PHYTOPHOTOLYSIS: A NEW MECHANISM FOR PLANT BASED REMEDIATION OF RDX
CONTAMINATION
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Craig Just and 2Jerald Schnoor
1,2
The University of Iowa, Department of Civil and Environmental Engineering, Iowa City, IA 52242; 1Phone:
(319) 335-5051, 1E-mail: craig-just@uiowa.edu; 2Phone: (319) 335-5647, 2E-mail: jerald-schnoor@uiowa.edu.
The EPA describes phytoremediation as the direct use of living green plants for in situ risk reduction for
contaminated soil, sludges, sediments, and groundwater, through contaminant removal, degradation, or
containment. Phytoremediation has been utilized at many hazardous waste sites where more traditional
remediation technologies are cost prohibitive. Uptake of explosive compounds, such as 2,4,6-trinitrotoluene
(TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), by plants has been shown to occur at some U.S. Army
installations including the Iowa Army Ammunition Plant (IAAP). An engineered wetlands system has been
installed at IAAP to remediate contaminated soil and groundwater near Brush Creek. Research into the fate of
RDX in wetland plants has been initiated to help determine what constitutes “success” in this application of
phytoremediation.
Regulators have been inclined to declare a remediation strategy successful when the final concentration of
the priority contaminant, in this case RDX, falls below a risk-based action level in the soil or water. For
phytoremediation of RDX, this criterion may be inadequate due to the possibility of accumulation of RDX and/or
RDX metabolites in plant tissues. Many plants, including poplar trees, reed canary grass, parrot feather, and water
plantain, have been shown to readily uptake and translocate RDX. Surveys by the University of Iowa at the IAAP
showed RDX concentrations ranging from 0.5 to 2.9 mg/kg in plants growing in contaminated soil. Reed canary
grass at the site showed the highest levels of RDX compared to other plant types. Reed canary grass has the
highest water transpiration rate of the species sampled, which could explain the elevated RDX levels. It’s also
possible that reed canary grass accumulates more RDX then other plant species. Studies in our laboratory with
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C-RDX and reed canary grass have provided evidence supporting the formation of formaldehyde within the
plants with subsequent loss of the atmosphere. The proposed mechanism of transformation, termed
“phytophotolysis,” involves the translocation of contaminants to the leaves of plants with subsequent photolysis
due to light exposure. This new phytoremediation mechanism could prove useful for treatment of photoactive
compounds in shallow groundwater that might otherwise not be exposed to sunlight.
Key words: phytophotolysis, RDX, wetlands, IAAP, formaldehyde