2. Technology Description
2.1 Technology Development and Application
Phytoremediation is an umbrella term that describes varied uses of plants for the purpose of
remediating soil or groundwater. Bioremediation is a form of phytoremediation that has been
defined by the US Environmental Protection Agency as "a treatment process that uses naturally
occurring microorganisms (yeast, fungi, or bacteria) to break down, or degrade, hazardous
substances into less toxic or nontoxic substances." Rhizosphere-enhanced remediation is based
on root exudation of excess plant-produced carbon compounds. The rhizosphere is the zone of
soil surrounding a plant root and influenced by the plant root. Typically, the root releases excess
carbon molecules produced by the plant and the excess carbon stimulates the nearby soil
microbial ecology. Researchers generally agree that the stimulated microbial activity near the
root in turn results in enhanced biotreatment.
Bioremediation is less expensive than more aggressive treatment technologies such as excavation
to bioreactors or land farms because contaminants can be treated on site, keeping down the costs
of operation and maintenance. Bioremediation is essentially a natural process and, as a result,
generally has a low environmental impact. Bioremediation also tends to have high public
acceptance because it is a "green" or "natural" approach. At some sites, there are simply no other
feasible alternatives to in situ bioremediation due to location, cost, or available resources.
Rhizosphere-enhanced remediation is especially applicable in the treatment of soils where low
temperatures, site inaccessibility, permafrost, and freeze-thaw cycles limit or, in many cases,
prevent cost-effective application of both traditional technologies and a number of emerging
innovative technologies. Petroleum compounds are ideal targets for rhizosphere-enhanced
bioremediation. Microbial degradation of petroleum compounds is well characterized and often
can be readily implemented under both aerobic and anaerobic conditions. In most cases, the key
is adjusting in situ conditions to promote degradation of petroleum compounds.
Prior to selecting rhizosphere-enhanced remediation as the treatment strategy, certain criteria
must be considered. In brief, the fundamental goal of all bioremediation strategies is to have the
contaminant, the proper microorganisms, and the correct soil conditions present simultaneously
for a period of time sufficient for the desired process to progress to a satisfactory endpoint. Field
implementation of rhizosphere-enhanced remediation includes selecting and adding appropriate
seeds and nutrients to the contaminated soil to stimulate rhizosphere activity. It requires minimal
equipment and costs for set up, operation and maintenance, or shut down. Demonstration plots at
the Campion Air Force Station are shown in Figures 1 and 2.
2.2 Process Description
Rhizosphere-enhanced remediation technology consists primarily of adding appropriate seeds
and nutrients to the contaminated soil to grow plants that, in turn, stimulate rhizosphere activity.
It thus requires minimal equipment and costs for setup, operation and maintenance, or shut
down. It is easy to "operate" and minimal training or safety requirements are needed. The
contaminated soil is not disturbed in the process beyond optional tilling, seeding, and fertilizing.