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PBT assessment

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PBT assessment: overall result

PBT status:
the substance is not PBT / vPvB
Justification:

The substance itself is not PBT. However an assessment has also been performed for the transformation products.

1.      Identification of biodegradation products

Based on the literature and results of industry testing, it can be concluded that, under anaerobic conditions, the main biodegradation pathway in the environment is the sequential debromination of TBBPA, ending with BPA. BPA is further mineralized under aerobic conditions. Details are presented in appendix 1 below.

The partially debrominated products formed during the debromination process are intermediates that do not accumulate. This can clearly be seen e.g. in Ravit et al. (2005), Ronen et al. (2000), Arbeli et al. (2003), Chu et al. (2005) and Voodeckers et al. (2005).

In the most recent industry studies testing the biodegradation in sediments and sludge (Schaefer et al. 2006), the intermediate products could not be definitively identified due to the low levels used for the study (50 µg/kg ¹⁴C- TBBPA). However, when comparing to the data from the above mentioned scientific literature, it can be concluded that indeed the debromination pathway was occurring in these studies as well.

2.      Identification of additional transformation products

Based on scientific literature and monitoring data (details in appendix 2 below), the o-methylation of TBBPA can occur in the environment. Although the significance of this pathway is not conclusive, monitoring data show that the methyl ether TBBPA can be found at low levels (µgs/kg) in suspended particular matter in rivers in Europe. However, the dimethyl ether TBBPA is much less frequently found and only in highly contaminated rivers (Mersey and Tees in the UK). In fish, the methylated ether was found at very low levels, mostly below the LOQ of 0.8 µg/kg, while the dimethyl ether was not detected in any of the samples.

3.      PBT assessment of biodegradation products

The available data show that as intermediate products, the partially debrominated TBBPA are not persistent molecules. In order to be able to provide values for the degradation process, calculations of half-life for degradation of TBBPA and formation of BPA in the industry studies (Schaefer et al. 2006) were performed. Assuming first order kinetics, the calculations were made for the two sediments tested, Turkey Creek (6% TOC) and Choptank River (0.9% TOC). The results are shown in the following table:

System

t½ TBBPA degradation (days)

t½ BPA formation (days)

Turkey creek sediments

42

24

Turkey creek whole system

28

24

Choptank river sediment

28

20

Choptank river whole system

24

21

 

As can be seen from the results, the short half-life value for the formation of BPA clearly indicates that the intermediates are formed and transformed quickly and therefore are not considered as persistent. Since these products are not persistent, the bioaccumulation and toxicity end points are not further discussed.

4.      PBT assessment of transformation products

There are currently no data available for the methyl ether TBBPA. QSAR calculations using EPIWIN (version 4.1) predict that the methylated ether TBBPA and the dimethyl ether TBBPA are both expected to be bioaccumulative and persistent (appendix 3 below). Therefore, additional work is needed in order to assess their PBT properties. The water solubility value for the dimethyl ether was measured and found to be 68 ng/L. The water solubility of the methyl ether TBBPA has not been measured but according to the EPIWIN program is expected to be at least 10 times higher than the solubility of the dimethyl ether TBBPA.

5.      Testing proposal

The first test to be run should address the persistency criterion. Since the hydrophobic methyl ether TBBPA was found in the suspended particular matter, which represent fresh sediments, and in core sediments samples, the most appropriate test would be the OECD 308 where the degradation rate in sediments is determined. The sediment compartment is also expected to be the final sink for the hydrophobic methylated forms. However, for best design of the study and for the development of the analytical method, the water solubility of the methyl ether TBBPA should be first determined.

Appendix 1: Identification of Degradation Products of TBBPA in sediments, digester sludge and soil

Several scientific publications have shown that Tetrabromobisphenol-A (TBBPA) could undergo dehalogenation under anaerobic conditions (e.g. Voodeckers et al. 2002, Ravit et al. 2005, Ronen et al. 2000, 2003). In industry’s three most recent anaerobic studies performed in aquatic sediments, digester sludge and soil (Schaefer 2006), three unknown HPLC peaks were observed between TBBPA and bisphenol-A (BPA). The kinetics of these peaks show an increase and then decrease in their concentrations during the experimental period, indicating that these are indeed intermediates.  Although not definitively identified, these peaks can be attributed to three dehalogenated intermediates: tribromobisphenol-A (Br3-BPA), dibromobisphenol-A (Br2-BPA) and monobromobisphenol A (Br-BPA).

Efforts were invested to identify the degradation products in the aquatic sediments and the digester sludge studies. However due to a low environmentally relevant starting concentration of 50 µg/kg the identity of the above mentioned degradation products could not be confirmed. Furthermore, background noise and extremely low levels of analyte precluded definitive identification of any of the unknowns. In the older studies (Fackler 1989), transformation products were observed but not identified.

More information on the dehalogenation mechanism in which TBBPA is degrading to BPA under anaerobic conditions is well described in the literature for the soil and sediment compartments, as described below. 

Supporting Information from Published Literature:

o  Ravit et al. (2005): TBBPA degradation was investigated in sediments taken from Marsh fields at two locations in New Jersey. The effect of two macrophyte species on the biodegradation of TBBPA was investigated during a period of up to 130 days. Under methanogenic conditions, TBBPA was reductively debrominated and a stoichiometric equivalent amount of BPA was produced. Debromination was more rapid in sediments planted with Spartina than in those planted with Phragmites or unvegetated sediments from both sites. The transient metabolites di and tri bromobisphenol A were identified.

o  Ronen and Abeliovich 2002: In this publication the anaerobic metabolism of TBBPA was examined in sediments collected from the vicinity of an industrial complex in the northern Negev in Israel. The experiment was performed at 30 °C for a period of 50 days.The intermediate metabolites of the biodegradation products of TBBPA were determined by HPLC and GC-MS methods. Br3-BPA and Br2-BPA were identified using HPLC. BPA was identified as the final metabolite under the anaerobic conditions. The further degradation of BPA under aerobic conditions was studied by using a pure culture of a bacterium that was isolated from soil. Degradation products of BPA under aerobic conditions were also identified by a mass spectral method. The first proposed mechanistic degradation pathway was for the full dehalogenation of TBBPA to BPA under anaerobic conditions. The second proposed mechanistic degradation pathway was for the cleavage and full mineralization of BPA.

o  Voordeckers et. al. (2002): Methanogenic and sulfidogenic conditions were applied to sediments and the degradation of TBBPA was investigated. Sediment grab samples were collected from the Arthur Kill tidal strait located between Staten Island and New Jersey. Sediments were exposed to TBBPA at 28 °C for a period of 120 days. Under both conditions, debromination of TBBPA to BPA was demonstrated after 120 days. Based on the study results, the authors claim that TBBPA seems to be readily dehalogenated to BPA under both methanogenic and sulfate-reducing conditions.

o  Arbeli and Ronen (2003): A full dehalogenation mechanism with identification of intermediate metabolites is described in this publication. Sediment samples were taken from the vicinity of an industrial complex in the northern Negev desert, Israel. The debromination and transformation of TBBPA to BPA was investigated under anaerobic conditions during 80 days. GC-MS and HPLC analytical methods were used for the identification of the metabolites.

The kinetics of TBBPA biodegradation was investigated with full identification of three intermediate metabolites: Br3-BPA, Br2-BPA and Br-BPA. In this paper, the debromination mechanism of TBBPA with full metabolites identification is presented. These results further support the dehalogenation mechanistic degradation pathway of TBBPA to BPA under anaerobic conditions

o  Chu et al. (2005): The investigation of the biodegradation mechanism of TBBPA in sludge and sediments is described in this publication. An HPLC-ESI(-)-MS-MS analytical method was developed for simultaneous determination of BPA, TBBPA and debrominated TBBPA derivatives.

Surface sediment samples were taken from Lake Erie in Canada and analyzed for TBBPA and its potential debromination metabolites. In 65% of the 55 surface sediment samples, BPA, TBBPA, and Br3-BPA were detected. Although, Br2-BPA and Br-BPA were not detected in these samples, the detection and quantification of BPA in these samples indicate that debromination processes are occurring. Analyses were also performed on sewage sludge samples collected from Little River Waste Water Treatment Plant and from the West Windsor Pollution Control in Canada. In these samples, BPA, TBBPA, Br3-BPA, Br2-BPA and Br-BPA were identified. The full debromination pathway is presented in this paper.

o  Liu et al. 2013: The degradation mechanism of radio-labelled TBBPA in soil was investigated. ¹⁴C-labeled TBBPA was added to soil under static anoxic (195 days) and sequential anoxic (125 days) oxic (70 days) conditions. During the anoxic incubation, TBBPA dissipated with a half-life of 36 days, yielding four debromination metabolites: BPA, Br3-BPA, Br2-BPA and Br-BPA. The substances were identified using LC-MS and liquid scintillation counter.

Conclusions from the literature review

The information presented in the reviewed papers above support the suggested debromination mechanism of TBBPA under anaerobic conditions. Therefore, although in the more recent GLP studies the final confirmation could not be done, the information in the papers supports the conclusions that TBBPA would undergo sequential debromination to BPA under anaerobic conditions.

In addition, BPA further biodegrades and undergoes mineralization under aerobic conditions as presented in the papers of Ronen and Abeliovich (2000) and Liu et al. (2013). The formation of BPA is also addressed in the EU risk assessments of both TBBPA and BPA. The conclusions were that TBBPA degradation would not increase the risk of BPA to the environment.

 

Appendix 2: Identification of additional transformation products of TBBPA

The following is a summary of the information available from scientific literature on the methylation of TBBPA in the environment. In addition, data from a monitoring study performed by industry (Kotthoff 2015) are presented. The o-methylation can occur in the environment, probably under oxic conditions, resulting in the formation of methyl ether TBBPA and much less frequently dimethyl ether TBBPA

o  Sun et al. (2014): The fate and formation of metabolites of ¹⁴C-TBBPA in a submerged soil with an anoxic-oxic interface, with or without rice (Oryza sativa) and reed (Phragmites australis) seedlings, are investigated in this paper. The concentration used was 5 mg/kg TBBPA and the test was run at 34 °C. Debromination occurred under anoxic/anaerobic conditions while o-methylation occurred under aerobic conditions. Approximately 10% of the radioactivity could be associated with o-methylated forms after 10 days in the presence of the reed and after 35 days in the presence of the rice, with no additional decrease in their levels by the end of the experimental period (65 days). However, no differentiation was made between the methylated and the dimethylated forms.

o  George and Häggblom (2008): The biodegradation of TBBPA in sediments was tested using initial concentrations of 5 mg/kg. The paper demonstrated o-methylation of tetrabromobisphenol-A (TBBPA) to its mono- and dimethyl ether derivatives by microorganisms present in different sediments; transformation rates to the monomethyl ether were 25 and 5% after 60 and 80 days, respectively, in the different sediments, and transformation rates to the dimethyl ether were 35 and 5%, respectively. 

o  Li at al. (2015a): In this paper the TBBPA transformation in soil was studied. Flasks containing soil and TBBPA at 5.0 mg per kg soil (dry weight) were incubated at 20 ± 1 °C. O-methylation was apparent in oxic sandy soils. The monomethyl ether TBBPA was formed continuously reaching 12% after 143 days of incubation. The dimethyl ether TBBPA was not formed during this period and started to appear only on day 143.

o  Peng et al. (2014): The transformations of TBBPA by 6 freshwater green microalgae and identification of the transformation products were investigated. Transformation experiments were conducted at a concentration of 0.8 µM (~ 440 µg/L) of TBBPA for 10 d. The results showed that 70-90% of the TBBPA could be transformed by Scenedesmus quadricauda and Coelastrum sphaericum by the end of the 10 days of incubation. Five transformation products were positively identified by mass spectrometry: TBBPA sulfate, TBBPA glucoside, sulfated TBBPA glucoside, tribromobisphenol-A and TBBPA monomethyl ether, which was formed only by Scenedesmus.

o  Li et al. (2015b): TBBPA transformation in nitrifying activated sludge was studied using a ¹⁴C-tracer. During the 31-day incubation, TBBPA transformation (half-life 10.3 days) was accompanied by mineralization (17% of initial TBBPA). Twelve metabolites, including those with single benzene ring, o-methyl TBBPA ether (mono methyl) and nitro compounds, were identified. When allylthiourea was added to the sludge to completely inhibit nitrification, TBBPA transformation was significantly reduced (half-life 28.9 days), formation of the polar and single-ring metabolites stopped, but o-methylation was not significantly affected. Three biotic (type II ipso-substitution, oxidative skeletal cleavage, and o-methylation) and one abiotic (nitro-debromination) pathways were proposed for TBBPA transformation in nitrifying activated sludge. Apart from o-methylation, ammonia-oxidizing microorganisms were involved in three other pathways.

o  Kotthoff (2015): A 7 year monitoring project (2007 - 2013) was undertaken by Industry and the full report is available. Samples of fish were collected annually from selected rivers in Europe and one lake.  In addition, suspended particular matter (SPM) was collected from the same sites. TBBPA, methyl ether TBBPA and dimethyl ether TBBPA were analyzed in these samples. The LOQs for fish were: 0.45, 0.8 and 1.6 µg/kg ww for TBBPA, methyl ether TBBPA and dimethyl ether TBBPA, respectively. The LOQs for the SPM samples were: 0.45. 0.8 and 0.7 µg/kg dw for TBBPA, methyl ether TBBPA and dimethyl ether TBBPA, respectively. The results showed no trend during the years. In the fish samples, TBBPA was found at all sites at levels close to LOQ, the methyl ether was detected at all sites at very low levels and the dimethyl ether was not detected in any of the sites. In the SPM samples, TBBPA and the methyl ether were found at most sites while the dimethyl ether was detected only at two sites at levels below LOQ.

 

Conclusion from literature on transformation products

The above data indicate that o-methylation of TBBPA is occurring in the environment, probably under oxic conditions in sediments. It seems that the formation of methylated ether derivative is more prevalent than the formation of the dimethylated form.

Appendix 3: QSAR analysis of TBBPA o-methylated transformation products

In order to evaluate the possible PBT properties of the transformation products of TBBPA, the US EPA EPI estimation program (version 4.1) has been used to estimate the key properties of the two methylated ether forms of TBBPA. The results are shown in the following table: 

Substance

P criteria

B criteria

T criteria

PBTconclusion

MM-TBBPA

Readily biodegradable – No

 

DT50water = 180 d

DT50sediment = 1620 d

DT50soil = 360 d

 

The substance fulfils P criteria

LogKow = 7.76

 

BCF = 5618 L/kg

 

The substance fulfils B criteria

WS = 0.000278 mg/L

 

Fish ChV = 0.00048 mg/L

Daphnia ChV = 0.0017 mg/L

Algae ChV = 0.031 mg/L

 

ChV value for fish is at the same order of magnitude as the WS value of the substance. Thus, the substance fulfils T criteria.

The substance fulfils the following criteria:

PBT

DM-TBBPA

Readily biodegradable – No

 

DT50water = 180 d

DT50sediment = 1620 d

DT50soil = 360 d

 

The substance fulfils P criteria

LogKow = 8.33

 

BCF = 2983 L/kg

 

The substance fulfils B criteria

WS = 0.000019 mg/L

 

Fish ChV = 0.00019 mg/L

Daphnia ChV = 0.00054 mg/L

Algae ChV = 0.014 mg/L

 

For all the species ChV values are very small but higher than the WS value of the substance byat least in an order of magnitude. Thus, the substance does not fulfil T criteria.

The substance fulfils the following criteria:

PB

The substance does not fulfil the following criteria:

T