Naphthenic acids, zinc salts

Bioaccumulation: aquatic / sediment

Description of key information

Zinc naphthenate consists of zinc salts of naphthenic acids and in the aquatic environment is expected to dissociate into zinc cations and naphthenic acid components.

 

Zinc is an essential element which is actively regulated by organisms, so bioconcentration/bioaccumulation is not considered relevant. The ranges of BCF values are the result of active regulation mechanisms that keep the internal zinc concentration of the organisms within an optimal range.

 

For the naphthenic acid component, QSAR estimates are provided showing the bioaccumulation potential of representative structures. The BCF of the representative structures of zinc naphthenate were estimated using a QSAR model (BCFBAF v3.01 in EPISuite v4.11, US EPA 2010). QSAR predictions are provided for representative structures which bracket the range of potential constituents present in the naphthenic acid components of zinc naphthenate with regard to the molecular weight and number of cyclic groups. The BCF of the representative constituents of zinc naphthenate are estimated by QSAR to be in the range of 3.162 to 207.7 L/kg ww.

Additional information

Zinc naphthenate consists of zinc salts of naphthenic acids and in the aquatic environment is expected to dissociate into zinc ions and naphthenic acid anions. As such, data are presented separately for the bioaccumulation potential of the zinc cations and the naphthenic acid anions.

 

For the naphthenic acids component, QSAR predictions have been provided for representative structures which bracket the range of potential constituents present in the naphthenic acid components of zinc naphthenate with regard to the molecular weight and number of cyclic groups. Zinc naphthenate consists of 70-95% zinc salts of naphthenic acids, C8-C20 and 0-3 rings, and 5-30% petroleum fraction, C12-C22, which remains unreacted from the naphthenic acid raw material. The BCF of the representative structures of zinc naphthenate were estimated using a QSAR model (BCFBAF v3.01 in EPISuite v4.11, US EPA 2010). The BCF values of the representative constituents of zinc naphthenate are estimated by QSAR to be in the range of 3.162 to 207.7 L/kg ww.

 

For the zinc component, bioaccumulation is not considered relevant for essential elements because of the general presence of homeostatic control mechanisms.

 

McGeer et al (2003) extensively reviewed the evidence on bioconcentration and bioaccumulation of zinc as a function of exposure concentration in a number of taxonomic groups (algae, molluscs, arthropods, annelids, salmonid fish, cyprinid fish, and other fish). The data clearly illustrated that internal zinc content is well regulated. All eight species taxonomic groups investigated exhibited very slight increases in whole body concentration over a dramatic increase in exposure concentration. In fact, most species did not show significant increases in zinc accumulation when exposure levels increased, even when exposure concentrations reached those that would be predicted to cause chronic effects. This suggests that adverse effects related to Zn exposure are independent of whole body accumulation. Due to the general lack of increased whole body and tissue concentrations at higher exposure levels, the zinc BCF data showed an inverse relationship to exposure concentrations. In all cases, the relationship of BCF to exposure was significant and negative. The slopes of the BCF/BAF – exposure relations were: algae: -1.0, insects: -0.79, arthropods: -0.73, molluscs: -0.83, salmonids: -0.92, Centrarchids: -0.80, Killifish: -0.84, other fish: -0.87. Overall, species mean slope was -0.85 ± 0.03 (McGeer et al 2003).

 

The physiological basis for the inverse relationship of BCF to zinc exposure concentration arises from Zn uptake and control mechanisms. At low environmental zinc levels, organisms are able to sequester and retain Zn in tissues for essential functions. When Zn exposure is more elevated, aquatic organisms are able to control uptake. There is clear evidence that many species actively regulate their body Zn concentrations, including crustaceae, oligochaetes, mussels, gastropods, fish, amphipods, chironomids by different mechanisms (McGeer et al 2003).