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The same authors [52] calculated the surface concentration of adsorbed species as rId where r is adsorption density and dis the diameter of adsorbed species. This diameter was assumed to be 0.5 nm for Cd and 0.6 nm for Zn, i.e., on the order of the size of hydrated ion. At X R = 1 the surface concentration exceeds the bulk concentration by a factor of 3.