Phytochemical Removal of Heavy Metal-Contaminated Soils

Abstract

16.1 Phytochemical Remediation Contamination of soils by heavy metals and metalloids has recently been a major concern particularly with the rapid expansion in industrial activities. It has become a global threat to plants, humans, animals, fish, and other living organisms. Such metals remain indefinitely in the soil (Bodar et al. 2006; Fotakis and Timbrell 2006). There are two methods for the remediation of the contaminated soils (i.e., chemo-remediation and phyto-remediation). The cleanup of soils contaminated with heavy metals is one of the most difficult tasks for environmental engineering. A number of techniques have been developed to remove heavy metals from contaminated soil, including ex situ washing with physical-chemical methods (Anderson 1993) and in situ phyto-extraction (McGrath 1998 and Salt et al. 1998). In the ex situ washing methods, chelating agents or acids are used to enhance heavy metal removal. 16.1.1 Chemo-remediation Chemo-remediation is remediating the soil through the use of some chemicals such as acids, chelators, and immobilizers of heavy metals. Treating contaminated soils 299 with acids and chelators can increase the mobility of heavy metals either as soluble metal ions or as complexes of metal chelates, rendering the heavy metals easily removed from soil through leaching, electrokinetic extraction, or plant accumula-tion (Lestan et al. 2008). Chelants can either be mixed with the soil or added close to the plant roots (Kayser et al. 1999). Chelates could be added in one operation or in different successive applications. The latter one is more effective (Shen et al. 2002). Successive application by different combinations of chelates can be highly efficient in remediation, following three different combination operations. The first one is using a mixture which reduces soil pH, consequently increasing the solubility of heavy metals in the soil (Blaylock et al. 1997). The second is using a mixture which increases heavy metal mobility via reaction between the mixture components and heavy metals in soil (Tandy et al. 2004). The third is using a mixture which increases metal uptake by plants (Luo et al. 2006; Mathis and Kayser 2001). The method causes immobilization of heavy metals in soil by adding particular soil amendments to the contaminated soil to increase adsorption of heavy metals or decrease their mobility, hence decreasing their harmful effect. These amendments comprise inorganic materials (e.g., lime and fly ash), organic materials (e.g., organic residues), and synthetic materials (e.g., zeolites and Fe/Al hydrous oxides) (Bolan and Duraisamy 2003). Mainly, there are four categories used in chemo-remediation: (a) synthetic aminopolycarboxylic acids, (b) natural biodegradable acids, (c) low-molecular-weight organic acids, and (d) humic substances including humic acids (Evangelou et al. 2007a). Synthetic aminopolycarboxylic acids are generally used to supply plants with micronutrients. They are used to increase the removed heavy metals either through uptake by plants grown on the soil or through leaching of the bare soil (Luo et al. 2005; Finzgar and Lestan 2007). They include ethylenediami-netetraacetic acid (EDTA), hydroxyethylene diamine tetraacetic acid (HEDTA), diethylene triamino pentaacetic acid (DTPA), trans-1,2-cyclohexylene dinitrilo tetraacetic acid (CDTA), ethylene glycol tetraacetic acid (EGTA), ethylenediamine-N,N 0 -bis (o-hydroxyphenyl)acetic acid (EDDHA), N-(2-hydroxyethyl)iminodia-cetic acid (HEIDA), and N,N 0 -bis(2-hydroxybenzyl)