Chromium kinetics

Figure 2-4. A Physiologically Based Model of Chromium Kinetics in the Rat ...

Flaherty EJ. 1996. A physiologically based model of chromium kinetics in the rat. Toxicol Appl Pharmacol 138 54-64. 

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). 

Cr(VI) compounds are corrosive as a result of their acidity and oxidizing potential. Oral, pulmonary, and dermal exposure to Cr(VI) may involve local irritation and corrosive action. Dermal irritation with ulceration can lead to systemic uptake of chromium, but this is an extreme situation. Perforation of the nasal septum, formerly common in industries using chromium, is rarely seen today. Dermal contact can also cause delayed sensitization and allergic dermatitis Cr(VI) appears to be more active than Cr(III) in this regard. These actions of Cr(VI) are local, not systemic, and do not involve any consideration of chromium kinetics. 

Cr(III) that occur in blood plasma, but there are no data at the present time to support or refute this possibility. The kinetics of biologically active Cr(III) would not be expected to have a detectable impact on the kinetics of total chromium at the typical total chromium doses given in experimental animal toxicity studies. However, the essentiality of chromium is an important consideration in the development of a global understanding of chromium kinetics and toxicity. 

The important features of chromium kinetics discussed in this section have been drawn from studies in the rat unless otherwise indicated. Table 1 contains a listing of the most important studies of the kinetics of Cr(III) and Cr(VI) in rats. The results of these studies form the core of the following discussion. 

A qualitative outline of chromium kinetic behavior emerges from these studies. Chromium(VI) is absorbed, distributed, and excreted substantially more readily than Cr(III). At the same time, reduction of Cr(VI) to Cr(III) occurs so rapidly in the lung, the gastrointestinal tract, and the body that to a large extent the kinetics of Cr(VI) have been thought of as the kinetics of Cr(III). This is not precisely correct. The rapidity with which Cr(VI) is reduced, compared to the rapidity of its absorption and excretion processes, controls a sensitive balance that determines overall absorption, distribution, and excretion of chromium as well as the amounts absorbed, distributed, and excreted as Cr(VI). In addition, the nature and rate of the reduction process itself appear to be linked with Cr(VI) pulmonary carcinogenicity. 

Several important data gaps relating to chromium kinetic behavior can be identified. 

Little is known about the importance of bone as a reservoir and continuing source of internal exposure to chromium. The mechanism(s) by which chromium is incorporated into bone, and the dependence of bone chromium uptake on age and physiologic status, are important features of any complete model of chromium kinetics. 

Nearly all of our understanding of chromium kinetics is based on single-dose animal studies. However, humans are generally chronically exposed to chromium, perhaps at varying rates over time. The impact of chronic higher-level as well as lower-level exposure on chromium kinetics should be investigated. 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

The reaction kinetics for the dehydrogenation of ethanol are also weU documented (309—312). The vapor-phase dehydrogenation of ethanol ia the presence of a chromium-activated copper catalyst at 280—340°C produces acetaldehyde ia a yield of 89% and a conversion of 75% per pass (313). Other catalysts used iaclude neodymium oxide and samarium hydroxide (314). 

Fig. 7.10 Kinetics of wustite growth on mild steel and low-chromium alloy steels in air and...

Mukherjee studied the gas phase equilibria and the kinetics of the possible chemical reactions in the pack-chromising of iron by the iodide process. One conclusion was that iodine-etching of the iron preceded chromis-ing also, not unexpectedly, the initial rate of chromising was controlled by transport of chromium iodide. Neiri and Vandenbulcke calculated, for the Al-Ni-Cr-Fe system, the partial pressures of chlorides and mixed chlorides in equilibrium with various alloys and phases, and so developed for pack aluminising a model of gaseous transport, solid-state transport, and equilibria at interfaces. 

DETERMINATION OF CHROMIUM(III) AND IRDN(III) IN A MIXTURE AN EXAMPLE OF KINETIC MASKING 10.66... 

Schlatter et al. found that their data with copper chromite agrees better with 0.7 order for CO concentrations (53). For crystals of nickel oxide and chromium oxide, Yu Yao and Kummer have found that the kinetics depend on CO or hydrocarbon around 0.55 order and depend on oxygen around 0.45 order (79). Hertl and Farrauto found evidence that CO adsorbs on copper as a carbonyl group, and adsorbs on chromium oxide as a unidentate carbonate. They found that the kinetics depends on CO to the first order, and depends on oxygen to the zero order (80). 

The kinetics of NO reduction by hydrogen and CO was studied by Ayen and Peters. Hydrogen reduction of NO over oxides of copper, zinc, and chromium was studied at 375-425°C. The products formed include... 

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