Phytoremediation: Where Does It Stand and Where Will It Go? Dr. Jorge 1. Gardea-Torresdey Department of Chemistry and Environmental Science and Engineering, The University of Texas-El Paso, El Paso, TX 79968; jgardea@utep.edu

There are currently thousands of sites containing hazardous wastes throughout the United States. Utilizing present technological clean-up methods will cost approximately more than $1.7 trillion dollars. The two most widely recognized technologies used in clean-ups are landfill disposal and incinera- tion. However, in addition to being costly, both methods have drawbacks. Landfill dis- posal does not treat waste, just transfers it to other sites. Contaminated ash and emission are consequences of incineration.

One technology that avoids the problems of both methods is the organic treatment of phytoremediation. It could be the safer and more successful clean-up process that mod- ern technology has been seeking.

Phytoremediation is a relatively new approach to the cost-effective treatment of wastewater, groundwater, and soils contami- nated by organic xenobiotics, heavy metals, and radionuclides. While there are numer- ous, wordy descriptions of the term, phy- toremediation can be summed up with one clear definition: the use of plants for in situ removal of pollutants from the environment. Plants have shown their ability to process high concentra- tions of organic compounds with no significant toxic effects. Some of these chemicals can be transformed very rapidly to less toxic metabolites. And, metal- accumulating plants and adequate soil amendments can be used to translocate and concentrate metals from the soil on surface shoots (phytoextraction), which can be harvested, and the tissue processed by drying, ashing, or composting. In addition, these plants can either filter metals from water onto root systems (rhizofiltration), or stabilize hazardous metal- containing sites (phytostabilization). In the end, the volume of toxic waste produced using this method is only a fraction of what is typically generated by cur- rent remediation technologies.

Successful phytoremediation field tests have been conducted at sites containing petroleum hydrocarbons, such as BTEX (benzene, toluene, et hylbenzene and xylene), PAHs (polycyclic aromatic hydrocarbons), chlorinated aliphatics, pentachlorophenol, PCBs (polychlorinat- ed biphenyls), ammunition wastes such as trinitrotoluene (TNT) and cyclo- trimethylenetrinitramine (also referred to as cyclonite, hexogen, or RDX), metals (Pb, Cd, Zn, As, Cr, Se, Cu), pesticides and runoff waste, radionuclides, and nutrient wastes. Plant species as different as grass (e.g., rye, and fescue), Salixspp. (e.g., hybrid poplars), legumes (e.g. , alfalfa), aquatic plants (e.g., cattail), and metal-hyperaccumulators (e.g., sunflow- ers, pennycresses or Thlaspi spp.) have been utilized in these tests.

Most recently, exciting new other applications of phytoremediation have been reported, such as “phyto-mining” and the actual formation of gold nanoparticles. As the Greek myth goes,

King Midas’ touch converted everything to a metallic gold. The myth contains a kernel of truth, as ordinary alfalfa plants have shown tendencies to accumulate small particles (nanoparticles) of metallic gold. The most widely known materials containing nanoparti- cles of metallic gold are gold colloids, which have been used to color materials, such as glass and enam- el, since the 16th century. In modern times, the most important application of gold colloids lies in the field of nanotechnology. The next question modern sci- ence is left to address is just how to extract gold nanoparticles from alfalfa with the King Midas “touch.”

Maximizing phytoremediation’s promising capabili- ty will require the use of plants that treat specific pol- lutants more effectively. Metals require plants that

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produce high quantities of biomass at high concen- trations of metal uptake, and better metal transloca- tion rates. Organic chemicals require plants with increased degradation potential. Several researchers are attempting to make plants perform more effi- ciently through genetic engineering. The genes of rate-limiting enzymes in plants are receiving constant attention, and additional metabolic pathways from pollutant hyperaccumulators are just now being introduced. While the utilization of genetically-modi- fied plants is possible, this approach must always take into account safety, environmental impacts, and the risks of translocation of pollutants to the food chain.

Several successful field applications of phytoreme- diation show its potential for future application. Using dense poplar groves, a research team from the University of Iowa has helped stabilize soil and reduce levels of toxic metals at a decommissioned smelter in Kansas, and at a South Dakota gold mine. Homestake Mining Company was a direct participant in this research, providing partial funding and buying the 8,000 poplar trees planted at the mine. A con- structed wetland near Aveiro, Portugal, has treated industrial effluents containing nitrogenous aromatic compounds from both an aniline and nitrobenzene production plant. A 100% reduction in aromatic com- pounds has been obtained using reed beds on a total planted area of 10,000 m2. The U.S. Army Corps of Engineers Waterways Experiment Station in Vicks- burg, MS, has recently developed a wetlands system to treat groundwater contaminated with 1 mg/L TNT, and up to 13 mg/L RDX. The Phytotech Company conducted a field trial on surface water contaminated with Cs-137 and Sr-90 from the Chernobyl nuclear accident and showed a great reduction in the levels of radionuclides in the water in a four- to eight-week period.

While such successes leave me very optimistic, I decided to ask the opinion of four of the most prominent specialists in this particular field. Dr. Jer- ald Schnoor, University of Iowa’s distinguished pro- fessor of environmental engineering and the new editor of Environmental Science G Technology, states that “Phytoremediation is an emerging technology for a wide variety of applications, including rhizos- phere bioremediation of petroleum hydrocarbons and other organics, phytotransformation of chlorinat- ed solvents, and phytostabilization and/or rhizofiltra- tion for metals and nutrients. It is cost-effective, aes- thetically pleasing, and it employs nature’s cycles to treat wastes. ’‘

Dr. Larry Erickson, professor of chemical engi- neering and director of the Center for Hazardous Substance Research at Kansas State University, and a member of the Environmental Progress Editorial Advisory Board, remarks, “Phytoremediation has a

bright future because it is efficient, effective, and economical. While phytoremediation can occur nat- urally, it is more effective when good design, plant- ing, and site management processes are properly car- ried out.” Dr. C. Herb Ward, Rice University Foyt Chair of Engineering and editor of Environmental Toxicology & Chemistry, has said, “Phytoremediation, like most remediation technologies, will fill a niche in the environmental clean-up marketplace. The task is to determine quantitatively when and how to use it most effectively. Good research by well-qualified sci- entists and engineers should provide the guidance needed for future development.”

Lastly, Dr. Gary Bafiuelos, a soil scientist at the U.S. Department of Agriculture’s Agricultural Research Service, remarks, “The largest challenge for future phytoremediation research is to realize that the selected plant species may only be one compo- nent of the overall remediation strategy for the potential contaminant. Successful phytoremediation requires an integrated approach for each specific site, which must consider plant selection, genetic engineering, soil and water management, soil amendments, microflora activity, economics, product utilization, social acceptance, and time available to bring contaminant to desired level. Most importantly, we should not lose sight that phytoremediation should be considered, but the green technology is only one tool in our remediation toolbox.”

The views of these experts, along with my own research and findings have led me to conclude, with great enthusiasm, that the cost-effective, safe, and successful method of phytoremediation will allow science to contribute towards a giant step in the treatment of this country’s hazardous waste sites.

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