Lianhong Wang

Degradation, metabolism, and bound-residue formation and release of Tetrabromobisphenol A in soil during sequential anoxic-oxic incubation.

Tetrabromobisphenol A (TBBPA) is one of the most commonly used flame retardants and has become an environmental contaminant worldwide. We studied the fate of (14)C-labeled TBBPA in soil under static anoxic (195 days) and sequential anoxic (125 days)-oxic (70 days) conditions. During anoxic incubation, TBBPA dissipated with a half-life of 36 days, yielding four debromination metabolites: bisphenol A (BPA) and mono-, di-, and tribrominated BPA. At the end of anoxic incubation, all four brominated BPAs completely disappeared, leaving BPA (54% of initial TBBPA) as the sole detectable organic metabolite. TBBPA dissipation was accompanied by trace mineralization (<1.3%) and substantial bound-residue formation (35%), probably owing to chemical binding to soil organic matter. Subsequent oxic incubation was effective in degrading accumulated BPA (half-life 11 days) through mineralization (6%) and bound-residue formation (62%). However, 42% of the anoxically formed bound residues was released as TBBPA and lower brominated BPAs, which were then persistent during oxic incubation. Our results provide the first evidence for release of bound residues during alteration of the redox environment and indicate that sequential anoxic-oxic incubation approaches-considered effective in remediation of environments containing halogenated xenobiotics-do not completely remove xenobiotics from environmental matrices.

Fate of Tetrabromobisphenol A (TBBPA) and Formation of Ester- and Ether-Linked Bound Residues in an Oxic Sandy Soil

Bound-residue formation is a major dissipation process of most organic xenobiotics in soil. However, both the formation and nature of bound residues of tetrabromobisphenol A (TBBPA) in soil are unclear. Using a 14C-tracer, 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; at the end of incubation, 19.6% mineralized and 66.5% formed bound residues. Eight extractable metabolites were detected, including TBBPA methyl ethers, single-ring bromophenols, and their methyl ethers. Bound residues (mostly bound to humin) rapidly formed during the first 35 days. The amount of those humin-bound residues then quickly decreased, whereas total bound residues decreased slowly. By contrast, residues bound to humic acids and fulvic acids increased continuously until a plateau was reached. Ester- and ether-linked residues accounted for 9.6-27.0% of total bound residues during the incubation, with ester linkages being predominant. Residues bound via ester linkages consisted of TBBPA, TBBPA monomethyl ether, and an unknown polar compound. Our results indicated that bound-residue formation is the major pathway of TBBPA dissipation in oxic soil and provide first insights into the chemical structure of the reversibly ester-linked bound residues of TBBPA and its metabolites.

Enhanced Transformation of Tetrabromobisphenol A by Nitrifiers in Nitrifying Activated Sludge

The fate of the most commonly used brominated flame retardant, tetrabromobisphenol A (TBBPA), in wastewater treatment plants is obscure. Using a (14)C-tracer, we studied TBBPA transformation in nitrifying activated sludge (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 dinitro-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. Our results are 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.

Inhibitory effects of carbon nanotubes on the degradation of 14C-2,4-dichlorophenol in soil.

Concerns on the potential risks of engineered nanoparticles to the environment are increasing; however, little is known about the effects of carbon nanotubes (CNTs) on the environmental fate of hydrophobic organic pollutants in soil. We incubated radioactive labeled 2,4-dichlorophenol ((14)C-2,4-DCP) in a soil in the presence of various concentrations (0, 2, 20, and 2000 mg kg(-1) dry soil) of single-walled (SWCNTs) and multi-walled (MWCNTs) carbon nanotubes, and determined the mineralization, degradation, and residue distribution of 2,4-DCP in the soil. CNTs were added to the soil either after the spiking of (14)C-2,4-DCP or together with (14)C-2,4-DCP as a mixture. CNTs at the concentration of 2000 mg kg(-1) significantly (P<0.05) inhibited the mineralization of (14)C-2,4-DCP and induced a 2.3- to 3.9-fold increase in the amounts of the non-degraded (14)C-2,4-DCP in the soil after 90 d of incubation. Pre-adsorption of (14)C-2,4-DCP on CNTs showed stronger inhibitory effects on the degradation of (14)C-2,4-DCP, already significant with CNTs at 20 mg kg(-1). In general, SWCNTs had a higher effect on the degradation and residue distribution of 2,4-DCP in the soil than MWCNTs. The inhibitory effects are supposed to be owing to limited activities of soil endogenous microorganisms, potential toxicities of CNTs to the microorganisms, and reduced bioavailability of 2,4-DCP in the presence of CNTs, even though a desorption hysteresis of 2,4-DCP on CNTs was not observed. Our results indicate that CNTs have more significant impacts on the environmental fate of the hydrophobic pollutants entering soil together with CNTs via strong sorption than the pollutants already present in soil.

Degradation and bound-residue formation of nonylphenol in red soil and the effects of ammonium.

Fate of nonylphenol (NP) in soils and the effects of nitrogen fertilizers are unclear. Using (14)C-tracer, we studied the aerobic and anaerobic degradation of 4-NP111 in a paddy red soil amended without and with ammonium chloride. Under oxic conditions, 4-NP111 had a half-life of 16.1 ± 1.6 days and minor mineralization (3.84 ± 0.02%), forming no extractable metabolite but abundant bound residues (60.9 ± 1.7%, mostly bound to humin) after 49 days of incubation. The ammonium amendment (8 mmol/kg soil) significantly inhibited the degradation (half-life of 68.0 ± 7.7 days), mineralization (2.0 ± 1.1%), and bound-residue formation (23.7 ± 0.2%). Under anoxic conditions, 4-NP111 did not degrade during 49 days of incubation and the ammonium amendment (40 mmol/kg soil) did not affect its persistence. Our results demonstrate that bound-residue formation was a major mechanism for NP dissipation in the red soil under oxic conditions and that chemical nitrogen fertilizer at average field application rate may already considerably increase NP recalcitrance in agricultural soils.

Effects of the geophagous earthworm Metaphire guillelmi on sorption, mineralization, and bound-residue formation of 4-nonylphenol in an agricultural soil

Effects of earthworms on fate of nonylphenol (NP) are obscure. Using (14)C-4-NP111 as a representative, we studied the fate of 4-NP in an agricultural soil with or without the earthworm Metaphire guillelmi and in fresh cast of the earthworm. Sorption of 4-NP on the cast (Kd 1564) was significantly higher than on the parent soil (Kd 1474). Mineralization of 4-NP was significantly lower in the cast (13.2%) and the soil with earthworms (10.4%) than in the earthworm-free soil (16.0%). One nitro metabolite of 4-NP111 (2-nitro-4-NP111) was identified in the soil and cast, and the presence of the earthworm significantly decreased its amounts. The presence of earthworm also significantly decreased formation of bound residues of 4-NP in the soil. Our results demonstrate that earthworms could significantly change the fate of 4-NP, underlining that earthworm effects should be considered when evaluating behavior and risk of 4-NP in soil.

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

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 93days under oxic conditions. TBBPA dissipated with a first-order kinetic constant kd of 0.0474±0.0017day-1 (t1/2 14.6±0.3days) and mineralized with a km of 0.0011±0.00002day-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 231days, 9.2±0.3% was mineralized (km 0.00047±0.00002day-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.

Fate and metabolism of the brominated flame retardant tetrabromobisphenol A (TBBPA) in rice cell suspension culture.

Tetrabromobisphenol A (TBBPA) is the brominated flame retardant with the highest production volume and its bioaccumulation in environment has caused both human health and environmental concerns, however the fate and metabolism of TBBPA in plants is unknown. We studied the fate, metabolites, and transformation of (14)C-labeled TBBPA in rice cell suspension culture. During the incubation for 14 days, TBBPA degradation occurred continuously in the culture, accompanied by formation of one anisolic metabolite [2,6-dibromo-4-(2-(2-hydroxy)-propyl)-anisole] (DBHPA) (50% of the degraded TBBPA) and cellular debris-bound residues (46.4%) as well as mineralization (3.6%). The cells continuously accumulated TBBPA in the cytoplasm, while a small amount of DBHPA (2.1% of the initially applied TBBPA) was detectable inside the cells only at the end of incubation. The majority of the accumulated residues in the cells was attributed to the cellular debris-bound residues, accounting for 70-79% of the accumulation after the first incubation day. About 5.4% of the accumulation was associated with cell organelles, which contributed 7.5% to the cellular debris-bound residues. Based on the fate and metabolism of TBBPA in the rice cell suspension culture, a type II ipso-substitution pathway was proposed to describe the initial step for TBBPA degradation in the culture and balance the fate of TBBPA in the cells. To the best of our knowledge, our study provides for the first time the insights into the fate and metabolism of TBBPA in plants and points out the potential role of type II ipso-hydroxylation substitution in degradation of alkylphenols in plants. Further studies are required to reveal the mechanisms for the bound-residue formation (e.g., binding of residues to specific cell wall components), nature of the binding, and toxicological effects of the bound residues and DBHPA.

Effects of the earthworm Metaphire guillelmi on the mineralization, metabolism, and bound-residue formation of tetrabromobisphenol A (TBBPA) in soil

Tetrabromobisphenol A (TBBPA) is one of the most widely used brominated flame retardants worldwide. The degradation and fate of this organic pollutant of soils is of great concern and can be strongly affected by geophagous earthworms through ingestion and burrowing activities. Using 14C-tracers, we studied the effects of the geophagous earthworm Metaphire guillelmi on the mineralization, metabolism, and bound-residue formation of TBBPA in a typical Chinese rice paddy soil during 30days of incubation in the laboratory. Earthworms significantly decreased both mineralization (from 3.9±0.3% of the initial amount to 2.6±0.2%) and dissipation (from 90.6±0.6% to 84.1±1.2%) of TBBPA in the soil, and stimulated the generation of O-methylation metabolites (TBBPA methyl ethers; from 1.4±0.4% to 15.4±0.6%). This resulted in a strong decrease in bound-residue formation of TBBPA and its metabolites in the soil (from 80.3±0.4% to 41.8±3.1%). Results from a first-order, two-compartment model that describes the fate of TBBPA in soil indicated that the TBBPA-derived bound residues were mainly attributed to the binding of metabolites to the soil matrix and not to the binding of TBBPA, and that earthworms reduced the kinetic rates of both polar metabolite generation and their bound-residue formation. Our results suggested that the geophagous earthworm Metaphire guillelmi strongly influenced the fate of TBBPA by altering the composition of metabolites and therefore bound-residue formation. The increased persistence of TBBPA and the formation of persistent O-methylation metabolites by M. guillelmi would increase the environmental risk of TBBPA.

Mineralisation of (14)C-labelled polystyrene plastics by Penicillium variabile after ozonation pre-treatment.

Large amounts of polystyrene (PS), one of the most widely used plastics in the world, end up in the environment through industrial discharge and littering, becoming one of the major components of plastic debris. Such plastics, especially the small-sized microplastics and nanoplastics, have received increasing concerns in terms of their potential environmental risks. Feasible approaches for the degradation of PS in waste materials and in the environment are highly desirable. Physicochemical pretreatments of PS may be applied to enhance biological degradation. In the present study, we synthesized 14C-labelled PS polymers, either uniformly labelled on the ring ([U-ring-14C]-PS) or labelled at the β-carbon position of the alkyl chain ([β-14C]-PS), and investigated the mineralisation of the 14C-PS polymers by the fungus Penicillium variabile CCF3219 as well as the effect of ozonation as a physico-chemical pre-treatment on the mineralisation by the fungi. Biodegradation of the 14C-PS polymers was studied in liquid medium (pH 7.5, without additional carbon substrate) with P. variabile for 16 weeks. During the incubation time, 14CO2 was captured to calculate the mineralisation of 14C-PS and the remaining polymers were analysed by means of scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectrometry and gel-permeation chromatography (GPC). The results showed that the fungi mineralised both labelled polymers, and that the [U-ring-14C]-PS with a lower molecular weight led to a higher mineralisation rate. Ozonation pre-treatment strongly enhanced mineralisation of [β-14C]-PS. SEM analysis showed that the surface of the ozonated [β-14C]-PS became uneven and rough after the incubation, indicating an attack on the polymer by P. variabile. FT-IR analysis showed that ozonation generated carbonyl groups on the [β-14C]-PS and the amount of the carbonyl groups decreased after incubation of the [β-14C]-PS with P. variabile. GPC analysis showed that the molecular weights of the ozonated [β-14C]-PS decreased after incubation. The present data suggest that ozonation pretreatment could be a potential approach for degradation of PS waste and remediation of PS-contaminated sites.