Ryan Townsend 11/27/07 CE/ENVS 521 Dr. Ellis Phytoremediation: A Study of Practical Applications and Current Research ABSTRACT Practical applications of phytoremediation are essential for the clean-up of moderately and severely contaminated sites in an economical and environmentally responsible way. The usage of plants to do a job that would otherwise be done by chemicals or energy intensive soil removal is a natural choice on several levels. First, by using plants for clean-up, pollutants are being absorbed by nature and cleansed in the most natural way, all while aiding in restoring the site. Second, and perhaps most importantly, phytoremediation is a low energy and low input process. Usage of fossil fuels and other chemicals are kept to a minimum and, in most cases, phytoremediation yields better results than that alternative. Economically, phytoremediation is also the method of choice in nearly all application scenarios. KEYWORDS Phytoremediation; Phytotransformation; Phytoextraction; Heavy Metals; Phytostabilization; Rhizofiltration; Rhizosphere Bioremediation; Bioconcentration Factor; Phytoextraction Efficiency. INTRODUCTION AND FORMS OF PHYTOREMEDIATION There exist five forms of Phytoremediation, all of which remediate pollutants from soil and water in different ways: Phytotransformation Phytotransformation occurs when a plant is able to uptake a pollutant or chemical into its roots and break it down into an environmentally benign substance. This is primarily done by the plants metabolism. Phytotransformation breaks down organic substances such as fuel oils, pesticides, explosives, solvents, and industrial chemicals. It is not effective in remediating inorganics. Phytotransformation is known as the “green liver” model because it breaks down chemicals much the same as the liver in a human breaks down chemicals and renders them harmless.Phytoextraction Phytoextraction is a form of phytoremediation which is used for the remediation of sites with high levels of inorganic compounds such as metal contaminants from the soil into the plant matter via plant roots. With this method, plants act as filters or traps for metals as they remove them for the contaminated soil. Unlike Phytotransformation, phytoextraction cannot break down compounds like heavy metals into environmentally benign substances. Rather, plants are harvested and incinerated, with waste ash disposed of in a hazardous waste landfill. Other options include harvesting biomass for metal extraction. With many precious metal prices on the rise, there have become an increased interest in “phytomining.” Phytomining is a form of phytoextraction that extracts nickel, cobalt, and other metals, including the platinum and palladium from known contaminated sites. Plants can be harvested, dried, and smelted to recover the metal. Phytostabilization Phytostabilization is the process of using plants and their roots to stabilize a contaminant in the soil. This interaction between plants and contaminants retards the rate of contaminants leaching into groundwater. Phytostabilization is achieved through several ways including pH control, reduction or rainfall infiltration through increased evapotranspiration. It is usually more successful in remediating metals with addition of nutrients such as phosphate or lime to the soil. Phytotransformation is primarily used at abandoned mining and smelting sites where levels of metal are too high to expect to phytoextract all or most of it. Rhizofiltration Rhizofiltration is a form of phytoremediation that refers to the use of plant roots to absorb, concentrate, and precipitate toxic metals from contaminated groundwater. Suitable plants with stable root systems are supplied with contaminated water to acclimate them to the conditions of the area in need of remediation. The plants are then transferred to the contaminated site to collect the contaminants, and once the roots are saturated, they are harvested. Rhizosphere Bioremediation Plants play an indirect role in Rhizosphere Bioremediation. They act as hosts for bacteria and other tiny organisms that perform the actual decontamination. The plants main purpose is to encourage the growth of these small organisms by providing a root structure and a supply of nutrients.LIMITING AND EXPERIMENTAL FACTORS Root Depth Perhaps the most limiting factor to the effectiveness of phytoremediation is the maximum depth to which the roots can reach. While phytoremediation yields a very positive result when the roots of a plant reach down to where the chemical to be remediated is located, they yield no result to any chemical that is below the root depth. Additionally, the plant used at a given site must be able to handle and have a positive effect on the chemical once it has reached the depth where it is located. (Merkl, 2005) Soil and Temperature Conditions The conditions that have a tremendous impact on plants used in phytoremediation are soil and temperature. These conditions play a major role in both a plants effectiveness in up taking contaminates from the soil and also its ability to thrive and even survive in a location. While a plant should be robust enough to uptake and remediate a soil of contaminates, factors such as soil pH, soil porosity and moisture content of the given soil will play a significant role in its life expectancy and effectiveness. Temperature at the site is also of importance and will affect how effective a plant is at remediating the soil as well as how long it will live. Colder temperatures in many locations will render phytoremediation less effective or non-useable as a remediation method. During usage, temperature must be controlled and recorded to every extent possible. (Merkl, 2005) Temperature in greenhouse during experiment (Merkl, 2005).Fig. 2. Fraction analysis (SARA) of TOG in soil. Left: Composition of TOG. Right: Corresponding absolute values. (Merkl, 2005). Biomass Production and Uptake Effectiveness A plants ability to grow well and thrive while up taking a significant amount of contaminates from a site is critical. In fact, a plants growth rate is directly tied to its effectiveness during phytoremediation, even it is possesses a slow metabolism. In turn, a plant that grows slowly will in turn not be able to take in as many chemicals as a plant that grows faster.Fig. 1 Assessment of Tropical Grasses Biomass production of shoot