Phytoremediation of Toxic Organics

Direct:• Plant may release enzyme to the environment that

break down the organic contaminants which result in detoxification

Indirect:• Plant provide a favorable environment in the

rhizosphere for microbial growth that result in break down of soil contaminants.

• Plant release organic acids or chelating agents that increase solubility of the contaminant, which result in increased degradation by soil microorganisms.

Plant derived degradative enzyme:

Five plant derived enzymes have been identified that can degrade soil contaminants, including nitrate and nitrite reductase, laccase, nitrilase, and dehalogenase.

(Note: over 50 soil enzymes have been detected. Presumably, they are mostly microbial origin.)

• Nitrate reductase: involved in reducing nitrate to nitrite

• Nitrite reductase: involved in reducing nitrite to ammonium

• Nitrilase: also named nitrile aminohydrolase. Acts on a wide range of aromatic nitriles and some aliphatic nitriles. (CN)

• Laccase: A group of multi-copper proteins of low specificity acting on both o- and p-quinols, and often acting on aminophenols and phenylenediamine and produce semiquinone.

• Dehalogenase: degrades hexachloroethane (Cl3C-CCl3) and TCE (ClHC=CCl2).

Example: Filed trials of phytoremediation of petroleum-contaminated soils by researchers at Kansas State Univ.

Objectives:

1. To evaluate effect of plant species2. To determine the effect of management

techniques3. To develop protocols for quantification of

contaminants in large numbers of soils.

Three field trials were chosen:

1. CRO. Crude oil site locate near the Gulf of Mexico

2. CAR. California refinery site3. VAD. Virginia diesel contaminated site

Schwab and Banks, 1999

Schwab and Banks, 1999

Schwab and Banks, 1999

Schwab and Banks, 1999

• CRO site has been returned to its original agricultural use and the others are still being treated.

• At least two growing season is required to return the site for agricultural use.

Potential and limitations of phytoremediation

• Not competitive if a quick remediation is needed.

• Not appropriate if immediate risk of groundwater contamination exist due to imperfect containment of pollutants against leaching

• Limited by conditions that inherently limit plant growth.

• Suitable for remediation of large surface contaminated areas. Contaminated soil could be used to produce energy and fiber crops.

Phytoremediation of TCE in Groundwater using Populus

Hybrid poplar (Populus charkowiiensis x incrassata, NE 308) at Edward Sears Property

Air Force Plant 4 Phytoremediation Site Layout, Carswell Air Force Base - Ft. Worth, TX