efsa-opinion-chromium-food-drinking-water

Chromium in food and drinking water
Cr(VI) compounds are reduced to the trivalent form in the presence of oxidizable substances
(reductants). In natural waters, often characterized by a fair degree of acidity (Kotaś and Stasicka,
2000), Cr(VI) compounds are generally more stable as the concentration of reducing materials is
relatively low. However, Fe(II) in solution or Fe(II)-bearing minerals, sulphides, and/or oxidizable
organic matter may cause a reduction of Cr(VI) to Cr(III) (Schroeder and Lee, 1975; Fendorf, 1995;
Loyaux-Lawniczak et al., 2001).
Figure 4: Dichromate ion
Conclusion
In conclusion, in aqueous media chromium generally occurs in the form of its two most stable
oxidation states, Cr(III) and Cr(VI), both existing as complex groups of interrelated chemical species.
As described, the distribution of species containing Cr(III) and Cr(VI) depends on the redox potential,
the pH, the presence of oxidizing or reducing substances, the kinetics of redox reactions, the formation
of Cr(III) complexes or insoluble Cr(III) compounds, and the total chromium concentration. In the
environment, and specifically in aqueous media, the two forms are involved in rather complex
equilibria, which may be easily altered if the ambient chemico-physical conditions are modified (for
the technical problems in Cr(VI) analysis, see Section 3).
1.2. Environmental fate and sources of food and drinking water contamination
1.2.1. Environmental fate
In the atmosphere, chromium occurs from natural sources (e.g. volcanic emissions) as well as from
many anthropogenic activities, including burning of fossil fuels and wood; the most important
industrial sources of airborne chromium are associated with ferrochrome production. Both Cr(III), and
Cr(VI) can be released into the air, although the latter to a lesser extent (WHO, 2003): due to
analytical difficulties, chromium speciation data in air are very limited. In air, chromium is present in
the form of aerosols that are removed by wet and dry deposition. Chromium particles of small
aerodynamic diameter (< 10 µm) may remain airborne for long periods and undergo long-range
transport. Under normal conditions, airborne Cr(0) and Cr(III) forms do not undergo any reaction,
whereas Cr(VI) eventually reacts with dust particles or other pollutants to yield Cr(III) (U.S. EPA,
1998a, b).
As observed in the preceding Section, in the aquatic environment Cr(III) and Cr(VI) occur mostly as
Cr(OH) n
(3 – n)+ and as CrO 4 2− or HCrO 4 − . In water, Cr(III) may form positive or negative ionic species
at low or high pH values, respectively, whereas at intermediate pH values the neutral hydroxide form,
Cr(OH) 3 0 , is predominant. In surface waters, relatively high concentrations of Cr(VI) forms can be
found locally (WHO, 2003). Surface runoff, deposition from air, and release of municipal and
industrial waste waters are sources of chromium in surface waters. Cr(III) is lost from the aquatic
environment primarily due to precipitation of hydrated Cr 2 O 3 followed by sedimentation. The Cr(VI)
anion species can persist in aquatic media, possibly for long periods, as water-soluble complexes;
however, they will react with organic matter or other reducing agents to form Cr(III). Therefore, in
surface waters rich in organic content, Cr(VI) will have a much shorter lifetime (U.S. EPA, 1998a, b).
EFSA Journal 2014;12(3):3595 16