Size, charge, ionization, ...

Size, charge, ionization, bonded pair, temperature.  Smaller molecules (sodium vs calcium) will be better rejected.  Divalent Ions are typically better rejected than similar sized monovalent ions. The ionization/dissocation of the molecule also has an important effect. Depending on the pH and temperature a large percentage of some molecules (ammonia, boron) will be in either a gas state or acid state) that is poorly rejected (gases are not rejected at all). Finally, bonded pair. Charge balance must be maintained across the membrane. A solution of Sodium Sulfate will be better removed than a solution of Calcium Chloride as the Sulfate is larger and better rejected and the sodium will be rejected to maintain the charge balance. The higher the temperature the more porus the membrane and the lower the feed viscosity both of which make it easier for salts to pass through the membrane.