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EC number: 232-140-5
CAS number: 7789-00-6
overview of the expected behaviour of the substance in natural waters,
sediment and soil is provided in the EU RAR and is discussed below
(VI) and chromium (III) are the most stable oxidation states of chromium
at the redox potential (Eh) and pH range of natural waters. The
prevalent species present at equilibrium depends both on the pH and Eh
of a given system (see figure on page 72 of the EU RAR).
major dissolved species of chromium (III) are Cr3+, CrOH2+, Cr(OH)30 and
Cr(OH)4-. Of these species, Cr3+ only exists in significant amounts at
pH 3.6-3.8 and similarly, Cr(OH)4- is prevalent only at high pH (pH >
c.a. 10-11.5). Between these pH values, CrOH2+ is though to be the
dominant species up to a pH of around 6.3-6.5, and Cr(OH)30is the
dominant species in solution at pH between 6.3-7 and 10-11.5. Polymeric
species such as Cr2(OH)24+, Cr3(OH)45+ and Cr4(OH)66+, although they
exist, are never significant in the environment. Overall, chromium (III)
species show a minimum solubility between pH 7-10. Over this range, the
solubility of Cr (OH)3 is ~10-6.84 mole/l (= 7.5 µg Cr/l). The chromium
(III) ion acts as a hard Lewis acid and so readily forms complexes with
ligands such as hydroxyl, sulphate, ammonium, cyanide, sulphocyanide,
fluoride and chloride, as well as natural and synthetic organic ligands.
from around 5-6 up to around 12, the solubility of chromium (III) in
aqueous systems is limited by the formation of Cr (OH)3. If iron,
particularly Fe (III), is also present, the chromium (III) can also form
insoluble iron complexes of the form CrxFe1-x(OH)3, the solubility of
which decreases with decreasing chromium (III) fraction, but all are
less soluble than Cr (OH)3. The mixed chromium/iron hydroxides also have
a lower free energy of formation than for Cr (OH)3 and so are expected
to preferentially form. This reaction is particularly important when
chromium (VI) is reduced to chromium (III) by iron (II) (which itself is
oxidised to iron (III)).
major dissolved species of chromium (VI) are HCrO4- and CrO42-. The
relative proportion of these two species depends on the pH of the
system. Although these two species could dimerise to form dichromate
anions (e.g. HCr2O7-or Cr2O72-) the equilibrium is such that the process
only becomes significant at high chromate concentrations (e.g. >0.08
mol/l = 0.4 g Cr/l). The chromium (VI) species present in the
environment are much more soluble than the chromium (III) forms,
however, a relatively insoluble barium salt (BaCrO4 or mixed
sulphate/chromate salt) could be formed if barium ions are present.
Formation of such salts could limit the solubility of chromium (VI) in
significant proportion of total chromium in aquatic systems is
associated with the solid phase. For example, around 90% of the total
chromium transported in the River Po (Italy) was found to be associated
with the particulate but at least 85% of the soluble or dissolved
chromium (~10% of total chromium) was found as chromium (VI).
of chromium (VI) to chromium (III) may also occur to some extent in
surface waters, particularly where oxygen-deficient conditions exist.
Iron (II) and organic matter-rich environments favour the reduction
(III) is not readily, or rapidly, oxidised to chromium (VI) under most
conditions found in the environment, but can be by oxidised by naturally
occurring manganese oxides. The extent of chromium (III) oxidation is
limited by anionic adsorption of chromium (VI) to the mineral surface in
acidic solutions and by precipitation of Cr (OH)3 in neutral to alkaline
solutions. The rate of reaction is slower than the reduction of chromium
(VI), possibly explaining why the distribution of chromium (VI) and
chromium (III) in natural waters often deviates from thermodynamic
of chromium (VI) to suspended and bottom sediment exhibits typical
anionic sorption behaviour where the adsorption occurs to positively
charged sites on mineral particles. The adsorption of chromium (VI) to
particulate matter decreases with increasing pH and when competing
dissolved anions are present. On the other hand, chromium (III) exhibits
a typical cationic sorption behaviour, where adsorption occurs onto
negatively charged sites on the mineral surface or onto organic matter.
The adsorption of chromium (III) increases with pH but decreases when
competing cations are present, however, in general, the adsorption of
chromium (III) to particulate matter is much higher than that of
chromium (VI) under the same conditions.
(VI) reduction occurs in solutions, particularly where the oxygen
concentration is low or reducing conditions exist, over a wide range of
pHs, indicating that chromium introduced into groundwater as chromium
(VI) will be reduced to chromium (III) by the residual amounts of Fe
(II) commonly contained in oxide and silicate materials. In such
environments, dissolved total chromium concentrations will be limited by
the solubility of (Cr,Fe)(OH)3(s) over the pH range of natural waters (4
iron contained in naturally occurring minerals (e.g., hematite, biotite)
is an important inorganic reductant for chromium (VI) to chromium (III)
in groundwater. Chromium (VI) reduction by iron (II) ions in solution is
nearly instantaneous, but when the iron (II) source is contained within
weathering minerals, the rate of reduction is dependent on the
dissolution rates of the iron (II) contents of these minerals, which is
increased at low pH or by high concentrations of anions that complex
iron (II). Following reduction, chromium (III) may precipitate as
(Cr,Fe)(OH)3, which limits the concentration of dissolved chromium to
less than 10-6 M between pH 4 and 12. Chromium (VI) reduction by
dissolved iron (II) has been demonstrated to occur even in oxygenated
solutions. Overall, the rates of chromium (VI) reduction are fastest at
pH 4, independent of pH over the range 6 to 9, and slower at pH>9.
presence of manganese oxides in groundwater would indicate the potential
oxidation of chromium (III) to the more soluble chromium (VI). In the
absence of significant concentrations of manganese oxides, the oxidation
of aqueous chromium (III) is unlikely to occur and all the chromium
(III) present will be adsorbed and relatively immobile.
same processes that govern the distribution of chromium in natural
waters, such as redox potential, precipitation and adsorption also
govern the distribution of chromium in sediments. Chromium (VI) exists
mainly as oxoanions in the environment and is expected to be highly
mobile under aerobic conditions. Under alkaline conditions, chromium
(VI) is not readily sorbed and remains highly mobile. In acidic oxidised
sediments with a high content of iron and manganese oxides or clay
minerals, chromium (VI) should be adsorbed more strongly onto the
sediment as the higher net positive charge present in acidic sediment
should provide more or stronger sites for adsorption of the chromium
(VI) anions. The adsorption is thought to occur with the mineral
fraction, especially those with exposed hydroxyl groups on their surface
such as iron and aluminium oxides and montmorillonite. Decreasing pH
results in increasing protonation of the mineral surface and hence
increasing adsorption of the chromium (VI)-containing anions. However,
other anions present in natural systems such as SO42- can also compete
with the adsorption of chromium (VI), resulting in lower adsorption of
chromium (VI) than might be expected. Overall, chromium (VI) anions can
be considered to be mobile in sediments in the environment, except
possibly under highly acidic conditions. Reduction of chromium (VI) to
chromium (III) is expected to occur in anaerobic sediments. Strong
adsorption of the insoluble chromium (III) species formed to sediment is
likely at pHs found typically in the environment. At very low pH (5)
more soluble chromium (III) cationic species may be formed, which may be
more mobile in sediments at these pHs. In general, once chromium (III)
is scavenged from the water column, it becomes part of the sediment
matrix and is thus less available for uptake from biota.
(III) may be oxidised by naturally occurring manganese oxides in
sediments to give chromium (VI). The extent of chromium (III) oxidation
is limited by anionic adsorption of chromium (VI) to the mineral surface
in acidic solutions and by the formation of insoluble Cr (OH)3 in
neutral to alkaline solutions.
behaviour of chromium (VI) in soils is likely to be similar to that in
sediment. Adsorption to the soil matrix is expected to increase with
increasing acidity of the soil, but under neutral to alkaline
conditions, chromium (VI) is expected to be highly mobile in soil, and
may leach into lower anaerobic layers where reduction to chromium (III)
would be expected to occur. In the environment, iron oxides are the
primary site of adsorption for chromium (VI) in acidic to neutral soils,
with some contribution also from minerals with aluminium-OH groups.
Since adsorption of chromium (VI) appears to be electrostatic in nature,
this implies that once the available adsorption sites are occupied
(either by chromium (VI) or other anions) then no further adsorption can
take place and increased mobility may occur.
(VI) added to soil will remain mobile only if its concentration exceeds
both the adsorbing and reducing capacities of the soil. In the presence
of organic matter, chromium (VI) is reduced rapidly to chromium (III).
Reduction is likely to be slower in soils with low organic matter
to the case with sediment, chromium (III) is expected to be rapidly and
strongly adsorbed onto soil, particularly by iron and manganese oxides,
clay minerals and sand. About 90% of added chromium has been found to be
adsorbed onto clay minerals and iron oxides in 24 hours. The adsorption
of chromium (III) onto soil follows the pattern typical of cationic
metals and increases with pH and the organic matter content of the soil
and decreases when other competing (metal) cations are present. Certain
dissolved organic ligands may also reduce the adsorption of chromium
(III) to the solid phase by forming complexes which enhance the
solubility of chromium (III) in the aqueous phase. Oxidation of chromium
(III) to chromium (VI) could also occur to a limited extent in soils
rich in manganese dioxide.
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