Page 196 - Plant-Based Remediation Processes

188 A. Branzini and M.S. Zubillaga

Phytostabilization: In Phytostabilization when certain plants immobilize soil
contaminants (USEPA 2000), they are absorbed and accumulated by roots,
adsorbed onto the roots, or precipitated in the rhizosphere. This reduces or even
prevents the mobility of the contaminants preventing migration into the groundwa-
ter or air and reduces the bioavailability of the contaminant, thus preventing spread
through the food chain. Plants for use in phytostabilization should be able to
(1) decrease the amount of water percolating through the soil matrix, which may
result in the formation of a hazardous leachate, (2) act as barrier to prevent direct
contact with the contaminated soil, and (3) prevent soil erosion and the distribution
of the toxic metal to other areas (Raskin and Ensley 2000). Phytostabilization can
occur through the process of sorption, precipitation, complexation, or metal
valence reduction. This technique is useful for the cleanup of Pb, As, Cd, Cr, Cu,
and Zn (Jadia and Fulekar 2008). It can also be used to reestablish a plant
community on sites that have been denuded due to the high levels of metal
contamination. Once a community of tolerant species has been established, the
potential for wind erosion is reduced, and leaching of the soil contaminants is
reduced. Phytostabilization is advantageous because disposal of hazardous mate-
rial/biomass is not required, and it is very effective when rapid immobilization is
needed to preserve ground and surface waters (Jadia and Fulekar 2009; USEPA
2000). Therefore, sometimes it is extremely difficult to distinguish between direct
and indirect responses if metal concentrations are too high or excessively
prolonged. If there are metabolic alterations, these might reflect general failure
of plant metabolism, but little is known about the earlier stages. Therefore, the
characterization of heavy metal stress perception mechanisms should be
undertaken in adequate experimental conditions, where we could learn about the
primary cellular components involved. In fact, during the initial germination stage,
there are many processes in which the presence of heavy metals will have a direct
impact on seed viability and normal development of plants (Sobrero and Ronco
2004). Therefore, this stage is considered a critical phase in the life cycle of an
individual (Veasey et al. 1999). Consequently, heavy metals’ effects on initial
germination stage might be assessed through chemical, biological, and toxicologi-
cal data as well (Gruiz 2005). The use of phytotoxicity tests may offer a simple
alternative to assess effects in early stage of plants.
The advantages of phytoremediation compared with classical remediation are as
follows (1) it is more economically viable using the same tools and supplies as
agriculture, (2) it is less disruptive to the environment and does not involve waiting
for new plant communities to recolonize the site, (3) disposal sites are not needed,
(4) it is more likely to be accepted by the public as it is more aesthetically pleasing
then traditional methods, (5) it avoids excavation and transport of polluted media,
thus reducing the risk of spreading the contamination, and (6) it has the potential to
treat sites polluted with more than one type of pollutant. The disadvantages are as
follows (1) it depends on environmental conditions (i.e., climate, geology, altitude,
and temperature), (2) large-scale operations require access to agricultural equip-
ment and knowledge, (3) success is dependent on the tolerance of the plant to
the pollutant, (4) contaminants collected in senescing tissues may be released