In situ examination of uranium contaminated soil particles by micro-X-ray absorption and micro-fluorescence spectroscopies

Abstract

Two complimentary spectroscopic techniques, X-ray absorption and fluorescence spectroscopy have been conducted at spatial scales of 1 to 25 ~tm on uranium contaminated soil sediments collected from two former nuclear materials processing facilities of the DOE: Fernald, OH and Savannah River Site, SC. A method of imbedding particles in a non-reactive Si polymer was developed such that individual particles could be examined before and after extraction with a wide range of chemicals typically used in sequential extraction techniques and others proposed for ex situ chemical intervention technologies. Using both the micro-X-ray fluorescence (XRF) and micro-X-ray Absorption Near Edge Structure (XANES) techniques, both elemental and oxidation state distribution maps were generated on individual particles before and following chemical extraction. XANES can determine the relative proportion of U(VI) and U(IV) in phases comprising individual particles before and after extraction and showed that greater than 85% of the uranium existed as hexavalent U(VI). Fluorescence spectra of contaminated particles containing mainly U(VI) revealed populations of uranyl hydroxide phases and demonstrated the relative efficacy and specificity of each extraction method. Correlation of XAS and fluorescence data at micron scales provides information of U oxidation state as well as chemical form in heterogeneous samples.

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…Uranyl oxyhydroxide minerals such as compreignacite are formed easily from oxidation of reduced U minerals (compared to uranyl phosphates and silicates) and precipitate readily from saturated solutions (Morris et al, 1996;Hunter and Bertsch, 1998;Kanematsu et al, 2014). Their fast dissolution rates in natural waters, especially in the presence of high dissolved carbonate, compared with uranyl phosphate and silicate phases emphasizes the need for quantification of coupled reaction and transport processes that control the concentration and fate of U in surface and subsurface environments.…”

“…In the absence of dissolved phosphate or silicate, U(VI) has been observed to form a variety of crystalline and amorphous oxyhydroxide mineral phases in oxic subsurface environments (Morris et al, 1996;Hunter and Bertsch, 1998) and in synthetic acidic-tocircumneutral wastewaters (Kanematsu et al, 2014). The first oxidized alteration phase from the weathering of synthetic or natural UO 2 (s) is typically a simple oxyhydroxide, either schoepite ((UO 2 ) 8 O 2 (OH) 12 Á12H 2 O) or metaschoepite ((UO 2 ) 4 O(OH) 6 Á5H 2 O) (Finch and Ewing, 1992).…”

Uranium co-precipitation with iron oxide minerals

“…and 380 nm (HUNTER and BERTSCH, 1998). Samples were mounted on glass coverslips over a 40x objective.…”

“…Schoepite is uranyl-rich and fluoresces strongly and characteristically between 500 to 600 nm at room and low temperature (DUFF et al, 2000;HUNTER and BERTSCH, 1998;MORRIS et al, 1996). However, U(VI)-containing uranate solids fluoresce much more so at low temperature than at room temperature (AZENHA et al, 1992).…”

Determination of trace elements bound to soil and sediment fractions (IUPAC Technical Report)

“…Trace metal sulfides in anoxic sediments have been identified by microbe techniques [168][169]. The mechanism of sorption of trace metals on hydrous Fe/Mn-oxides and calcite have been recently revealed by speciation analysis using XANES and EXAFS [44], XANES [170]. Laser ionization breakdown spectrometry is still a rather "exotic" method having a certain potential to analyze soils directly in the field.…”