The role of electrolyte anions (ClO4-, NO3-, and Cl-) in divalent metal (M2+) adsorption on oxide and hydroxide surfaces in salt solutions

Adsorption of divalent metal ions (M2+) onto oxide and hydroxide surfaces from solutions of strong electrolytes has typically been inferred to take place without the involvement of the electrolyte anion. Only in situations where M2+ forms a strong enough aqueous complex with the electrolyte anion (for example, CdCl+ or PbCl+) has it been frequently suggested that the metal and the electrolyte anion adsorb simultaneously. A review of experimental data for the adsorption of Cd2+, Pb2+, Co2+, UO22+, Zn2+, Cu2+, Ba2+, Sr2+, and Ca2+ onto quartz, silica, goethite, hydrous ferric oxide, corundum, gamma-alumina, anatase, birnessite, and magnetite, from NaNO3, KNO3, NaCl, and NaClO4 solutions over a wide range of ionic strengths (0.0001 M-1.0 M), reveals that transition and heavy metal adsorption behavior with ionic strength is a function of the type of electrolyte. In NaNO3 solutions, metal adsorption exhibits little or no dependence on the ionic strength of the solution. However, in NaCl solutions, transition and heavy metal adsorption decreases strongly with increasing ionic strength. In NaClO4 solutions, metal adsorption exhibits little dependence on ionic strength but is often suggestive of an increase in metal adsorption with increasing ionic strength. Analysis of selected adsorption edges was carried out using the extended triple-layer model (Sverjensky and Sahai, 1996; Sahai and Sverjensky, 1997a,b; Koretsky, Sverjensky, and Sahai, 1998) and aqueous speciation models that included metal-nitrate, metal-chloride, and metal-hydroxide complexes. In the ex-tended triple-layer model, activity coefficients for all aqueous ions are calculated using the extended Debye-Huckel expression (Helgeson, Kirkham, and Flowers, 1981). Using this approach, transition and heavy metal adsorption is best described by metal adsorption on the 0-plane of the triple-layer model, a result similar to those of previous investigators (for example, Hayes and Leckie, 1987). However, in contrast to previous investigations, we find that transition and heavy metal adsorption onto solids such as goethite, gamma-Al2O3, corundum, and anatase (with dielectric constants epsilon = 10-22), is best described by surface complexes of the metal and the electrolyte anion. Under these circumstances, adsorption of M2+ from NaNO3 solutions is described by >SOH + M2+ + NO3- = >SOHM2+ - NO3- where the nitrate ion adsorbs onto the beta-plane. adsorption of M2+ from NaClO4 solutions is described by >SOH + M2+ + ClO4- = >SOHMClO4+ where both the metal and the perchlorate are on the 0-plane. Similarly, Cd2+ adsorption from NaCl solutions is described by >SOH + M2+ + Cl- = >SOHMCl+ Adsorption of these bivalent metals onto solids Like quartz and silica (with low epsilon = 4-5) may be accompanied by the electrolyte anion in NaClO4 solutions, but in NaNO3 and NaCl solutions metal adsorption occurs as >SOM+ or >SOMOH. Under these circumstances, the low dielectric constants of the solids result in such large Born solvation free energies opposing adsorption of the electrolyte anions that only the perchlorate ion can bind to the surface.