Effect of Heavy Metals
AMD contains a variety of dissolved heavy metals from abandoned and active mining areas (Gaikwad and Gupta, 2008). Appenroth (2010) argues that the definition of heavy metals should be more unequivocal and that it should be based on the periodic system of elements than how it is commonly defined. In this context, Appenroth (2010) defines heavy metals by the following groups of the periodic table of elements: (1) all transition elements, except La and Ac (transition metals), (2) rare earth elements, subdivided in the series of lanthanides and the series of actinides including La and Ac themselves (tare earth metals), and (3) a heterogenous group of elements including the metal Bi, the amphoterous oxides forming elements Al, Ga, In, Tl, Sn, Pb, Sb and Po, and the metalloids Ge, As and Те (= Lead-group elements as termed by Appenroth, 2010).
It is noted also that many other scholars define heavy metals as elements with atomic density greater than 6 g/cm3 (Gardea-Torresdey et al., 2005; Akpor and Muchie, 2010), i.e., density of five times or more greater than that of water (Asati et al., 2016) or are conventionally defined as elements with metallic properties and an atomic number greater than 20 (Tangahu et al.,
2011) and have serious implications on human life due to their acute and long-term toxicity (Ndlovu et al., 2013). In fact any toxic metal may be called a heavy metal, irrespective of its atomic mass or density (Singh et al., 2011). However, Appenroth (2010) also argues that heavy metals are not toxic per se, but only when a certain threshold of internal concentrations is exceeded. In fact, heavy metals are intrinsic components of the environment which are either essential or non-essential (Asati et al., 2016). The eight most common heavy metal pollutants listed by the Environment Protection Agency (EPA) are: As, Cd, Cr, Cu, Hg, Ni, Pb and Zn (Athar and Vohora, 2001; Khayatzadeh and Abbasi, 2010).
Effect of Heavy Metals on Plants
The influence of heavy metals on plants and their metabolic activities has been studied extensively (Cheng, 2003; Jiwan and Ajay, 2011; Asati et al.,
2016). The toxicity of heavy metals to plants varies with plant species, specific metal, concentration of the metal, chemical form of the metal, soil composition and prevailing pH (Asati et al., 2016). In other words, high concentrations and/or certain mixtures of heavy metals in plant tissues can affect plant growth in different manners (Gardea-Torresdey et al., 2005). However, many heavy metals are considered to be essential for plant growth (Asati et al., 2016). For example, Cu and Zn can either serve as cofactor and activators of enzyme reactions (Mildvan, 1970; Asati et al., 2016). Some of the heavy metals such as As, Cd, Hg, Pb or Se are not essential for plant growth, since they do not perform any known physiological function in plants (Jiwan and Ajay, 2011). Other heavy metals such as Co, Fe, Mn, Mo, Ni and V are essential in minute quantities, but become harmful in excessive amounts (Asati et al., 2016). Specifically, if plentiful amounts of heavy metals are accumulated in plants, they adversely affect the absorption and transport of essential elements, disturb the metabolism and have an impact on growth and reproduction (Xu and Shi 2000; Cheng, 2003). In general, plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage and disturbance of cellular ionic homeostasis (Yadav, 2010; Singh et al., 2011), thus disrupting the physiology and morphology of plants (Gardea-Torresdey et al., 2005). Table 5.2 is a summary of the main undesirable effects of some of the heavy metals on plants (Loneragan, 1988; Elamin and Wilcox, 1986; Marin et al., 1993; Wu, 1994; Barrachina et al., 1995; Cox et al., 1996; Sinha et al., 1997; Abedin et al., 2002; Gardea-Torresdey et al., 2005; Jayakumar et al., 2007; Solomon, 2008; Li et al., 2009; Akpor and Muchie, 2010; Yadav, 2010; Asati et al., 2016).