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Phytotechnology with Biomass Production
on the biodegradation of TNT (2,4,6-trinitrotoluene), RDX (Royal Demolition
Explosive; cyclotrimethylene-trinitramine; 1,3,5-trinitroperhydro-1,3,5-triazine), and HMX (high-melting explosive; octogen; cyclotetramethylene-tetranitramene; 1,3,5,7-tetranitro-1,3,5,7-tetrazocane). Water solubility is largest for
TNT (130 mg L −1 ), followed by RDX (42 mg L −1 ), and HMX (5 mg L −1 ) facilitating their transfer to organisms. Biodegradation pathways are presented in the
Kalderis’ et al. (2011) review. The greatest success to date with bioremediation
and phytoremediation has been with TNT, the simplest and most soluble of
the common explosives. Lists of bacteria and fungi that biodegrade TNT, RDX,
and HMX are provided in the Kalderis et al.’s (2011) review along with data on
toxicity of these explosive compounds to microorganisms and invertebrates.
Phytoremediation field studies of TNT have been reported, and a good summary of the few reported, is provided by Via (2020), along with degradation
pathways of TNT within plants, and a list of plants with results. A field-scale
wetland treatment system for TNT and RDX in water was designed and implemented at the Iowa Army Ammunition Plant (McCutcheon & Schnoor, 2003).
The concept of phytoremediation with biomass production on sites with
explosive contaminants needs further development. One tropical biomass crop,
vetiver grass (Chrysopogon zizanoides), a relative of sorghum, has been tested for
its capacity to take up and degrade TNT (Das, 2014). When urea was added to the
soil system, uptake increased as much as 90% of the input 100mg kg −1 amount of
TNT in 22days. Miscanthus may be a good plant for field-scale phytoremediation
of soils contaminated with explosive compounds; however, further research is
needed on the fate of TNT, RDX, and HMX in soils where Miscanthus is grown.
In many cases, the actual degradation is microbially driven, and hence many
different plant species may facilitate the process (Esteve-Núñez et al., 2001). On
the other hand, TNT metabolism and detoxification within plants may differ
between plant species. The process has been well characterized in the dicot
Arabidopsis (Gandia-Herrero et al., 2008), but thus far not in grasses.
3.2.3 Remediation of Chlorinated Hydrocarbons
Many chlorinated organic compounds have found their way into soil and
ground water. The liquid forms are especially challenging to deal with
because they are often denser than water and move to the bottom of an
aquifer, gradually contaminating that water by slow diffusive dissolution.
Chlorinated solvents such as trichloroethylene have been used for a number
of beneficial applications in machine shops and dry-cleaning operations, and
they are found in soil and groundwater at many locations. Volatile solvents
dissolved in water can be taken up into plant roots. Plant evapotranspiration
releases water into the atmosphere and the chlorinated solvents in the water
are released into the atmosphere. Because of the low vapor pressure of water
at ambient temperatures, only about 18 mg L −1 of water (1 mM) can be evaporated into the air phase. In consequence, huge volumes of air are needed for
evapotranspiration, and thus the concentrations of the volatile chlorinated