Zinc stable isotope fractionation during its adsorption on oxides and hydroxides

• Published in: Journal of colloid and interface science 2005-11, Vol.291 (1), p.192-200
• Main Authors: Pokrovsky, O.S., Viers, J., Freydier, R.
• Format: Article
• Language: English
• Subjects: Adsorption; Aluminum; Chemistry; Exact sciences and technology; Fractionation; General and physical chemistry; Hydroxide; Iron; Manganese; Oxide; Stable isotopes; Surface complexation; Surface physical chemistry; Zinc
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2005.04.079

Adsorption of Zn on goethite, hematite, birnessite, pyrolusite, corundum, and gibbsite was studied using a batch adsorption technique as a function of pH, zinc concentration in solution, and time of exposure. Adsorption from 0.01 M NaNO 3 solutions undersaturated with respect to zinc (hydr)oxide at 3 < pH < 8 was found to be reversible and equilibrium was achieved in less than 24 h. A 2p K surface complexation model that assumes the constant capacitance of the electric double layer (CCM) and postulates the formation of positively charged >MeO Zn + complexes, where Me = Fe, Mn, and Al, was used to describe the dependence of adsorption equilibria on aqueous solution composition in a wide range of pH and Zn concentration. The logarithms of surface stability constant for Zn interaction with metal oxy(hydr)oxides (>MeOH 0 +Zn 2+ ⇒ MeO Zn +) vary from −2.5 to 0.5. They are higher for oxy(hydr)oxides than for anhydrous oxides. Stable isotopes of zinc in several filtrates were measured using an ICP-MS Neptune multicollector which made it possible, for the first time, to assess the degree of isotopic fractionation between 66Zn and 64Zn during zinc adsorption on mineral surfaces. The isotopic offset between aqueous solution and mineral surfaces (Δ 66/64Zn soln/solid= δ( 66/64Zn) solution− δ( 66/64Zn) surface) was found to be weakly dependent on percentage of adsorbed metal and equals 0.20 ± 0.03 , 0.17 ± 0.06 , − 0.10 ± 0.03 , − 0.10 ± 0.09 , and − 0.13 ± 0.12 ‰ for goethite, birnessite, pyrolusite, corundum, and Al(OH) 3. For hematite, Δ 66/64Zn varies from − 0.61 ± 0.10 ‰ at pH 5.5 to − 0.02 ± 0.09 ‰ at 5.8 < pH < 6.7. Overall, zinc stable isotopic fractionation induced by adsorption on most mineral surfaces does not exceed 0.2‰. We do not observe any correlation between the sign and magnitude of isotopic offset and the chemical nature of solid phase (hydrous versus anhydrous minerals), zinc surface adsorption constants (surface complexation model of the present work), and coordination and first-neighbor distances of surface >MeO Zn(H 2O) n complexes (available literature data on X-ray absorption spectroscopy). Apparently, the fine structure of surface complexes and the position and bond strength for second neighbors of zinc are likely to control its isotopic fractionation during adsorption on mineral surfaces. Our results strongly suggest that inorganic processes controlling zinc isotope adsorption on soil and sediment minerals should be of second-order importanc