Preferential ion adsorption in mixed electrolytes plays a crucial role in many practical applications, such as ion sensing and separation and in colloid science. Using all-atom molecular dynamics simulations of aqueous NaCl, CaCl2, and NaCl-CaCl2 solutions confined by charged amorphous silica, we show that Na+ ions can adsorb preferentially over Ca2+ ions, depending on the surface structure. We propose that this occurs when the local surface structure sterically hinders the first hydration shell of the Ca2+ ion. Introducing a protrusion metric as a function of protrusion of deprotonated silanols, ion-specificity is successfully predicted on isolated, vicinal, and geminal silanols alike, provided that no other deprotonated silanols are found nearby. Furthermore, we introduce a new strategy to analyze the results as a function of distance from the surface. This approach effectively removes surface roughness effects allowing for direct comparison with classical electric double layer theory and distinction of specifically adsorbed ions and electrostatically adsorbed ions.