Chromium trioxide

Endpoint summary

Description of key information

Toxicity to soil macroorganisms except arthropods

Key_Eisenia fetida, in water at pH 6, 7, and 8, 10 different soils and an organic substrate. Six concentrations of 100, 150, 200, 250, 300, and 350 mg Cr/kg were applied to the test animals in the definitive test. The 14-day LC50 values of Cr(VI) in all soils ranged from 222 mg/kg to 257 mg/kg at pH 6, 7 and 8. The LC50 values of Cr(VI) was 219 mg/kg in the organic substrate.

Toxicity to terrestrial plants

1) Key_Toxicity to terrestrial plants by summary of data from European Union Risk Assessment Report: chromium trioxide, sodium chromate, sodium dichromate, ammonium dichromate and potassium dichromate; 3rd. Priority List; Volume 53: lowest NOEC: 0.35 mg Cr(VI)/kg dry weight soil for several species from various studies with different test durations

2) Key_Toxicity to terrestrial plants: 14d-EC50: 11-31 mg Cr/kg dry weight soil (humic sand), 1.8-7.4 mg Cr/kg dry weight soil (loam); 14d-NOEC: 10->11 mg Cr/kg dry weight soil (humic sand), 0.35-3.5 mg Cr/kg dry soil weight (loam) for the parameter growth of three species (Lactuca sativa, Avena sativa, Lycopersicon esculentum) exposed to potassium dichromate according to OECD TG 208

Toxicity to soil microorganisms

Key_Crommentuijn et al. (1997) reviewed the toxicity of chromium (III) to soil processes. The results of 51 determinations were reported, covering arylsulphatase, nitrification, N-mineralisation, phosphatase, respiration and urease.

A total of 37 values were obtained, and a further selection was made giving preference to longer exposure times in the same studies, resulting in a final data set of 30 values. The statistical extrapolation method has been used to derive an HC5-50% value of 5.9 mg/kg. In this case it is considered that an assessment factor of 1 is sufficient, hence the PNEC for soil processes with chromium (III) is 5.9 mg/kg.

Additional information

A substantial amount of information is available for the toxicity of chromium (VI) to terrestrial organisms. In the environment, it is likely that chromium (VI) will be reduced to chromium (III) in soil, and it is also likely that such conversion would have taken place in many of the toxicity tests. The follwoing data are taken from the EU RAR:

For chromium (VI), long-term toxicity data are available for three trophic levels (plants, earthworms and soil processes/microorganisms), with plants generally being the most sensitive species (although a clear NOEC has not been determined for earthworms, the EC50 values are generally higher than those found in the plant experiments). The lowest NOEC from these studies is around 0.35 mg/kg dry weight of soil for plants. According to the Technical Guidance Document, an assessment factor of 10 is appropriate and so the PNECsoil can be estimated as 0.035 mg/kg dry weight. Using the water content of soil from the Technical Guidance Document of 11.8% by weight (20% by volume), this is equivalent to a PNECsoil of around 0.031 mg/kg on a wet weight of soil basis.

Chromium (III) has generally been shown to be less toxic than chromium (VI) to soil organisms. One exception to this may be on the effects seen in some experiments using growth media (no soil) where reduction in root growth was seen at similar concentration as found for chromium (VI). Since chromium (III) adsorbs more strongly onto soil than chromium (VI), it would again be expected that in soils, chromium (III) would be less toxic than chromium (VI). From the available data, the NOEC for chromium (III) to plants is of the order of 100 mg Cr/kg soil, with a NOEC of 32 mg Cr/kg dry soil being reported for earthworms, and a NOEC/LOEC of ~100-330 mg Cr/kg soil also being reported. Applying an assessment factor of 10 to the lowest of these NOECs gives a PNEC for chromium (III) of approximately 3.2 mg Cr/kg dry soil, which is equivalent to a PNEC of around 2.8 mg/kg on a wet weight of soil basis. This value is also lower than the HC5 value for soil processes.

According to the Technical Guidance Document, an equilibrium partitioning approach can also be used in the derivation of the PNECsoil. However, such an approach for chromium (VI) should be considered very tentative in nature as chromium (VI) is likely to be reduced to chromium (III) under the conditions found in most soils, and the chromium (III) formed is likely to be of much lower water solubility (and bioavailability).

For chromium (VI), a PNECwater of 3.4 µg/l has been derived. For chromium (III) a worst-case PNEC of 4.7 µg/l was derived.

According to the Technical Guidance Document, the PNECsoil can be estimated from:

PNEC_soil = K_soil-water/RHO_soil x PNEC_water x 1000

where RHO_soil = density of soil = 1,700 kg/m³

The following values for Ksoil-water were derived:

Chromium (VI) Ksoil-water = 75 m³/m³ (acid conditions); Ksoil-water = 3.2 m³/m³ (neutral/alkaline conditions)

Chromium (III) Ksoil-water = 1,200 m³/m³ (acid conditions); Ksoil-water = 22,500 m³/m³ (neutral/alkaline conditions).

Using these values, the PNECsoil can be estimated as follows:

For chromium (VI), PNECsoil = 0.15 mg/kg wet weight for acid conditions, and 0.006 mg/kg wet weight for other conditions. Similarly, for chromium (III), PNECsoil = 3.3 mg/kg wet weight for acid conditions and 62 mg/kg wet weight for other conditions.

The PNECsoil estimated for chromium (III) for acidic conditions using the equilibrium partitioning method is in very good agreement with the values obtained above using the available toxicity data. For the risk assessment the PNECs obtained from experimental data will be used. Thus the PNECsoil for chromium (VI) is taken as 0.031 mg/kg wet weight. The PNECsoil for chromium (III) is taken to be 2.8 mg/kg wet weight.