Page 194 - Plant-Based Remediation Processes

186 A. Branzini and M.S. Zubillaga

10.3.1.1 Organic and Inorganic Amendments

There are different strategies for chemical immobilization of heavy metals in
degraded soil. The use of soil amendments has been proposed as a low input
alternative for remediation of metal-polluted soils. The primary role of
immobilizing amendments is to alter the original soil metals to more geochemically
stable phases via sorption, precipitation, and complexation processes (Hashimoto
et al. 2009). The mostly applied amendments include clay, cement, zeolites,
minerals, phosphates, organic composts, and microbes (Finz ˇgar et al. 2006). In
fact, in situ chemical immobilization decreases the concentration of dissolved
contaminants by sorption or precipitation (Basta and McGowen 2004). It is well
documented that some amendments (lime, phosphates, and organic and inorganic
waste products) are effective in reducing mobility and availability of heavy metals
in soils (Brown et al. 2005). In addition, their toxicity could be minimized by
reducing their availability using organic and inorganic amendments (Adriano 2001;
Basta et al. 2001). Generally, formation of insoluble metal element chemical
species reduces both leaching through the soil profile and the labile pool available
for biological interaction (Geebelen et al. 2003).
In particular, organic amendments, like mature compost, contain a high propor-
tion of humified organic matter (humin and humic and fulvic acids). They could
adsorb heavy metals temporarily through quelates’ formation or by the formation of
stable complexes sorbing them for a longer period (Basta et al. 2005). However, the
abundant literature concerning the use of such amendments for metal immobiliza-
tion is not conclusive, as contradictory results have been reported by different
authors, depending upon various soil conditions, specific metals involved, origin,
molecular size, and concentration of the organic matter, etc. For example, Yang
et al. (2006) found that some organic ligands inhibited desorption of previously
1
adsorbed Pb in soils at low ligand concentrations (<10 3 mol l ), whereas a
greater desorption was found at greater ligand concentrations. An extra important
effect of organic amendment in soil is that it allows the recycling of nutrients and of
organic matter present in them, and the improvement in soil physical properties
(Sa ´nchez-Monedero et al. 2004). On the other hand, inorganic amendments, such as
water-soluble phosphates, provide long-term remediation through direct metal
adsorption by the phosphate and precipitation of metals with solution phosphate
(Adriano et al. 2004). In addition, inorganic amendments can also be used as a
fertilizer to provide plant nutrients (Sharpley et al. 1999).

10.3.1.2 Phytoremediation

Due to their sessile nature, terrestrial plants have restricted mechanisms for stress
avoidance, but during the course of evolution, some plant species have developed
tolerance mechanisms to ensure the survival and breeding ability under elevated
metal concentrations (Pastori and Foyer 2002) (Fig. 10.3). These adaptative
responses of plants to heavy metal-contaminated environments are efficient