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Phytoremediation

Field Setup, process involved

The general process outlined below, operates in a mainly linear fashion, with the possibility of two “loops” depending on the effect of the treatment.

Figure 1. General phytoremediation process flow (adapted from Sharma and Reddy, 2004)

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Plant Selection and Positioning

Because the entire process takes so much time, it is critical to correctly determine the proper type and quantity of plants. The soil type generally determines which plants are effective, but additional laboratory testing is often performed to ensure toxins do not kill the plants. Varieties of plants with huge biomass yields (more than 3 tons/ acre) are generally chosen, in a general attempt to deplete the soil. Plants called “hyperaccumulators” are necessary for inorganic pollutants, as they will take up more than ten times background soil concentrations. When groundwater remediation is of concern, deep-rooting trees such as poplar, cottonwood, and willow trees are generally planted in rows perpendicular to flow, and monitoring wells are placed nearby (Sharma and Reddy, 2004).

Table 2. Hyperaccumulating plant species for various metals (Chaney, et al, 2000)

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Table 3. Applications of phytoremediation (Rock, 1998)

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Irrigation and Soil Amendment

Often soils need to be amended and routinely irrigated to ensure plant viability (Sharma/Reddy, 2004). Soil is sometimes flooded to encourage the dissolution of pollutants and increased evapotranspiration. If the same plant species is used in succession, the same rate-limiting nutrient may need to be continually replaced. The removal of nutrients and pollutants can drastically affect the pH, which may also need adjustment. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA), (Baylock, 1999) can aide in keeping radionuclides and heavy metals in solution, increasing the ability to be taken up by vegetation.

Monitoring

Sampling of soil and/or groundwater should be routinely practiced. The changing contaminant concentration ensures that the current process is effective, and can resultingly be utilized to modified the current treatment plan. If the process is  too slow, a different type of plant can be substituted, or soil amendments can be adjusted to increase performance (Sharma and Reddy, 2004). This is the first “feedback loop” that can necessitate the modification of a previous process step.

Harvesting

As the last potential step, harvesting can also be the most controversial. It involves the removal of the plant, sometimes including the roots. Through soil sampling, a mass balance can be performed to approximate the efficiency of contaminant removal (Sharma/Reddy, 2004). Depending on the nature of the contaminant and concentrations within the plant tissue, the vegetation may be composted, or require more expensive disposal. One emerging option includes incineration, in which the biomass provides electricity and heat, while pollutants are either oxidized or captured in flue-stack scrubbers. Some “contaminant-laced” biomass actually has economic value. A crop used to remediate selenium can be sold to livestock farmers as “bio-enhanced” and defray their costs of purchasing selenium salts to add to their normal livestock feed (Chaney, et al, 2000). Ash from the nickel-accumulator “Alyssum murale” can be economically processed into ore if the content is above 20% by weight (Chaney, et al, 2000). Generally hyperaccumulators retain inorganic compounds in the following “compartments” (in decreasing order): root, leaf, stem, fruit. Nightshade plants, such as cucumbers and tomatoes, however,  accumulate compounds in their leaves and stems more readily than roots, which make them better for above-ground harvest (Gupta et al, 2000). Following annual or semi-annual harvesting, testing determines whether additional treatment through another growing season is warranted.

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