efsa-opinion-chromium-food-drinking-water

Chromium in food and drinking water
accommodate electron pairs. Therefore, ligands must have at least one pair of electrons suitable for
being donated to the metal. The metal-ligand bonding can have various degrees of covalent nature
even when both chromium and ligands are formally ionic species.
Figure 2: Chromium hexa-carbonyl
Figure 3: Chromate ion
DIVALENT CHROMIUM. All Cr(II) compounds are energetic reducing agents and under environmental
conditions Cr(II) (chromous state) is relatively unstable. In aqueous media, the chromous ion is readily
oxidized to the stable Cr(III) species (Cr 3+ + e − → Cr 2+ ; E 0 = - 0.41 V), for instance by the dissolved
molecular oxygen, O 2 (Kotaś and Stasicka, 2000). Therefore, Cr(II) solutions can only be preserved if
degassed (anaerobic conditions). The only coordination number observed for Cr(II) is six, in the form
of a twisted octahedral geometry.
TRIVALENT CHROMIUM. Cr(III) is the most stable and important oxidation state of the element, in
particular in relation to its aqueous chemistry (Kotaś and Stasicka, 2000). This state is characterized
by the formation of a very large number of relatively kinetically inert complexes, in which Cr(III) is
always hexacoordinate (octahedral geometry). This kinetic inertness allows many complex species to
be isolated as solids and to persist for relatively long periods of time in solution, even if their
thermodynamical condition is unstable. In aqueous media and in the absence of specific ligands,
Cr(III) is present as Cr(H 2 O) 6 3+ (hexa-aquachromium (3+) , a moderately strong acid), Cr(OH) 3
(chromium trihydroxide), and their reaction products. Therefore, the aqueous compositions of these
groups of substances are complex and depend on environmental conditions and their influence on
processes such as hydrolysis, complexation, redox reactions, and adsorption. Even at naturallyoccurring
concentrations and substantially neutral pHs, Cr(III) compounds in aqueous systems may be
actively oxidized to Cr(VI) by strong oxidants such as chlorine or hypochlorous acid, ozone, or
potassium permanganate — used, for instance, in water purification treatments (Schroeder and Lee,
1975; Lai and McNeill, 2006; Saputro et al., 2011; Lindsay et al., 2012).
HEXAVALENT CHROMIUM. Cr(VI), or chromate, is the second most stable state: its compounds, whose
aqueous chemistry is of particular relevance, primarily arise from anthropogenic sources (Shanker et
al., 2005; Johnson et al., 2006). In addition to its occurrence in rare minerals, naturally occurring
Cr(VI) has also been occasionally detected in groundwater (McNeill et al., 2012a). In its highest
oxidation state, chromium forms oxy-compounds that are fairly potent oxidizing agents (Kotaś and
Stasicka, 2000). In basic solutions (pH > 6.5), it exists predominantly as the yellow chromate ion
(CrO 4 2− ), exhibiting a coordination number of four and a tetrahedral geometry (Figure 3). As the pH is
lowered (pH < 6), the solution of chromate ions turns orange owing to the formation of dichromate
ions (Cr 2 O 7 2− ). In Cr 2 O 7 2− two chromium atoms are linked by an oxygen bridge and exhibit a slightly
distorted tetrahedral geometry (Figure 4).
Acid solutions of dichromate are quite powerful oxidizing agents, the Cr(VI) reduction process
yielding Cr(III). In basic solution, the chromate ion exhibits a much lower oxidizing power as the
CrO 4 2− species undergoes a relative stabilization.
EFSA Journal 2014;12(3):3595 15