Formation, characterization, and mineralization of bound residues of tetrabromobisphenol A (TBBPA) in silty clay soil under oxic conditions

- In the oxic soil, TBBPA had four major metabolites, two of which were methyl ethers.
- Up to 80% of TBBPA formed bound residues (BRs) in the silt-rich oxic soil.
- The majority of BRs were localized in humin and were releasable in alkali solution.
- Alkali-releasable BRs were mineralized and converted to stable BRs in fresh soil.
- Amendment of rice root exudates to soil did not significantly affect BR release.

The nature and stability of bound residues (BRs) derived from the widely used brominated flame retardant tetrabromobisphenol A (TBBPA) in fine-textured soil is unknown. We incubated 14C-labeled TBBPA in silty clay rice paddy soil for 93 days under oxic conditions. TBBPA dissipated with a first-order kinetic constant kd of 0.0474 ± 0.0017 day−1 (t1/2 14.6 ± 0.3 days) and mineralized with a km of 0.0011 ± 0.00002 day−1. At the end of the incubation, four metabolites, including two methylation products (TBBPA monomethyl and dimethyl ether), accounted for 7.9 ± 0.1% of the initial TBBPA. The BRs continuously increased in amount to a maximum of 80.1 ± 3.6%. About 86.3 ± 0.9% of the BRs localized in the humin fraction and 55.9 ± 1.5% was hydrolyzable with strong alkali (SAH-BRs), which represents reversible BRs. Together with results previously reported for coarse-textured soil, these results indicate that the absolute amounts of both BRs and SAH-BRs of TBBPA as well as the relative contribution of SAH-BRs to total BRs in fine-textured soil are markedly higher than in coarse-textured soil. When BRs-containing soil was incubated with fresh soil for 231 days, 9.2 ± 0.3% was mineralized (km 0.00047 ± 0.00002 day−1) and SAH-BRs decreased to 34.1 ± 1.1%, accompanied by transformation into other BR forms. These indicate that BRs are bioavailable in the soil. Amendment with rice root exudates did not effectively affect the mineralization, release, and distribution of BRs, suggesting that bioavailability of BRs but not microbial activity limits the degradation of BRs in the silty clay soil. This study provides first insights into the nature and stability of TBBPA-derived BRs in fine-textured soil under oxic conditions and indicates the significant role of reversible BRs in the environmental risk of TBBPA.

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