The operation of ion exchange at high ionic concentration has shown high uptake of the co-ion species in addition to the effect of accumulation in pores giving rise to the possibility of exploitation of a selective co-ion exchange for industrial applications. This phenomenon includes some kind of ion exchange which is the subject of this article; in particular, equilibrium and kinetics were studied in view of the application in fixed bed column operation. Anionic exchange (Cl-/SO4(2-)) was noticed to take place during the recovery of potassium from a concentrated industrial wastewater using a cationic resin and a concentrated solution containing sodium ions for regeneration. The equilibrium of co-ions under the operating conditions was analyzed in terms of constant separation factor, and the kinetics by the pore diffusion model. Both models led to good fits for cation exchange. The co-ion retention in fixed bed could also be predicted from previous results using the axial dispersion model.