General Aspects/Case Studies on Sources and Bioremediation Mechanisms of Metal(loid)s 153

Figure 9.1. A description of the interactions between plants and metal contamination as well as potential outcomes for the

metal contaminants (Ojuederie and Babalola 2017).

metal(oid) removal also have some shortcomings. These include the requirements for nutritional

resources, certain climatic conditions, and appropriate soil properties for normal plant growth

(Karami and Shamsuddin 2010). The most significant disadvantages of phytoextraction are the

lengthy time required, which has hampered the widespread implementation of phytoremediation.

An overview of plant-metal contaminants interaction and the possible fate of the metal contaminants

is shown in Figure 9.1.

9.5.1 Valence Reduction/Oxidation

The mechanism involved in the reduction of Se, Hg, Cr, and As, among other metal(loid)s, was

developed as a result of the oxidation/reduction processes carried out by beneficial microorganisms.

Metal(loid) speciation and mobility are influenced by redox processes. As(III) gets converted to

As(V) by microorganisms in sediments and soils (Bachate et al. 2012, Battaglia Brunet et al. 2002).

As(V) has a strong attraction for inorganic soil elements, it gets immobilized following its oxidation.

In well-drained soils, As(V) is the dominant form of As, whereas in poorly drained soils, As(III)

predominates, however arsine gas (H2As) [As(0)] and elemental arsenic can also be found. In most

–

cases, the breakdown of organic materials by bacteria, the reduction is followed by the role of SO4

as the terminal e^- acceptor, and then reduction, mediates the reduction and methylation processes

in sediments (Kim et al. 2002). In the instance of Cr, its mobility and bioavailability improve with

its oxidation into Cr(VI). Oxidizing agents like Mn(IV), Fe(III) to a small extent, while the Cr(VI)

- Cr(III) reduction is mediated by mechanisms that are both abiotic and biotic (Choppala et al.

2015). The settings having an accessible available electron source Fe(II), chromate Cr(VI) can get

reduced into Cr(III). When organic fraction serves as an e^- donor, reduction via microbial Cr(VI) is

boosted, while a marked enhancement of Cr(VI) reduction can be observed under acidic conditions

(Choppala et al. 2015, Hsu et al. 2009). The metal(loid)s are usually reduced rather than oxidized

in most biological systems. Chemical reductants like sulfide or hydroxylamine, or glutathione

reductase biochemically reduce Se (Zhang et al. 2004). Microorganisms have a major bearing in

converting the reactive Hg(II) species into its non-reactive counterpart Hg(0), which is susceptible

to losses by volatilization. Mercuric reductases have been known to reduce Hg(II) into Hg(0), and

the bacteria Shewanella oneidensis, which carries out dissimilatory metal(loid) reduction, has been

demonstrated to reduce Hg(II) into Hg(0) when electron donors are present (Wiatrowski et al. 2006).