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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Soil biodegradation data include an aerobic and anaerobic study (Schafer 2006), an aerobic study (Fackler 1989) and an anaerobic study (Fackler 1989).

Key value for chemical safety assessment

Half-life in soil:
6.4 d
at the temperature of:
20 °C

Additional information

The biodegradation potential of TBBPA in soil (simulation tests) was investigated in 3 studies:

1) Schaefer and Stenzel (2006): The transformation of 14C-TBBPA was tested for 6 months in aerobic and anaerobic soil systems at a nominal concentration of 50 µg/kg dry soil. Four types of soils were used to prepare various test chambers: loamy sand, sandy clay loam, silt loam and silty clay loam. All test chambers were maintained under aerobic conditions for the first 30 days after dosing. On day 30, test chambers designated for anaerobic conditions for the remainder of the test were flooded with water and headspaces were purged with nitrogen for the remainder of the test.

Volatile metabolites were identified (CO2 at 17.2 to 21.4% after 6 months) as were residues (declining from 89.6 to 92.5% at day 0 to 1.7 to 2.4% after 6 months). The radioactive mass balance for the four aerobic soils ranged from 99.7-100.6% of the dose at study initiation and 82.2-104.0% at termination (Month 6). Similarly, the mass balance in anaerobic soils ranged from 101.7-107.3% at initiation and 88.5-98.7% at termination. In all soil types under both aerobic and anaerobic conditions, the amount of radioactivity available for extraction declined with time. In the aerobic soil systems, 93.7-98.5% of the dose was extractable at study initiation whereas only 6.1-8.7% was extractable at study termination. The percent of dose remaining in these soil solids, e.g. that not extractable, increased markedly at Month 1 (55.3-83.6%) compared to study initiation (1.8-6.2%) and remained generally stable thereafter. The loamy sand soil consistently yielded the greatest amount extracted at each interval. Thus, TBBPA appeared to readily bind to soil matrices during the first 30 days, and the amounts bound appeared correlated with soil organic carbon content. The percent of dose mineralized in the aerobic soils increased slowly over the course of the study (from 8.5-13.7% at Month 1 to 17.5-21.6% at Month 6).

Anaerobic conditions were not achieved in any of the four soil types in the transformation vessels, therefore the anaerobic results were limited to the mineralization chambers collected on Month 6. At Month 6, the percent of dose not extractable from soil (58.2-65.6%) was similar to that in the aerobic soils whereas the amount extractable (18.5-30.0% of the dose) was greater.

The disappearance of TBBPA was calculated based on the percent TBBPA found in the soil extracts; the amount of 14C-activity remaining in the soil solids after extraction was considered bound and not considered in the TBBPA totals. In the aerobic soil extracts, TBBPA’s initial concentration ranged from 44.6-46.3 µg/kg and declined to 0.9-1.2 µg/kg by 6 months. The calculated disappearance time for 50% of the starting material (DT50) was 5.3-7.7 days and the DT90 was 20.8-31.3 days. Similar information could not be calculated for anaerobic soils since reliable anaerobic data were only available for Month 6.

 

2) Fackler (1989b): The biodegradability of 14C-TBBPA was tested under anaerobic conditions in three soil types: Massachusetts sandy loam, Arkansas silty loam, and California clay loam. Thin layer chromatography showed biodegradation of TBBPA in all soil types. Less than 0.5% of the radiolabel was recovered in the volatile traps, indicating little degradation to CO₂. The recovered radioactivity in all traps was almost exclusively CO₂. Results of the TLC analysis indicated variable degradation rates that were dependent on the soil type. After 64 days, the amount of TBBPA remaining in the soils was as follows: MSL: 43.7-57.4%; ASL: 53.4-65%; and CCL: 89.5-90.6%. Radioactivity recovered from the water ranged from 0.5 to 2.5%. Degradation products (2 or 3 depending on soil type) were not specifically identified, but the dimethyl and diethyl derivatives of TBBPA were ruled out based on TLC characteristics of authentic standards. 

 

3) Fackler (1989c): The biodegradability of14C-TBBPA was tested under aerobic conditions in three soil types, i.e., Massachusetts sandy loam, a California clay loam, and Arkansas silty loam. Thin layer chromatography (TLC) showed biodegradation of TBBPA in all soil types. Less than or equal to 6% of the applied radioactive TBBPA was recovered in the volatile traps, indicating partial degradation to CO₂. Results of the TLC analysis indicated variable degradation rates of TBBPA which were dependent on soil type. After 64 days, the amount of TBBPA remaining in the soils ranged from 36 to 82% with the highest level in sandy loam soil and the lowest in the silty loam soil. Degradation products (2 or 3 depending on soil type) were not specifically identified, but the dimethyl and diethyl derivatives of TBBPA were ruled out based on TLC characteristics of authentic standards. 

Summary of degradation and o-methylation of TBBPA in the literature

- Liu et. al. (2013): The degradation mechanism of radiolabelled TBBPA in soil was investigated. 14C-labelled 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 counting.

- 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) or reed (Phragmites australis) seedlings are investigated in this paper. The concentration used was 5 mg/kg 14C-TBBPA and the test was run at 34 °C. Debromination occurred under anoxic/anaerobic conditions while O-methylation occurred under aerobic conditions. In unplanted soil, TBBPA dissipation (half-life 20.8 days) was accompanied by mineralization (11.5% of initial TBBPA) and the substantial formation (60.8%) of bound residues. Twelve metabolites (10 in unplanted soil and 7 in planted soil) were formed and identified. The presence of the seedlings strongly reduced 14C-TBBPA mineralization and bound-residue formation and stimulated debromination and O-methylation. Considerable radioactivity accumulated in rice (21.3%) and reed (33.1%) seedlings, mainly on/in the roots. 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

- Li et al (2015): The degradation of TBBPA in oxic sandy soils was investigated. TBBPA initial concentration was 5.0 mg per kg soil (dry weight). While TBBPA was removed rapidly, metabolites resulting from the O-methylation of TBBPA could be observed. 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.