during Pu production, has leaked into the subsurface from underground storage tanks at the U.S. Department of Energy's Hanford site. The leaking solutions contained 137Cs and were of high ionic strength. Such a tank leak was simulated experimentally in steady-state flow experiments with packed Hanford sediments. The initial leak was simulated by a 1 M NaNO3 solution, followed by a decrease of ionic strength to 1 mM NaNO3. Cesium breakthrough curves were determined in both 1 M and 1 mM NaNO3 background. Colloidal particles were mobilized during the change of ionic strength. Mobilized colloids consisted mainly of quartz, mica, illite, kaolinite, and chlorite. Electrophoretic mobilities of colloids in the eluent solution were -3(microm/s)(V/cm) and increased to less negative values during later stages of mobilization. Mobilized colloids carried a fraction of the cesium along. While transport of cesium in 1 M NaNO3 background was much faster than in 1 mM NaNO3, cesium attached to colloids moved almost unretarded through the sediments. Cesium attached to mobilized colloids was likely associated with high affinity sorption sites on micas and illites.

Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media

“…The experimental evidence also suggests that, upon disturbances of ionic strength, in situ colloid mobilization may be an important transport mechanism for contaminant species [31]. If the disturbance of ionic strength occurs in combination with the presence of sorbing contaminants, the mobilized particles may also likely act as vectors for accelerated contaminant movement [41]. Saiers and Hornberger [31] also experimentally found that under conditions of low pore water ionic strength, the kaolinite colloids significantly accelerate 137 Cs transport through column packed with quartz sand.…”

“…A comprehensive amount of literature exists on the topic of colloid mobilization and transport in model and natural systems [7,10,14,22,24,27,41,87,88]. Considerable advances have been made on the prediction of colloidal transport using experimental laboratory and field studies as well as numerical models [87,89,90].…”

“…Besides the well-documented effects of clay mineralogy (e.g. Seta and Karathanasi [34]) and solution ionic strength and pH [27,41] on colloid dispersion, total clay content [94], soil moisture conditions [95], soil management [96] and interactions among these properties have also been shown to affect colloid dispersibility. Actually, interparticle forces such as vander Waals attraction and electrolytic repulsion determine colloidal fines stability.…”

Geological Disposal of Nuclear Waste: Fate and Transport of Radioactive Materials

“…These contaminant-attached soil particles may themselves become mobile and move through the soil and eventually reach the water bodies (fig 2). This is commonly known as colloid-facilitated contaminant transport [9,11,14,16,20,23,41,52,56]. Colloids are a ubiquitous component of subsurface systems and play an important role in radioactive contaminant fate and transport.…”

Colloid Formation at Waste Plume Fronts

“…Subsequent colloid transport might have contributed to the deep migration of contaminants (7,8). Under the DOE's initiative on cleanup of the Hanford Site, recent studies have resulted in a variety of findings relevant to contaminant transport, including identifying the mechanisms for sorption of Cs + in sediments (9,10), the formation of cancrinite as a secondary mineral phase (11,12), the geochemical evolution of waste plumes (13,14), the effect of cation exchange on Cs + sorption (15,16), and the effect of colloids on Cs + transport (17).…”