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Sediment toxicity

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sediment toxicity: long-term
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Description of key information

Key value for chemical safety assessment

Additional information

In accordance with Column 2 of REACH Annex X, there is no need to further investigate the effects of this substance to sediment dwelling organisms because, as indicated in REACH guidance R7b Section R.7.8.14 (ECHA, 2017), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is below 1, therefore the risk is already adequately controlled and further testing is not justifiable. An additional factor of 10 is applied to the screening assessment when undertaking risk characterisation based on equilibrium partitioning PNEC for sediment for substances which have log Kow >5. This is to account for additional exposure via ingestion of adsorbed substance on particles and it is therefore scientifically appropriate to consider making a similar adjustment for substances which have binding or adsorption properties indicating highly adsorbing behaviour, equivalent to a high log Kow. Phosphonates are highly adsorbing even though the log Kow value is extremely low, as discussed in IUCLID Section 5.4. The tendency to bind irreversibly to the inorganic substrate, the generalised lack of uptake and rapid clearance from the body associated with oral exposure (see IUCLID Section 7 for further information), and the low bioaccumulation potential, are strong arguments against a need to make such an adjustment to RCR values. On this basis, the extra factor of 10 is not applied in this assessment. The substance is involatile and highly adsorbing and low toxicity was observed in short and long-term aquatic tests, and there is no reason to expect effects in the sediment compartment that were not expressed in the aquatic compartment. The phosphonate ligand binds strongly and irreversibly to various minerals present in sediment and so bioavailability to sediment organisms is extremely limited. The PNEC calculated by Equilibrium Partitioning has been derived for the purpose of conducting a chemical safety assessment, and the risk characterisation ratios are below 1. Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0 Ecotoxicological Information and Chapters 9 and 10 of the Chemical Safety Report, respectively.

Additional information on the considerations for the applicability of the Equilibrium Partitioning Model to chelating substances:

Taken from ECHA (2008, R. “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.”

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 Kp (susp-water)= 2000 l/kg, equivalent to K(susp-water) of approximately 500 m3/m3 have 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 Koc or Kow.

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 = Kd at equilibrium, and when total loading rate = Cs*Ms+Cw*Mw at equilibrium (also expressed by Equation R.16-7 in ECHA guidance: Kcomp-water = Ctotal.comp/Cporewater.comp= Fair.comp*Kair-water+ Fwater.comp+ Fsolid.comp*(Kp.comp/1000)*RHOsolid) the distribution of HEDP may be modelled. Using this method, it can be seen that a nominal 43.4 mg HEDP/kg wwt (equivalent to 200 mg/kg dwt) 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 70.1 mg HEDP/kg wwt (equivalent to 79.4 mg/kg dwt) 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, PEC values are reported in the ranges of 5.67E-03 to 883.8 mg/kg dwt (equivalent to 1.23E-03 to 192.13 mg/kg wwt) for freshwater sediment, 0.799 to 88.13 mg/kg dwt (equivalent to 0.1737 to 19.16 mg/kg wwt) for marine water sediment and 1.02E-08 to 4.387 mg/kg dwt (equivalent to 8.976E-09 to 3.870 mg/kg wwt). These values can be expressed in terms of the interstitial water, based on the equations above and in ECHA guidance R.16: 2.83E-06 to 0.443 mg/L freshwater sediment interstitial/overlying water, 4.00E-04 to 0.044 mg/L marine water sediment interstitial/overlying water and 1.28E-11 to 5.52E-03 mg/L soil interstitial water, respectively. 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 active acid/L as HEDP (acid equivalent). The PNECaquatic (freshwater) value is 0.675 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 Kd data.


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