Cobalt(2+) C3/C10 carboxylates salts

Endpoint summary

Additional information

Read-across statement:

Metal carboxylate complexes are salts comprised of a metal atom and one or more organic acid (RCOOH) moieties. The metal ions can be monovalent or divalent and cover several groups from the periodic table: alkali metals (e.g. K), alkaline earth metals (e.g. Ca) and several groups of transition metals (e.g. Co, Zn, Mo, Mn and Zr). The carboxylic acids vary, depending on the compound, in terms of chain length, differences in the degree of saturation, and differences in branching. For example, the common carboxylic acids range in chain length from a short C3 alkyl chain (propionic acid) to the longest C18 (stearic acid). Additionally, the acids can have either straight chains (e.g. stearic acid) or branched chains (e.g. neodecanoic acid).

Measured data on the toxicity of such metal carboxylates are scarce and a read-across approach from data for both constituent ions is generally selected for the effects and risk assessment of these substances in the environment.

Under environmentally-relevant conditions, metal carboxylate salts readily dissociate from an ionic bonded salt into metal ions and free carboxylic acid upon dissolution in aqueous media. Dissociation is a reversible process and the proportion of dissociated salt present is dependent on the pH and composition of the solution and the metal-ligand dissociation constant of the salt. Predictions of stability of metal carboxylates (propionate, valerate, isovalerate and benzoate salts for Ca, K, Zn, Mn and Co) in a standard ISO 6341 medium (2 mMCaCl2, 0.5 mM MgSO4, 0.77 mM NaHCO3 and 0.077 mM KCl, pH 6 and 8) clearly show that monodentate ligands such as carboxylic acids have no potential for complexing metal ions in solution (<<1% of metal complexed at 0.001 mM; Visual minteq. Version 3.0, update of 18 October 2012. http://www2.lwr.kth.se/English/OurSoftware/vminteq/index.html). Complexation of the liberated free metal ion with ligands present in the natural environment (e.g. dissolved organic matter) will further increase the degree of dissociation of the metal-carboxylate salts.

Upon dissolution and dissociation of metal carboxylates into the metal ion and carboxylate anion, both constituent ions will each show its proper partitioning, degradation, and bioaccumulation behaviour in the environment. The environmental fate and behaviour for the metal and organic moieties is predicted to be clearly different from each other, resulting in a different relative distribution over the environmental compartments (water, air, sediment and soil). Because the relative exposure to both constituent ions in the various environmental compartments is hence predicted to be different from the original composition of the metal carboxylate, data for the ecotoxicological properties of metal carboxylates tested as such are also considered less relevant and a read-across approach to separate data for both the metal cation and carboxylate anion is preferred.

Physicochemical and ecotoxicological data for the individual dissociation products (i.e., metal and carboxylic acids) are therefore essential to understanding the environmental fate and toxicological characteristics of the metal carboxylate salts. Environmental fate and effects data developed with the free acid, or a simple salt that would readily dissociate (e.g. the sodium salt), can serve as surrogate data for the carboxylic acid component of each cobalt carboxylate salt. Similarly, data for the metal ions can be represented by fate and toxicity data generated with simple metal salts (e.g. chloride or nitrate salts). For example, the potential hazards associated with cobalt acetate can be estimated through the evaluation of the cobalt free ion, tested as cobalt dichloride, and the acetate moiety, tested as sodium acetate. Thus, data for each individual carboxylic acid (tested as the free acid or Na, K or Ca salt) and the individual metal (tested as the free metal, metal chloride, other simple metal salt) can be used to “read-across” to characterize the hazard of a cobalt carboxylate compound.

In waters, cobalt has two common oxidation states, +2 and +3 (Baes and Mesmer, 1976). Under most environmental conditions including natural waters, Co exists as the divalent cation Co(II) and is able to form strong complexes with organic ligands (McLaughlin and Batley 2010). Recent modelling suggests that Co(II) is thermodynamically stable under the Eh-pH conditions of seawater and most natural freshwaters (Glasby and Schulz, 1999; Krupka and Serne, 2002). Key conclusions from the available geochemical data indicate:

- Cobalt has two main oxidations states (Co(II) and Co(III)) in the environment. The divalent cobalt species is highly soluble and is readily available for uptake by organisms, while the trivalent cobalt species is relatively insoluble and usually found as insoluble oxides or hydroxides.

- In a well-oxygenated system of neutral to acidic pH, aqueous Co(II) is the dominant species. As the pH increases (beyond pH 7), if cobalt is present in high concentrations, Co(II) can precipitate as either cobalt hydroxide (Co(OH)2), cobalt carbonate (CoCO3)or cobalt oxide (Co3O4), depending upon the redox potential of the system.

- Under reducing conditions, at high cobalt concentrations, cobalt sulphide (CoS) can precipitate. At low cobalt concentrations, cobalt solubility is also low at high pH due to strong retention by sediments and soils through sorption processes.

- Cobalt is present as Co(II) in fresh and marine waters, however, a significant percentage (e.g., 50-90 %) of cobalt may be complexed by dissolved organic carbon (DOC). Depending on the ligand, a portion of the Co may also be present as Co(III), in biogenic complexes such as cobalamin (vitamin B12).

- Available data for Co(III) species are limited but do not suggest toxicity greater than that shown for Co(II) species and, based on the fact that vitamin B12 is essential for the growth of many aquatic organisms, the limiting toxicity of cobalt in aquatic systems is most likely to be that of Co(II).

- In the absence of speciation data, ecotoxicity data derived for cobalt using soluble Co(II) compounds should provide a conservative estimate of the toxicity of other sparingly soluble cobalt compounds.

As previously discussed, cobalt salts readily dissociate in the aquatic environment from an ion pair into a free cobalt ion and free anion(s) (depending on the metal salt) under environmentally relevant conditions. Dissociation is a reversible process and the proportion of dissociated salt present is dependent on the pH of the solution and the pKa of the salt. The pKa is the dissociation constant, which is the pH at which 50% dissociation occurs. With any cobalt salt, the transport and bioavailability of the cobalt cation and associated anion are determined by their solubility in environmental media (i.e., water, soils, sediments) and biological fluids (e.g., gastric fluid, blood), which is dictated by environmental parameters such as pH. Under most environmentally relevant conditions, cobalt salts will be present as the free metal and free anion. Under increasingly acidic conditions, the relative proportions will shift until all of the salt is present as free metal and free anion.

The derivation of environmental fate data like adsorption/desorption coefficients and bioconcentration/ bioaccumulation factors is based on measured cobalt concentrations and reflect the properties of the cobalt ion. The cobalt ion is the only form under which cobalt originating from the cobalt salt will occur. Therefore, the reported elemental-based environmental fate data in this section of the dossier are considered relevant for the behaviour of the cobalt ion that is released into the environment from the cobalt salt.

With reference to the RAAF (ECHA, 2017), criteria for a category approach Scenario 5 with Appendices C are met for the cobalt category substances as detailed in Appendix 1.3 of the CSR. Due to the absence of substance specific information for the majority of substances within the cobalt category, the approach will read-across data from representative source substances to all other members of the read-across group. Salt reaction of cobalt(2+) and C3/C10 carboxylates can be considered as part of this read-across group.