Page 35 - Plant-Based Remediation Processes

22 M. Barbafieri et al.

capacity to modify the mobility/bioavailability of heavy metals in the rhizosphere
and seem to have access to basically non-plant-available heavy metal pools in the
soil as well. “Bioavailable Contaminant Stripping” (BCS) firstly discussed by
Hamon and McLaughlin (2003) can be further developed as a remediation approach
which is focusing at the removal of all actually and potentially bioavailable heavy
metal fractions (see Chap. 13).
Monitoring the mobility and bioavailability of inorganic pollutants (including
heavy metals) in contaminated soil provides important information regarding the
fate of these contaminants in the environment, time-dependent changes in heavy
metal speciation, mobility towards the water table, and ecotoxicological risks
(Environmental Agency 2004; Mulligan and Yong 2004). Some authors tend to
promote that risk assessment of soils should consider both mobile and bioavailable
fractions of heavy metals, which of course depends on the definition of bioavail-
ability (Wahle and Kordel 1997). Despite such considerations, it remains clear that
total concentrations of heavy metals in soils are poor indicators of heavy metal
toxicity since heavy metals exist in different solid-phase forms that vary consider-
ably in terms of (potential) bioavailability (Nolan et al. 2003). Phytoextraction has
proved to be effective, relatively straightforward, and inexpensive compared to
other procedures for extracting bioavailable metal fractions from soils. Bioavailable
heavy metal fractions, removed by plants, probably correspond to fractions of soil
heavy metals that are most prone to affect the soil ecosystem. However, there are
surprisingly few reports which show that bioavailable fractions of heavy metals in
soils are indeed reduced after concluding a phytoextraction project in the field
(Ban ˜uelos et al. 2011; Willschera et al. 2012). As other bioavailable heavy metal
fractions can be slowly released by nonmobile heavy metal fractions in the soil
(aging), a longer term decrease of the bioavailable fraction might be difficult to
observe experimentally. Moreover, this is an argument for considering both mobile
and immobile (bioavailable and potentially bioavailable) heavy metal fractions in
the soil, when estimating risks.
More data are available regarding phytostabilization; Phytostabilization often
uses chemical additives to immobilize heavy metal mobile fractions, especially at
heavily polluted sites, which are initially without vegetation due to heavy metal
phytotoxicity. Such immobilization is a prerequisite for plant growth. Immobiliza-
tion therefore has to be measured and monitored.

2.2 How Can We Use Phytoextraction?

2.2.1 Technology Description

Phytoextraction refers to the translocation of metal contaminants from soil up to the
above-ground tissues by the root system. After plants have grown for a certain
period, they are harvested and may be incinerated to recycle the metals.