Neodecanoic acid, zirconium salt

Endpoint summary

Description of key information

The fate of neodecanoic acid, zirconium salt in the environment is most accurately evaluated by separately assessing the fate of its moieties zirconium cations and neodecanoate anions. Abiotic and biotic degradation in respective compartments do not seem to affect the fate of zirconium cations and neodecanoate anions.

 

Zirconium

Abiotic degradation (hydrolysis) is not relevant for inorganic substances including zirconium. In general, (abiotic) degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

Biotic degradation is not relevant for inorganic metals and metal compounds. Zirconium as an element is not considered to be (bio)degradable.

Transport and distribution: Zirconium has a very low mobility under most environmental conditions, mainly due to the stability of the mineral zircon (Zr(SiO4)) and the low solubility of the hydroxide Zr(OH)4. This limits the concentration of Zr in most natural water to <0.05 μg/L even in saltwater. Depending on the solution pH, Zr4+ and different zirconium hydroxides (i.e. Zr(OH)(3+), Zr(OH)2(2+), Zr(OH)3(+), Zr(OH)4) exist in solution. At pH 7, a Zr(OH)2(CO3)2(2-) complex can form, but this is unstable and decomposes with decreasing pH to form Zr(OH)4. The hydro-bicarbonate (Zr(OH)4-HCO3-H2O) complex may be the most significant Zr complex in natural water. Colloidal zirconium is also readily adsorbed by organic matter, macroplankton and siliceous material. Zirconium is considered to be only slightly mobile in soil with organic acids the main transporting agents for its transport and distribution (Salminen et al. 2005 and references therein).

Batch equilibrium experiments with solutions of soluble ZrOCl2 seem to indicate a very fast adsorption of zirconium to soil (1/k = ~ 3 min) and resulted in Kd values of 6,000 and 30,000 L/kg (dw) for an acidic and calcareous soil, respectively. The most important process appears to be adsorption to ferric oxides. However, very low soil/solution ratios favoring an adsorption were applied and resulting Kd values may be overestimated. Desorption experiments indicated very limited desorption, suggesting that non-reversible adsorption processes such as innersphere complexation or surface precipitation are involved. Considering the low solubility of zirconium in environmental solutions, a precipitation of zirconium hydroxides may also (at least in parts) explain the derived Kd values. Thus, zirconium is expected to be rather immobile in soils.

 

Neodecanoic acid

Abiotic degradation is not considered to significantly affect the environmental fate of neodecanoic acid since neodecanoic acid is lacking hydrolysable functional groups and does not absorb light within a range of 290 to 750 nm. Neodecanoic acid is not readily biodegradable (11% biodegradation in 28 d) based on results from a standard OECD ready biodegradation test. Studies are not available to assess the biodegradability of neodecanoic acid under simulated conditions or in soil, but given the limited biodegradation in water, biodegradation under simulated conditions, or in soil is not expected to occur to a great extent.

 

Transport and distribution: The estimated adsorption coefficient is expected to be sensitive to pH. Neodecanoic acid is ionizable and has a pKa of 5.17 and is expected to dissociate to the ionised form at neutral pH, which is typical of most natural surface waters and therefore, remain largely in water. Estimated neo-decanoic acid log Koc is 2.5 using the measured log(Kow) value of 4.3 for the neutral species (at pH 2.5). The log Koc is 1.32 (Koc = 21) using the measured log(Kow) value at pH 6.7.

The  vapor pressure for neo-decanoic acid is 2.0 Pa, which suggests limited volatilization from the terrestrial compartment. The estimated Henry’s Law constant for neo-decanoic acid is also low (0.54 Pa m³/mol at 25 °C), which indicates that volatilization from water is not expected to occur at a significant rate. Volatilization is not expected to be a significant transport process for neodecanoic acid.

Additional information

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Dissociation of dissolved neodecanoic acid, zirconium salts resulting in zirconium cations and neodecanoate anions 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.

Based on analysis of thermodynamic stability of aqueous zirconium species, the concentration of free Zr4+ ions under environmental conditions is barely detectable. Ionic zirconium (Zr4+) at relevant pH conditions (pH 7 - 8) of aquatic and terrestrial environments will rapidly transform to zirconium-oxide and -hydroxide complexes, precipitate and not be bioavailable to aquatic organisms. Therefore, zirconium has a very low mobility and bioavailability under most environmental conditions whereas neodecanoate is rather mobile.

Thus, it may reasonably be assumed that the respective behaviour of zirconium cations and neodecanoate anions in the environment determine the fate of neodecanoic acid, zirconium salts 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 ecotoxicological potential.

Thus, in the assessment of environmental fate and pathways of neodecanoic acid, zirconium salts, read-across to the assessment entities zirconium ions and neodecanoate is applied since the individual ions of neodecanoic acid, zirconium salts determine its environmental fate and toxicity. Since zirconium ions and neodecanoate ions behave differently in the environment, regarding their fate and toxicity, a separate assessment of each assessment entity is performed. 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 Assessment Report for neodecanoic acid, zirconium salts.