A nanocomposite solid material includes nanoparticles of a metal coordination polymer with CN ligands comprising Mn+ cations, in which M is a transition metal and n is 2 or 3; and anions [M′(CN)m]x− in which M′ is a transition metal, x is 3 or 4, and m is 6 or 8. The Mn+ cations of the coordination polymer are bound through an organometallic bond to an organic group of an organic graft chemically attached inside the pores of a support made of porous glass. The material can be used in a method for fixing (binding) a mineral pollutant, such as radioactive cesium, contained in a solution by bringing the solution in contact with the nanocomposite solid material.