Method of preparing purifying agents consisting of activated siliceous porous mineral substances

Method of preparing activated siliceous porous mineral substances so as to increase the ability thereof to purify waste waters and the like, which consists essentially of impregnating an adsorptive siliceous porous mineral substance containing water insoluble SiO.sub.2 and a water insoluble metal oxide and being selected from the group consisting of diatomaceous earth and foamed perlite with at least one mineral acid selected from the group consisting of hydrochloric acid and sulphuric acid in an amount sufficient to convert a predetermined quantity of said insoluble metal oxide to the corresponding water soluble salt, baking the thus impregnated porous mineral substance at a temperature of from 100.degree. C to 500.degree.C for a time sufficient to convert said predetermined quantity of said water insoluble metal oxide into the corresponding water soluble salt which remains within the porous structure of said porous mineral substance and there acts as coagulant for impurities in waste water and the like to increase the purifying action of said adsorptive siliceous porous mineral substance while said SiO.sub.2 remains in its water insoluble adsorbent form, and retaining said water soluble metal salt in said porous structure of said activated siliceous porous mineral substance until the same is used for the purification of waste waters and the like.