Phytostabilization
Background
Phytostabilization involves the reduction of the mobility of heavy metals in soil. Immobilization of metals can be accomplished by decreasing wind-blown dust, minimizing soil erosion, and reducing contaminant solubility or bioavailability to the food chain. The addition of soil amendments, such as organic matter, phosphates, alkalizing agents, and biosolids can decrease solubility of metals in soil and minimize leaching to groundwater. The mobility of contaminants is reduced by the accumulation of contaminants by plant roots, absorption onto roots, or precipitation within the root zone. In some instances, hydraulic control to prevent leachate migration can be achieved because of the large quantity of water transpired by plants.
Applicability
The use of phytostabilization to keep metals in their current location is particularly attractive when other methods to remediate large-scale areas having low contamination are not feasible. Remediation is difficult at locales having high metals concentration because of soil toxicity. Plants should be able to tolerate high levels of contaminants, have high production of root biomass with the ability to immobilize contaminants, and the ability to hold contaminants in the roots [2].
Field work has shown that phytostabilization is efficient at lowering levels of Pb in a sand/Perlite mixtures and low-level radionuclides may be stabilized. Studies also suggest that phytostabilization may reduce metal leaching by converting metals from a soluble oxidation state to an insoluble oxidation state. Plants have reduced available and toxic Cr(VI) to unavailable and less toxic Cr(III) [1].
Limitations
Phytostabilization is useful at sites with shallow contamination and where contamination is relatively low. Plants that accumulate heavy metals in the roots and in the root zone typically are effective at depths of up to 24 inches [1]. Metals that are readily translocated to leaves in plants may limit the applicability of phytostabilization due to potential affects to the food chain.
Performance
Results from two phytostabilization projects are summarized in Table 1. The objectives of the field demonstrations were to stabilize the soil and to decrease the vertical migration of leachate to groundwater.
Table 1. Phytoremediation field investigations [3].
Site/Media
Contaminants
Plants
Performance
Location
1 acre test plot - abandoned smelter, barren land (demonstration)
Pb, Zn, Cd
(Pb and Zn at >20,000 ppm)Poplars
50% survival after 3 years; site successfully revegetated
Dearing, KS
1-acre test plot - mine wastes
(demonstration)As, Cd
Poplars
5% survival; inclement weather, deer browse, toxicity blamed
Whitewood Cr., SD
Data Requirements
Depth of contamination, types of heavy metal present, and level of contamination must be determined and monitored. Hydraulic control, soil stabilization, and immobilization also are critical for design considerations [3]. Treatability and toxicology investigations in small plots should be performed.
Cost
Phytostabilization is a cost efficient method when compared to other technologies.
Status of Technology
Research to validate and develop phytostabilization is ongoing and are aimed at: (1) elucidating key mechanisms; (2) investigating methodologies and test protocols; (3) improving capabilities to predict performance; (4) validating the effectiveness of the technology; and (5) preparing guidelines for implementation [4].
References
1. Blaylock, M., B. Ensley, D. Salt, N. Kumar, V. Dushenkov, and I. Raskin, 1995, Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants, Biotechnology, 13 (7), pp.468-474.2. Miller, R., 1996, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series O, Vol. 3, October.
3. Schnoor, J.L., 1997, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series E, Vol. 1, October.
4. Cunningham, S.D., J.R. Shann, D.E. Crowley, and T.A. Anderson, 1997, Phytoremediation of Contaminated Soil and Water, in Phytoremediation of Soil and Water Contaminants, E.L. Kruger, T.A. Anderson, and J.R. Coats, Eds., ACS Symposium Series 664, American Chemical Society, Washington, DC.