Fate And Metabolism Of Tetrabromobisphenol A In Soil And Nitrifying Activated Sludge Under Oxic Conditions

Tetrabromobisphenol A [4,4’-isopropylidenebis(2,6-dibromophenol)] (TBBPA) is the most abundantly used brominated flame retardant, which is added to a wide range of products worldwide. TBBPA is an ubiquitously distributed environmental contaminant, and has been frequently detected in sewage sludge, air, dust, water, sediment, soil, and biological matrices. It has been identified as a disrupter of thyroid and reproductive hormone function, cytotoxicity, neurotoxicity, and an inducer of oxidative stresses and damages in animals and plants. Based on the character of TBBPA, it may persist in the environment, resulting in the threat to the environment and human health.TBBPA could be accumulated in the soil via various ways, resulting in that soil became one of the major sinks for TBBPA in the environment. TBBPA can sorbs onto sludge flocs, leading to its release in soils in regions where the sludge is applied as fertilizer on agricultural lands. In previous study, wastewater treatment plants operated for nitrogen removal may enhance the biotransformation of micropollutants. TBBPA already became one the most important pollutants in the world; However, detailed studies on the fate of TBBPA in soil environments under oxic conditions and activated sludge in wastewater treatment plants are scarce.Using a 14C-tracer, we studied TBBPA fate and metabolism in the oxic soil and the mechanism of bound-residues formation. TBBPA transformation in nitrifying activated sludge (NAS), and the effect of ammonia-oxidizing microorganisms on TBBPA transformation were also studied. In addition, the proposed transformation pathway in oxic soil and NAS were discussed. The results were shown as follows:(i) Transformation of TBBPA was studied in an oxic soil slurry with and without amendment with Sphingomonas sp. strain TTNP3, a bacterium degrading bisphenol-A. During the 20-day incubation, TBBPA degradation was accompanied by mineralization and formation of metabolites and bound-residues. The biotransformation was stimulated in the slurry bio-augmented with strain TTNP3, via a mechanism of metabolic compensation, although this strain did not grow on TBBPA. In the absence and presence of strain TTNP3, six and nine metabolites, respectively, were identified. The initial O-methylation metabolite (MeO-TBBPA) and hydroxytribromobisphenol-A were detected only when strain TTNP3 was present. Four primary metabolic pathways of TBBPA in the slurries are proposed:oxidative skeletal rearrangements, O-methylation, ipso-substitution (with hydroxyl group attacking a quite uncommon position connected to a quaternary a-carbon), and reductive debromination.(ii) In the study on TBBPA metabolism in NAS, 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, 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. Abiotic experiments confirmed the generation of mono-and di-nitro-brominated forms of bisphenol A in NAS by the abiotic nitration of TBBPA by nitrite, a product of ammonia-oxidizing microorganisms (AOMs). Three biotic (type Ⅱ ipso-substitution, oxidative skeletal cleavage, and O-methylation) and one abiotic (nitro-debromination) pathways were proposed for TBBPA transformation in NAS. Apart from O-methylation, AOMs were involved in three other pathways.(iii) According to the results from the above two points that a substantial amount of the bound residues of TBBPA and its metabolites were formed in both soil slurry and NAS during the incubation, the reaserach about bound residues was further extended. We studied the fate of TBBPA in an oxic soil during 143 days of incubation. TBBPA dissipated with a half-life of 14.7 days, accompanied with mineralization (19.6%) and substantial bound-residue formation (66.5%) at the end of incubation. Eight extractable metabolites including TBBPA methyl ethers, single benzene-ring bromophenols and their methyl ethers were detected. During the first 35 days, bound residues were rapidly formed and the majority was humin-bound residues that rapidly decreased afterwards, being consistent with dynamics of the total bound-residue formation, whereas humic acid- and fulvic acid-bound residues increased continuously to constant amounts. The residues via ester- and ether-linkages accounted for 9.6-27.0% of the total bound residues during the incubation, with more contribution from the ester-linkages. The ester-linkage bound residues consisted of TBBPA, an unknown polar compound, and TBBPA monomethyl ether.The metabolic pathways of TBBPA in oxic soil and NAS were discussed in our study, providing detailed information on TBBPA transformation. In the study on TBBPA degradation in oxic soil slurry with and without amendment of Sphingomonas sp. strain TTNP3, the results showed that strain TTNP3 could stimulate TBBPA degradation in the soil. The proposed metabolic pathways of TBBPA provides for the first time the information about the complex metabolism of TBBPA in oxic soil and suggests that type Ⅱ ipso-substitution could play a significant role in the fate of alkylphenol derivatives in the environment; In the case of TBBPA transformation in NAS, our results are also the first to provide information about the complex metabolism of TBBPA in NAS and they are consistent with a determining role for nitrifiers in TBBPA degradation, by initiating its cleavage into single-ring metabolites that are substrates for the growth of heterotrophic bacteria. In addition, our results indicate that bound-residue formation was the major pathway for TBBPA dissipation and provide for the first time information on chemical structure of the bound residues, being important for risk assessment of TBBPA in soil.It is worthy to be noted that TBBPA could be transformed and degraded in oxic soil and NAS, forming various metabolites. The lipophilic O-methylated metabolites could physically sorb to soil organic matter, while the hydroxylated metabolites are chemically reactive and could chemically react with soil organic matter forming bound residues in the soil, thus reducing their bioavailability.O-methylated metabolites have higher bioaccumulation factors and tend to accumulate in lipids, while active metabolites may attack biomolecules within the organisms; nevertheless, both may trigger adverse toxicological effects.Furthermore, nitro-aromatic compounds that produced during TBBPA transformationin NAS may be genotoxic and carcinogenic. Further studies are needed on the environmental fate and biological effects of the reported metabolites.