Chrome and Environment

 Human Health

Chromiumoccurs widely in nature, almost exclusively in the trivalent oxidation stateand most commonly in the mineral chromite.

Other common valency states occuras a result of manufacturing processes:

-    Zero valency for metallic chromium and many chromium-containing alloys,including stainless steels. Note that in these cases the chromium on thesurface is oxidised spontaneously to the trivalent state creating a passivefilm which prevents further oxidation and which is responsible for corrosion resistance. This layer is alsoresponsible for determining the health effects-see FIOH, in press.

-    Hexavalentchromium occurs predominantly in chemical manufacturing processes and to a muchlesser extent in metallurgical processessuch as ferrochromium and stainless steel production, stainless steel welding and in some high temperature furnaceoperations which use chromium-containing refractories

-    Trivalent chromium is recognised as an essential trace element in humanand animal diets and important in glucose metabolism. Most diets areconsidered to be deficient in chromium for which the recommended dailyintake is 200 [ig for adults. For more detailed information, refer to Anderson,1999.

The most significant occupational health effects are related tohexavalent chromium compounds. Exposure to such compounds may result in acuteeffects such as skin irritation, ulceration and sensitisation, nasalirritation, ulceration and nasal septum perforation and respiratorysensitisation. The most serious health effect is respiratory cancer.Epidemiological studies have confirmed that long-term exposure to high levelsof hexavalent chromium as encountered historically in chromate chemicals andchromate pigments manufacture and electrolytic plating processes using chromicacid has led to a measurable excess incidence of respiratory cancer with alatency period in excess of 15 years.

In addition toepidemiology, many studies have been reported in which laboratory animals,mainly rats or mice, have been exposed tovarious chromium species by a variety of techniques. These studies are reviewed comprehensively in the followingpublications: ATSDR,2000; DECOS, 1998; Health and Safety Executive, 1989; IARC, 1990, ICDA, 1997,

The IARCclassification for carcinogenicity of chromium compounds as follows:

Final IARC Evaluations

Degree of evidence                     Overallevaluation

for carcinogenicity

Chromium/ChromiumCompounds                 Human                   Animal

·         Chromium (VI)                                                                                                                                  1

·         Chromium (VI) compoundsas              Sufficient

encountered inthe chromate, chromate pigment production and chromate plating industries:

·      Barium Chromate                                          Inadequate

·         Calcium Chromate                                           Sufficient

·         Chromium Trioxide                                          Limited

·      Lead Chromates                                             Sufficient

·         Sodium Dichromate                                         Limited

·         Strontium Chromate                                        Sufficient

·         Zinc Chromates                                               Sufficient

As with human health, the environmental toxicity of chromium is species dependent. In nature, chromium is bound to other elements forming different compounds, such as chromite. Generally, trivalent chromium compounds are neutral or essential to organisms, but hexavalent compounds are toxic. Chromium is typically a metal which does not accumulate in the terrestrial or marine food chain or animals

Natural processes such as erosion and leaching of minerals actuates chromium to be physically present in soil, water and air. In the trivalent state, chromium has low solubility, and also man made chromium metal, chromium-containing alloys and insoluble trivalent chromium products such as chromic oxide are essentially inert and not bio-available. Some water-soluble trivalent chromium compounds of strong mineral acids exhibit aquatic toxicity but this may be associated with their highly acidic properties. Under normal ambient conditions in aquatic and terrestrial environments, trivalent chromium also forms relatively stable complexes with many

naturally occurring organic species thus limiting its bio-availability. In some cases complexation may enhance chromium bioavailability depending on the solubility of the ligand (Saleh et al, 1989). Unless complexed, trivalent chromium is readily removed from aqueous effluents by precipitation and filtration and rendered insoluble in solid waste.

Because of the above considerations, the majority of trivalent chromium species are insoluble (Bartlett & James 1988), very poorly absorbed and do not bio-accumulate through the food chain. This characteristic is very important, taking into account that chromium is an essential element in human nutrition..

On the other hand, hexavalent chromium compounds are classified as dangerous for the environment and releases into all environmental compartments are regulated. It should be noted that there are also differences in solubility among hexavalent chromium species (James 1996). The presence of hexavalent chromium is rare in nature and it is mainly released as a result of anthropogenic activity. Environmental effects of hexavalent chromium are related to its nature as a relatively mobile anion and a strong oxidiser. The action can persist both in acid and alkaline soils (Yassi & Nieboer 1988) that are sandy or have low organic content (Cary 1982). So, the anion will remain mobile only if its concentration exceeds both the absorbing and reducing capacities of the soil (Cary 1982; Smith et al. 1989). Once in contact with organisms, the strong oxidiser reduces rapidly forming intermediates and reactive oxygen species that can cause adverse effects (Costa 1997; Goyer & Clarkson 2001)

Redox reactions can convert Cr(VI) to Cr(III) and vice versa (Rai et al. 1989; Kimbrough et al. 1999; Kotas & Staticka 2000). The Cr interconversion is controlled at the same time by several factors, including the presence and concentrations of Cr species and oxidising or reducing agents, ambient temperature, light, sorbents, acid-base reactions, pH, complexing agents and precipitation reactions (Saleh et al. 1989; Kotas & Staticka 2000). In the presence of iron (II) compounds, sulphides and organic matter, Cr (VI) is readily reduced to Cr (III) (Rai et al. 1989; Fendorf 1995). Only few oxidising agents exist in environmental situations at high enough concentrations to actually oxidise Cr (III) to Cr (VI) (Rai et al. 1989; Kimbrough et al. 1999). The presence of natural reducing agents in air, water and soil normally ensures that hexavalent chromium entering the environment is reduced to the trivalent state.

Hexavalent chromium is phytotoxic. Normally it is chemically reduced in the plant roots and is deposited as trivalent chromium there rather than in the above-ground foliage. The lowest chromium concentration in above ground plant tissues is observed in fruit with increases in the stem and the leaf (Smith et al. 1989; Zayed & Terry 2003). The concentrations of chromium in plants vary widely among different species, tissues and stages of growth (Kabata-Pendiasa & Pendias 2001). Generally, only a small fraction of the total chromium content of soils is available to plants. Plant to soil ratio has been shown to vary between 0.1 and 0.3 (Zayed &Terry 2003). The chromium concentration in plants is controlled mainly by the soluble chromium content of the soils (Kabata-Pendias & Pendias 2001)

Techniques are available to ensure that the hexavalent chromium content of releases to the environment is either eliminated or reduced to levels, which are consistent with no realistic risk. Such techniques relate i.a. to reduction of Cr6+ by e.g. Fe2+ followed by stabilisation/solidification for soluble chromates or in-situ remediation of e.g. contaminated soils, again with suitable reducing agents such as FeCl2.