2,6-di-tert-butylphenol

Endpoint summary

Description of key information

The most sensitive endpoint is the NOEC for plants (shoot fresh weight, oats) with 6.97 mg/kg dw.

Additional information

The experimentally determined most sensitive endpoint is the NOEC for plants (shoot fresh weight, oats) with 6.97 mg/kg dw. This value will be used for the risk assessment.

In the past, the PNEC soil was calculated based on the comparison of a read across approach and the results from the EPM method. The more conservative approach (EPM method) was used and delivered a PNEC soil that was more conservative than the new, test based, value by a factor of >10. The approach is described below for information only. Please be aware that the now applicable PNEC soil is based on the experimental test data as described under "Description of key information".

Read Across approach

PNEC calculation based on terrestrial Read-Across data

Versonnen et al. 2014 (Sci Total Environ.2014 Mar 15;475:123-31. doi: 10.1016/j.scitotenv.2013.10.058. Epub 2013 Nov 14. Analysis of the ecotoxicity data submitted within the framework of the REACH Regulation: part 4. Experimental terrestrial toxicity assays. Versonnen B, Tarazona JV, Cesnaitis R, Sobanska MA, Sobanski T, Bonnomet V, De Coen W.) describe that also for terrestrial endpoints “Standard REACH information requirements can be adapted on the basis of REACH column 2 rules for adaptation mentioned above, as well as on the basis of the 'general rules for adaptation' listed in Annex XI of the REACH Regulation. These general rules are applicable to all endpoints and include weight of evidence (WoE) approaches, qualitative or quantitative structure-activity relationship ((Q)SAR), in vitro methods, grouping of substances and read-across, indications that testing is technically not possible, and tailored exposure-driven approaches”.
Therefore read across data from structural analogues in the “phenols” category, using the OECD QSAR Toolbox 3.2.0.103 was generated:

Read across data

Test/OECD TG	Predicted NOEC for 2,6-DTBP, mg/kg soildw	Range of NOEC values used for read across, mg/kg soildw
Long-term toxicity to terrestrial invertebrate (earthworm)/OECD 222	800	125 – 1250
Long-term toxicity to terrestrial plants: seedling emergence and growth/OECD 208	251	113-370
Long-term effects on soil microorganisms: nitrogen transformation test/OECD 216	399	2 – 1250

(see below under “Data basis- Newly generated data” for more details)

The read across assessments were conducted according to the relevant guidance documents and 2,6-DTBP falls in all three cases into the applicability domain (please see chapter “Data basis” and Annex 2 for detailed prediction reports in TPRF (Toolbox Prediction Reporting Format)). Therefore the consortium regards this data to be scientifically valid.
Following ECHA guidance on PNECsoil calculation using assessment factors (Table R.10-10 in Chapter R.10) the PNEC soil can be calculated using an assessment factor (AF) of 10 (with three long-term NOEC values for three trophic levels available) on the lowest of the three NOECs (251 mg/kg soil dw).
Accordingly the PNEC_{dry soil}would be 25.1 mg/kg soil dw.
Using as a worst case the lowest experimental NOEC value for the analogues (2 mg/kg soildw) with an AF of 10 the respective PNEC_{dry soil}calculates out as 0.2 mg/kg soil dw.
All NOECs over all trophic levels fall into a relatively narrow range and the NOECs predicted for 2,6-DTBP are well inside this range.

 

PNEC calculation based on newly generated aquatic toxicity data using the EPM method

 

Calculation of PNEC for sediments or soil using the equilibrium partitioning method (EPM)

To calculate PNEC for freshwater sediment, marine sediment and soil the guidance on how to do this is on Chapter R.10 and R.16.

The equations R.10-2, R.10-3, R.10-5 in ECHA guidance Chapter R.10

The equations use default values and data you have already on the substance.

PNEC_comp= K_comp-water/RHO_comp* PNEC_water* 1000

where,

comp' = environmental compartment, e. g. freshwater sediment, marine sediment, suspended matter or soil;

PNEC_comp= Predicted No Effect Concentration in wet comp, mg/kg of wet comp;

K_comp-water= comp-water partitioning coefficient, m³/m³;

RHO_comp= bulk density of wet comp, kg/m³ (= 1150 given); and

PNEC_water= Predicted No Effect Concentration in water (freshwater or marine), mg/L.

The unknown term in the above equation is the partitioning coefficient, K_comp-water, which is derived from equation R.16-7 in ECHA guidance Chapter R.16:

K_comp-water= F_aircomp* K_air-water+ F_watercomp+ F_solidcomp* (Kp_comp/1000) * RHO_solid

where,

F_aircomp= fraction of air in comp, m³/m³ -- only relevant for soil; F_airsoil= 0.2 m³-air/m³-soil, from Table R.16-9

K_air-water= air-water partitioning coefficient;

F_watercomp= fraction of water in comp, m³/m³; F_watersusp= 0.9 m³-water/m³-suspended matter, F_watersed= 0.8 m³-water/m³-sediment, F_watersoil= 0.2 m³-water/m³-soil, from Table R.16-9

F_solidcomp= fraction of solids in comp, m³/m³; F_solidsusp= 0.1 m³-soilds/m³-suspended matter, F_solidsed= 0.2 m³-soilds/m³-sediment, F_solidsoil = 0.6 m³-soilds/m³-soil, from Table R.16-9

Kp_comp= solids-water partitioning coefficient in comp, L/kg;

Kp_comp= Foc_comp* Koc                  equation R.16-6

where,

Foc_comp= weight fraction organic carbon in comp solids, kg-oc/kg-solids;

Foc_susp= 0.1 kg-oc/kg-solid

Foc_sed= 0.05 kg-oc/kg-solid

Foc_soil= 0.02 kg-oc/kg-solid

Koc = partitioning coefficient organic carbon-water (4493 L/kg -- from data set for the substance being registered)

RHO_solid= density of the solid phase, kg/m³ (= 2500 given).

 

Compartment	PNECcomp	Kcomp-water	RHOcomp	PNECwater	Faircomp	Kair-water	Fwatercomp	Fsolidcomp	Kpcomp	RHOsolid	Foccomp	Koc	HLC	v. p	MW	water solubility
	mg/kg of wet comp	m3/m3	kg/m3	mg/L	m3/m3		m3/m3	m3/m3	L/kg	kg/m3	kg/kg		Pa-m3/mole	Pa	g/mole	mg/L
freshwater sediment	0.0689	113	1150	0.0007	NA	NA	0.9	0.1	477	2500	0.1	4493
marine sediment	0.00689	113	1150	0.000070	NA	NA	0.9	0.1	477	2500	0.1	4493
soil	0.0556	135	1150	0.0007	0.2	0.0145	0.2	0.6	95.4	2500	0.02	4493	34.3	1.01	206.33	4.11

 

PNEC_freshwater= 0.0007 mg/L

PNEC_{marine water}= 0.000070 mg/L

Koc = 4493

MW = 206.33

water solubility = 4.11 mg/L

vapour pressure = 1.01 Pa

PNEC_{freshwater wet sediment}= 0.0689 mg/kg ww

PNEC_{freshwater dry sediment}= 0.317 mg/kg dw

PNEC_{marine wet sediment}= 0.00689 mg/kg ww

PNEC_{marine dry sediment}= 0.0317 mg/kg dw

PNEC_{wet soil}= 0.0556 mg/kg ww

PNEC_{dry soil}= 0.063 mg/kg dw

 

Summary of Read Across approach and EPM results on sediment toxicity

Comparison of PNEC_{dry soil}:

Data base	PNECdry soil
Based on the terrestrial Read-Across data	25.1 mg/kg soil dw.
Based on a worst case the lowest experimental NOEC value for the analogues in the terrestrial Read-Across assessments (2 mg/kg soildw)	0.2 mg/kg soil dw.
Based on the new Daphnia magna reproduction test data (M.H.J. Migchielsen 2014, DAPHNIA MAGNA, REPRODUCTION TEST WITH 2,6-DI-TERT-BUTYLPHENOL (FLOW-THROUGH)) using the EPM method	0.063 mg/kg dw

 

The comparison of PNEC_{dry soil} values shows that using the Daphnia magna reproduction test data in combination with the EPM method leads to the lowest PNEC_{dry soil}value. The comparison with the respective value derived from the terrestrial Read-Across data clearly indicates that the former value is very conservative. Comparison with the experimentally derived PNEC soil (0.697 mg/L, using an AF of 10) show that the results obtained by the Read across approach (NOEC-data) and EPM-method would have been more conservative. However, these data are overruled by the experimental data newly generated upon request of ECHA.

The results from the read across approach and EPM method will not be used for the derivation of the PNEC soil since experimental data are available for the registered substance.