The International Information Center for Geotechnical Engineers

Phytoremediation

Theoretical Background

The word “phyto” means plant in Greek. This process involves utilizing plants in remediating environmental contaminants. It generally refers to the use of plants without additional excavation or soil processing. Many different actions occur to absorb or degrade contaminants across a variety of scales. The plant’s root zones must be in contact with the contaminated soil, as this is where the contaminant removal occurs. The root membranes act as a filter in a process termed “rhizofiltration” and eventually absorb the pollution. Two driving forces then determine the contaminant fate depending on the nature of the chemical. “Phytodegradation” occurs when metabolic processes within the plant break down the organic chemicals, while “phytoaccumulation” occurs when typically inorganic compounds are locked into the plant’s structure (Sharma and Reddy, 2004). A somewhat combined process, called “rhizodegradation” occurs in root rhizomes in which mutually beneficial bacteria or fungus breaks down and/or incorporates pollutants on the surface of or within the plant’s roots. The driving energy force can be likened to a miniature pump and treatment system in which evapotranspiration during summer months causes large amounts of soil moisture to be processed. This natural process of “phytoextraction” can draw the soil’s pollution from the ground all the way to the leaves, however the bulk of the material can either be degraded, or accumulates where a local metabolic process is unable to break it down (Sharma and Reddy, 2004). The soil-moisture removal creates a small “cone of depression”, and limits the migration of a plume in a very miniature way. The organic attraction combined with the groundwater immobilization effectively limits its movement in a process called “phytostabilization”. “Phytovolatilization” is an emerging process called out by Mudhoo (2011), which involves the conversion of contaminants to volatile forms and directly releasing them to the atmosphere.

Table 1. Phytoremediation processes, mechanisms, and related pollutants/plant species (Gupta et al, 2000)

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Through tracing organic pollutants with radioactive carbon, it has been found that plants like poplar trees transform compounds into trichloroethano, trichloroacetic acid, and dichloroacetic acid metabolites, sorbed by the roots, and “mineralized” to carbon dioxide (Sharma and Reddy, 2004). Around 15% of pesticides are able to be permanently affixed to the plant material, and almost every pollutant released to the atmosphere quickly break down due to hydroxyl radicals.

The efficiency of the phytoremediation process is ultimately determined by the fraction of organic matter in the soil and the pollutant’s hydrophobicity (Sharma and Reddy, 2004). If the pollutant strongly prefers organic material, or if there is an overabundance of organic content, it may be nearly impossible for plants to remove enough of the compound. Extremely hydrophilic pollutants will remain in solution and often pass through plant tissues without significant uptake. Contaminants with octanol-water (KOW) ratios of 1 to 3.5 have the greatest potential for phytoremediation.

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