New Combination of EXAFS Spectroscopy and Density Fractionation for the Speciation of Chromium within an Andosol

• Published in: Environmental science & technology Vol. 40; no. 24; pp. 7602 - 7608
• Main Authors: Dœlsch, Emmanuel, Basile-Dœlsch, Isabelle, Rose, Jérôme, Masion, Armand, Borschneck, Daniel, Hazemann, Jean-Louis, Saint Macary, Hervé, Bottero, Jean-Yves
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.12.2006
• Subjects: Applied sciences; Biological and physicochemical properties of pollutants. Interaction in the soil; Characterization of Natural and Affected Environments; Chromium; Chromium - analysis; Chromium - classification; Continental interfaces, environment; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fractionation; Pollution; Pollution, environment geology; Sciences of the Universe; Soil and sediments pollution; Soil Pollutants - analysis; Soil testing; Spectrum analysis; Spectrum Analysis - methods; Reunion island; Indian Ocean Islands; ISW, Indian Ocean, Mascarene Is., Reunion; Organic matter; Pollutant behavior; Mineralogy; Sequential extraction; Andosols; Densimetry; Soil pollution; EXAFS spectrometry; Heavy metal; X ray absorption spectrometry; Volcanic soils; Chromium; Speciation
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es060906q

Studying speciation of heavy metals instead of their total concentration in a complex matrix such as soil is a scientific challenge that requires a combination of different analytical techniques. We compared the speciation of Cr within an andosol (island of Réunion) by using X-ray absorption spectroscopy (XAS) and sequential extraction. Contradictory results were obtained since the fraction of Cr bound to organic matter was detected only with the latter method. As bulk soil is rather complex, its fractionation by a densimetric method decreased its heterogeneity. We found that 60% Cr was within chromite-type primary minerals. Weathering of these phases led to Fe and Cr release, eventually resulting in either coprecipitation as mixed Fe−Cr oxyhydroxide (16% total Cr) or precipitation of a Cr oxyhydroxide (5% total Cr). Our results also revealed that 13% Cr was bound to organic matter. The organic matter was complexed with mineral phases to form organomineral complexes with a density ranging from 1.9 to 2.6. The use of an original density fractionation-based sample preparation allowed identification of the role of organic matter in chromium speciation within an andosol and to overcome the difficulties of EXAFS to detect light elements in the vicinity of heavy elements.