Plants for BioRemediation
Plants can be used to clean up contaminants of the soils, water, and the air in a process called phytoremediation. These can be used to remove, destroy, or sequester these materials, including heavy metals, temporarily or permanently to mitigate its harmful effects on the environment.
Mechanisms of phytoremediation are as follows:
- Phytostabilization is the process by which the plant reduces the mobility, bioavailability, and toxicity of the heavy metal by adsorption and/or absorption and retaining these in roots (Cheng, 2003). This is influenced by the soil properties such as pH, organic matter, and soil type which influences its cation exchange property. In particular, the heavy metal has low mobility and becomes more inavailable if soil is high in pH, organic matter, and clay type, the heavy metals are (Jung and Thornton, 1996; Rosselli et al., 2003).
- Phytoextraction is the process which the plant absorbs the heavy metal and accumulates these in its shoots, resulting to higher levels in the plant than in the soil (Reeves, 2003; Reeves, 2006; Cheng 2003). This is sometimes known as hyperaccumulation. The concept of phytoextraction was introduced by Chaney (1983) and further explained in relation to hyperaccumulation by Chaney et al. (2005).
- Phytovolatization uses plants to absorb volatile pollutants, including selenium and mercury from soils, transform, and release these into the atmosphere (Cheng, 2003).
- Phytofiltration, also known as rhizosphere accumulation, rhizofiltration, or blastofiltration (Sarma, 2001) involves aquatic, wetland, and terrestrial plants to absorb heavy metals from aqueous environments (Tang 1993, Cheng et al., 2002, Cheng 2003).
Plants, therefore responding in different ways to varying levels of heavy metals and their potential can be estimated quantitatively using the following parameters, as applied by Nazir et al. (2011), Sarma (2001), Bu-Olayan and Thomas (2009), Robinson (1998):
- Biological Accumulation Factor (BAF) or Biological Accumulation Coefficient (BAC) is a measure of the plant’s ability to retain the heavy metal in its shoots. It is computed as the ratio of the heavy metal in the shoots and the soils.
- Biological Concentration Factor (BCF) is a measure of the plant’s ability to absorb and retain the heavy metal in its roots. It is computed as the ratio of the heavy metal in the roots and the soils.
- Translocation Factor (TF) or Efficiency Factor (EF) is a measure of the plant’s ability to absorb, translocate and retain the heavy metals in its shoots. It is computed as the ratio of the heavy metal in the shoots and the roots.
Based on the specific responses of plants, these can then be classified according to their phytoremediation potential as:
- Phytostabilizers are plants which have BCF >1 and TF < 1. This is also referred to as excluders, a classification and its physiology explained by Baker (1981) and McGrath et al. (1999).
- Phytoextractors are plants which have BAF >1.
- Hyperaccumulators are plants which have BAF>1, TF >1, and heavy metals at least given their hyperaccumulation levels (van der Ernt et al. 2012). These are also referred to as plants which contain from 1- 4 orders of magnitude higher than in adjacent plants (Baker, 1981, Baker and Brooks, 1989; Reeves and Baker, 2000; Reeves and Brooks, 1983).
Pollard et al. (2014) classifies plants with potential phytoremediation properties based on their occurrence in metalliferous soils and normal soils as follows:
Metallophytes are plants that occur on metal-enriched soils or metalliferous soils.
Metallophytes found exclusively in metalliferous soils are indicator, endemic, bodenstet, eumetallophytes, absolute metallophytes, or strict metallophytes, or obligate metallophytes.
Metallophytes found in both metalliferous soils and normal soils are also called termed bodenvag/ bodenwag/bodenwaag (meaning “soil-wandering”;), pseudometallophytes and facultative metallophytes. These can be further classified as follows:
- Elective pseudometallophytes which occur with greatest vigor on metalliferous soils
- Indifferent pseudometallophytes which grow indiscriminately and equally well on metalliferous and normal soils and
- Accidental pseudometallophytes which occur sporadically and with reduced vigor on metal-contaminated soils such as weeds and ruderals.
Among these, facultative metallophyte or pseudometallophytes are effective metal bioindicators in providing easy to detect clues of soil contamination as they can manifest changes in the morphology, which deviate from what is usually observed when one is found in normal soils. These can also be adopted for restoration of the vegetation as these can be sourced and grown in both metalliferous and normal soils, or as where it may be needed.