Nature and Extent of the Contamination Toxicity of the Contaminants

An overall evaluation of the particular contamination issue and site must be made before starting a phytoextraction program. Indeed, sites to be remediated are very seldom contaminated with single metals. Multicontamination with organic and inorganic pollutants (and sometimes with physical constraints, such as compacted soil) is common and may induce toxicity to plants that are not tolerant to all the contaminants present. Plants are potentially sensitive to a broad range of elements if their concentrations in the soil are above a given threshold. However, the most toxic metals for higher plants and certain microorganisms are Hg, Cu, Ni, Pb, Co, Cd, and, possibly, Ag, Be, and Sn [26].

The effect of organic pollutants (toxicity) on plants and their fate in plants is poorly known. Attempts have been made to use plants to reduce their concentrations in soils. However, their uptake by roots, translocation to shoots, volatilization, and/or degradation seem to be highly plant, soil, and component dependent [27-29]. It is also clear that the presence of herbicide residues may reduce or even prevent plant establishment.

In general, critical limits in plant can be quickly reached when a large amount of a metal contaminant is in a form available in the soil for plant uptake, especially when it is combined with a large soil-plant transfer coefficient [30,31] or when the threshold is low. For example, copper is a widespread contaminant and no example of successful extraction of Cu by Salix species from contaminated soil has been reported thus far [32]. The remediation efforts are mostly directed towards Cu immobilization through the use of additives [33]. However, its toxicity has been found to reduce phytoextraction efficiency because, for most of the plants, the toxic level is already reached when the Cu concentration in the plant is above 15 to 20 mg kg-1 DM [31] or when the concentration in the soil solution reaches 0.02 to 0.06 mg L-1 [34]. As a result of Cu toxicity, the biomass is reduced and the metal uptake is impaired [35-37].

In contrast, Punshon et al. [38] have shown that Salix species (S. caprea, S. cinerea, and their hybrids with S. viminalis) originating from contaminated sites exhibit Cu tolerance, but that there is a large variability between willows at the species and the population levels. Thus, soils with undesirable contaminants might not be easily decontaminated and a soil pretreatment might be necessary to remove or inhibit the toxic compounds. Alternatively, specific willow clones may need to be selected for these sites. Additionally, metals may have antagonistic or synergic effects on their toxicity and uptake as demonstrated for Cd by Costa and Morel [39]. Heterogeneity of the Contamination

Contamination is highly heterogeneous at polluted sites; this includes spatial variation in composition and concentration [4,17] as well as variation with depth [40]. An example of surface and at depth heterogeneity is displayed in Figure 30.3 for HNO3-extractable Cd and Cu concentrations at Dornach. The standard deviations (SD) increased with depth, illustrating that the thickness of the contaminated layer varied between 0.2 and 0.7 m within a 400-m2 area. For Cd, the SD was smaller and similar along the soil profile because, although Cd was brought to the soil by atmospheric deposition along with Cu and Zn, it also had a geogenic origin [41,42]. The high heterogeneity in the thickness of the contaminated layer prevents an optimal root colonization of the contaminated layer (see Table 30.4, third column) because phytoextraction can only be achieved when roots are within the contaminated layer. The question of root colonization will be developed later. Limited Availability of Metals

From a different perspective, low availability of a metal to be remediated may reduce the potential for its phytoextraction. Although the metal available fraction in soil may be determined by different methods, it is indeed usually better correlated to plant uptake than total metal content in soil. In Figure 30.4, the low Cd uptake by S. viminalis is explained by the low NaNO3-extractable (s) Cd

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