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Environmental fate & pathways

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In the assessment of the environmental fate of zinc dipropionate, read-across to the assessment entities soluble zinc substances and propionate is applied since the ions of zinc dipropionate determine its environmental fate. Since zinc ions and propionate ions behave differently in the environment, in respect of 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. In brief, metal partition coefficients for distribution between different fractions e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) are the only parameter relevant for the fate of zinc in the environment and biotic degradation is the only fate process expected to influence the fate of propionate in the environment. For both moieties, bioconcentration/bioaccumulation is considered not relevant.

Additional information

Metal carboxylates are salts consisting of metal cation and carboxylic acid anion. Based on the solubility of substance zinc dipropionate in water (40.5 g/L at 30°C), a complete dissociation upon dissolution resulting in zinc and propionate ions may be assumed under environmental conditions. The respective dissociation is in principle 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. However, under environmental conditions, a reunion of the dissociated ions is highly unlikely and it may reasonable be assumed that the respective behaviour of the dissociated zinc cations and propionate anions in the environment determine the fate of zinc dipropionate upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently the (eco)toxicological potential. 

In general terms, the stability constant of a metal complex can be calculated as follows: K = [ML] / [M][L], where K is the stability constant (expressed as a logarithm); M is the amount of metal ion such as Zn2+ion, and L is the amount of a ligand such as propionate. The total concentration of metal CM can be computed with specialized computation programs. The basic equation CM = [M] + [ML] with [ML] = K [M] [L] becomes CM = [M] (1 + K [L]); hence [M] = CM / (1 + K [L]) shows that the concentration of the metal depends on the stability constant of the complex and free concentration of the ligand which is dependent upon corresponding pK and pH values. The metal chelate stability constant (log K1) for zinc dipropionate is 1.01. Very low stability constant numeric values (between negative values and 1) mean that the metal-ligand is not only soluble in water but readily dissociates into the metal ionic form and the ligand, yielding essentially all metal in ionic form at pH as low as stomach acid (about pH 2 to 3) to as high as physiologic pH 7.4 (e.g. the pH of the main extracellular body fluids such as serum and lymph). All metal-ligand compositions release increasingly more metal ion as pH is lowered towards increased acidity, and more metal hydroxides are released as pH is raised into the basic end of the pH scale (Martell E.A. NIST Critically Selected Stability Constants of Metal Complexes). 

Thus, it may reasonably be assumed that based on the stability constant for zinc dipropionate, the respective behaviour of the dissociated zinc cations and propionate anions under physiological conditions and in the environment determine the fate of zinc dipropionate upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently its (eco)toxicological potential.

Therefore, in the assessment of the (eco)toxicity of zinc dipropionate, a read-across to data for propionic acid and soluble zinc substances is applied since only the ions zinc and propionate are available in the environment and systemically and determine the (eco)toxicological potential of zinc dipropionate.

Please refer to the data as submitted for each individual assessment entity.

For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely "Read-across concept Category approach for zinc dipropionate".