Tris(methylphenyl) phosphate

Bioaccumulation: aquatic / sediment

Description of key information

Discussions on bioaccumulation potential

Key value for chemical safety assessment

BCF (aquatic species):

800 L/kg ww

Additional information

An assessment of the propensity towards bioaccumulation was undertaken, utilizing appropriate techniques such as:

·       Literature study data

·       QSAR derivation, using recognized tools

 

The results are as follows:

 

Literature data

 

A study by Bengtsson et al (1985) is presented as the key study. In a comprehensive study, bioaccumulation properties of the constituents tris(methylphenyl) phosphate, triphenyl phosphate and cresydiphenyl phosphate were investigated using the saltwater species bleak (Alburnus alburnus) as test organism. The experiment was conducted during a period of 28 d with a 14-d accumulation phase and a 14-d depuration phase. No mortality or abnormal behavior in bleak was observed in the test aquaria during the experiment. Analytical monitoring was carried out via gas chromatography. For these compounds the specific BCF values were: triphenyl phosphate (BCF = 400), cresyldiphenyl phosphates (BCF = 100 - 220), and tricresyl phosphates (BCF = 400 - 800).

The study used shorter uptake durations than what is recommended for the OECD 305 method; however it is deemed suitable on a weight of evidence basis, as the results match other data available. Although uptake is demonstrated, subsequent elimination is ongoing of the substance from the organisms. In addition the BAF values are not indicative of bioaccumulation. The substance is therefore proposed to not be bioaccumulative on the basis that it does not meet the threshold values listed in the regulation.

A single study by Muir et al was also evaluated.  The BCF values based on total radioactivity should be used with care as they do not reflect the concentration of the substance itself but may also contain metabolites and radioactive CO2 metabolised in the fish body. BCF values of the hexane extracts may be used as data for the parent substance itself as the substance is soluble in hexane whereas more polar metabolites are likely not hexane-soluble.

Uptake, clearance and extent of metabolism of 14C-labelled, tri-m-cresol phosphate (mTCP) and tri-p-cresol phosphate (pTCP)as contained in commercially available TCPwere studied in rainbow trout (Salmo gairdneri) and fathead minnows (Pimephales promelas) using short-term static exposures (50 and 5 μg/l). Bioconcentration factors for pTCP and mTCP are assessed using various techniques.

 

1. “Initial rate”

2. Static test

3. Biofac

4. hexane extractable radioactivity

 

Differences in rates of clearance and biotransformation among the compounds assessed were more closely related to their ease of hydrolysis than to hydrophobicity.  On the basis of the values obtained the substance does not appear to be bioaccumulative.

Uptake and clearance of radioactivity: Uptake of each chemical by rainbow trout was linear and proportional to exposure concentration over the initial 6 hrs and did not reach a steady state within 24 hrs.T-aryl phosphates(TAP) concentrations in minnows increased linearly (0-6 hrs) and generally reached maximum values after 12 hrs (TPP, tBPDP and pTCP) exposure in static systems. During the exposure period water concentrations declined exponentially and could be described by equations of the form ln Cw = ln A – Bt (hrs). Water concentrations in minnow exposures decreased more rapidly because the loading density was 7.2 g/l due to the large size of the minnows, compared to 3.0 g/l for rainbow trout fry.

Clearance of radioactivity by both species was well described by a single exponential decay curve of the form ln CFish vs t (hrs). However, TPP and mTCP elimination was biphasic with more rapid rates of clearance in the first 6 days after transfer to clean water, especially for rainbow trout.

Uptake (K1) and clearance (K2) rate constants were calculated by simple linear regression techniques using total 14C levels in whole fish to allow comparisons among compounds, exposure concentrations and species. Uptake rate constants of all 4 compounds were similar at each concentration. Large confidence limits on the rates were due to the small sample size over the time interval used (0 – 6 hrs). Major differences among the 4 TAPs studied were noted in the clearance rate constants in order of mTCP>TPP>tBPDP>pTCP for rainbow trout and mTCP>TPP>pTCP>tBPDP for fathead minnows over the 0 – 144 hr interval. Differences between rate constants calculated over the 0 – 432 hr interval showed a similar trend but were not as great. Clearance rate constants for rainbow trout were greater than those for fathead minnows by about 50%. Clearance rate for each chemical were similar at each exposure concentration, i.e. independent of initial concentration, suggesting that depuration was best described by first order kinetic rate equations.

 

The BCF values resulting from the study are not indicative of bioaccumulation for the m and p isomers assessed. The substance is therefore proposed to not be bioaccumulative on the basis that it does not meet the threshold values listed in the regulation.

 

A study by Veith et al (1979) determined that the BCF after 32-days exposure was 165 l/kg. It is not entirely clear from this paper if the measured concentrations are based on 14C measurements or parent compound measurements. It is considered to support a weight of evidence approach.

A study by Sitthichaikasem (1978) The mean BCF value determined was 1,589 l/kg. Owing to the method of feeding used in this study, the results probably reflect uptake via both water and food and so may overestimate the true BCF (via water alone) for this substance. Elimination experiments (28 days in clean water) showed a two-stage elimination of the test substance. The first stage showed rapid elimination of the radiolabel over the first three days, with little or no further elimination until day seven. From day seven onwards, elimination of the radiolabel occurred at a slower rate with a half-life of around 39 days. By day 28 of the elimination period the total body burden of the radiolabel had fallen to around 32µg/kg.

Although available only as a summary, the results are considered suitable to support the weight of evidence that the substance is proposed to not be bioaccumulative on the basis that it does not meet the threshold values listed in the regulation.

 

QSAR derivation, using recognized tools

 

·       CAESAR (version 2.1.13)

·       BCF Read-Across (version 1.0.2)

·       US EPA On-Line EPI Suite™ v4.0 model BCFBAF

·       US EPA (T.E.S.T) v4.1

·       BCF model (Meylan) (version 1.0.2) - CAESAR Tool

 

It is understood that Annex  XI, section  1.3  allows  adaptation of the standard testing requirements by making use of (Q)SAR only if the following conditions are met:

(i) results are derived from   a  (Q)SAR  model  whose  scientific validity has  been  established,

(ii) the substance falls within the applicability domain of the (Q)SAR model,

(iii) results are adequate for the purpose of classification and labelling and/or risk assessment, and

(iv) adequate and  reliable documentation of  the  applied method is  provided.

 

Within the four models used, the conditions (i) through (iv) are considered to be met. As the substance has 3 main structures, an assessment of these was undertaken. The structures assessed were deemed to fall within the applicability domain of the model, and this is demonstrated within the relevant QPRF’s. The models are recognized, and are referenced within ECHA’s own guidance and/or have relevant QMRF’s which are also detailed.

 

As a result, the QSAR’s conducted all generally agree with each other that the substance is not bioaccumulative.

 

Overall

On the basis of a weight of evidence approach, there is sufficient information available to state that the substance is not bioaccumulative. Whilst it is not possible to provide a definitive BCF value for the substance, due to the variation in the results, none of these are above the threshold value quoted in the Regulation of 2000 or 5000 which indicates the potential to bioaccumulate.  A summary of the results is as follows:

  

Endpoint study	Result (BCF)	Notes
EPIWIN BCFBAF Results [L/kg]	568.5	None
BCF Read-Across (version 1.0.2) [L/kg]	127-128	None
BCF model (CAESAR) (version 2.1.13) [L/kg]	77	None
BCF model (Meylan) (version 1.0.0)	1064	None
US EPA (T.E.S.T) v4.1	83.09 – 95.92
Bengtsson et all - Bioaccumulation and effects of some technical triaryl phosphate products in fish and Nitocra spinipes - Alburnus alburnus	>= 400 — <= 800
Muir Et Al; Environmental Dynamics Of Phosphate Esters. Comparison Of The Bioconcentration Of Four Triaryl Phosphates By Fish	>= 1162 — <= 1653*	Initial rate (mTCP)
	>= 596 — < 784	static test (mTCP)
	>= 385 — <= 1102	Biofac (mTCP)
	>= 2199 — <= 2768*	Initial rate (pTCP)
	>= 928 — <= 1420	static test (pTCP)
	>= 588 — <= 1466	Biofac (pTCP)
	>= 709 — <= 770	static test (p-TCP)
	>= 310 — <= 462	static test (m-TCP)
Veith et al (1979) – assessment of commercial TCP in Fathead minnows (pimephales promelas)	165
Sitthichaikasem (1978) – assessment of p-isomer in Bluegill (Leptomis macrochirus)	1589

 

*The authors states the results of the initial rate as less valid as the substance decreased rapidly in the water phase.

 

In addition, this material has been assessed by the UK Environment Agency Member State Authority, in their report references as “Environmental risk evaluation report: Tricresyl phosphate (CAS no. 1330-78-5)”. This report is available at:https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/290861/scho0809bquj-e-e.pdf

 

The position of theUK Environment Agency Member State Authority is that this substance is not bioaccumulative.

 

On the basis of the available evidence, the substance is not considered to be “bioaccumulative” or “very bioaccumulative” on the basis of the available data.

 

For the purposes of assessment, a BCF value of 800 is applied.