  We present a study of the elemental depletion in the interstellar medium. We
combined the results of a Galatic model describing the gas physical conditions
during the formation of dense cores with a full-gas-grain chemical model.
During the transition between diffuse and dense medium, the reservoirs of
elements, initially atomic in the gas, are gradually depleted on dust grains
(with a phase of neutralisation for those which are ions). This process becomes
efficient when the density is larger than 100~cm$^{-3}$. If the dense material
goes back into diffuse conditions, these elements are brought back in the
gas-phase because of photo-dissociations of the molecules on the ices followed
by thermal desorption from the grains. Nothing remains on the grains for
densities below 10~cm$^{-3}$ or in the gas-phase in a molecular form. One
exception is chlorine, which is efficiently converted at low density. Our
current gas-grain chemical model is not able to reproduce the depletion of
atoms observed in the diffuse medium except for Cl which gas abundance follows
the observed one in medium with densities smaller than 10~cm$^{-3}$. This is an
indication that crucial processes (involving maybe chemisorption and/or ice
irradiation profoundly modifying the nature of the ices) are missing.
