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It is notable that the processes and exposure scenarios discussed in this CSR do not result in the introduction of any hexavalent chromium into the environment and there is no oxidation of Cr (III) to Cr (VI) under foreseeable environmental conditions. Experience shows that Cr (III) species rapidly age to form insoluble chromium (III) oxide or, in the presence of iron, chromite. Even under conditions of very high naturally occurring levels of soil chromium, levels of soluble Cr (III) in groundwater are low. For example groundwater monitoring at a mine inreports levels of <10 µg/L Cr (III), where the soil levels are >30%. Data therefore indicate the low potential for water contamination with soluble chromium (III), even under extreme conditions.

No proprietary studies are available, however an overview of the expected behaviour of Cr (III) species in natural waters and sediment is provided in the ERA report from ICDA. 

Surface water chromium (III) is a 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. The major dissolved species of chromium (III) are Cr (III)+, CrOH2+, Cr(OH)30and Cr(OH)4-. Of these species, Cr (III)+only exists in significant amounts at pH 3.6-3.8 and similarly, Cr(OH)4-is prevalent only at high pH (pH > ca. 10-11.5). Between these pH values, CrOH2+is thought 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+, Cr (III)(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)3is ~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)3and so are expected to preferentially form. 

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 () was found to be associated with the particulate phase. Chromium (III) exhibits 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 very high.

Groundwater

Chromium (III) in groundwater may precipitate as (Cr, Fe)(OH)3, which limits the concentration of dissolved chromium to less than 10-6M between pH 4 and 12. The oxidation of aqueous chromium (III) is unlikely to occur and all the chromium (III) will be adsorbed and consequently 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. Strong adsorption of the insoluble chromium (III) species formed to sediment is likely at pH levels 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) forms of insoluble Cr (OH)3in neutral to alkaline solutions.

Soil

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.

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