Chromium transport in an acidic waste contaminated subsurface medium: the role of reduction

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 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 relatively large fraction of tightly-bound Cr. Results from column experiments showed that effluent Cr concentrations were low and only a small percentage of the total Cr inventory was removed from the sediments. EMPA inspections indicated that Cr contamination was spread throughout sediment matrix and high-concentrated Cr spots were absent. XPS analyses confirmed that most surface Cr occurred as reduced Cr(III), which was spatially associated with Fe(III). Collectively, the results from macroscopic experiments and microprobe and spectroscopic measurements demonstrated that reduction of Cr(VI) have occurred in these sediments, limiting dramatically the mass flux from this contaminated source. The most likely mechanism of Cr(VI) reduction is the 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), and subsequently, formation of insoluble Cr(III) phases or [Cr(III) Fe(III)] solid solutions. The results from this study will improve our fundamental understanding of Cr(VI) behavior in natural heterogeneous subsurface media and may be used as a basis for developing or selecting potential remedial measures.