Partition uptake of a brominated diphenyl ether by the edible plant root of white radish (Raphanus sativus L.)☆
Graphical abstract
Introduction
Often employed as flame retardants, polybrominated diphenyl ethers (PBDEs) were found in many consumer products around the world prior to their bans by EU in 2008. These brominated chemicals have now been detected in various environmental matrices, such as sludge, soil, water, air and even in human blood (Bi et al., 2006, Harrad et al., 2006, Hellström, 2000, Lacorte et al., 2003). The chemicals are persistent and bioaccumulative and may act as endocrine disruptors, carcinogens, and neurodevelopment toxicants (Darnerud et al., 2001). Some farms are cultivated or irrigated with wastewater and sludge containing these pollutants, and therefore many of these PBDE congeners in the environment may enter the food chain and impact food safety (Bocio et al., 2003).
Among many remediation technologies, phytoremediation has been developed to resolve many environmental problems (Burken and Schnoor, 1996, Burken and Schnoor, 1998, Garbisu et al., 2002). Previous studies suggested that large green plants are capable of transpiring large masses of soil water through their roots to their leaves and the atmosphere (Burken and Schnoor, 1996, Dietz and Schnoor, 2001, Hinchman et al., 1996, Kling, 1997, Salt et al., 1998). Through this process, the contaminants in soil water are taken up and sequestered, metabolized, or vaporized into the atmosphere along with the transpired water, which constitutes a friendly remediation technology for pollutants in soil. However, the uptake and sequestration of contaminants from soils by plants also causes a serious concern regarding crop contamination by toxic substances. The contaminant accumulation by plants and the subsequent distribution within plants depends in principle on the source input, the plant species, and contaminant properties.
Plant uptake through roots is a crucial pathway for the subsequent translocation of pollutants to all tissues. Although the contaminant uptake by plants from air is important for volatile organic chemicals (Orwell et al., 2004, Zuo et al., 2006), the uptake from soil and water by plant roots is a very important pathway for sparingly water-insoluble organic compounds (Collins et al., 2006). Chen et al. (2009) suggests that plant root cells have a high sorption affinity for organic chemicals. Many pollutants, such as PBDEs (Chow et al., 2015, Li et al., 2015), chlorinated organic compounds(Trapp, 2015), and polycyclic aromatic hydrocarbons (PAHs)(Samsøe-Petersen et al., 2002), have been found to be readily absorbed by plant roots and translocated into shoots. Dietz and Schnoor (2001) recognized that the efficiency of the contaminant uptake from a cultivated medium by plants depends on the contaminant octanol-water partition coefficient (Kow). However, because of the system diversity, it is difficult to come up with a general rule to estimate the uptakes of different contaminants by different plants.
The transport of nonionic organic compounds through plant roots into various plant tissues is found to closely follow a partition-limited process according to Chiou et al. (2001). For highly lipophilic organic compounds, the plant lipid phase is considered to be the dominant reservoir and the partition coefficients of these compounds with lipids (Klip) may be approximated by the corresponding Kow values. The contributions to the pollutants uptake by other plant components, such as carbohydrates, are also considered. Up to now, the partition-limited model has been widely used to explain the uptakes of various contaminants by a variety of plants and crops (Card et al., 2012, Chen et al., 2009, Li et al., 2009, Zhang and Zhu, 2009). Card et al. (2012) used the partition-limited model to evaluate the uptake of synthetic estrogens by maize seedlings. Yang and Zhu (2007) evaluated the uptake of PAHs (acenaphthene, fluorene, phenanthrene, and pyrene) by ryegrass after a 96 h exposure using the model. Moreover, the partition-limited model provided a satisfactory account of the passive transport of pharmaceutical chemicals (oxytetracycline, sulfamethoxazole, and ketoconazole) from soil into the grass and watercress (Chitescu et al., 2013).
At this time, more mechanistic information about PBDE uptake by root crops would be helpful to further substantiate the capability of the partition-limited model. The purposes of this study are to investigate (1) whether the extent of the plant-component uptake toward the limiting capacity (i.e., the α value) relates with the order of the contaminant transport distance and (2) whether the DBE-3 uptake is significantly influenced by the medium through which the contaminant enters the plant. Meanwhile, in consideration of the lack of studies for the potential of food crops to be contaminated by PBDEs, we have investigated the uptake of 4-bromodiphenyl ether (BDE-3) by white radish (Raphanus sativus) to generate the required data for analysis. The accumulated BDE-3 concentrations in several parts of the white radish from various external media (hydroponics, silica sand, and soil) were analyzed and compared.
Section snippets
Chemicals
4-Bromodiphenyl ether (BDE-3) was purchased from Aldrich-Chemie GmbH & Co. KG (Steinheim, Germany). Aqueous solutions of BDE-3 were prepared with methanol as an intermediate solvent (methanol in water ≤ 2% v/v) and diluted with purified water (18.2 MΩ cm). The molecular formula of BDE-3 is C12H9BrO and molecular weight is 249.11; the log KOW value is 4.8 (Hayward et al., 2006) and water solubility is about 5 mg-L−1 (Tittlemier et al., 2002).
Plant species and determination of compositions
Fresh white radishes (Raphanus sativus L.) and the
Isotherms of the soil and the residue of white radish
The sorption isotherm of BDE-3 on soil is highly linear (Fig. 1) indicating that partition processes dominate in the soil system. Table 1 shows the measured composition for each component of the white radish on a fresh weight basis. As shown, any radish part (root, peel, or leaf) contains tremendously more carbohydrates than lipids. The lipid contents in the components range from 0.05 to 0.11% by weight, with the highest content being found in the roots. Based on the compositions of the white
Acknowledgment
The authors would like to thank Agriculture and Food Agency from Council of Agriculture in Executive Yuan in Taiwan for the financial support under Contract No. 101AS-14.2.3-FD-Z1(1).
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