Effect of moisture content on the desorption of carbon tetrachloride from Hanford silt

Sachin Mervin Saldanha

The manufacture of plutonium at the Hanford Nuclear Reservation site in Richland, WA, resulted in the contamination of the vadose zone with carbon tetrachloride among other contaminants. Carbon tetrachloride, used as a solvent in the recovery of plutonium, was disposed of in unlined trenches and made its way into the subsurface. Although most of the contamination occurred in the late seventies, cleanup using soil vapor extraction (SVE) in the vadose zone began only in 1992. For SVE operations, the efficiency lowered significantly within a few weeks after initial high recoveries and resulted in high operation costs. The latest update on cleanup operations at Hanford states that approximately 78,000 Kg of the carbon tetrachloride has been removed from the vadose zone. In addition 24,000 lbs of carbon tetrachloride have been removed from the groundwater using pump and treat systems. Several independent studies conducted indicate that the prolonged cleanup times are a result of the rate limited intra-particle diffusion of carbon tetrachloride from the vadose zone. This thesis investigates the impact of moisture content on the cleanup of carbon tetrachloride from Hanford silt. Several SVE experiments were conducted in a simulated vadose zone environment comprising a silt lens (10 cm x 20 cm x 10 cm) embedded in sand within a 2.44m x 0.61m x 0.152m stainless steel box. The moisture content of the silt was varied (2%, 7% and 14%), while the quantity of liquid carbon tetrachloride (50 mL in the center of the silt lens) and airflow through the system (309 mL/min) were kept constant. The results do not present conclusive evidence to suggest that the presence of moisture has an impact on the removal of adsorbed carbon tetrachloride. There was no evidence either of any effect of the presence of moisture on the removal of carbon tetrachloride from the pure phase. When the pure phase had been depleted, removal rates were slightly higher at lower moisture contents and lower concentrations of carbon tetrachloride vapors were measured in the exit stream over the same time period. With the understanding that the rate of diffusion through water is slower than diffusion through air, it is likely that the presence of water decreases the rate of diffusion. The slow desorption of carbon tetrachloride and the subsequent lengthy cleanup times may also be a result of the intra-particle adsorption and diffusion.

Effect of moisture content on the desorption of carbon tetrachloride from Hanford silt

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