Stephen+Louie

Phytoremediation

flat = = = **Introduction** =

Phytoremediation is a form of bioremediation, a waste management system that utilizes organisms to neutralize or remove pollutants from a contaminated site. Phytoremediation deals with the specific use of plants that can provide restoration to environments that suffer from air, soil, and water pollution. Typically, these types of plants will be applied to areas that are heavily impacted by industrial, mining, and urban activities. Common pollutants that plants may be required to deal with include heavy metals, pesticides, herbicides, crude oils, and solvents. [1]

= **Benefits and Costs** =

**Advantages**
 * Conventional remediation of one ton of contaminated soil can cost between $500-$1000. To treat an acre that is 3-feet deep would cost approximately $250,000. Phytoremediation treatments can cost less than a quarter of this expense.
 * Phytoremediation minimizes the amount of disruption caused to the ecosystem as opposed to traditional methods which can disrupt soil quality and organism ecology.
 * In addition to removing organic and inorganic pollutants, phytoremediation plants can also facilitate the growth of other plants. These plants can provide essential macronutrients such as nitrogen and phosphorous to nearby plants. They can also break down tough soil for the roots of the other plants to grow into.
 * Disadvantages**
 * Phytoremediation can take longer than conventional methods.
 * Phytoremediation is limited to areas in which these plants can colonize [1]

= **Interactions with the Environment** =

Phytoremediation can occur through a variety of processes depending on the plant and the environment. Phytoremediation is a relatively new process and as such has not seen widespread use. Currently different plants are being tested to assess their phytoremediation potential for various pollutants and environments.

Phytoextraction
Phytoextraction uses hyperaccumulators to extract contaminants from the environment. These contaminants are sequestered in the plant tissue until they can be properly disposed of. Phytoextraction usually deals with heavy metals as they are have been commonly been shown to be collected by plants as opposed to organic compounds. These heavy metals can potentially be harvested later from the plant tissue in a process known as "phytomining" for economical purposes. Because the extent of a plant's reach is limited by the depth of its roots, multiple crops cycles may be necessary before the site can be officially deemed clean. [3]

**Phytostabilization**

Phytostabilization relies on sequestering the pollutants at its current position as opposed to removing or neutralizing it. Using its root system, a plant can prevent external forces such as wind or rain from moving the pollutant to another area. Unlike the other phytoremediation methods that seek to provide remediation to the contaminated are, the main purpose of phytostabilization is to limit the exposure of the pollutant to prevent it from spreading in areas that may be deemed more sensitive or important. [1]

**Phytotransformation**

Phytotransformation is the process in which plants uptake the environmental pollutants and convert them into nontoxic substances. Phytotransformation specifically utilizes the metabolic processes of the plants and/or its mycorrhizal bacteria to reduce the toxicity of the pollutant. While the process may vary, phtotransformation usually occurs in three phases.
 * 1) Phase 1: Addition of hydroxyl groups (-OH) to change the polarity of the substance
 * 2) Phase 2: Conjugation of the molecule through the addition of amino acids and other molecules
 * 3) Phase 3: Transformation and sequestration of substances. [4]

**Phytostimulation**

Phytostimulation is highly reliant on the plant-microbe interactions. Unlike the other methods which use plants as the main agent of phytoremediation, phytostimulation relies heavily on the capabilities of the microbes within the plant's rhizosphere. Plants function to stimulate these microbes through various means:
 * 1) Provide nutrients such as sugars and carbohydrates
 * 2) Provide oxygen to allow for aerobic metabolism
 * 3) Increase availability of organic carbon
 * 4) Allow use of unique enzymatic pathways that are not normally available to bacteria
 * 5) Use as a habitat for microbes to grow

Cyanobacteria is a phylum of bacteria that have been associated with phytostimulation properties. Their ability to fixate nitrogen is of particular importance, since this provide surrounding plants with a usable form of nitrogen (NH 3 ) that would not otherwise be available. This ability has been shown to be greatly stimulated in the presence of phytohormones. One research study has shown that Chroococcidiopsis sp. Ck4 and Anabaena sp. Ck1 show improved performance when phytostimulated by wheat, which in turn also shows increased growth and performance. [2]

Phytovolatization
Phytovolatization refers to the plant's ability to uptake the pollutants and volatilizing them. Similar to both phytoextraction and phytotransformation, phytovolatization involves the uptake of the pollutant compounds and their transformation into more volatile compounds. These compounds are then released from the leaves via the respiratory process. [4]

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One particular study focuses on the phytoremediation of soil through the use of the plant //Pteris vittata//. Arsenic is a heavy metal of particular concern due to its widespread prevalence and potent toxicity. It was determined that vapor released from of P. vittata included arsenic compounds, arsenite and arsenate. The result suggested that P. vittata effectively volatilizes Aresenic and removed a maximum ratio of 90% of the total uptake of As from As-contaminated soils in greenhouse. One concern was that the volatilized Arsenic could act as a secondary pollution source. [4]

**References**

1. Cunningham, S. D. and Ow, D. W. Promises and Prospects of Phytoremediation // Plant Physiology // (1996) 110: 715-719

2. Hussain, A., and Hasnain, S. Phytostimulation and biofertilization in wheat by cyanobacteria //Journal of Industrial Microbiology and Biotechnology// (2010)

3. Cunningham, S. D., Betri, W. R., and Huang, J. W. Phytoremediation of contaminated soils //Trends in Biotechnology// (1995) 13: 393-397

4. Sakakibara, M., Watanabe, A., Inoue, M., Sano, S., and Kaise, T. Phytoextraction And Phytovolatilization Of Arsenic From As-Contaminated Soils By Pteris vittata," //Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy// (2007) Vol. 12, Article 26.

Page authored by Stephen Louie, student of Dr. Michelle Lum at Loyola Marymount University, Los Angeles.