Surface charges and Np(V) sorption on amorphous Al and Fe silicates

Abstract

Sorption onto Si-rich alteration layers of crystalline minerals and nuclear glasses, and onto amorphous secondary silicates of rocks and soils, are expected to retard the migration of actinides in the near- and far-field of high-level waste repositories. In this work, we present experimental and modeling studies on the effects of silicate structure and bulk chemistry, and of solution chemistry, on charges and adsorption of neptunyl ions at surfaces of synthetic, amorphous or poorly ordered silica, Al silicates and Fe silicates. The Al silicates display similar pH-dependent surface charges characterized by predominant Si–O − sites, and similar surface affinities for neptunyl ions, irrespective to their Si/Al molar ratio (varying from 10 to 4.3). Such experimental features are explained by incorporation of Al atoms in tetrahedral position in the silicate lattice, leading to only trace amounts of high-affinity Al–OH surface groups due to octahedral Al. By contrast, the structure of the Fe silicates ensures the occurrence of high-affinity Fe–OH surface groups, whose concentration is shown by proton adsorption measurements to increase with decreasing of the silicate Si/Fe molar ratio (from 10 to 2.3). Nevertheless, experimental data of the adsorption of neptunyl and electrolyte ions show unexpectedly weak effect of the Si/Fe ratio, and suggest predominant Si–OH surface groups. A possible explanation is that aqueous silicate anions, released by dissolution, adsorb at Fe–OH surface groups and/or precipitate as silica gel coatings, because experimental solutions were found at near-equilibrium with respect to amorphous silica. Therefore, the environmental sorption of Np(V) onto Si-rich, amorphous or poorly ordered Al silicates may primarily depend on pH and silicate-specific surface areas, given the low overall chemical affinity of such phases for dissolved metals. By contrast, the sorption of Np(V) on natural, amorphous or poorly ordered Fe silicates may be a complex function of silicate bulk chemistry and solution chemistry, i.e., of pH and aqueous Si concentrations. Simple conceptual models of the surface chemistry of the Al and Fe silicates are developed here, based on the wealth of experimental data of silicate surface charges. The surface complexation models predict reasonably the effect of solution chemistry on the sorption of neptunyl ions on poorly ordered silicates of various compositions, and can thus be useful in extrapolating neptunyl mobility in many geochemical systems.