Sequestration, Phytoreduction, and Phytooxidation of Halogenated Organic Chemicals by Aquatic and Terrestrial Plants

ABSTRACT Laboratory data from plant-mediated transformation of chlorinated and brominated alkanes, alkenes, and chlorinated pesticides, including phytotransformation data from field plants currently used in phytoremediation of trichloroethylene (TCE), were reviewed for the purpose of identifying important phytoprocesses and their respective roles in phytoremediation of halogenated organic compounds (HOCs). The results of the laboratory experiments indicated that the initial very rapid removal of hydrophobic HOCs from water or the gas phase by aquatic and terrestrial plants is primarily due to sequestration. The amount of HOC sequestered is controlled by the plant species and the physicochemical properties (e.g., Kow, aqueous solubility, volatility) of the contaminant. Phytodegradation studies conducted in both the gas and aqueous phases indicated that hexachloroethane (HCA) is dechlorinated to the same metabolites by sterilized and axenically cultivated aquatic plants and an isolated plant dehalogenase factor. Similar results were obtained in experiments conducted with o,p'-DDT and p,p'-DDT in aqueous solution. The sterilized and axenically cultivated aquatic plants also oxidized HCA to similar chloroacetic acids. The metabolism of HOCs to the corresponding oxidative and reductive transformation products identified in the plant rhizosphere, stems, and leaves suggested that more than one pathway, requiring different enzymes, may be involved in phytotransformation reactions. Four phytoprocesses (mechanisms) were found to be important in the removal of the probe HOCs from water by aquatic plants, namely, (1) rapid sequestration by partitioning to the lipophilic plant cuticles; (2) phytoreduction to less halogenated metabolites; (3) phytooxidation to haloethanols, haloacetic acids, and unidentified metabolites; and (4) assimilation into the plant tissues as nonphytotoxic products, presumably produced by covalent binding with the plant tissues. Laboratory and field data indicate that the distribution of metabolites of perchloroethylene (PCE) and TCE in cottonwood and willow trees is determined by the growth stage or age of these vascular plants, the plant species, and the duration of exposure to the compound. For terrestrial plants, the predominant phytoprocesses by which HOCs are attenuated in the environment include sequestration, rhizodegradation, uptake, phytodegradation, and phytovolatilization. Using PCE as a model chlorinated organic solvent, possible phytotransformation pathways are proposed to account for the different metabolites identified in the rhizosphere and tissues of laboratory and field plants. The proposed pathways also combine phytoreduction reactions that occur in plant tissues and are likely catalyzed by plant dehalogenase(s) for example, enzyme(s) such as glutathione-S-transferase and Fe-S clusters in chloroplast ferredoxin, with phytooxidation and covalent binding (phytoassimilation) reactions mediated by oxidative-enzymes (possibly cytochrome P-450 with monooxygenase activity, glutathione or laccase). Depending on the characteristics of the field site, the phytoprocesses identified in this study are vital in the design and implementation of phytoremediation of halogenated organic contaminants.