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Ecotoxicological information

Toxicity to microorganisms

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Link to relevant study record(s)

Description of key information

Information is based on the EU RAR (2005): European Union Risk Assessment Report: chromium trioxide, sodium chromate, sodium dichromate, ammonium dichromate and potassium dichromate, 3rd. Priority List; Volume 53.

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:
0.21 mg/L

Additional information

There are a number of studies that indicate the chromium (VI) is toxic to single species of bacteria. However, it is also clear that many bacteria are tolerant of high concentrations of chromium (VI). Both single species and mixed population tests can be used to derive a PNEC for wastewater treatment plants. The available data were summarised in the review (see endpoint study record) From these data

those which are relevant to the assessment of the wastewater treatment plant have been selected, and are listed in the table below with the appropriate assessment factor and resulting PNEC.

Species

Endpoint

Value (mg/L)

Assessment factor

PNEC (mg/L)

Chilomonas paramecium

NOEC

1.0

1

1

Colpidium campylum

IC50

2.8

10

0.28

Microregma heterosoma

NOEC

0.21

1

0.21

Activated sludge

IC50

30

100

0.3

The lowest of the PNEC values in the table is 0.21 mg/L, and this will be used in the risk characterisation.

There is evidence from studies on pilot-scale activated sludge plants that once acclimated to the presence of chromium (VI), plants can tolerate up to 10 mg Cr (VI)/L in the influent, with only minor reductions in efficiency seen at substantially higher concentrations. This observation indicates that the PNEC derived above may be overprotective of wastewater treatment plants that regularly receive, and are therefore acclimated to, chromium (VI) in the influent.

Chromium (III) appears to be much less toxic to micro-organisms than chromium (VI). A concentration of 10 mg/L promoted growth in some species of bacteria, and 100 mg/L produced only a small amount of growth inhibition. The study did not provide a formal NOEC; it is proposed to use 10 mg/l as the PNEC for chromium (III).