I062 M. Kosmulski: Co-Adsorotion of Mono- and Multivalent Ions on Silica and Alumina zyxw Co-Adsorption of Mono- and Multivalent Ions on Silica and Alumina Marek Kosmulski Institute of Catalysis and Physical Chemistry of Interfaces, Laboratory of Adsorption and Physical Chemistry of Interfaces, Polish Academy of Sciences, PI. M. C. Sklodowskiej 3, 20031 Lublin, Poland zyxwv Key Words: Adsorption zyxwvut / Oxides / Surfaces / Thermodynamics Adsorption of calcium and europium from 0.001 - 1 M solutions of different 1 : 1 electrolytes on silica and alumina was studied as a function of pH, surface charge density go, ionic strength and temperature. Adsorption of multivalent cations on a negatively charged surface is competitive with respect to monovalent cations, i.e., the increase of the ionic strength leads to a decrease of multivalent ions adsorption. In contrast, adsorption of multivalent cations on a positively charged surface is insensitive to the concentration of monovalent cations or even increases with the ionic strength. 1. Introduction The electric properties of an oxide surface in contact with an aqueous solution depend on the nature and concentra- tion of the ions in the solution. The H+/OH- ions have a special meaning and they are often called potential deter- mining ions. In contrast, many 1-1 electrolytes like alkali perchlorates, nitrates and most often also halides exert a very weak effect, namely, the position of point of zero charge P.Z.C. and isoelectric point i.e.p. of a given oxide does not depend of their concentration. Therefore they are called indifferent or inert electrolytes. The distinction be- tween potential determining and inert ions follows from their different adsorption properties. It can be shown by a potentiometric titration that beyond p. z. c., the equilibrium concentrations of H’ and OH- ions in contact with an ox- ide may differ from the initial concentrations by an order of magnitude and more. On the other hand adsorption of the anion or the cation of an inert electrolyte is low and specially designed experimental conditions are necessary to measure depletion of the solution. This topic was discussed in detail earlier [l]. Positive adsorption of counterions and negative adsorption of co-ions from the inert electrolytes is often observed. Therefore, the electrostatic attraction/ repulsion is probably the major driving force. Most multivalent ions show completely different adsorption pro- perties towards oxides. Their presence leads to a shift of i. e. p. and p. z. c. and their adsorption at low initial concen- tration increases from 0 to practically 100% adsorption within a range of 2 ~ 3 pH units. Sometimes they adsorb on the surfaces of like sign, i.e. against the electrostatic repul- sion. Therefore, some chemical interaction between the sur- face and the ion is probably involved and this process is called specific adsorption. Although specific adsorption is typical for multivalent ions, the valency is certainly not a crucial factor. For example, silver(1) shows similar adsorp- tion properties towards silica as cadmium(I1) [2]. Apparently, the distinction between inert and specific electrolytes is obvious and most surface chemists “feel” the difference. Various aspects of specific adsorption of ions have been pointed in the literature. Corey [3] has discussed surface precipitation as a possible mechanism of the loss of multivalent metal cations from the solution to conclude that usually it is difficult to distinguish between adsorption and formation of a new three-dimensional phase (surface pre- cipitation) unless the experiment is specially designed for this purpose. In other words many cases reported in the literature as specific “adsorption” may be due to surface precipitation. Fokking et al. have discussed specific adsorp- tion in terms of the proton stoichiometry factor r [4], i.e., the number of OH- ions co-adsorbed (or protons desorb- ed) per one adsorbing cation. For anion adsorption, r is defined as a number of protons co-adsorbed per one adsor- bing anion, respectively. The factor zyxw r usually ranges from 1 to 2 and it depends on the pH for a given metal ion-oxide adsorbent combination. It is possible that the ratio of 1 cor- responds to a formation of surface compounds like =SOCa+ while the ratio of 2 represents bidental com- plexes, e.g., - ’O>Ca and fractional values are obtained with a mixture of mono- and bidental complexes. With nonspecific adsorption, r does not exceed 1. Kosmulski [5] has noticed that at constant cro adsorption of the ions of inert electrolytes on a given oxide does not de- pend on the temperature while specific adsorption of ca- tions increases with the temperature. Thus, specific adsorp- tion of the cations is endothermic, probably due to dehydra- tion of the ions and/or the surface. Gray and MatijeviC [6] emphasize the role of hydrolysis of the ions in the solution in the adsorption behavior of multivalent ions and the effect exerted by the adsorbing ions on the surface properties of the solid. On the other hand, a term “surface-induced hydrolysis” is often used [7] suggesting that pre-hydrolysis of adsorbing ions in the solution is of secondary impor- tance. In some of the quoted papers [4, 71 and in many other publications, the standard notation used for adsorption of ions from inert electrolytes on oxides [S] is also applied for specific adsorption, namely a formation of surface com- plexes like = SO - Na or = SO - Ca+ is proposed. Such a treatment suggests that the difference between specific and nonspecific adsorption in only quantitative, namely, there is a difference in complexation constants for particular ions. If this is true, adsorption of mono- and multivalent ions of the same sign should show a competitive character, i.e. the increase of the ionic strength should lead to a de- - - - = so L~CT zyxwvutsrqponmlkjihgfedc Bunsenges. Phm Chern. 98, 1062-1067 (1994) No. 8 8 VCH Verlugsgesellschuft mbH, 0-69451 zyxwvu Weinheim, 1994 0005-9021/94/0808-1062 $ 10.00+ ,2510