Page 36 - Plant-Based Remediation Processes

2 Protocols for Applying Phytotechnologies in Metal-Contaminated Soils 23

This procedure, repeated several times, brings soil contaminant levels down to
below legally acceptable limits (Chaney et al. 1997). The time required for remedi-
ation depends on the type and extent of heavy contamination, the duration of the
growing season, the amount and characteristics of the produced biomass, and the
plants natural capability for heavy metal accumulation. Two different strategies can
be used (Lombi et al. 2001; Robinson et al. 2003a): continuous phytoextraction—
using natural metal hyperaccumulator plants which absorb, translocate, and accu-
mulate an enormous amount of metals during their entire life period without visible
toxicity symptoms (Baker and Brooks 1989; Brooks 1998); assisted
phytoextraction—the accumulation process is induced in tolerant plants by the
increased contaminant bioavailability in soil (Blaylock et al. 1997). Synthetic
amendments such as chelates (e.g., EDTA, EDDS, NTA—Cooper et al. 1999;
Evangelou et al. 2007), organic acids (e.g., citric acid), or ion competitors (e.g.,
phosphate—Tassi et al. 2004) added to the soil enhance metal bioavailability,
although the soil microbial community is usually neglected and there is a potential
risk of leaching of metals to groundwater (Dickinson et al. 2009; Evangelou et al.
2007).
Generally, phytoextraction is only applicable to sites containing low-to-moderate
levels of metal contamination. Effective phytoextraction requires both plant genetic
ability and optimal soil and crop management practices (Di Gregorio et al. 2006; Tassi
et al. 2008;Pedron etal. 2009). Thlaspi caerulescens (Cd and Zn hyperaccumulator)
and Brassica juncea (heavy metal accumulator) are examples of species that well
represent the two phytoextraction strategies described above. Metals such as Ni, Zn,
Cu, and As are the best candidates for removal by phytoextraction, although Cd, Pb,
etc., have been extensively studied as well. Genetic engineering studies have been
performed to manipulate plant accumulation with the overexpression or knockdown
of membrane transporter proteins (Rogers et al. 2000).
The accumulation of hazardous plant biomass must be disposed of, in order to
minimize environmental risk. The waste volume can be reduced by thermal,
microbial, physical, or chemical means such as composting, compaction, or
thermo-chemical conversion processes (combustion, gasification and pyrolysis).
Recycling the biomass from phytoextraction for fuel and other uses cuts down on
the need for landfills and provides the contaminated site with an economical value.
Added value to the phytoextraction process could be obtained by combining the
biomass produced as an energy source, resulting in an ore after incinerating the
residual biomass. This would be possible in the case of phytomining, a particular
example of phytoextraction. Phytomining involves the exploitation of subeconomic
ore bodies using hyperaccumulating plants. For instance, the species Alyssum
bertolonii, Berkheya coddii have a high potential in extracting Ni because of their
high biomass and a Ni concentration of 1 % in the dry matter (Robinson et al.
2003b). Other metals such as gold, thallium, and cobalt have been exploited from
tailings or other residues of low commercial value (LaCoste et al. 2001; Keeling
et al. 2003). Heavy metal phytoextraction refers to the use of plants that can remove
contaminants from soil and accumulate them in a harvestable part in a process
alongside water and nutrient absorption by roots. Therefore plant biomass