Promises and Prospects of Phytoremediation

We often think of plants primarily as a source of wood, food, and fiber. Secondarily we may also appreciate their presence for aesthetic reasons as well as for "altruistically" providing habitat for other species. Increasingly, however, their value as an environmental counterbalance to industrialization processes is being appreciated. These processes include the burning of fossil fuels, generation of wastes (sewage, inorganic and organic solids, and effluents), and general water flow and processing. Plants have long been recognized for their consumption of CO2 and, more recently, of other gaseous industrial byproducts (Simonich and Hites, 1994). Recently, their role in slowing the rate of global warming has been further appreciated in both the scientific and popular press. Their use as a final water treatment and for disposal of sludge resulting from waste water treatment is centuries old (Hartman, 1975). The extensive literature concerning water and sludge treatment and the emerging field of air pollution abatement with plants will not be discussed here. Instead, we focus on an emerging concept, "phytoremediation," the use of plants to remediate contamination of soil with organic or inorganic wastes. Remediation of soil contamination by conventional engineering techniques often costs between $50 and $500 per ton. Certain specialized techniques can exceed costs of $1000 per ton. With an acre of soil (to a 3-foot depth) weighing approximately 4500 tons, this translates to a minimum cost of about a quarter million dollars per acre (Cunningham et al., 1995). It is not surprising that the cleanup of contaminated sites has not been proceeding at a rapid pace. There is an active effort to develop new, more costeffective technologies to remediate contamination of such soils. For the most part these efforts are being led by engineers and microbiologists. More recently, however, green plant-based processes have begun receiving greater attention. It has long been known that the life cycle of a plant has profound effects on the chemical, physical, and biological processes that occur in its immediate vicinity. In the process of shoot and root growth, water and mineral acquisition, senescence, and eventual decay, plants can profoundly alter the surrounding soil. The effects of many of these processes are apparent on the restoration of land at physically and chemically altered sites, ranging from road cuts to the site of the Mount St. Helen's eruption. These same plant-driven processes also occur in areas heavily impacted by industrial, mining, and urban activities. One of the greatest forces driving increased emphasis on research in this area is the potential economic benefit of an agronomybased technology. Growing a crop on an acre of land can be accomplished at a cost ranging from 2 to 4 orders of magnitude less than the current engineering cost of excavation and reburial. There have been perhaps two dozen field tests to date; however, in many ways phytoremediation is still at its initial stages of research and development. A comforting thought for plant biologists is that much of the research effort will be expected to center on a deeper understanding of basic plant processes. So how do we envision phytoremediation working? The theory appears to be simple. Agronomic techniques will be used to ready the contaminated soil for planting and to ameliorate chemical and physical limitations to plant growth. Plants will then directly or indirectly absorb, sequester, and/or degrade the contaminant. Plants and irrigation, fertilization, and cropping schemes will be managed to maximize this remedial effect. By growing plants over a number of years, the aim is to either remove the pollutant from the contaminated matrix or to alter the chemical and physical nature of the contaminant within the soil so that it no longer presents a risk to human health and the environment. As people who work in the remediation, herbicide development, and farming industries will attest, many weed species are remarkably tolerant of a wide range of organic and inorganic toxins. Plants can thrive in soil contaminated to levels that are often orders of magnitude higher than current regulatory limits. These limits are often set relatively independent of plant tolerance limits and are most often derived from human health and aquatic toxicology end points. Ironically, many remediation plans begin with the destruction of the existing vegetation.