2 The 4 Technology Solutions
PHYTOREMEDIATION: ENHANCING NATURAL ATTENUATION PROCESSES
An On-Line Version of a Columns First Published in:
The National Environmental Journal Jan./Feb. 1996 Vol. 6 No. 1 Pages 30-31
Note: The cover of this issue used one of my photographs
by: David Vance
In a previous groundwater column (July/August 1995) in-situ mechanisms responsible for natural attenuation of organic contaminants in groundwater were discussed, as well as the rates of degradation that could be expected under those conditions. This column will focus on phytoremediation as a method to proactively enhance those degradation rates, as well as provide mechanisms for the remediation of metals from near surface soils and groundwater. Phytoremediation acts through two fundamental remediation processes, enhancement of saturated zone in-situ biodegradation and phytoextraction.
The rhizosphere is the zone in the subsurface occupied by plants root system. Root depths of 6 to 30 feet are common, with some trees and shrubs capable of root penetration to 60 feet. Phreatophytes are deep rooted plants that draw water from beneath the water table, xerophytes are shallow rooted and depend more directly on infiltrating rainwater. Depending on site conditions and contaminant distribution, either type of plant can be of value for phytoremediation.
Within the rhizosphere plants contribute to enhanced in-situ biodegradation through the supply of carbonaceous substrate and oxygen transfer. Rhizodeposition is partially the result of the decay of dead roots and root hairs. Also important are carbonaceous root exudations such as: leakage from epidermal cells; secretions resulting from metabolic activity; mucilages from root tips (which act as lubricants for root penetration); and lysates from sloughed cells. Exudates are composed of a wide range of chemicals that include sugars, amino acids, organic acids, fatty acids, and numerous other compounds. It is estimated that 7% to 27% of the total plant mass is annually deposited as carbonaceous material in the rhizoshpere, amounting to 85 to 155 tons per acre. This carbonaceous material stimulates overall bacterial activity as well as providing substrate to support cometabolic degradation of xenobiotic hydrocarbons.
The capability to support oxygen transfer can be divided into three categories of plants: nonwetland herbaceous and woody plants with poor oxygen transfer capacity; wetland woody plants with moderate capacity; and wetland herbaceous plants with high oxygen transport capacity. Plants of the first variety will ultimately die with their roots under saturated conditions. Wetland plants adsorb oxygen through their leaves, twigs, stems, bark and unflooded roots. This oxygen is in turn transported to the roots where it diffuses out into the rhizosphere. Consequently wetland plants are able to support oxidation in the rhizosphere at rates that are significantly greater than that seen through saturated zone diffusion alone. Due to the processes described above it is not uncommon to find bacterial populations in the rhizoshpere elevated to a level of magnitude or more above surrounding undeveloped soils.
The primary in-situ remediation potential of plants for hydrocarbons lies in their capacity to enhance oxidation rates in the subsurface and provide cometabolic substrate. However, plants also have the ability to remove compounds, a process termed phytoextraction, which can be applied to organic or metal contaminants. In the case of hydrocarbons, the compound must be water soluble and have a moderate degree of lipid solubility. Lipid solubility is a function of the octanol-water partition coefficient (Kow) for the compound.
- Compounds most readily mobilized by plants have log Kow values in the range of 1 to 3.
- Compounds with values of Kow in this range include BTEX hydrocarbons, chlorinated solvents, and other short chain aliphatic hydrocarbons.
Once in the interior of the plant the adsorbed hydrocarbons may be: stored via lignification; volatilized; partially degraded through metabolization; or completely mineralized.
- Compounds with values of log Kow higher than 3 such as PNA's are incapable of entering the root,
- those with log Kow values lower than 1 are rejected by the root membranes.
It is also important to remember that as with any in-situ biodegradation process the desorption and mass transfer of contaminant hydrocarbons from the geologic matrix may be the rate limiting step in the remediation process.
In the instance of the phytoremediation of metals the dominant active mechanism is phytoextraction and accumulation in the tissues of the plant. This is a process that has long been familiar to exploration geologists in the mining industry. Geobotany is concerned with the identification of plants or plant conditions common to metal rich soils and biogeochemistry is concerned with actual metal concentrations in parts of plants.
The mechanisms for metal accumulation includes:
- compartmentalization, and
These same mechanisms often contribute to the metal tolerance of the plant. To date accumulators of lead, cadmium, chromium, nickel, cobalt, zinc and selenium have been identified. To successfully apply this technology to a metal contaminated site it is important that pH, organic complexes, and interfering elements be assessed and that plant species with the appropriate metal selectivity be utilized. In some instances it may be necessary to apply soil amendments to enhance the process.
A key part of the metal extraction process are phytochelatins. These are low molecular weight peptides that have the capability to bind metals. Their presence in plants has likely evolved because toxicity to the plant is reduced by having the metals bound by the phytochelatins. Currently there are several hyper-accumulating plants that have been discovered such as Indian Mustard and Pennycress. However, these plants are small and slow growing, and thus lack enough biomass to remove significant amounts of metals. What is required, and is the focus of current research, are plants that are resistant to heavy metals, have the capacity to hyper-accumulate those metals, and exhibit desirable properties of common crops such as high growth rates and easy harvesting.
Lastly, plants have other properties with potential remediation applications. In semiarid climates sufficient numbers of trees are capable of depressing the water table through transpiration, up to the equivalent of 3 feet of rainfall per year. In tight soils root penetration can improve the overall mass transport properties. Plants can also aid in the surface stabilization of soils, preventing the windblown migration of soil with adsorbed contaminants.
Planting costs have been estimated to be in the range of $ 10,000 per acre, with monitoring costs parallel to those associated with other remediation technologies. Total phytoremediation costs are estimated to range from $ 60,000 to $ 100,000 an acre. The potential of phytoremediation is significant given proper design for its application. Contaminant type and distribution, soil chemistry, and climate are all important factors that must be considered.
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