Honglin Huang

Behavior of Decabromodiphenyl Ether (BDE-209) in the Soil−Plant System: Uptake, Translocation, and Metabolism in Plants and Dissipation in Soil

Deca-bromodiphenyl ether (BDE-209) is the major component of the commercial deca-BDE flame retardant. There is increasing concern over BDE-209 due to its increasing occurrence in the environment and in humans. In this study the behavior of BDE-209 in the soil-plant system was investigated. Accumulation of BDE-209 was observed in the roots and shoots of all the six plant species examined, namely ryegrass, alfalfa, pumpkin, summer squash, maize, and radish. Root uptake of BDE-209 was positively correlated with root lipid content (P < 0.001, R(2) = 0.81). The translocation factor (TF, C(shoot)/C(root)) of BDE-209 was inversely related to its concentration in roots. Nineteen lower brominated (di- to nona-) PBDEs were detected in the soil and plant samples and five hydroxylated congeners were detected in the plant samples, indicating debromination and hydroxylation of BDE-209 in the soil-plant system. Evidence of a relatively higher proportion of penta- through di-BDE congeners in plant tissues than in the soil indicates that there is further debromination of PBDEs within plants or low brominated PBDEs are more readily taken up by plants. A significant negative correlation between the residual BDE-209 concentration in soil and the soil microbial biomass measured as the total phospholipid fatty acids (PLFAs) (P < 0.05, R(2) = 0.74) suggests that microbial metabolism and degradation contribute to BDE-209 dissipation in soil. These results provide important information about the behavior of BDE-209 in the soil-plant system.

Plant uptake and dissipation of PBDEs in the soils of electronic waste recycling sites

Plant uptake and dissipation of weathered PBDEs in the soils of e-waste recycling sites were investigated in a greenhouse study. Eighteen PBDE congeners (tri- through deca-) were detected in the plant tissues. The proportion of lower brominated PBDEs (mono- through hexa-) in plant roots was higher than that in the soils. A concentration gradient was observed of PBDEs in plants with the highest concentrations in the roots followed by the stems and lowest in the leaves. Reduction rates of the total PBDEs in the soils ranged from 13.3 to 21.7% after plant harvest and lower brominated PBDEs were associated with a higher tendency to dissipate than the higher brominated PBDEs. This study provides the first evidence for plant uptake of weathered PBDEs in the soils of e-waste recycling sites and planting contributes to the removal of PBDEs in e-waste contaminated soils.

Relationships between aging of PAHs and soil properties

Sequestration and diffusion of three polycyclic aromatic hydrocarbons (PAHs) in seven Chinese soils were investigated for up to 200 days in sterile soil microcosms as functions of soil property and aging time. The aging of the PAHs, assessed using a mild extractant that removes primarily the labile fraction, showed a biphasic behavior. The rapid diffusion from labile to nonlabile domains was mainly dependent upon the distribution of meso- and micropore fraction and total organic carbon content. Meanwhile, the slow diffusion was found to decrease with the increase of the content of soil organic carbon, particularly of hard organic carbon (p < 0.01) and the meso- and micropore fraction, as well as with the increasing molecular size of PAHs. This work offers evidence that analyses of organic carbon fractionation and porosity are important to adequately assess the mechanistic basis of sequestration and diffusion of organic contaminants in soils.

Uptake, translocation and metabolism of polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) in maize (Zea mays L.)

A hydroponic experiment was conducted in the present study to investigate and compare plant uptake, translocation and metabolism of polybrominated diphenyl ethers (PBDEs) of BDE-15, BDE-28 and BDE-47 and polychlorinated biphenyls (PCBs) of PCB-15, PCB-28 and PCB-47 in maize. Root concentrations of BDE-15, BDE-28 and BDE-47 were consistently higher than PCB-15, PCB-28 and PCB-47, respectively. A significantly positive correlation was found between logRCF (root concentration factor) and logKow of these PBDEs and PCBs, suggesting a control role of their partitioning in plant uptake. The translocation factors (TFs, Cstem/Croot) of PBDEs were generally lower than those of PCBs of the same halogen-substitutions, demonstrating easier transport of PCBs than PBDEs. Metabolites mono-, di- and tri-BDEs and PCBs were detected, suggesting the existence of in vivo metabolism of PBDEs and PCBs in maize. Dehalogenation and rearrangement of halogen atoms were identified, and some similarities but also significant differences existed between the PBDEs and PCBs. PBDEs in maize were, in general, more susceptible to metabolism compared with PCBs of the same halogen-substitutions. This is the first comparative report on the uptake, translocation and metabolism of PBDEs and PCBs in plants.

Arsenic Accumulation and Speciation in Maize as Affected by Inoculation with Arbuscular Mycorrhizal Fungus Glomus mosseae

Effects of inoculation with arbuscular mycorrhizal (AM) fungus (Glomus mosseae) on arsenic (As) accumulation and speciation in maize were investigated by using As spiked soil at the application levels of 0, 25, 50, and 100 mg kg(-1). Inorganic As was the major species in plants, and mycorrhizal inoculation generally decreased concentrations of arsenite [As(III)] in maize roots and concentrations of As(III) and arsenate [As(V)] in the shoots. Dimethylarsenic acid (DMA) concentrations (detected in every plant sample) were higher in maize shoots for mycorrhizal than for nonmycorrhizal treatment, but no significant differences were observed for roots. Monomethylarsenic acid (MMA) was only detected in roots with mycorrhizal colonization. The uptake of As(V) was much lower by excised mycorrhizal than nonmycorrhizal roots, and the differences for the uptake of As(III) were negligible. Arsenate reductase (AR) activity was detected in maize roots, and it was reduced with mycorrhizal inoculation. Activities of peroxidase (POD) and superoxide dismutase (SOD) were detected in both maize shoots and roots, and they were suppressed by mycorrhizal inoculation. AM inoculation inhibited the uptake of As(V) and its reduction to As(III), reducing oxidation stress and thereby alleviating As toxicity to the host plant.

Characterization of polybrominated diphenyl ethers (PBDEs) and hydroxylated and methoxylated PBDEs in soils and plants from an e-waste area, China.

In order to characterize polybrominated diphenyl ethers (PBDEs), and hydroxylated and methoxylated PBDEs (OH-PBDEs and MeO-PBDEs) in the soil-plant system, soil and plant samples were collected from an e-waste recycling area in China. Forty one PBDEs, twelve OH-PBDEs and MeO-PBDEs were detected in the soil and plant samples. Concentrations of PBDEs in roots were significantly correlated to their concentrations in the soils, but the percentages of lower brominated congeners in the plants were higher than those in the soils. Significant positive linear relationships exist between concentrations of ∑OH-PBDEs and ∑MeO-PBDEs with higher levels of ∑MeO-PBDEs than those of ∑OH-PBDEs in the soils, plant roots and leaves. A majority of the OH-/MeO-PBDEs had the hydroxyl or methoxy group at the ortho-positions to the biphenyl bond for most of the plant species. However the occurrence of meta- and para- substituted OH-/MeO-PBDEs in soils and plants were also confirmed.

Debrominated, hydroxylated and methoxylated metabolism in maize (Zea mays L.) exposed to lesser polybrominated diphenyl ethers (PBDEs)

A hydroponic experiment was conducted to investigate the debrominated, hydroxylated and methoxylated metabolism of polybrominated diphenyl ethers (PBDEs, BDE-15, -28 and -47) in maize. A total of six debrominated metabolites (de-PBDEs), seven hydroxylated PBDEs (OH-PBDEs, including two unidentified OH-di-PBDEs and one unidentified OH-tri-PBDE) and four methoxylated PBDEs (MeO-PBDEs) were determined in the exposed plants. The metabolic products were detected in maize only after 12h of exposure to the PBDEs. However, the concentration of each type of the metabolites (de-PBDEs, OH-PBDEs or MeO-PBDEs) decreased at the later exposure time, possibly due to further metabolism. The removal of a bromine atom or the introduction of a hydroxyl/methoxy group was easier at the ortho-positions on the biphenyl structure than at the para-positions. Concentration ratios of the total debrominated, hydroxylated or methoxylated metabolites to the parent congener (BDE-28 or -47) generally followed the order of leaves>stems>>roots, and MeO-PBDEs>de-PBDEs>>OH-PBDEs. These results suggest that metabolism occurred preferentially in leaves and stems than in roots. Less transformation and shorter elimination half-life of OH-PBDEs would contribute to the lower concentrations of OH-PBDEs than of de-PBDEs or MeO-PBDEs in maize.

Enhanced dissipation of phenanthrene in spiked soil by arbuscular mycorrhizal alfalfa combined with a non-ionic surfactant amendment.

Experiments were conducted to assess the role of colonization of alfalfa roots by an arbuscular mycorrhizal (AM) fungus (Glomus etunicatum) in conjunction with a non-ionic surfactant (Triton X-100) in dissipation of phenanthrene in a soil spiked with phenanthrene at 0, 2.5, 5.0 and 10.0 mg kg(-1). After plant harvest the residual phenanthrene concentration in the soil decreased markedly. Mycorrhizal treatment enhanced phenanthrene dissipation in the rhizosphere and bulk soils irrespective of phenanthrene application rate. Addition of Triton X-100 resulted in the highest phenanthrene concentration in the rhizosphere soil among the treatments, while the lowest phenanthrene concentration in the bulk soil was obtained by AM inoculation and amendment with Triton X-100. AM inoculation and addition of the surfactant consistently promoted phenanthrene dissipation in the soil and decreased the microbial biomass based on phospholipid fatty acid (PLFA) analysis. PLFA profiles demonstrated that AM inoculation together with addition of Triton X-100 altered the microbial community structure in the rhizosphere soil. The results of this study provide a reference value for phytoremediation of soil contaminated by organic pollutants.

Behavior of decabromodiphenyl ether (BDE-209) in soil: effects of rhizosphere and mycorrhizal colonization of ryegrass roots.

A rhizobox experiment was conducted to investigate degradation of decabromodiphenyl ether (BDE-209) in the rhizosphere of ryegrass and the influence of root colonization with an arbuscular mycorrhizal (AM) fungus. BDE-209 dissipation in soil varied with its proximity to the roots and was enhanced by AM inoculation. A negative correlation (P < 0.001, R(2) = 0.66) was found between the residual BDE-209 concentration in soil and soil microbial biomass estimated as the total phospholipid fatty acids, suggesting a contribution of microbial degradation to BDE-209 dissipation. Twelve and twenty-four lower brominated PBDEs were detected in soil and plant samples, respectively, with a higher proportion of di- through hepta-BDE congeners in the plant tissues than in the soils, indicating the occurrence of BDE-209 debromination in the soil-plant system. AM inoculation increased the levels of lower brominated PBDEs in ryegrass. These results provide important information about the behavior of BDE-209 in the soil-plant system.

Phenanthrene uptake by Medicago sativa L. under the influence of an arbuscular mycorrhizal fungus

Phenanthrene uptake by Medicago sativa L. was investigated under the influence of an arbuscular mycorrhizal fungus. Inoculation of lucerne with the arbuscular mycorrhizal fungus Glomus etunicatum L. resulted in higher phenanthrene accumulation in the roots and lower accumulation in the shoots compared to non-mycorrhizal controls. Studies on sorption and desorption of phenanthrene by roots and characterization of heterogeneity of mycorrhizal and non-mycorrhizal roots using solid-state (13)C nuclear magnetic resonance spectroscopy ((13)C NMR) demonstrated that increased aromatic components due to mycorrhizal inoculation resulted in enhanced phenanthrene uptake by the roots but lower translocation to the shoots. Direct visualization using two-photon excitation microscopy (TPEM) revealed higher phenanthrene accumulation in epidermal cells of roots and lower transport into the root interior and stem in mycorrhizal plants than in non-mycorrhizal controls. These results provide some insight into the mechanisms by which arbuscular mycorrhizal inoculation may influence the uptake of organic contaminants by plants.