We study the extraction of strontium by sodium nonatitanate powder from nitrate strontium and acetate sodium mixture. Experiments show that adsorption is quantitative. The excess Gibbs free energy has been modeled by various models (ideal, 2D Coulomb, regular solution model) for the solid phase. We find that the free energy of the solid phase is controlled by short-range interactions rather than long-ranged Coulombic forces. The selectivity is the consequence of a competition between the liquid and solid phases: both phases prefer strontium rather than sodium but the solid contribution is predominant.