PURPOSE: To offer the titled sealed body applicable for various uses such as easy and economical oxidizing clarification of waste water with high efficiency, by enclosing an activated aluminum plate, prepared by forming a thin amalgam layer on its surface temporarily and then separating the amalgam layer from the surface, in a sealed vessel.

CONSTITUTION: After the surface of an aluminum plate is cleaned, the aluminum plate is dipped in mercury or a mercury salt solution, e.g. mercuric acetate or corrosive sublimate, to form a thin amalgam layer. The treated plate is washed with water in another vessel to separate the amalgam layer from the surface of the plate. Thus, the surface is activated. Water may be removed from the surface by a dehydrating agent such as acetone or ethanol, if necessary. Thereafter, the aluminum plate 1, optionally together with a siccative 8, is enclosed in a sealed vessel 7. On usage, the aluminum plate 1 is located in waste water 10 containing an oxidizing agent. The oxidizing agent is decomposed by the strong reducing power of the plate 1, and the waste water 10 is clarified by the resulting generated oxygen.