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Description of key information

Regarding the partitioning of chromium (III) in soil, suspended matter and sediments, data are available from published studies and reviews by U.S. EPA, RIVM and of the previous EU risk assessment:

According to the EU RAR on chromates (ECB, 2005), “chromium (III) appears to be much more strongly adsorbed to soils and sediments than chromium (VI). The adsorption of chromium (III) onto soil follows the pattern typical of cationic metals and increases with increasing pH (lowering pH results in increased protonation of the adsorbent leading to fewer adsorption sites for the cationic metal) 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”.

In sum, the following median Kd ranges are derived for soluble chromium (III) substances taking into account different pH regimes based on available reliable data and conclusions of U.S. EPA and the EU RA on chromates (ECB, 2005):

log Kp(solids-water in soil)

Range: 3.2 (median pH = 4.8) - 4.4 (median pH = 6.64).

Due to the wealth of data taken into account by U.S. EPA (n=43) and similar partition coefficients being reported in the EU RA on chromates (ECB, 2005) and in other additional studies, a median log Kp(solids-water in soil) of 3.9 is derived.

log Kp(solids-water in sediment)

Range: 4.04 (pH<6) - 5.08 (pH>6).

Due to the wealth of data taken into account by U.S. EPA (n=25) and similar partition coefficients being reported in the EU RA on chromates (ECB, 2005) and in other additional studies (taking into account different pH regimes), a median log Kp(solids-water in sediment) of 4.5 is derived.

log Kp(solids-water in suspended matter)

Range = 4.47 (acidic conditions) - 5.47 (alkaline conditions). Based on the limited data available and conclusions drawn by U.S. EPA and the EU RA on chromates (ECB, 2005), a log Kp(solids-water in suspended matter) of 5.1 is derived.

Thus, partitioning data indicate that chromium (III) species have a strong preference for the solid phase of suspended matter, sediment and soil.

Key value for chemical safety assessment

Additional information

Based on spiking experiments performed with soluble chromium (III) substances, a median log Kp(solids-water in soil) of 4.14 was derived based on non-standard batch experiments performed by Hassan et al. (1996) with three American soils: Kp values for EPA no.2 soil ranged from 298 to 15,382 L/kg (logKp = 3.45-4.19, median = 3.25) over a pH range of 4.70 to 6.69, whereas for EPA no.9 soil Kp values ranged from 19,716 to 55,918 L/kg (logKp = 4.29-4.75, median = 4.44) over a pH range of 6.03 to 7.41. In EPA soil no.13, Kp values ranged from 330 to 27,151 L/kg (logKp = 3.45-4.19, median = 3.2, pH range of 3.63 to 6.16). Thus, Kp values by Hassan et al. (1996) are in the range of 800 L/kg (logKp = 2.90) and 15,000 L/kg (logKp = 4.17) derived for acidic and alkaline conditions in the EU RA on chromates (ECB, 2005), respectively. In addition, similar partition coefficients were reported by U.S EPA (2005) based on an extensive literature review on chromium (III) substances. U.S EPA (2005) derived a median log Kp(solids-water in soil) of 3.9.

Regarding the partitioning of soluble total chromium in soil (without differentiating between Cr(III) and Cr(VI)), a median log Kp of 4.12 is proposed by Sauve et al. (2000) based on a comprehensive meta-analysis of approx. 70 studies. In addition, a log Kp of 2.04 was derived by RIVM (Crommentuijn et al., 1997), however, based on secondary literature.

Regarding sediment partitioning of chromium (III), in reliable studies. on river sediments, Young et al. (1987, 1992) derived logKp values of 4.04 (pH < 6), 5.08 (pH > 6) and 4.48 (pH 8.3). Reported Kp values are in agreement with partition coefficients proposed by U.S. EPA (2005) and the EU RA on chromates (ECB, 2005), i.e. logKp values of 4.5 and 4.04 - 5.08, respectively.

Partitioning data for chromium (III) in suspended matter are scarce. A median logKp of 5.02 based on 35 individual samples of the Po river was derived by Pettine et al. (1994) without, however, differentiating between Cr(III) and Cr(VI). Crommentuijn et al. (1997) report a logKp of 5.46 based on secondary literature also without stating the respective chromium valence. Nevertheless, the partition coefficients are similar to Cr(III) partition coefficient (logKp of 5.1) derived by U.S. EPA (2005) based on a comprehensive literature review and in the EU RA on chromates (ECB, 2005) (logKps of 4.47 (acidic conditions) and 5.47 (alkaline conditions)).