PNNL

Abstract

A series of wet chemical extractions and column experiments, combined with electron microprobe analysis (EMPA) and X-ray photoelectron spectroscopy (XPS) measurements, were conducted to estimate the extent of contaminant Cr(VI) desorption and determine the mechanism(s) of Cr(VI) attenuation in contaminated and naturally aged (decades) Hanford sediments which were exposed to dichromate and acidic waste solutions. Results from wet extractions demonstrated that contaminated sediments contained a large fraction of tightly-bound Cr. Results from column experiments showed that effluent Cr(VI) concentrations were low and only a small percentage of the total Cr inventory was released from the contaminated sediments, demonstrating again low Cr mobility. EMPA inspections showed that Cr contamination was spread throughout sediment matrix and high-concentrated Cr spots were not apparently present. XPS analyses showed that most surface Cr was Cr(III). Reduced Cr(III) was spatially associated with Fe, which occurred mostly as Fe(III). Neoprecipitates were most likely Cr(III) oxyhydroxides or [Cr(III) Fe(III)] solid solutions. Extensive reduction to Cr(III) limited dramatically mass flux from this source. The following multiple-step mechanism was most likely operational in the contaminated sediments: acid promoted dissolution of Fe(II)-bearing soil minerals and/or their surface coatings, release of Fe(II) in the aqueous phase, abiotic homogeneous and/or heterogeneous Cr(VI) reduction by aqueous, sorbed and/or structural Fe(II), formation of insoluble Cr(III) phases or [Cr(III) Fe(III)] solid solutions.