Oxidant systems, chromium toxicity

A chronic oral RfD of 1.5 mg chromium(III)/kg/day has been derived and verified by EPA for insoluble salts of chromium(III) (e.g., chromium oxide and chromium sulfate) (IRIS 2000a). The RfD is based on a NOAEL for systemic effects in rats fed 1,800 mg chromium(III)/kg/day for 5 days/week for 600 feedings (840 total days) in the study by Ivankovic and Preussmann (1975). EPA has determined that the data are inadequate for the development of an RfC for chromium(III) due to the lack of relevant toxicity study addressing the respiratory effects of chromium(III) (IRIS 2000a). 

Chromium (III) and Pu (III/IV) cations are sorbed to soil constituents and, thus, immobile in most aqueous and soil environments. On the other hand, Cr(VI) and Pu(VI) are quite mobile in soils and aqueous systems, because they are not sorbed by soil components to any extent. Therefore, in the hexavalent form, these elements are readily bioavailable (Amacher and Baker, 1982) and are of concern in food chain contamination. Chromium(III) and Pu(in/IV) can be oxidized to Cr(VI) and Pu(VI) by Mn(III/IV) oxides (Cleveland, 1970 Amacher and Baker, 1982). Manganese oxides can, thus, enhance the mobility and toxicity of Cr and Pu in soil and associated environments. 

The presence of Cr(VI) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. They are also the only forms that undergo valence changes in the Eh-pH ranges encountered in natural systems. The solubility and potential toxicity of the chromium that enters into wetlands and aquatic systems is governed, to a large extent, by oxidation-reduction reactions. Cr(Vl) reduces to Cr(lll) at approximately +300 mV (Figure 12.2). 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

The use of chromium(VI) reagents as oxidants is limited by their inherent toxicity, the need to prepare them in various complex forms (with acetic acid or pyridine), and complicated work-up procedures. Chromium trioxide (Cr03) immobilized on pre-moistened alumina affords efficient oxidation of benzyl alcohols to carbonyl compounds by simple mixing. Remarkably, neither the over-oxidation to carboxylic acids nor the usual formation of tar, a typical occurrence in many CrOj oxidations, is observed [108]. The reagent system is also used for the preparation of acyclic a-nitro ketones by the oxidation of nitroalkanols under solvent-free conditions [109]... 

Change in the oxidation state of an element can have a profound effect on bioavailability and toxicity. Fe(II) is biologically the main usable form of iron, Fe(III) in excess can cause stomach lesions.When chromium appears as a pollutant in a sample of river water or in a Cr complex dye, its valency is of relevance in ascertaining the relative toxicity of the sample. Trivalent chromium is an essential element in mammalian systems, hexavalent chromium is considered to be a moderate to severe industrial hazard. 

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