The Influence of Colloidal Kaolinite on Th(IV) Transport in Saturated Porous Media
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Many laboratory and field studies have indicated that colloid mobilization was the most important potential source of enhancing the transport of a variety of contaminants in groundwater. However, limited research studied the possibility of colloid-associated retardation. Therefore, this thesis is to investigate the influence of colloidal kaolinite on the migration of Th(IV) (an analog for Pu(IV)) in saturated porous media, including both facilitation and retardation effects. Column experiments were conducted under constant conditions of 0.001 M ionic strength, a pH of 4.0, and specific discharge of 0.2 cm/min. When 100 mg/L colloidal kaolinite was pumped through uncontaminated quartz sands, it took approximately one pore volume (PV) to break through, and reached to maximum C/C0 of 0.91 attributed to small and irreversible deposition of colloids onto solid phase by physical-chemical collection between kaolinite colloids and sand particles. The transport of kaolinite was much similar to that of the conservative tracer bromides, indicating high mobility of stable colloids. In contrast, Th(IV) transport showed a significant retardation in colloid-free columns, and the desorption of the actinide was a prolonged, gradual decreasing process. The breakthrough of 1.0 mg/L Th(IV) (20 PVs) required twice as many pore volumes as the 2.65 mg/L Th(IV) transport (10 PVs), and based on mass balance, very close amount of Th(IV) (5mg/kg and 5.7 mg/kg) retained in columns, suggesting that there exists maximum sorption sites on sand surfaces for Th(IV). The transport of 1 mg/L Th(IV) adsorbed onto 100 mg/L kaolinite exhibited no retardation, but 2.65 mg/L Th(IV) co-transporting with colloids was completely retained in the column. Therefore, the conclusion that kaolinite plays an important role in accelerating transport of Th(IV) is effective only under the condition that Th(IV) and colloidal kaolinite formed mobile pseudo-colloids or the amount of actinides was not enough to destabilize colloids. However, once colloids and actinides transport separately, both mobilities were reduced in different extent due to surface complexation reaction. The kaolinite transport in sand media containing 6.5 or 9.3 mg/kg Th(IV) showed more than 10 times retardation compared to its transport on non-Th(IV) conditions. Furthermore, no increase of Th(IV) concentration was observed when kaolinite broke through and increased, indicating that kaolinite failed to stripe Th(IV) off the sand matrix to enhance the contaminant transport. Whereas 1 mg/L and 2.65 mg/L of Th(IV) mobilization triggered a release of about 38% and 45% kaolinite entrapped in pore constrictions due to physical-chemical forces. Moreover, both initial breakthroughs of 1 mg/L and 2.65 mg/L Th(IV) movement were delayed approximately twice in the column containing 302 mg/kg kaolinite with 5 mg/kg Th(IV) and 340 mg/kg kaolinite with 6 mg/kg Th(IV) compared to both transports in the absence of colloids. Hence, kaolinite deposition onto sand media could increase retention of Th(IV) transport attributed to increasing the accessibility of adsorption sites for Th(IV).
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