Chromium Chemistry At Soil Oxidation-Reduction Interfaces Defined By Iron and Manganese Oxides.

Soil interfaces, including those defined by horizons, are a largely unstudied area in soils research, especially in terms of oxidation-reduction properties. Despite this lack of research, understanding these interfaces is essential to the complete understanding of chromium remediation practices. Chromium is most commonly found in its trivalent (Cr(III)) and hexavalent (Cr(VI)) forms. Trivalent chromium is non-toxic and largely insoluble, rendering it immobile in soils (Kozuh et al., 1999), while Cr(VI) is highly toxic and soluble, making it an element of concern (Bartlett, 1991). In soils, both Cr(III) and Cr(VI) can undergo oxidation-reduction processes and can exist as soluble ions in the soil solution, sorbed ions on the soil surface, soluble organic complexes, or precipitated compounds (James and Bartlett, 1983, 1983b). Quantitatively determining each of these fractions can isolate key interfacial properties (Bartlett, 1991).      

This study attempted to address the question of how oxidation/reduction of chromium changes throughout mineralogically different soil horizons using miscible displacement column experiments. The A and B horizons of sampled soil materials encompassing a wide range of soil properties, were used in a series of leaching experiments which were set up as follows. Using a mechanical vacuum extractor, trivalent and hexavalent chromium solutions were leached through A horizons, B horizons, and A horizons stacked on B horizons; the A horizon leachate was also leached through the B horizons in an attempt to dissect the mechanisms of interfacial oxidation-reduction processes. Soluble and sorbed Cr(VI) were measured using the diphenyl carbazide (DPC) spectrophotometric method (Bartlett and James, 1979) and total soluble chromium was measured using flame atomic absorption spectroscopy (FAAS) (Sahuquillo et al., 1995) to illustrate the speciation of chromium. The results of the study indicate that there are significant interfacial properties affecting oxidation and reduction of chromium in soils.

References:

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Bartlett R., James B. (1979) Behavior of chromium in soils .3. Oxidation. Journal of Environmental Quality 8:31-35.

James B.R., Bartlett R.J. (1983) Behavior of Chromium in Soils. VI. Interactions Between Oxidation-Reduction and Organic Complexation1. J. Environ. Qual. 12:173-176.

James B.R., Bartlett R.J. (1983b) Behavior of Chromium in Soils: VII. Adsorption and Reduction of Hexavalent Forms1. J. Environ. Qual. 12:177-181.

Kozuh N., Stupar J., Gorenc B. (1999) Reduction and Oxidation Processes of Chromium in Soils. Environmental Science & Technology 34:112-119.

Sahuquillo A., Rubio R., Rauret G., Griepink B. (1995) Determination of total chromium in sediments by FAAS. Fresenius Journal of Analytical Chemistry 352:572-576.