Acid-base surface chemistry and sorption of some lanthanides on K+-saturated marblehead illite: II. A multisite-surface complexation modeling

The surface reactivity and sorption of Nd and Eu onto K+-saturated Marblehe ad illite at 25 degrees C, measured in aqueous 0.01, 0.1, and 1.0 M KCl sol utions, were interpreted with a multi-site-surface complexation model. Mode l potentiometric titration and sorption curves (computed using the Gibbs fr ee energy minimization code, Selektor-A) resolve into reactions on variable -charge amphoteric sites on edge surfaces and on permanent-charge siloxane surfaces (phi(x)). Standard partial molal Gibbs free energy of formation fr om elements (g(298)(0)) for surface complexes were derived from oxide (SiO2 ,am and gamma-Al2O3) surface deprotonation K-A1(0), K-A2(0), and electrolyt e adsorption constants K-Cl(0), K-Na(0). Because surface complexation react ions on siloxane basal surfaces are negligible in 1 M KCl, models of surfac e charge and adsorption edges of Nd and Eu presumed that C-1 is equal to 1. 6 Fm-2 for amphoteric site types, and a maximum site density of 1.2 +/- 0.2 sites nm(-2) for the outer-sphere species. (Al>OH2+Cl-). To obtain values of g(298)(0) for exchangeable cations and charged (XREE)-R-similar to compl exes, ion exchange sites were assumed to be fully deprotonated in 1.0 M KCl solutions (pH > 2.7). Proton release and REE3+ uptake on ion exchange site s were then simulated (pH < 5 and I less than or equal to 0.1 M KCl) using a nonelectrostatic model and assuming a 50% contribution to the total surfa ce area at Gamma(max,X) of 3.0 sites nm(-2) whereas the contributions of th e silanol (phi(Sil) = 30%) and aluminol (phi(Alu) = 20%) surface types were described using a TLM. At pH < 4.5 and I less than or equal to 0.1 M KCl, "frayed edges" of interlayer site (Y-similar to sites, Gamma(max,Y) = 3.0 s ites nm-2) play a dominant role in controlling surface reactions on ion exc hange sites; the contribution to total surface area of "frayed edges" (phi( Y)) decay exponentially (phi(Y) less than or equal to 0.1% at pH > 4.0) fro m initial values of 20 to 48%. The application of Gibbs free energy minimiz ation to sorption processes is innovative in that simultaneous treatment of surface complexation reactions and minerals stability is feasible in any s ystem without introducing mass-balance constraints particular to surface sp ecies. Copyright (C) 2000 Elsevier Science Ltd.