Solid nanocomposite material comprising nanoparticles of a hexacyanometallate or octacyanometallate of an alkali metal and of a transition metal, of formula [Alk+x]Mn+[M′(CN)m]z− in which Alk is an alkali metal, x is 1 or 2, M is a transition metal, n is 2 or 3, M′ is a transition metal, m is 6 or 8, z is 3 or 4, attached to at least one surface of a porous inorganic solid support, in which the nanoparticles are attached by adsorption to the at least one surface of the solid support, and in which the surface is a basic surface. Method for preparing this material. Method for extracting at least one metal cation from a liquid medium containing it, wherein the liquid medium is brought into contact with the material.