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Environmental fate & pathways

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An overview of the expected behaviour of the substance in natural waters, sediment and soil is provided in the EU RAR and is discussed below

Surface water

Chromium (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).

The 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.

At pHs 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)).

The 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 the environment.

A 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).

Reduction 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 processes.

Chromium (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 predictions.

Adsorption 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.

Groundwater

Chromium (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 to 9).

Ferrous 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.

The 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.

Sediment

The 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.

Chromium (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.

Soil

The 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.

Chromium (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 contents.

Similar 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.