Acid-base surface chemistry and sorption of some lanthanides on K{sup +}-saturated Marblehead illite: 2. A multisite-surface complexation modeling
The surface reactivity and sorption of Nd and Eu onto K{sup +}-saturated Marblehead illite at 25 C, measured in aqueous 0.01, 0.1, and 1.0 M KCl solutions, were interpreted with a multi-site-surface complexation model. Model potentiometric titration and sorption curves (computed using the Gibbs free energy minimization code, Selektor-A) resolve into reactions on variable-charge amphoteric sites on edge surfaces and on permanent-charge siloxane surfaces ({phi}{sub x}). Standard partial molal Gibbs free energy of formation from elements (g{sub 298}{sup 0}) for surface complexes were derived from oxide surface deprotonation K{sub A1}{sup 0}, K{sub A2}{sup 0} and electrolyte adsorption constants K{sub Cl}{sup 0}, K{sub Na}{sup 0}. Because surface complexation reactions on siloxane basal surfaces are negligible in 1 M KCl, models of surface charge and adsorption edges of Nd and Eu presumed that C{sub 1} is equal to 1.6 Fm{sup {minus}2} for amphoteric site types, and a maximum site density of 1.2 {+-} 0.2 sites nm{sup {minus}2} for the outer-sphere species. To obtain values of g{sub 298}{sup 0} for exchangeable cations and charged X{sup {approximately}}REE complexes, ion exchange sites were assumed to be fully deprotonated in 1.0 M KCl solutions (pH > 2.7). Proton release and REE{sup 3+} uptake on ion exchange sites were then simulated using a nonelectrostatic model and assuming a 50% contribution to the total surface area at {Gamma}{sub max.X} of 3.0 sites nm{sup {minus}2} whereas the contributions of the silanol and aluminol surface types were described using a TLM. At pH < 4.5 and I {le} 0.1 M KCl, frayed edges of interlayer site play a dominant role in controlling surface reactions on ion exchange sites; the contribution to total surface area of frayed edges decay exponentially from initial values of 20 to 48%. The application of Gibbs free energy minimization to sorption processes is innovative in that simultaneous treatment of surface complexation reactions and minerals stability is feasible in any system without introducing mass-balance constraints particular to surface species.
- Research Organization:
- National Academy of Sciences, Kiev (UA)
- OSTI ID:
- 20014545
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 64, Issue 2; Other Information: PBD: Jan 2000; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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