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The primary cooling water system of a nuclear reactor contains Co-60 which requires treatments to avoid adverse effects on society and the environment. Experiment of Co-60 adsorption by using silica xerogel has been done. This experiment was intended to study the isothermal adsorption equilibrium and to determine thermodynamic parameters. Silica xerogel was made from rice husk ash using sol-gel method. The prepared of silica xerogel has a specific surface area of 142.034 m2/g. The adsorption experiments were carried out in a batch wise until it reaches an equilibrium state with variation of the initial concentration and temperature. The results show that Co (II) adsorption by silica xerogel following the Langmuir isotherm than Fruendlich model. Based on the experiment at three different temperatures, the maximum adsorption capacities depend on the temperature there are 25.71; 26.25; and 26.81 mg/g successively at a temperature of 30; 35; and 40 °C. The thermodynamic parameters showed that the cobalt-60 adsorption process by silica xerogel is spontaneous and endothermic with values of ΔH 117.084 kJ / mol and ΔS 35.582 kJ/K/mol. The use of the silica xerogel on the adsorption of reactor cooling water system containing Co-60 showed a decreasing of activity of the waste until 85% from the initial activity. These results indicate that the silica xerogel can be utilized as material to treat radioactive liquid waste containing Co-60.