Page 281 - Plant-Based Remediation Processes

13 Phytoremediation Towards the Future: Focus on Bioavailable Contaminants 275

13.2.1 The Impact of Soil Properties

Before selecting a phytoextraction process, it is necessary to consider the specific
characteristics of the soil at the contaminated site in order to evaluate how the soil
properties will influence contaminant bioavailability and thus the final result of
remediation in the field.

13.2.1.1 pH

In plant–soil–metal interactions, pH affects the uptake of metals in different ways
for hyperaccumulator and nonaccumulator plants (Li et al. 2003; Chaney et al.
2005). pH is the most important parameter that determines the concentrations of
metals in soil solutions by regulating precipitation–dissolution processes. pH values
also regulate the specific adsorption and complexation of inorganics in the soil
environment. Metal hydrolysis is also regulated by pH and beyond a threshold pH
level (which is specific for each metal) these reactions drastically reduce the
concentration of most metal ions in the soil pore water. At low pH levels, on the
other hand, sorption processes are reduced due to the acid-catalyzed dissolution of
oxides and their sorption sites, whereas complexation by organic matter tends to
decrease with increasing acidity.

13.2.1.2 Clay Content

The influence of clay content on phytoremediation has been reported for specific
species (Abdullah and Sarem 2010), but, in general, clay minerals regulate the
amounts of metals in soil solutions. Ion exchange and specific adsorption are the
mechanisms by which clay minerals adsorb metal ions from the soil liquid phase.
This is done through the adsorption of hydroxyl ions followed by the attachment of
the metal ion to the clay by linking it to the adsorbed hydroxyl ions or directly to
sites created by proton removal. Highly selective sorption occurs at the mineral
edges. However, notable differences exist between clay minerals in terms of their
ability to retain heavy metals, which are more strongly adsorbed by kaolinite than
montmorillonite. This is probably due to a higher amount of weakly acidic edge
sites on kaolinite surfaces. In expandable clays (vermiculite and smectite), sorption
essentially involves the interlayer spaces and is greater than in non-expandable
clays such as kaolinite. The importance of clay minerals, and of soil texture, in
determining the distribution of heavy metals between the solid and the liquid phases
of soil has a direct consequence on the metal bioavailability of plants and
phytoextraction efficiency.