Chromium, oxidation-reduction behavior

James, B.R. and R.J. Bartlett. 1983b. Behavior of chromium in soils. VI. Interactions between oxidation-reduction and organic complexation. Jour. Environ. Qual. 12 173-176. 

Surface oxide formation undoubtedly is involved in the Fe(II)-dichromate titration curves, which Smith and Brandt found to be different when the direction of titration was reversed (Figure 15-2, right). Kolthoff and Tanaka found that the rate of oxidation with dichromate was slow, whereas the rate of reduction with Fe(II) was fast. Ross and Shain found the same sort of behavior and noted also that the rates of oxidation and reduction decreased in more dilute solutions. The oxidized surface in a dichromate solution may be largely covered with adsorbed dichromate, as chromium surfaces have been shown to be in some experiments with radio-chromium, so that it is relatively ineffective as an electron-transfer surface for the Fe(III)-Fe(II) system. 

In contrast to Cr(III), the soil redox condition strongly influences sorption of Cr(VI). Under oxidized and moderately reduced (+500 to +100 mV) soil conditions, chromium behavior is dominated by Cr(VI) sorption and reduction of Cr(VI) to Cr(III) (DeLaune et al., 1998). Under more reduced soil redox levels (<+100 mV), chromium chemistry and solubility is controlled by the reduction of Cr(VI) by soluble ferrous iron. 

Chromium (Cr) toxicokinetic is the toxicokinetics of two different oxidation states, Cr(III) and Cr(VI), linked by reduction processes that are ubiquitous in body fluids and tissues. The kinetic behaviors of these two major oxidation states of chromium are very different. Reduction of Cr(VI) to Cr(III) in the body, the lung, and the gastrointestinal tract is sufficiently rapid that bulk chromium kinetics may be considered to be the kinetics of Cr(III). However, certain detectable differences in chromium disposition depend upon whether exposure is to a Cr(III) or a Cr(VI) salt. In addition, the reduction process itself is of interest relative to the carcinogenicity of Cr(VI) in the lung. Therefore, a comprehensive understanding of the toxicokinetics of chromium must include the disposition of both Cr(III) and Cr(VI). 

---