When is phytoremediation appropriate?

There are many factors to consider when exploring the possibility of a phytoremediation project:

Bioavailability of a pollutant: the extent to which a pollutant is available to living things. Some compounds are very bioavailable to plants and animals because they are soluble in water; different compounds are soluble to different degrees. If a substance is insoluble in water, or hydrophobic, it is generally not bioavailable. Hydrophobic compounds have an affinity for lipids and thus do not associate with water. In the case of petroleum hydrocarbons, and  most organic pollutants, bioavailability is often measured by log Kow. Log Kow  is the octanol/water coefficient and is used to assess solubility. The optimal log Kow for plant uptake is between .5 - 3. If the log Kow>3 then the compound is too hydrophobic and will get caught in the lipid bilayer of the cell wall. If log Kow<.5, the compound is too hydrophilic (water-loving) and will not be able to cross the cell membrane. Petroleum hydrocarbon log Kow vary, depending on composition of the pollutants, but in general, alkanes are not very soluble in water, relative to other compounds, and solubility decreases by a factor of about 3 or 4 for every carbon added (Frick et al., 1999).  Therefore it would follow that alkanes and BTEX compounds (single ring structures), would be more readily phytoremediated. Most PAHs, (multiple ring structures), which are very hydrophobic, and therefore cannot be taken up by the plant, may not be suitable for phytoremediation.

Bioavailability will vary with soil structure and organic matter content. In general, the higher the clay content of a soil, the more likely a pollutant is to be bound to the soil and therefore unavailable for plant uptake. Clay soils have a smaller particle size, and thus more surface area. This high surface area provides for more exchange sites. Organic compounds that have a charge are subject to these exchange sites and become less bioavailable. High organic matter content in the soil will bind lipophilic (i.e. hydrophobic) compounds, making petroleum hydrocarbons less available.

Lastly, but certainly not least important, is the ability of the plant roots, and the rhizosphere, to come in contact with the pollutant. If the contamination is too deep for the roots to penetrate, usually plant roots will not penetrate more then 20cm  -1m deep, phytoremediation is not possible without some alterations. Plants may be deep planted or the soil may have to be excavated in some situations to allow for phytoremediation. Plants with fibrous root systems, most grasses and legumes, are often preferred for phytoremediation due to the greater surface area for contaminant contact they provide.

Toxicity or contaminant levels: The toxicity of a pollutant is a factor in choosing phytoremediation or an alternative method. Obviously if the compound is highly toxic to humans or wildlife it should be dealt with immediately. Often phytoremediation is a long term process, and thus not a viable option for compounds which pose an immediate threat. The concentration of a pollutant should also be considered. Phytoremediation may be inhibited if concentrations of the pollutant are too high and cause toxicity in plants and microbes. If the concentrations are too low however, the uptake and degradation by microbes and/or plants may cease as the pollutant becomes unavailable. The microbes will not be able to reduce the concentrations below a certain level and will, in effect, starve to death.
 

Risks to humans and/or wildlife: The risks to humans and wildlife must be assessed before implementing any phytoremediation project. If the plants are going to translocate the pollutant in some form into their tissue and then either store it there or transpire it into the atmosphere, there may be risks associated. The degradation products of the contaminants may be even more toxic if released into the air or ingested by animals. Pilot studies and lab research must be conducted to predict any repercussions associated with phytoremediation on the surrounding ecosystems.
 

Climate and soil conditions: The climate and soil conditions are obviously key in determining the suitability of phytoremediation. The warmer the climate, the longer the growing season, and the longer biological process will take place. Most degradation processes have an optimal temperature in which the enzymes are most active. In climates where the temperatures are less then optimal much of the time, phytoremediation may not be an option. The same can be said for soil moisture, optimal levels need to be maintained for the degradation processes to remain active. Soil fertility can often be a limiting factor in the breakdown of pollutants. Plants and microbe often compete for the same inorganic nutrients and thus slow the remediation processes. Fertility must be monitored and maintained in order to optimize  organic phytoremediation, this often includes annual fertilization with inorganic nutrients.
 

Time scale and economics: As mentioned above, phytoremediation is often a much longer process then traditional remediation methods. The time line may be one of years rather then months. Again, issues of human health and pollutant toxicity come into play. If there is immediate danger phytoremediation may not be the answer. It is often hard to predict exactly how long it will take to clean up a site. There has been some mathematical modeling to estimate clean up rates and time to remediation goals. See Schnoor, GWRTAC Report, 1997 for details. Economically speaking however, phytoremediation is almost always the clear choice. The costs of phytoremediation are often less then 25% of traditional methods. This may include the long term monitoring that accompanies a phytoremediation project. Public reactions tend to be favorable for phytoremediation as well. It is perceived as a beautification process as well as a remedy for an environmental problem.

Phytoremediation Decision Tree

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