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