PURPOSE: To capture uranium efficiently, by adjusting the pH of seawater to a specific value with calcium oxide or calcium hydroxide or both added thereto.

CONSTITUTION: Ca(OH)2 or CaO powder capable of forming Ca(OH)2 by the hydration thereof or a mixture thereof is added to adjust the pH of seawater to 10W11. Uranium complexes in the seawater are converted into hydroxide complexes and (co)precipitated on positively charged Mg(OH)2 to be precipitated in the seawater. The Mg(OH)2 begins precipitating just on addition of the Ca(OH)2, and deposited almost quantitatively up to the addition of a certain amount. The relation between the amount of the added calcium hydroxide and the pH is as shown in the figure. The uranium recovery ratio is low below the added amount of about 150mg/l. However, the recovery ratio is sufficiently high above about 1g/l. Therefore, the uranium content in the total coprecipitated amount is reduced to increase the separating cost in the next step.