Naphthenic acids, bismuth salts

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

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

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

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 microorganisms, results for bismuth 3+ cations were assessed as well as data for a surrogate to 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), represented by a specific derivative as model compound (cyclohexyl carboxylic acid) and bismuth hydroxy nitrate oxide (as surrogate for bismuth cations) are appropriate surrogates for assessing the acute toxicity of the target substance to microorganisms.

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 hardly effects were seen (no EC50 could be established and the NOEC was set to 300 mg Bi/L). The second source substance cyclohexyl carboxylic acid, a model substance used by researchers for naphthenic acids allows to assess the anionic hydrolysis product of the target substance, and for cyclohexyl carboxylic acids an EC50 (vibrio fisheri, brackish water microorganism) of 13 ±1.6 mg/L (equivalent to 100 mM/L) was found. 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 microorganisms and an EC50 (3 h) of ≥ 300 mg Bi/L was found, equivalent to more than 1.4 mol/L. Thus, determining for the toxicity towards microorganisms will be any naphthenic acid anions formed upon hydrolysis of the target substance. Considering an EC50 (15 min, vibrio fisheri) of ~13 mg/L for cyclohexyl carboxylic acid, this result was re-calculated to approx. 100 mmol/L (calculated based on a molar mass of 128.17 of cyclohexanecarboxylic acid) and considering the 3:1 molar ratio 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 28.3 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 microorganisms.

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

>= 1.44 mol/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

bismuth

Basis for effect:

inhibition of total respiration

Remarks on result:

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

Duration:

15 min

Dose descriptor:

EC50

Effect conc.:

ca. 100 mmol/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth inhibition

Remarks on result:

other: value for cyclohexylcarboxylic acid

Duration:

15 min

Dose descriptor:

EC50

Effect conc.:

28.3 mg/L

Nominal / measured:

estimated

Conc. based on:

test mat.

Basis for effect:

growth inhibition

Remarks on result:

other: value for naphthenic acids, bismuth salts

Validity criteria fulfilled:

not applicable

Conclusions:

Based on read-across from naphthenic acids surrogate and bismuth toxicity data to microorganisms, a theoretical EC50 value of ~28.3 mg/L was estimated for naphthenic acids, bismuth salts; this value is approx. three orders of magnitudes above the threshold for water solubility (< 0.036 mg/L) and thus, no acute toxicity to microorganisms is expected upon exposure to naphthenic acids, bismuth salts.

Description of key information

Based on read-across from naphthenic acids surrogate and bismuth toxicity data to microorganisms, a theoretical EC50 value of ~28.3 mg/L was estimated for naphthenic acids, bismuth salts; this value is approx. three orders of magnitudes above the threshold for water solubility (< 0.036 mg/L) and thus, no acute toxicity to microorganisms is expected upon exposure to naphthenic acids, bismuth salts. Nevertheless, the theoretical value is conservatively taken forward for hazard and risk assessment.

Key value for chemical safety assessment

EC50 for microorganisms:

28.3 mg/L