Naphthenic acids, bismuth salts

Toxicity to aquatic algae and cyanobacteria

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Study period:

2010

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
The substance naphthenic acids, bismuth salts is manufactured from bismuth oxide/hydroxide and 3 equivalents of naphthenic acids, resulting in the bismuth tri-naphthenate. Thus, to assess acute toxicity to aquatic algae, results for bismuth 3+ cations were assessed as well as data for naphthenic acids, the two potential hydrolysis products of the substance. The undissociated substance is considered uncritical, as its molecular mass of ~850 Dalton makes it unlikely that such compounds efficiently pass biological membranes. Thus, the acute aquatic toxicity on naphthenic acids bismuth salts will be dominated by its ions, derived from hydrolysis.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
The source compound naphthenic acids is the starting material for the manufacturing of naphthenic acids, bismuth salts. Naphthenic acids do contain mainly hydrocarbon acids with a carbon range from 10 - 15 (other naphthenic acids may have wider ranges), with a variable number of cyclics contained (n = 0, 1, 2 and rarely 3). To a minor extent also aliphatics may be present as "impurities".
The naphthenic acids are reacted in a slight excess of >3 equivalents of naphthenic acids with bismuth oxide to derive naphthenic acids, bismuth salts with a slight excess of naphthenic acids, the target substance. Thus, the starting compound for the synthesis of the target compound is actually the source substance. Whereas the bismuth oxide used has a purity of 99% by weight typically, the naphthenic acids, being a UVCB-type substance, are of 100% purity, by definition.
The second source substance used here is bismuth hydroxy nitrate oxide, a soluble form of bismuth compounds, suitable to achieve high Bi3+ concentrations in aqueous systems.
Thus, the two source compounds naphthenic acids (as surrogate for naphthenate anions) and bismuth hydroxy nitrate oxide (as surrogate for bismuth cations) are ideal surrogates for assessing the acute toxicity of the target substance to aquatic algae and cyanobacteria.

3. ANALOGUE APPROACH JUSTIFICATION
Although the two source substances are different in respect of properties compared to the target substance, the read-across is justified, as the source substance bismuth hydroxy nitrate oxide allows to achieve excessively high bismuth concentration in aqueous systems compared to the target substance (water solubility of < 0.036 mg/L) and still no EC50 was observed at 137 mg/L, Actually, the NOEC was set to the highest concentration tested being 137 mg/L bismuth hydroxy nitrate oxide (equivalent to 100 mg Bi/L). The second source substance naphthenic acids allows to assess the anionic hydrolysis product of the target substance, and for naphthenic acids an EC50(growth rate) of 29.6 mg/L (based on dissolved fraction) was found; The NOEC (growth rate) in this study was set to 7.38 mg/L. Thus, it can be concluded, that both results of the two source substances do consider a worst-case approach with respect to the target substance, especially considering the very low water solubility of the target substance that was determined being < 0.036 mg/L.

4. DATA MATRIX
The source substance bismuth hydroxy nitrate oxide was found being uncritical to aquatic algae and cyanobacteria and an EC50 (48 h) of > 137 mg/L was found (NOEC 137 mg/L), equivalent to more than 463 mmol/L. Thus, determining for the toxicity towards aquatic algae and cyanobacteria will be any naphthenic acid anions formed upon hydrolysis of the target substance. Considering an EC50 (96 h, pseudokirchneriella subcapitata) of 29.6 mg/L naphthenic acids determined in an acute algae toxicity study, this result was re-calculated to approx. 118 mmol/L (calculated based on an average of C14, 1-ring naphthenic acid as average value) and considering the 3:1 molar ration of naphthenic acid and bismuth in the target substance (on average estimated as Bi(O2C13H24)3) as well as the molecular average mass of 850 Dalton, the calculated theoretical EC50 is derived as 33.4 mg/L. This value is almost three orders of magnitude above the value for water solubility (determined as < 0.036 mg/L – see study on water solubility), and in consequence, it is expected that naphthenic acids, bismuth salts are not acutely toxic to invertebrates. The theoretical NOEC for naphthenic acids, bismuth salts was calculated as being 9.24 mg/L, accordingly.

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Qualifier:

according to guideline

Guideline:

OECD Guideline 201 (Alga, Growth Inhibition Test)

Duration:

96 h

Dose descriptor:

EC50

Effect conc.:

118 mmol/L

Nominal / measured:

nominal

Conc. based on:

test mat. (dissolved fraction)

Basis for effect:

growth rate

Remarks on result:

other: value for naphthenic acids

Duration:

96 h

Dose descriptor:

EC50

Effect conc.:

> 463 mmol/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: value for Bi(OH)(NO3)O

Duration:

96 h

Dose descriptor:

EC50

Effect conc.:

33.4 mg/L

Nominal / measured:

estimated

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: estimated EC50 for naphthenic acis, bismuth salts

Duration:

96 h

Dose descriptor:

NOEC

Effect conc.:

32.6 mmol/L

Nominal / measured:

nominal

Conc. based on:

test mat. (dissolved fraction)

Basis for effect:

growth rate

Remarks on result:

other: NOEC for naphthenic acids

Duration:

96 h

Dose descriptor:

NOEC

Effect conc.:

463 mmol/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: value for Bi(OH)(NO3)O

Duration:

96 h

Dose descriptor:

NOEC

Effect conc.:

9.24 mg/L

Nominal / measured:

estimated

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: estimated NOEC for naphthenic acids, bismuth salts

Validity criteria fulfilled:

yes

Conclusions:

For naphthenic acids, bismuth salts an EC50 to algae, such as pseudokirchneriella subcapitata, of approx. 33.4 mg/L is estimated based on data from naphthenic acids, considered relevant for the acute algae toxicity. Soluble bismuth compounds such as Bi(OH)(NO3)O have been tested and were found being non-toxic to pseudokirchneriella subcapitata (NOEC of 100 mg Bi/L). Therefore, the inorganic moiety is considered uncritical for acute toxicity to algae and naphthenic acids, released by hydrolysis, will be the determining ion for aquatic toxicity to algae. However, the EC50 values of source substances as well as the estimated EC50 value for the target substances are approx. 3 orders of magnitude above the water solubility of the target substance, and therefore, no acute toxicity effect is expected from the target substance.

Description of key information

For naphthenic acids, bismuth salts an EC50 to algae, such as pseudokirchneriella subcapitata, of approx. 33.4 mg/L is estimated based on data from naphthenic acids, considered relevant for the acute algae toxicity. Soluble bismuth compounds such as Bi(OH)(NO3)O have been tested and were found being non-toxic to pseudokirchneriella subcapitata (NOEC of 100 mg Bi/L). Therefore, the inorganic moiety is considered uncritical for acute toxicity to algae and naphthenic acids, released by hydrolysis, will be the determining ion for aquatic toxicity to algae. However, the EC50 values of source substances as well as the estimated EC50 value for the target substances are approx. 3 orders of magnitude above the water solubility of the target substance, and therefore, no acute toxicity effect is expected from the target substance. The NOEC of naphthenic acids, bismuth salts for algae was assessed on the basis of the NOEC value for naphthenic acids to be 9.24 mg/L.

Key value for chemical safety assessment

EC50 for freshwater algae:

33.4 mg/L

EC10 or NOEC for freshwater algae:

9.24 mg/L