The International Information Center for Geotechnical Engineers

# Phytoremediation

#### Other important considerations

##### Regulatory Concerns

As with other remediation techniques, cleanup time and the extent of the cleanup is often under regulatory considerations. The treatment objectives are usually selected at the onset of the process, and with the extended amount of time, care must be taken to incorporate any changing regulations. Many studies have shown that atmospheric releases are not of great concern, as they are mostly metabolized and in almost immeasurable concentrations (Sharma and Reddy, 2004). The biggest emphasis has been on control of the potentially toxic plant material, as it may have significant amounts of organic compounds and need additional treatment.

##### Cost

Each case is unique, but soils with low levels of contaminants can generally be cleansed for $10-$35 per ton (Sharma and Reddy, 2004). Removal, off site treatment, and the return of processed residue can cost $200-$600 per ton (Gupta et al, 2000). This is dependent on the amount of soil amendment and irrigation, number of growing cycles, and difficulty of disposal. Because the method uses no industrial fuel or products, it is considered to be passive, saving operating and maintenance costs. Localized waste treatment options determine where the plant biomass is processed. Zodrow (1999), makes a first order comparison of remediating 500 ppm lead contaminated soil, and states that excavation and disposal costs $300,000 per acre, while phytoextraction costs approximately$110,000 per acre.

Table 4. Cost comparison with other methods (Rock, 1998)

##### Due Care

Care should be taken to prevent wind or water-based soil erosion, as soil contaminants may migrate off site and cause additional problems. In addition, pests and rodents must be monitored and deterred, as they can limit plant growth and cause unnecessary exposure of pollutants.

##### Predictive Modeling

Four main modeling techniques have been developed based on first-order kinetics and different schemes of partitioning. They are listed below with their associated assumptions and limitations.

Table 5. Phytoremediation modeling techniques (adapted from Sharma and Reddy 2004)

##### Combination of Phytoremediation With Other Methods

Phytoremediation relies on a complex interaction between large plants and their associated microbes. Because bioremediation and phytoremediation both rely on natural, living processes, they are often used in conjunction for a synergistic effect (Sharma and Reddy, 2004). Sometimes soil is inoculated with microbes before plants begin growing to either prep the soil or encourage increased growth within the root zone. The two methods are most often combined when rhizodegradation is the driving process in treatment. As the table 6 below shows, enzymes play a vital role in degrading organic compounds. These enzymes are also often found in microbes that mutually benefit the plant, and through encouraging this relationship, treatment efficiencies can be increased (Gupta et al, 2000).

Table 6. Plant enzymes with degrading compounds (Gupta et al, 2000)