Copper

Endpoint summary

Additional information

4.0 General summary on environmental fate and pathways

Copper is a natural element and transition metal with more than one oxidation state. Copper in its metallic form (Cu°) is not available. Copper needs to be transformed to its ionic forms (copper ions) to become available for uptake by living organisms. This section summarizes available information relevant to the environmental fate of copper ions. Information is available on the release of soluble copper ions from metallic copper (Cu°) into environmental media (transformation-dissolution tests), on the speciation of copper ions in aqueous media, on removal processes that govern the fate of copper ions in the environment (water, sediments, and soils), on partitioning of copper to suspended particles, and on bioaccumulation.

The information on transformation-dissolution and on removal from the water column is only briefly summarized here. A more complete discussion is available in the report "The environmental hazard classification of copper", which is attached to IUCLID section 13.

4.0.1.  Stability and Biodegradation

4.0.1.1   Transformation/dissolution of Cu°

Copper metal (Cu°) is insoluble and needs to be transformed to solubilised (dissolved) species in order to be available to the aquatic environment. Therefore, transformation-dissolution tests were conducted according to the GHS protocol (GHS, 6^thedition, Annex 10) and are subsequently used for assessing the classification of copper for environmental hazard. The transformation-dissolution studies are described in details in the IUCLID file, section “additional information on fate and pathways”, and are summarized in the separate report “The environmental hazard classification of copper” which is attached in IUCLID section 13. The various studies and experiments have different pH (6, 7, or 8), duration (7 or 28 days), copper forms tested (massive copper, coarse powders, or fine powders), experimental set-ups (materials tested as such or embedded in epoxy resin), and loadings (expressed per unit mass or per unit exposed surface area). These studies are used for the classification of the various copper forms for environmental hazards, and they also show supportive evidence for the environmental fate of copper.

In summary, the transformation-dissolution data show that, in relevant environmental media, only very limited dissolution of copper occurs within 7 or 28 days. In addition, the copper release depends on the exposed surface area. Expressing copper release in transformation-dissolution tests relative to the exposed surface area (i.e. the “specific surface area approach”) therefore is a scientifically sound method to assess copper release in environmental media.

 

4.0.1.2   Transformation of Cu-ions released in the environment - Copper speciation

Once released to the environment, copper ions have more than one oxidation state and copper is thus characterized as transition metal. The principal ionic forms are cuprous (Cu(I), Cu⁺) and cupric (Cu(II), Cu²⁺). The trivalent form (Cu(III), Cu³⁺) occurs but is relatively unimportant in physical and biological systems. Cu⁺is unstable in aqueous media and soluble Cu¹⁺compounds readily transforms into soluble Cu²⁺ions, compounds and/or insoluble Cu²⁺ions, compounds (eg copper sulfides) that precipitate. This transformation of Cu⁺to Cu²⁺is a result of a redox reaction initiated through atmospheric water vapour as well as in aqueous solution. However, monovalent copper cations are only susceptible to such transformation when they are not chemically bound in insoluble compounds or stabilised in complexed forms.

The transformation of Cu(I) to Cu (II) can be described by:

 (1) 2 Cu₂O + 2H₂O = 4Cu⁺+ 4OH^-, and

 (2)  4Cu⁺+ O₂+ 4H⁺= 4Cu²⁺+ 2H₂O

 Both sub-reactions are summarised as:

2Cu₂O(s) + O₂(g) + 4H⁺= 4Cu²⁺+ 4OH^-

Among the copper species released/transformed, Cu (II) is thus the most environmental relevant species. It is further recognised that Cu (II) ions - commonly named free cupric ions- are the most active copper species and that total Cu or Cu(II) concentrations are usually not directly related to ecological effects since exposure of biota may be limited by processes that render Cu unavailable for uptake (ICPS, 1998). Assessing the species of Cu (II) therefore has ecotoxicological relevance. After being released into the environment, the Cu(II) ions typically bind to inorganic and organic ligands contained within water, soil, and sediments. In water Cu(II) binds to dissolved organic matter (e. g., humic or fulvic acids). The Cu(II) ion forms stable complexes with -NH₂, -SH, and, to a lesser extent, -OH groups in these organic acids. Cu(II) will also bind with varying affinities to inorganic and organic components in sediments and soils. For example, Cu(II) binds strongly to hydrous manganese and iron oxides in clay and to humic acids, but much less strongly to aluminosilicates in sand. In all environmental compartments (water, sediment, soil), the binding affinities of Cu(II) with inorganic and organic matter is dependent on pH, the oxidation-reduction potential in the local environment, and the presence of competing metal ions and inorganic anions.

Some key papers on copper speciation in freshwater, marine waters, sediments and soils are provided in the section "additional information on environmental fate"

4.0.1.3   Copper attenuation, removal from water column, geochemical cycling

As described above, after the release of Cu(II) as copper ions in the environment, further transformations occur which convert the copper ions into other, nontoxic forms. The concentrations of copper ions that are available for uptake by biota depends on different processes: precipitation, dissolution, adsorption, desorption, complexation and competition for biological adsorption sites (ligands). These processes are critical for the fate of copper in the environment. For copper, modelling, laboratory, mesocosm and field tests are available to document these processes. These papers are available from the section "additional information on environmental fate". This information is summarized here separately for the water, sediment, and soil compartments. A full discussion of these studies is available from the report "The environmental hazard classification of copper" which is attached to IUCLID section 13.

 

Copper removal rates from the water column have been quantified from laboratory experiments, field experiments, and modelling approaches under a range of environmentally relevant conditions. These datasets consistently demonstrate that copper is removed from natural aquatic systems. The modelling assessment with the Ticket unit world model (Rader 2013) validated the above findings and allowed to assess more broadly spatial and temporal variability as applicable to European waters.

 

In the sediment compartment,copper binds to the sediment organic carbon (particulate and dissolved) and to the anareobic sulfides, resulting in the formation of CuS. CuS has a very low stability constants/solubility limit (LogK=-41 (Di Toro et al.,1990) – see sectionadsorption/desorption) and therefore the “insoluble” CuS keeps copper in the anaerobic sediment layers, limiting the potential for remobilization of Cu-ions into the water column. Following the rapid removal from the water-column, evidence on change in speciation to non-soluble, non-available copper sulphides and the “irreversibility” of the formation of such “non-available forms” is available from modelling, laboratory experiments, field experiments, and monitoring data.

 

In view of the ample available evidence, it can therefore be concluded that under most “environmentally relevant” conditions, dissolved copper ions are transformed and removed from natural waters within 28 days. Remobilisation of Cu to the water column is not likely to occur. This is based on the extensive weight-of-evidence which is summarized in Burton et al. (2018) for metals in general, and above and in Mutch Associates (2018) for copper specifically, together with the data obtained through the extended OECD 29 T/DP method by CanmetMINING (2018). Copper is therefore considered rapidly removed, conceptually equivalent to “rapid degradation” for the purposes of hazard classification of organic substances. This information is also used in the context of classification for environmental hazards.

In soils,decreases in copper solubility and in copper bio-availability are observed following copper spiking in the laboratory and from long-term field copper exposure experiments. Short term attenuation and long term ageing of copper, spiked in soluble forms to soils was demonstrated from laboratory and field experiments (Ma et al., 2006a and 2006b) and reported in the section “adsorption/desorption”. The soil environmental factors governing short term attenuation and ageing rates are soil pH, organic matter content, incubation time and temperature with soil pH being the key factor for ageing of Cu added to soils. From a range of laboratory and field experiments an ageing factor of 2 was derived as a reasonable worst case when considering field exposure data.

 

4.0.2   Transport and distribution

Relevant partitioning coefficients are available from literature.

-Aquatic compartment

Partition coefficient in freshwater suspended matter         Kp_susp= 30,246 l/kg (log Kp (pm/w) = 4.48) (50^thpercentile)

Partition coefficient in freshwater sediment                       Kpsed = 24,409 l/kg (log Kp(sed/w) = 4.39) (50th percentile)

Partition coefficient in estuarine suspended matter            Kp_susp= 56,234 l/kg (log Kp (pm/w) = 4.75) (50^thpercentile)

Partition coefficient in marine suspended matter               Kp_susp= 131,826 l/kg (log Kp (pm/w) = 5.12) (50^thpercentile)

-Terrestrial compartment

Partitioning coefficient                                                   Kd value soil: 2120 L/kg(log Kp (pm/w) = 3.33) (50^thpercentile)

 

4.0.3. Bioaccumulation

Because copper is an essential nutrient, all living organisms have well developed mechanisms for regulating copper intake, copper elimination and internal copper binding. The information in the accumulation section demonstrates that copper is well regulated in all living organisms and that highest/ BAF values are noted when copper concentrations in water, sediments and soils are low and for organisms/ life stages with high nutritional needs. The BCF/BAF values therefore have no ecotoxicological meaning. It should be mentioned that the non-applicability of BCFs for metal and especially for essential metals was already recognized in the regulatory framework of aquatic hazard classification (OECD,2001).

Importantly, the literature review demonstrates that copper is not biomagnified in aquatic or terrestrial ecosystems.

The section further includes critical data related to (1) the accumulation of copper on critical target tissues (eg gills in aquatic organisms); (2) the influence of environmental parameters (eg Organic Carbon, pH, Cationic Exchange Capacity) as well as food intake on the accumulation of copper. 

This information is relevant to the understanding of the accumulation as well as the mechanism of actions, described in the section ecotoxicological information

More detailed summaries on respectively aquatic and terrestrial bioaccumulation are available from the aquatic and terrestrial bioaccumulation summary sections

The information relevant to assessing copper toxicity from dietary exposure - of relevance to secondary poisoning assessments is included in the section "ecotoxicological information". 

The summary record “ecotoxicological information “ further provides an overall summary of the rationale for the absence of bio-accumulation and no-concern for secondary poisoning (see alos section 7.5.3 – secondary poisoning)