Abstract
The thermal desorption of tritium (3H, T) and radiocarbon (14C) from spent activated carbon was investigated and three thermal desorption steps were established: the vaporization of homogeneously condensed molecules, the desorption of molecules physically binding with the carbon surface, and the decomposition of chemisorbed molecules. A model-free kinetic analysis was conducted to establish the optimum condition of vacuum thermal desorption. Physisorbed species, including tritiated water (HTO) and 14CO2, were effectively removed by vacuum thermal desorption. However, a fraction of 14C, which may take the form of carbon molecules in pyrocarbon form during the heating process, was not removed, even at a high temperature of 1000 °C under a vacuum of 0.3–0.5 Pa. Oxidative peeling of the pore surfaces by filling the evacuated pores with pure oxygen via vacuum thermal desorption and heating to 700 °C was found to be effective for reducing the level of 14C to a level below the established free-release criterion (1 Bq/g) when treating spent activated carbon with 14C radioactivity levels of 162 and 128 Bq/g. The reactivation of the spent granular activated carbon (GAC) by vacuum thermal desorption followed by surface oxidation was also confirmed by the slightly enhanced pore volumes when compared to those of virgin spent activated carbon.
Highlights
Activated carbon (AC), a porous carbon material, acts as a sponge for different types of gases.The gases are attracted to the carbon material via Van der Waals forces
Could be established by observing the desorption rates at elevated temperatures. These were the vaporization of homogeneously condensed molecules, the desorption of molecules physically binding with the carbon surface, and the decomposition of chemisorbed molecules
Three steps of a desorption reaction of spent activated carbon contaminated with 14 C and 3 H
Summary
Activated carbon (AC), a porous carbon material, acts as a sponge for different types of gases.The gases are attracted to the carbon material via Van der Waals forces. Activated carbon (AC), a porous carbon material, acts as a sponge for different types of gases. AC beds are commonly used to adsorb radioactive noble gases such as 86 Kr and 134 Xe as well as radioiodine sources such as 131 I and 133 I from gaseous effluents [2,3]. AC beds retain those short-half-life radioactive gases while they rapidly decay to nonradioactive species. Spent AC are generated as a type of radioactive waste to be disposed of in a radioactive waste repository. Adsorbed radioactive noble gases and radioiodine decay to nonradioactive elements, spent AC retains radioactive species with a long half-life, such as 3H (tritium, T) and 14 C (radiocarbon), and these must be disposed of in a radioactive waste repository
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