Phytoextraction limitation

Higher phytoextraction coefficients indicate higher metal uptake. The effectiveness of phytoextraction can be limited by the sorption of metals to soil particles and the low solubility of the metals however, metals can be solubilized through the addition of acids or chelating agents and so allow uptake of the contaminant by the plant. Ethylene diamine tetra-acetic acid (EDTA), citric acid, and ammonium nitrate have been reported to help in the solubilization of lead, uranium, and cesium... 

Phytoextraction has several advantages. The contaminants are permanently removed from the soil and the quantity of the waste material produced is substantially decreased. In some cases, the contaminant can be recycled from the contaminated biomass. However, the use of hyperaccumul-ating plants is limited by their slow growth, shallow root systems, and small biomass production. In order for this remediation scheme to be feasible, plants must tolerate high metal concentrations, extract large concentrations of heavy metals into their roots, translocate them into the surface biomass, and produce a large quantity of plant biomass. 

These maximum depths are not likely to occur in most cases. The effective depth for phytoremediation using most nonwoody plant species is likely to be only 30 or 61 cm (1 or 2 ft). Most accumulators have root zones limited to the top foot of soil, which restricts the use of phytoextraction to shallow soils. The effective depth of tree roots is likely to be in the few tens of feet or less, with one optimistic estimate that trees will be useful for extraction of groundwater up to 9 m (30 ft) deep.41-58... 

However, phytoremediation does have certain disadvantages and limitations. This technology is limited by depth (roots) and also by the solubility and the availability of the pollutant. Although it is faster than natural attenuation, phytoremediation requires long time periods and is restricted to sites with low contaminant concentrations. The plant biomass obtained from phytoextraction requires proper disposal as hazardous waste. Phytoremediation depends on the climate and season. It can also lose its effectiveness when damage occurs to vegetation from disease or pests. The introduction of inappropriate or invasive plant species should be avoided (non-native species may affect biodiversity). Contaminants may be transferred to another medium, the environment, and/or the food chain. Amendments and cultivation practices may have negative consequences on contaminant mobiUty. 

Lim et al. (2004) optimized the time and electric field applied to each Indian mustard Brassica juncea) plant in a remediation experiment performed in 1.2-kg pots of lead- and arsenic-contaminated soil. The electric potential of 30-40V applied 1 h per day were the optimal conditions to reach maximum phytoextraction after 9-day treatment. In this case, the increase of EDTA availability by the electrical current caused a rapid toxicity response of plants that limited the remediation process. 

Contaminant attenuation mechanisms involved in phytoremediation are complex and not limited only to the direct metabolism of contaminants by plants. Certain indirect attenuation mechanisms are implicated in phytoremediation, such as the metabolism of contaminants by plant-associated microorganisms, and plant-induced changes in the contaminated environment. In terrestrial species, transport of contaminants to the plant is dominated by uptaking water by roots, and distribution within the plant relies on xylem or phloem transport (Macek et ak, 2000). Various terms, reflecting each specific attenuation mechanism, have been extensively used to better describe specific applications of phytoremediation. These include phytoextraction, phytodegradation, phytotransformation, phytovolatilization and rhizodegradation (Burken and Ma, 2006). 

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