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ICPEP-3 (2005) Souvenir

Phytoremediation of Selenium and Other Toxic Trace Elements

Norman Terry and Danika LeDuc

Department of Plant and Microbial Biology, 111 Koshland Hall,

University of California, Berkeley, CA 94720-3102, U.S.A.


Soils and waters in many areas of the world are polluted with toxic trace elements. These include metalloids such as selenium (Se) and arsenic (As), as well as heavy metals, e.g., cadmium (Cd), lead (Pb), chromium (Cr) and mercury (Hg). Because of the acute toxicity of these elements, there is an urgent need to develop technologies to remove or detoxify them. Phytoremediation, the use of plants and their associated microbes, offers an effective, low-cost and sustainable means to achieve this end. For contaminants present in soils, there are several different approaches of which three are presented here. Phytoextraction utilizes the ability of certain plants to take up contaminants from the soil and water and accumulate them in their tissues, which can then be harvested and removed from the site. Phytovolatilization makes use of the plant's ability to convert pollutants into volatile forms, which then escape to the atmosphere (e.g., Se, mercury). Phytodetoxification involves the ability of plants to change the chemical species of the contaminant to a less toxic form, as occurs when plants take up toxic hexavalent chromium (Cr) and convert it to non-toxic trivalent Cr. For contaminants present in wastewaters, provide an effective, low-cost treatment system to remove a wide-range of contaminants from municipal wastewater as well as effluents from electricity generating facilities and oil refineries. Although this is not phytoremediation in the sense used above, plants nevertheless play a critical function in this form of pollutant removal. In this brief review, we highlight different aspects of our research on the phytoremediation of Se, as well as some heavy metal pollutants.

Constructed wetlands

Constructed wetlands are artificial wetlands specifically designed to improve water quality. Like natural wetlands, they are a complex mixture of water, sediments, living and dead plant materials, fauna and microbes. In essence, constructed wetlands act as giant biogeochemical filters able to remove contaminants present at very low concentrations from very large volumes of wastewater (e.g., Se from oil refinery wastewater). The filtration of contaminants that occurs in a wetland ecosystem takes place mostly in the layer of dead, partially decomposed plants, known as .fallen litter, and in the fine sediment layer beneath the litter layer. These two layers provide habitat for microbes and other organisms able to transform contaminants into less bio available and therefore less toxic chemical forms. In addition to their role in generating the fallen litter and fine sediment layers, plants provide the fixed carbon that supports these microbial populations.

Much of our research has focused on Se, a pollutant present in wastewater from industrial and agricultural enterprises. In California, soils on the west side of the San Joaquin Valley were formed from the natural weathering of Se-bearing Cretaceous shales of the surrounding California Coast Range and are therefore enriched in Se. Since these Se- and salt-laden soils must be heavily irrigated to leach away the salts, large amounts of subsurface drainage water are produced, which is highly contaminated with dissolved Se. This drainage-water Se has been shown to be responsible for mortality, developmental defects, and reproductive failure in migratory aquatic birds and fish. In an experiment designed to test whether constructed wetlands could be used to filter Se from irrigation drainage water, we constructed 10 quarter-acre wetland cells at the Tulare Lake Drainage District in the San Joaquin Valley. Specifically, we wanted to know whether varying plant species influences the efficiency of wetlands for Se removal. Each cell was planted with one or a combination of the following plant species: sturdy bulrush (Schoenoplectus robustus (Pursh) M.T. Strong), baltic rush (Juncus balticus Willd.), smooth cordgrass (Spartina alterniflora Loisel.), rabbitfoot grass (Polypogon monspeliensis (L.) Desf.), saltgrass [Distichlis spicata (L.) Greene], cattail [Typha latifolia L.], tule [Schoenoplectus acutus (Muhl. ex Bigelow) Á. Löve & D. Löve), and widgeon grass (Ruppia maritime L.). One cell was left unplanted as a control. On average, the wetland cells removed approximately 69% of the total Se mass from the inflow. The presence of plants improved the efficiency of Se removal over the unvegetated cell but the particular choice of plant species was found to have no significant effect on Se removal. Rates of biological Se volatilization on the other hand, varied substantially with the type of species planted, with the highest rates of biological Se volatilization being recorded when the wetland cell was planted with rabbitfoot grass.

Most of the Se filtered out by the wetland cells was trapped in the upper sediment layers, which could present a serious ecotoxic risk over the long term. One way of alleviating the buildup of Se in the upper sediment layers of constructed wetlands is to increase the efficiency by which wetlands convert the Se into volatile forms. Plants and microbes are both capable of absorbing inorganic and organic forms of Se and metabolizing them to volatile forms, e.g., dimethylselenide (DMSe), which is 500-700 times less toxic than SeO4-2 or SeO3-2 . In this way Se is removed from the sediments, and its entry into the food chain diminished. Even though the volatilized Se may eventually be redeposited in other areas, this is not a problem in California where much of the state is deficient in Se with respect to the nutrition of animals, which require Se in low concentrations.

Genetic modification of plants to enhance phytoremediation

Because of the potential importance of biological Se volatilization to Se phytoremediation in both wetlands and uplands, research in our laboratory has focused heavily on genetic engineering strategies to improve Se volatilization by plants, as well as on increasing the ability of plants to extract and remove Se from contaminated soil. A useful approach for increasing phytoremediation efficiency is to overexpress enzymes catalyzing rate- limiting steps. Tagmount et al. (2002) showed that S-adenosyl-L-methionine:L-methionine S-methyltransferase(MMT) is the enzyme responsible for the methylation of selenomethionine to Se-methylselenomethionine and that this enzyme was rate limiting with respect to the production of volatile Se. Overexpression of MMTin Arabidopsis and Indian mustard (Brassica juncea) increased the rate of Se phytovolatilization approximately twofold over wild type.

Another, quite different approach is to use hyperaccumulator plant species as a source of plant genes to enhance phytoremediation. Milk vetch (Astragalus bisulcatus) is a hyperaccumulator that accumulates Se in its leaves to concentrations in excess of 4000 ppm. However, because it grows very slowly, it is of limited value for phytoremediation. It is able to accumulate Se to very high levels partly through the presence of the gene encoding selenocysteine methyltransferase (SMT). SMT converts the amino acid, selenocysteine (SeCys), which can cause toxicity when incorporated into protein, into the non-protein amino acid, methylselenocysteine (MetSeCys), thereby diminishing Se toxicity. Transgenic plants overexpressing SMT show enhanced tolerance to Se, particularly selenite, and produced 3- to 7-fold more biomass than wild type. SMT plants accumulated up to 4-fold more Se than wild type, with higher proportions in the form of MetSeCys. Additionally, SMT Arabidopsis and SMT Indian mustard volatilized Se 2 to 3 times faster when treated with SeCys and selenate, respectively.

Our greatest success in using genetically engineered plants for phytoremediation has been in the phytoextraction of Se from soil with high levels of Se, boron (B), and salt. Three transgenic lines of Indian mustard were used in the first transgenic phytoremediation trial in the U.S. One line overexpressed the enzyme ATP sulfurylase (APS), which facilitates the reduction of selenate to selenite and is rate limiting with respect to the production of reduced, organic Se compounds. Indian mustard plants overexpressing APS have increased tolerance and accumulation of Se. APS Indian mustard may tolerate metals better because it has higher concentrations of the thiol, glutathione (GSH), than wild type. Glutathione (Y-Glu-Cys-Gly) plays an important role in heavy-metal detoxification. GSH can directly form GSH-metal complexes and, as part of the active oxygen-scavenging system, can protect the plant cell from oxidative stress. GSH is also the direct precursor of phytochelatins (PCs), which bind, detoxify, and sequester metal ions to the vacuole. The other two lines tested in the field trial expressed genes in the GSH synthesis pathway, Y-glutamylcysteine synthetase (Y-ECS) and glutathione synthetase (GS). Overexpression of these enzymes in Indian mustard was first shown to confer increased tolerance to Cd in solution culture. This tolerance was correlated with 1.5 to 2.5 higher levels of GSH and PCs. In the field, all three lines accumulated more Se in their shoots than wild type. In fact, the APS line accumulated over four times as much Se as wild type. Additionally, the GS line was significantly more tolerant of the high Se, B, and salt sediment than wild type.


The toxicity of the heavy metal, chromium (Cr), is of increasing concern because of its heavy use in many different industries, including metallurgy, electroplating, production of paints and pigments, tanning, wood preservation, Cr chemicals production, and pulp and paper production. It is especially toxic in its hexavalent form, Cr(VI). In this form, it is a highly toxic carcinogen, which may cause death to animals and humans if ingested in large doses. Chromium, in the trivalent form (Cr(III)), on the other hand, is an important component of a balanced human and animal diet, and its deficiency disturbs the metabolism of glucose and lipids in humans and animals. Plants have the ability to detoxify Cr(VI) by reducing it to nontoxic Cr(III). Using high energy X-ray absorption spectroscopy, research in our laboratory showed that many plants including water hyacinth and various vegetable crop species can convert Cr(VI) to Cr(III) in root and shoot tissues within hours after uptake. The ability of E. crassipes (water hyacinth) to reduce Cr(VI) to non-toxic Cr(III) could provide a significant approach for the in situ detoxification of Cr(VI) contaminated wastewater. Water hyacinth has an excellent potential for the phytoremediation of Cr in wastewater because (i) it has the ability to absorb and remove toxic Cr(VI) and then reduce it to nontoxic Cr(III); (ii) it can accumulate Cr in high concentrations in its tissues, especially roots; (iii) it produces very large amounts of biomass (106-165 t ha-1 yr-1); and (iv) since the whole plant can easily be harvested, Cr accumulated in both shoots and roots can be removed.


Our research has shown that plant-based technologies for removing and detoxifying toxic trace elements from contaminated soil and water are effective, and more importantly, can be improved substantially using modern scientific approaches. One such approach is genetic engineering. Laboratory-produced transgenic plants can be effective in field scale cleanup, as shown by our recent research with transgenic Indian mustard, which more efficiently removed Se from highly contaminated sediments. Thus, genetic improvement of plants for phytoremediation has a great potential for improving environmental quality. Nevertheless, there are very few examples where genetic approaches have reached the stage of practical exploitation. It is clear that further research and technical implementation is needed to carry phytoremediation projects beyond the pilot stage. Furthermore, there are regulatory and public acceptance barriers to overcome in applying transgenic plant technology to real-world, field situations. Such constraints have spurred researchers to innovate new methods of creating transgenic plants that will be more palatable to the public and pose less potential risk of hybridizing with nearby plants or adversely affecting wildlife. One such technique is the use of chloroplast transformation, the use of which prevents the escape of transgenes via pollen to related weeds and crops.

Unlike upland phytoremediation, which as yet has relatively limited use in environmental cleanup, plant-based approaches in wetland ecosystems (e.g., constructed wetlands) are widely used. The presence of plants in the wetland ecosystem facilitates an inexpensive, solar-driven treatment system for industrial wastewater, drainage waters, storm-water management, and treatment of surface water. Plants confer tolerance against fluctuations of flow, provide habitats for many wetland organisms, and create an aesthetic, natural system for cleanup. Aquatic plants and constructed wetlands have been used to treat municipal wastewater and remove inorganic nutrients like phosphate and nitrate. This technique has also been applied to industrial effluents containing toxic metals or recalcitrant organic pollutants. In all of these approaches, whether upland or wetland, there is clearly an urgent need for research aimed at the fundamental understanding of all the physical, chemical, and biological mechanisms involved. On a final note, more field demonstration projects are also urgently required to optimize phytoremediation approaches and to provide recommendations for the regulators, decision-makers and the general public to convince them of the suitability of the green approach for environmental cleanup.

This article has been reproduced from the Souvenir released during the Third International Conference

on Plants & Environmental Pollution (ICPEP-3) held at Lucknow from  28 November to 2 December 2005.

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