PNNL

Abstract

Individual 1 – 3 mm biotite and muscovite clasts from Hanford sediment were contacted with 0.001 – 0.08 M CsNO--3, then either left whole or sectioned perpendicular to their basal planes for examination by electron or X-ray microprobe. Cs+ was observed to preferentially sorb to mica edges, steps on mica surfaces, or to fractured regions. The observed localization conformed to hypothesized strong binding to frayed edge sites in preference to sites on basal planes. In section, Cs+ was found to penetrate the mica interior, forming discrete zones of concentration, particularly in muscovite. In biotite, Cs was more abundant, permeating the clasts at low abundance and forming also discrete zones of higher concentration. Concentrated Cs on both clast edges and within clast interiors corresponded to microscopic but relatively extensive zones where K was depleted. The localization of sorbed Cs in areas where K was depleted suggested that weathering reactions caused the formation of frayed edges sites within the micas. Cs+ accessed crystal interiors by diffusion along channels following crystal defects, cracks, or partings where pore fluids had previously migrated to form the interior alteration zones. At nm scale, areas with localized Cs were disordered, confirming that FES were developed in clast interiors. Cs+ that was adsorbed in this manner could be very resistant to desorption, as has been often observed.