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Description of key information

The equilibrium partitioning method (EPM) has provisionally been used to calculate the PNEC for benthic organisms using a Kp susp-water of 100000 L/kg

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.  For 50% of the substances the ratio predicted/observed NOEC/EC10 lies between 0.5 and 0.9. For the other 50% 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.

It should be noted that the Kp susp-water of the substances evaluated ranged from 1000 to 17000 L/kg and that despite of these high sorption constants a conservative hazard for the benthic compartment was predicted.


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          =    14.24 µg/L

                                                            Kpsusp                      =    100000 L/kg

                                                            SUSPwater                 =    15 mg/L (DefR.16)

The PNECsed is then calculated using the equations detailed in the TGD:

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

                                              Where:    PNECaquatic dissolved     =    5.7 µg/L

                                                            Ksusp-water                 =    25000 m3/m3

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

                           PNECsed-EPM             = 123.8 mg/kg ww

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

= 12.4 mg/kg ww or 57 mg/kg dw