Phytoextraction of Metals from Contaminated Soil: A Review of Plant/Soil/Metal Interaction and Assessment of Pertinent Agronomic Issues

Abstract

Phytoremediation is an emerging technology that employs the use of higher plants for the cleanup of contaminated environments. Fundamental and applied research have unequivocally demonstrated that selected plant species possess the genetic potential to remove, degrade, metabolize, or immobilize a wide range of contaminants. Despite this tremendous potential, phytoremediation is yet to become a commercial technology. Progress in the field is precluded by limited knowledge of basic plant remedial mechanisms. In addition, the effect of agronomic practices on these mechanisms is poorly understood. Another limitation lies within the very biological nature of this novel approach. For example, potential for phytoremediation depends upon the interaction among soil, contaminants, microbes, and plants. This complex interaction, affected by a variety of factors, such as climatic conditions, soil properties, and site hydro-geology, argues against generalization, and in favor of site-specific phytoremediating practices. Thus, an understanding of the basic plant mechanisms and the effect of agronomic practices on plant/soil/contaminant interaction would allow practitioners to optimize phytoremediation by customizing the process to site-specific conditions.Remediation of metal-contaminated soil faces a particular challenge. Unlike organic contaminants, metals cannot be degraded. Commonly, decontamination of metal-contaminated soils requires the removal of toxic metals. Recently, phytoextraction, the use of plants to extract toxic metals from contaminated soils, has emerged as a cost-effective, environment-friendly cleanup alternative. In this paper, we review the processes and mechanisms that allow plants to remove metals from contaminated soils and discuss the effects of agronomic practices on these processes.

Vegetable species for phytoextraction of boron, copper, lead, manganese and zinc from contaminated soil

“…Also, the immobilization of elements by root adsorption may be of some importance. This mechanism avoids element's translocation from roots to shoots in some species working as a defense barrier, decreasing the phytoextraction potential of such species since only a small part of ions associated to the roots are effectively absorbed (Lasat, 2000). Silva et al (2007) used soybean and rice to evaluate the absorption and capacity of translocation of heavy metals in the same soil studied here and observed that the roots limited the translocation of copper and lead to the aerial parts of both cultures.…”

“…These indicators allowed estimating also the period of time necessary to remove all soil contamination, considering four annual cultivation cycles. Soil decontamination is considered successful when the plants are able to extract up to 1 -2% of the metal from soil (Lasat, 2000) or up to 1% of Zn and Cu, 0.6% of Pb total content from soil (Garbisu and Alkorta, 2001).…”

“…It consists in the removal of metals by plants through uptake and accumulation into biomass (Nascimento and Xing, 2006). Interestingly, phytoremediation was recognized and documented by humans more than 300 years ago, however the scientific study and development of suitable plants was not conducted until the early 1980's (Lasat, 2000).…”

Effects of earthworm activity on fertility and heavy metal bioavailability in sewage sludge

“…Unlike organic compounds, metals cannot be degraded (Lasat, 2000). To decrease the contents and bioavailability of heavy metals usually requires their removal.…”

“…The addition of chemical immobilization amendments may only immobilize lethal levels for restricted periods of time and introduce some other chemicals. Phytoremediation is recognized as the most commercially and environmental friendly available technology (Lasat, 2000), but up to now, only a limited number of plants have been found to have the phytoaccumulation ability and much less can be used for field phytoremediation because of low biomass production.…”