Reductant systems, chromium toxicity

In general, chromium(VI) compounds are more toxic than chromium(III) compounds. The toxicity of hexavalent chromium is in part due to the generation of free radicals formed during reduction to chromium(III) in biological systems. 

While hexavalent chromium is reduced to its trivalent form in treatment systems mainly so that the metal can be precipitated, this also lowers its toxicity by a factor of 1000. Ferrous sulfate can be used for this reduction, but is not popular due to its inefficiency, high sludge generation rate, and expense. Sulfur dioxide gas is a far more economical reducing agent, although it is efficient only at low pH, preferably below 2. There can also be problems with atmospheric emission of S02 in this process. 

A study examining the chromium(VI) reduction in microsomes noted that the level of iron in the test system markedly influenced the Vmax of chromium(VI) reduction, suggesting that coexposure to chromium(VI) and agents that increase intracellular iron might lead to increased risk for chromium(VI) toxicity (Myers and Myers 1998). 

Pandey AK, Pandey SD, Misra V and Srimal AK (2003) Removal of chromium and reduction of toxicity to Microtox system from tannery ffluent by the use of calcium alginate beads containing humic add. Chemosphere 51 329-333. 

The carcinogenicity and mutagenicity of chromium(VI) are well established.The toxicity is usually considered in terms of the uptake/reduction modeP since chromium(VI) readily passes into the cell, via anion channels, and once within the cell it is eventually reduced by cellular components to chromium(III) species. Figure 6 illustrates the likely fate of chromate within a mammalian cell. As can be appreciated from Figure 6 it is complexes trapped within the cell that are the agents responsible for the toxic effects of chromate. The systems which reduce the chromiumfvi) are as yet unknown, as are the final products of the reaction. However, microsomes are capable of reducing chromium(VI) as are various nucleotides and even fulvic acids.In these cases chromium(V) species of considerable stability have been observed using EPR spectroscopy. Within the cell reduction by a sulfide is the most probable reaction. 

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. 

There are various oxidation states of chromium ion ranging from -2 to +6, but only the trivalent and hexavalent states are the most stable under most natural environments and are more prevalent in aqueous phases. These two stable states, Cr[lII) and Cr(Vl), exhibit very different toxicides and mobilities. It is well known that chromium (III) is relatively insoluble in aqueous systems and exhibits a little toxicity and mobility [74-76]. On the contrary, chromium (VI) occurs as a highly soluble and more toxic species [31, 32]. Hence, the more conventional methods for removing the Cr (VI) from wastewater were based on the chemical reduction of Cr (VI) present using suitable chemical reducing agents. Here, the cited safe, nontoxic, and biodegradable polysaccharides may be used efficiently as reductants for this purpose. 

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