Burning Toxic Plants for Green Energy:
The Merging of the Phytoremediation and Biomass Energy Crop Industries
In September 2002, researchers from Belarus presented a paper at the BioEnergy 2002 conference in Boise, Idaho, proudly making the U.S. audience aware of their project produce "renewable" energy from burning trees that have been used to suck up radioactive Cesium-137 and Strontium-90 from Chernobyl fallout. See: Feasibility of Application of Short Rotation Willow to Remediation of Contaminated Land.
For an update on this, see the Biofuels could clean up Chernobyl 'badlands' article in New Scientist (6/27/2009). The article explains that: "The heavy radioactive residues [trees] will be burned in a power station, producing a concentrated 'radioactive ash'."
Unfortunately, this isn't a new concept to U.S. "biomass" researchers, since scientists are Florida have already been working hard to justify burning trees that have been used to suck up arsenic from land contaminated by wood treatment chemicals.
The following report documents efforts by University of Florida scientists to merge the phytoremediation industry with the biomass energy crop industry. This means that trees grown on lands contaminated with toxic chemicals like arsenic (referred to below as "As") could be burned in coal-fired power plants as "biomass" and would be considered "green, renewable energy," despite the fact that arsenic (or other toxic chemicals in question) would be redistributed to the environment through air emissions and ash disposal.
"Recent research conducted by the SFRC has focused on the development of Short Rotation Woody Crops (SRWC). Species such as Populus and Eucalyptus are currently being improved by selection and breeding to increase biomass production. Although the aim of tree improvement is to develop feedstocks for energy production, mulchwood, and pulpwood, the objective of increasing yields also complements phytoremediation research objectives, because in the absence of hyperaccumulating trees, increasing yields potentially result in increased contaminant removal. Furthermore if the tree species, planting techniques, harvesting techniques, silvicultural options, etc. are the same for bioenergy production and phytoremediation, not only will this reduce phytoremediation development costs, but also increase the ability to utilize crops grown on contaminated sites as feedstocks to the energy, mulch, and paper industries."
"However, unlike hyperaccumulator, the plant tissue concentrations found in CW [Cottonwood] were considerably lower than the concentration required to classify the plant tissues as toxic waste according to the toxic characteristics leaching potential (TCLP). Therefore, CW grown on CCA contaminated land would not require specialist treatment or disposal and may provide an income for the landowner in combination with a gradual cleanup of the site."
When considering planting density, harvest frequency must also be considered. To maximize the coverage of root systems, and potentially increase the amount of soil receiving phytoremediation treatment, it may be beneficial to plant at very high densities. However, at high densities, trees rapidly compete with each other for light, as leaves overlap and block out light reaching the lower canopy during canopy closure. When canopy closure occurs, the conversion efficiency of light into biomass is greatly reduced. Severe competition between trees for light and nutrients can result in self-thinning whereby the smaller trees receive too little light to survive and the density of the plantation is reduced. Since biomass production is a driving force controlling uptake of contaminants, high densities may therefore lead to a reduction in the efficacy of phytoremediation. Harvesting has the effect of opening the canopy to light and since harvested trees often generate multiple stems, harvesting can greatly increase the rate of biomass production. However harvesting can also result in increased mortality, due to the introduction of infection into the cut stump, and therefore harvest frequency should not be too high.
Thus, planting and harvesting recommendations for a cottonwood phytoremediation system need to be a compromise between maximizing soil coverage by planting at high densities and planting at lower densities to avoid early canopy closure.
Plant tissue concentrations observed at Archer and Quincy were well below the levels required to classify the plant tissues as toxic waste according to the Toxic Characteristics Leaching Potential (TCLP). For example, the highest tissue concentration observed at either site was 6.7-mg kg -1 . The TCLP test method requires that waste samples be extracted with an amount of fluid equal to 20 times the weight of the solid phase. Even if all As in the tree tissue containing 6.7 mg kg -1 was leached out to the extracting fluid, the maximum possible As extract concentration would be 0.335 mg l -1 , significantly lower than the TCLP standard for As of 5 mg l -1 . Therefore, cottonwood biomass grown on As contaminated soil should not require statutory treatment or disposal as hazardous waste and may be disposed of in a municipal landfill or used for pulp, mulch or energy production.