Toxic species chromium

Data on toxicity of chromium to terrestrial invertebrates are sparse. Studies conducted in India showed that a concentration of 10 to 15 mg/L of CrM in irrigation water, when applied to soils for agricultural purposes, was lethal to two species of earthworms in 58 to 60 days (Soni and Abbasi 1981 Abbasi and Soni 1983). 

Thus the metabolic reduction of chromium(VI) may represent bioactivation and/or detoxification. If a bioactivation process, intracellular reduction of chromium(VI) would lead to the ultimate toxic species. Conversely, if chromium(VI) is the toxic agent, effects would be elicited only if the amount of chromium(VI) entering target cells saturates the reducing mechanisms. 

The California Air Resources Board (ARB) (1989) has developed a list of substances of concern in California, called Status of Toxic Air Contaminant Identification." This list and the organization of substances within it are subject to periodic revision, as needed. The February 1989 Status List groups substances into three categories. Category I includes identified toxic air contaminants asbestos, benzene, cadmium, carbon tetrachloride, chlorinated dioxins and dibenzofurans (15 species), chromium (VI), ethylene dibromide and... 

Chromium is a controversial element with important essentiaUty and toxicity, depending on its different species. Chromium in the oxidation state 111 is an important essential element. In contrast, hexavalent chromium compounds (chromates and dichromates) are toxic and may be responsible for certain allergenic, mutagenic and carcinogenic effects. 

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. 

Further oxidation of an aldehyde product to the corresponding carboxylic acid does not take place. Moreover, the SM>ern oxidation reaction does not require the use of toxic and pollutant chromium reagents. The activated DMSO species, however, are stable only at low temperature, which might in some cases be a drawback of this method. 

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

Whereas it is desirous to utilise analytical schemes that permit elucidation of the various chromium species particularly since CrVI is acknowledged to be a toxic form of this element, it is also useful to have the capability of rapid, total chromium measurement. 

Among warm-blooded organisms, hexavalent chromium was fatal to dogs in 3 months at 100 mg/kg in their food and killed most mammalian experimental animals at injected doses of 1 to 5 mg Cr/kg body weight, but it had no measurable effect on chickens at dietary levels of 100 mg/kg over a 32-day period. Trivalent chromium compounds were generally less toxic than hexavalent chromium compounds, but significant differences may occur in uptake of anionic and cationic CL3 species, and this difference may affect survival. 

Sauter, S., K.S. Buxton, K.J. Macek, and S.R. Petrocelli. 1976. Effects of exposure to heavy metals on selected freshwater fish. Toxicity of copper, cadmium, chromium and lead to eggs and fry of seven fish species. U.S. Environ. Protection Agen. Rep. 600/3-76-105. 75 pp. 

Taylor, D., B.G. Maddock, and G. Mance. 1985. The acute toxicity of nine grey list metals (arsenic, boron, chromium, copper, lead, nickel, tin, vanadium and zinc) to two marine fish species dab (Limanda limanda) and grey mullet (Chelon labrosus). Aquat. Toxicol. 7 135-144. 

One of the questions that arises in soil analysis is whether a determination of the total amount of a component in soil is desired or if just the biologically available amount is more relevant. Related to this is the question of which species of the component is present. In some cases, speciation is of utmost importance. For instance, chromium can be present as Cr(III) or Cr(VI). Cr(VI) is more toxic and thus of greater concern [15], This concern is also related to the biological availability of a specific species. In this case, while knowing the total chromium content (i.e., the sum of Cr(III) and Cr(IV)... 

It is often assumed that if something is in the soil, it will be in plants. This is incorrect. Plants do not take up all of the elements or molecules present in their immediate environment. However, there are some plants, called hyperaccumulators, that accumulate higher than normal levels of some toxic elements. These plants still do not take up all the elements in their environment and they are often small, so the total amount of toxic elements removed from soil is limited. In addition, not all species of an element are toxic and some are not biologically available and thus do not enter biological systems. For example, chromium as Cr(VI) is more toxic and more biologically available than is Cr(IH) [1]. 

Chromium occurs in soils predominantly as the immobile - -3 chromic cation (Cr ), but may be oxidized to or added as +6 chromate species (Cr04, HCr04 ). Chromate is weakly sorbed on soils and is highly toxic to... 

The chemical or physical form of trace metals in water is often of interest. The form in which a specific element is present will often influence is toxic effects. For instance the chemical state of chromium affects its toxicity i.e., Cr+6 is more carcino genic than Cr+3, Kopp (48) has described the various forms in which metals may he present. The categories include dissolved metals, suspended metals, total metals, extractable metals and organometallics. In addition, Kopp describes sample preparation requirements for each category. Gihhs (20) has also studied metal species in river water. It should be obvious that the desired analytical result has to he considered beforehand. For example, if dissolved metal concentrations were desired and normal acid preservation performed, suspended metals could possibly be solubilized to a large extent. Both Hamilton (25) and Robertson (81) have shown vast differences between acidified and non-acidified samples. Many other publications have dealt with this subject (16, 37, 80, 30). 

---