Phytoextraction Vs Phytostabilization

From the evidence of the BCFs listed in Table 20.2, only Cd and Zn will accumulate in sufficiently high concentrations in the above-ground tissues of trees (BCF > 1) to be candidates for phytoremediation. When the amounts accumulated are compared with the amounts of metal in the soil, only removal of Cd from marginally contaminated soil is possible within a realistic timescale. Zinc is usually present in high concentrations, of which the amount of metal in the plant tissues represents a very small proportion. The other metals commonly studied (Cr, Cu, Ni, Pb) are poorly bioavailable in the soil or are poorly translocated out of the root.

Willows accumulate Zn and Cd more readily than other trees and woody plants do and it has been suggested that this may pose a threat of transfer of metals to the food web [120,121]. It has also been demonstrated that Zn can be transferred to aphids via plant uptake, resulting in Zn concentrations in aphids four times greater than in the soil [122]. Cadmium concentrations in aphids reflected those in plants, but neither metal appeared to be transferred to predatory ladybirds. High Cd concentrations in tissues of small mammals have been recorded at similar sites where willow is grown on dredged sediments [123].

Alternatives to phytoremediation include ex situ washing methods with strong chelating agents such as EDTA [124] or acids [125]. Attempts to use the same chemicals in situ to enhance metal uptake into plants have had limited success [126], are often prohibitively expensive, and may be destructive to fertility and soil biota; also, residual concentrations in soil pore water may pose a risk of ground water contamination. Nevertheless, BCFs can vary by an order of magnitude over a relatively short pH range from 5.5 to 7.0 in leaf and root vegetables [127], and Cd uptake into Salix has been found to be highly pH dependent in field stands [90]. It may be possible to enhance Cd uptake with low-cost organic or inorganic acid soil amendments.

Considerable potential benefits can be gained from the growth of trees on contaminated land, resulting from the stabilization of the soil and/or the contaminant. The protection afforded simply by the presence of the large above-ground biomass of trees can result in a decrease in wind- and water erosion of the soil [128]. Leaf litter can accumulate on the soil surface, forming a barrier over the contaminated soil, which can also help its physical stabilization. Tree roots form a significant below-ground biomass that can effectively bind the soil [129]. They also take up large amounts of water lost from the leaf surface in the transpiration stream. This represents a significant upward movement of water from the soil, via the plant, to the atmosphere, which decreases the potential for metals leaching from the soil. It was estimated in one study that leaching under a tree cover was about 16% less than under grass [130].

In addition to the physical stabilization, chemical stabilization of certain elements may occur. For example, it has been shown that Cr is strongly held in the roots, regardless of the form supplied to the plant (CrIII or CrVI). CrIII is highly insoluble and thus poorly bioavailable, and it has been suggested that CrVI can be reduced to CrIII in the rhizosphere [131]. Lead is another element that is strongly bound in root tissues, possibly due to the formation of lead phosphate

In addition to the use of various soil amendments, including liming, zeolites, beringite, and organic matter, tree cover provides a potentially sustainable vegetation that may allow the contaminants to remain permanently immobilized in soil or woody biomass. At the present time, evidence demonstrating that this happens is insufficient. It is known that metals can become vertically mobile in soil profiles under mature woodlands [22,79] and, clearly, this may threaten underground aquifers in the longer term. Other studies of highly contaminated mature woodlands

[133] have demonstrated very low metal mobility and relatively steady-state conditions. Further studies are required to evaluate the long-term feasibility of using trees to stabilize metal contaminants in soil.

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