Acid-base surface chemistry and sorption of some lanthanides on K+-saturated marblehead illite: I. Results of an experimental investigation

The surface reactivity and sorption of Nd and Eu onto K+-saturated, Marbleh ead illite has been investigated in 0.01, 0.1, and 1.0 M KCl solutions at 2 5 degrees C; the potentiometric titrations were conducted using back-titrat ion techniques. Batch experimental protocols were used in both series of st udies. The ionic strength-dependent, proton surface charge density (sigma(H )) varies from -1500 to -1700 mC/m(2) in 1.0 M KCl solutions and from -1800 to -2200 mC/m(2) in 0.01 and 0.1 M KCl solutions. An isoelectric point was not defined by the sigma(H) vs. pH curves, which reflects the multi-phase nature of natural illitic materials. The functional dependence of REE bindi ng constants (log K-c) on surface coverage (log Gamma(REE)) indicates the e xistence of a multiplicity of energetically distinct surface types. These s urface site types include amphoteric silanol and aluminol sites, basal plan ar surfaces, and "frayed edges"; the frayed edges are observed only in low ionic strength solutions (I less than or equal to 0.1 M KCl). Copyright (C) 2000 Elsevier Science Ltd.