Neodecanoic acid, zirconium salt

Endpoint summary

Description of key 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. Thus, the transport and distribution of neodecanoic acid, zirconium salt in the environment is most accurately evaluated by separately assessing the fate of its constituents zirconium and neodecanoate anions. 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.

 

Zirconium, however, 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.

 

The estimated adsorption coefficient for neodecanoic acid 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.