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Environmental fate & pathways

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Lead di(acetate) consists of a lead cation and an acetate anion. Based on the solubility of lead di(acetate) in water, a complete dissociation of lead di(acetate) resulting in lead cations and acetate anions may be assumed under environmental conditions. Thus, fate and toxicity of lead di(acetate) in the environment is most accurately evaluated by separately assessing the fate of its constituents lead and acetate, and read-across to the assessment entities soluble lead substances and acetic acid / salts is applied for the assessment. Please also refer to the data as submitted for each individual assessment entity.

Degradation: In the assessment of enviromental fate and behaviour of lead di(acetate), data available for the lead cation indicate that abiotic and biotic degradation in respective compartments does not contribute significantly to its fate in the environment. However, the removal of lead ions from the water column is rapid and its remobilization from the sediment is relatively insignificant (Rader, 2010). Thus, lead ions are considered equivalent to ‘rapidly degradable”. Acetate is readily biodegradable.

Transport and distribution: Partition coefficients for suspended matter in fresh, estuarine and marine water of 295,121 L/kg; 667,954 L/kg and 1,518,099 L/kg; and for sediments in fresh and marine water of 153,848 L/kg and 457,088 L/kg, respectively, indicate that lead is not very mobile and tends to bind to solid phases. For soil, a solid-water partitioning coefficient of 6,400 L/kg was determined. Since acetate and its degradation products decompose rapidly in the environment., adsorption/desorption is not expected to significantly affect the fate of acetate in the evnironment.

Bioaccumulation and biomagnification: Lead is not an essential nutrient, but the observed inverse relationship between internal and external Pb concentrations indicates that Pb is actively regulated. Within the range of typical environmental Pb concentrations, the median BAF values for fish, molluscs, insects, crustaceans of 23 L/kg ww, 675 L/kg ww, 1,830 L/kg ww and 3,440 L/kg ww indicate that lead poses the highest risk to insects and crustaceans, compared to fish or molluscs. Further, lead does not appear to biomagnify. Acetate is the most common building unit for biosynthesis, i.e. is converted into more complex organic molecules in organisms. Based on its ready biodegradability and its essentiality for eukaryotic and prokaryotic cells, a low potential for bioaccumulation of acetate is assumed.

 

Additional information

Read-across

Lead di(acetate) is an inorganic solid salt at room temperature and consists of lead cations and acetate anions. Based on the solubility of lead di(acetate) in water (443 g/L at 20°C, CRC handbook, 2008), a complete dissociation of lead di(acetate) resulting in lead and acetate ions may be assumed under environmental conditions. The respective dissociation is reversible, and the ratio of the salt /dissociated ions is dependent on the metal-ligand dissociation constant of the salt, the composition of the solution and its pH.

The metal-ligand complexation constant of lead di(acetate) of 1.93 ± 0.04 (Bunting & Thong, 1969) points to a relatively low strength of the monodentate bond between lead and acetate. Based on an analysis by Carbonaro & Di Toro (2007) of monodentate binding of lead to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as acetate anions are not expected to bind strongly with lead. Accordingly, protons out-compete lead ions for complexation of monodentate ligands given equal activities of free lead and hydrogen ions. The analysis by Carbonaro & Di Toro (2007) suggests that the following equation models monodentate binding to negatively-charged oxygen donor atoms of carboxylic functional groups:

log KML = αO * log KHL + βO; where

KML is the metal-ligand formation constant, KHL is the corresponding proton–ligand formation constant, and αO and βO are termed the Irving–Rossotti slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of acetic acid of 4.75 results in:

log KML = 0.442 * 4.75 + 0.631

log KML = 2.73 (estimated lead- acetate formation constant).

Thus, it may reasonably be assumed that based on the lead-acetate formation constant, the respective behaviour of the dissociated lead cations and acetate anions in the environment determine the fate of lead di(acetate) upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and determine subsequently its (eco)toxicological potential.

In the assessment of the environmental fate and toxicity of lead di(acetate), read-across to the assessment entities soluble lead substances and acetic acid / salts is applied since only the ions of lead and acetate are available in the environment and determine its environmental fate. Since lead cations and acetate anions behave differently in the environment, including processes such as stability, degradation, transport and distribution, a separate assessment of the environmental fate of each assessment entity is performed. Please refer to the data as submitted for each individual assessment entity.

To evaluate the environmental fate and toxicity of lead di(acetate), information on the assessment entities lead cations and acetate anions were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for lead di(acetate).