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The equilibrium partitioning method (EPM) can be used as a screening tool to help determine whether sediment toxicity tests with benthic organisms are required. This equilibrium partitioning concept has been shown to be relevant for nonpolar hydrophobic contaminants, but the validity of this concept for ionic compounds has hardly been studied.

Chen, Y et al 2014 therefore studied if the equilibrium partitioning concept is also applicable to the cationic surfactant C12 benzalkonium chloride. Chen found that based on the results of 4 different bioassays, the equilibrium partitioning is also valid for this specific cationic surfactant.

To evaluate the applicability of the EPM for predicting the hazard for benthic organisms when exposed to cationic surfactants the available test results from long term daphnia NOEC/EC10 and long term lumbriculus/chironomid NOEC/EC10 for the available cationic surfactants were compared using the observed mean Kd sorption data for soil (not normalized for OC content) and the EPM. The comparison in this working document shows that for all substances evaluated the EPM predicted hazard for sediment organisms is more conservative than what is actually observed. For 43% of the substances the ratio predicted/observed NOEC/EC10 lies between 0.5 and 0.9. For the other 57% this ratio lies between 0.02 and 0.5. These findings confirm that the EP approach conservatively predicts the benthic hazard due to exposure to cationic surfactants and that it can safely be used as a screening tool for cationic surfactants.

It should be noted that for the EPM calculations the observed mean observed Kd soil value was used which was not normalized to the OC content. Sorption of cationic surfactants is mainly driven by ionic interaction (Droge&Goss 2013), normalisation using the OC content alone will due to this lead to erroneous extrapolations. For suspended sediment the Kd value was doubled to compensate for the higher number of small particles (higher CEC) present in suspended sediment. Finally, using this approach, the Ksusp-water of the substances evaluated ranged from 1000 to 20000 L/kg and for all the EPM predicted hazard for sediment organisms was more conservative than actually observed without using the additional assessment factor of 10 for strongly sorbing substances.

Key value for chemical safety assessment

Additional information

 The equilibrium partitioning method (EPM) can be used as a screening tool to help determine whether sediment toxicity tests with benthic organisms are required. This equilibrium partitioning concept has been shown to be relevant for nonpolar hydrophobic contaminants, but the validity of this concept for ionic compounds has hardly been studied. Chen, Y et al 2014 therefore studied if the equilibrium partitioning concept is also applicable to the cationic surfactant C12 benzalkonium chloride. Chen found that based on the results of 4 different bioassays, the equilibrium partitioning is also valid for this specific cationic surfactant.

To evaluate the applicability of the EPM for predicting the hazard for benthic organisms when exposed to cationic surfactants the available test results from long term daphnia NOEC/EC10 and long term lumbriculus/chironomid NOEC/EC10 for a number of cationic surfactants were compared using the observed sorption/desorption data (not normalized for OC content) and the EPM.

The comparison shows that for all substances evaluated the EPM predicted hazard for sediment organisms is more conservative than what is actually observed (without applying the additional safety factor of 10 for strongly sorbing substances). For 43% of the substances the ratio predicted/observed NOEC/EC10 lies between 0.5 and 0.9. For the other 57% this ratio lies between 0.02 and 0.5. These findings confirm that the EP approach conservatively predicts the benthic hazard due to exposure to cationic surfactants and that it can savely be used as a screening tool for cationic surfactants. A position paper is currently under preparation.

It should be noted that the Kp susp-water of the substances evaluated ranged from 1000 to 20000 L/kg and that despite of these high sorption constants a conservative hazard for the benthic compartment was predicted.

 

The EPM calculation for oleyltriamine:

According to REACH guidance R.16 the EPM uses the PNECaquaticand the sediment/water partitioning coefficient as inputs. However, since the available PNECaquaticis based on the bulk concentration present in surface water a re-calculation is necessary first:

                           PNECaquatic, dissolved          =     PNECaquatic bulk/ (1 + Kpsusp* SUSPwater* 10-6)

                                              Where:    PNECaquatic bulk          =    10.36 µg/L

                                                            Kpsusp                      =    100000 L/kg

                                                            SUSPwater                 =    15 mg/L (DefR.16)

The PNECsedis then calculated using the equations detailed in the TGD:

                           PNECsed                 =  Ksusp-water* PNECaquatic dissolved* 1000 * 1 / RHOsusp

                                              Where:    PNECaquatic dissolved     =     4.14 µg/L

                                                            Ksusp-water                 =    25000 m3/m3

                                                            RHOsusp                    =    1150 kg/m3(TGD, eq. 18)

                           PNECsed-EPM             = 90 mg/kg ww

To compensate for additional exposure via ingestion an additional factor of 10 can be applied.

= 9 mg/kg ww or 41.4 mg/kg dw