Etidronic acid

Sediment toxicity

Description of key information

In accordance with Column 2 of REACH Annex IX the study does not need to be conducted.

Additional information

Testing for sediment toxicity is not considered necessary because:

- PNECsediment has been calculated from PNECfreshwater on the basis of the equilibrium partitioning method; the risk characterisation ratios (RCR) based on PNECsediment derived from this method are <1.

Considerations for applicability of the Equilibrium Partitioning to chelating substances:

Taken from ECHA (2008, R.10.5.2.1) “In the partitioning method, it is assumed that the:

• sediment-dwelling organisms and water column organisms are equally sensitive to the chemical;

• concentration of the substance in sediment, interstitial water and benthic organisms are at thermodynamic equilibrium: the concentration in any of these phases can be predicted using the appropriate partition coefficients;

• sediment/water partition coefficients can either be measured or derived on the basis of a generic partition method from separately measurable characteristics of the sediment and the properties of the chemical (for the derivation of the sediment-water partition coefficient and the limits of the calculation methods see Section R.16.4.3.3).”

Based on the aquatic toxicity data available, including read-across data, there is no indication that the first assumption does not hold true for the substances in the HEDP category.

The concentration of the substance in sediment, interstitial water and benthic organisms can be predicted using appropriate coefficients. For the derivation of equilibrium partitioning sediment PNECs the K_{p(susp-water)}= 2000 l/kg, equivalent to K_(susp-water)of 500 m³/m³have been derived from available experimental evidence, consistent with adsorption behaviour of other phosphonate complexing agents. The use of substance specific measured adsorption values result in partition coefficient that take into account the inorganic substrate chelating properties of phosphonates, i.e. the resulting partition coefficient values are higher than if they had been calculated based on a true K_oc or K_ow.

An important factor when conducting the risk assessment of phosphonate substances to sediment (and soil) organisms is their bioavailability in these substrates. The substances are considered to be virtually unavailable due to their strong chelating potential. The equilibrium is strongly in favour of the bound form, as discussed in the adsorption/desorption section, such that the adsorption is considered effectively irreversible. As such, the concentration in interstitial water using this method represents a maximum value.

-          Using the relationships that Cs/Cw = K_dat equilibrium, and when total loading rate = C_s*M_s+C_w*M_wat equilibrium (also expressed by Equation R.16-7 in ECHA guidance: K_comp-water= C_total.comp/C_{porewater.comp}= F_air.comp*K_air-water+ F_water.comp+ F_solid.comp*(K_p.comp/1000)*RHO_solid) the distribution of HEDP may be modelled. Using this method, it can be seen that a nominal 100 mg HEDP/kg wwt in a standard sediment with the compositional characteristics defined in ECHA guidance part R.16 and relevant OECD testing guidelines (e.g. OECD TG 233) is equivalent to a concentration of approximately 0.1 mg/l concentration in the sediment interstitial/overlying water. Similarly, in a typical soil a nominal 100 mg HEDP/kg wwt would be equivalent to a concentration of approximately 0.1 mg/l in the soil interstitial water.

-          Additionally, based on local exposure concentrations in the exposure assessment of the HEDP Category, PECs in the range 1E-10 to 1E-02 mg/l in soil interstitial water and in the range 1– 60 mg/kg wet weight sediment (equivalent to 0.05 – 0.3 mg/l in sediment interstitial water) are predicted by EUSES. Aquatic testing does not suggest that effects are expected to be observed at these concentrations. The lowest NOEC in long-term aquatic testing is 6.75 mg/l as HEDP (acid equivalent). The PNEC_{fresh water} value is 0.14 mg/l.

-          The adsorption value of HEDP to inorganic substrate is considered to be as strong as EDTA (CAS 60-00-4, Risk Assessment 2004)a known complexing agent, which is considered to be non-toxic and to reduce bioavailability of metals and has been considered as a remediation process.

-          Different feeding patterns will amount to different exposures. Lumbriculus variegatus and Tubifex tubifex feed through ingestion of sediment particles, and therefore will be more susceptible to exposure of phosphonate bound to the inorganic matter; while organisms such as the Chironomus spp. feed on organic matter therefore exposure is likely to be very limited.

-          The adsorption of chemicals from the gut of sediment invertebrates is mediated through the presence of amino acids and surfactants at near neutral pHs (National Research Council, 2003), which will not dissociate the phosphonates from the inorganic substrate. Therefore phosphonates are not expected to become more bioavailable to sediment-ingesting organisms.

         

Based on the above information, substances will be very minimally bioavailable to sediment organisms, due to the substances’ substrate binding capacity; hence the Equilibrium partitioning model may be overestimating toxicity. Therefore, the use of Equilibrium partitioning is considered to be a conservative approach, when the coefficients are calculated based on measured K_d data.

References:

National Research Council, 2003, Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications Committee on Bioavailability of Contaminants in Soils and Sediments, National Research Council