Naphthenic acids, bismuth salts

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption: screening

Type of information:

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

Adequacy of study:

key study

Study period:

2006

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods with acceptable restrictions

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. Inorganic components such as bismuth cations are not expected to adsorb to organic matter and thus, the concept of adsorption/desorption to organic matter is only applicable to the anionic moiety of the substance, i.e. the naphthenic acids. Naphthenic acids have been investigated for adsorption to soil, and thus these data are used for assessing the adsorption behaviour of the organic moiety of the substance.

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.

3. ANALOGUE APPROACH JUSTIFICATION
Not applicable, as the source substance fully characterizes the organic moiety of the target substance, which is the corresponding bismuth salt. The target substance is expected to slowly hydrolyse to insoluble bismuth hydroxide, altering to bismuth oxide in biota, whereas the organic parts (the source substance) upon hydrolysis potentially may be adsorbed to organic matter as shown by the data on the source substance.

4. DATA MATRIX
The source substance was assessed for sorption to soil with 2.7 and 1.6% organic carbon. Also, samples of two different ionic strength were assessed. The results indicate that naphthenic acids in the carbon number range of 13 to 17 do sorb preferentially. Thus, higher naphthenic acids with lower ionic strength do sorb to soil less than lower naphthenic acids with higher ionic strength. Soils with higher organic content appeared to cause higher levels of sorption.
The target compound was analysed for the carbon distribution and found being mainly in the range of C10 – C15 with small amounts of higher carbon numbers present that were not quantified. Thus, the chain length distribution of the target compound is well represented by the range of the source carbon chain distribution.

Reason / purpose for cross-reference:

read-across source

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 121 (Estimation of the Adsorption Coefficient (Koc) on Soil and on Sewage Sludge using High Performance Liquid Chromatography (HPLC))

Type:

Kd

Value:

1.9 dimensionless

pH:

8

Temp.:

4 °C

% Org. carbon:

2.7

Remarks on result:

other: low ionic strength components

Type:

Kd

Value:

17.8 dimensionless

pH:

8

Temp.:

4 °C

% Org. carbon:

2.7

Remarks on result:

other: high ionic strength components

Type:

Kd

Value:

1.3 dimensionless

pH:

8

Temp.:

4 °C

% Org. carbon:

1.6

Remarks on result:

other: low ionic strength components

Type:

Kd

Value:

3.7 dimensionless

pH:

8

Temp.:

4 °C

% Org. carbon:

1.6

Remarks on result:

other: high ionic strength components

Validity criteria fulfilled:

yes

Conclusions:

Naphthenic acids obtained from oil sands tailings pond water (OSTPW) as a concentrate were observed to undergo rapid sorption to the soils tested. The Kd values determined ranged from 1.3 mL/g in Milli-Q water to 17.8 mL/g in synthetic groundwater. Selective sorption of naphthenic acids was observed, with components in the carbon number range of approximately 13 to 17 being adsorbed more readily than others. Soils with higher organic content appeared to cause higher levels of sorption. Similar results may be expected for naphthenic acid moieties resulting from hydrolysis of naphthenic acids, bismuth salts.

Executive summary:

Naphthenic acids obtained from OSTPW as a concentrate were observed to undergo rapid sorption to the soils tested. The values of Kd ranged from 1.3 mL/g in Milli-Q water to 17.8 mL/g in synthetic groundwater. The results also indicate that compounds in the Z = 0 to Z = -12 families in the carbon number range of 13 to 17 appear to sorb preferentially. This could have implications to the toxicity and transport in water, since different naphthenic acids are known to have varying degrees of toxicity. In the presence of synthetic groundwater (SGW), the adsorption coefficient (Kd) of the mixture of naphthenic acids on soil 1 with a higher organic carbon fraction (Foc) was an order of magnitude higher than that observed with the same soil and the Milli-Q water mixture, increasing from 1.9 ±0.2 mL/g to 17.8 ±1.5 mL/g. The adsorption coefficient of the mixture of naphthenic acids on soil 2, with a lower Foc, was also observably higher in the SGW mixture, increasing from 1.3 ±0.15 mL/g to 3.7 ±0.2 mL/g.

Description of key information

Naphthenic acids obtained from OSTPW as a concentrate were observed to undergo rapid sorption to the soils tested. The values of Kd ranged from 1.3 mL/g in Milli-Q water to 17.8 mL/g in synthetic groundwater. The results also indicate that compounds in the Z = 0 to Z =-12 families in the carbon number range of 13 to 17 appear to sorb preferentially. In the presence of synthetic groundwater(SGW), the adsorption coefficient (Kd) of the mixture of naphthenic acids on soil 1 with a higher organic carbon fraction (Foc) was an order of magnitude higher than that observed with the same soil and the Milli-Q water mixture, increasing from 1.9 ±0.2 mL/g to 17.8 ±1.5 mL/g. The adsorption coefficient of the mixture of naphthenic acids on soil 2, with a lower Foc, was also observably higher in the SGW mixture, increasing from 1.3 ±0.15 mL/g to 3.7 ±0.2 mL/g. The worst-case value found was Kd 17.8 at 2.7% org. carbon in soil (Koc = Kd * 100/% org. carbon = 659). This result is also used for hazard and risk assessment of naphthenic acids, bismuth salts, considering that the organic moieties of source and target substance are widely the same.

Key value for chemical safety assessment

Koc at 20 °C:

659