General Aspects/Case Studies on Sources and Bioremediation Mechanisms of Metal(loid)s 149

depend on pH, as a rise in pH attributes to a –ve charge on the adsorbent’s surface (Martı́nez-

Villegas et al. 2004). Pb precipitates to form hydroxide ions and structures hydroxyl species which is

more strongly bound in comparison to free metal ions (Martı́nez-Villegas et al. 2004). Multinuclear

metal-hydroxyl species in aqueous forms can precipitate metal hydroxyl in homogenized solutions

(Martınez-Villegas et al. 2004, Park et al. 2011).

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Components of the soil, such as organic matter, silicate clay, manganese oxides, iron and

aluminium, influence the metal(loid)s’ adsorption process (Merdy et al. 2009). These soil components

form inorganic and organic complexes with metal(oid)s (Bolan et al. 2003). Surface functional

groups form semi-covalent bonds with dissolved ions (Bradl 2004). The rise in pH dissociates H⁺ ions

from functional groups like phenolic, carboxyl, carbonyl and hydroxyl, increasing the metal cation’s

affinity (Bolan et al. 2003). The soil’s exterior surface, which has a number of hydroxyl groups,

creates stable surface complexes with heavy metalloids (Hanlie et al. 2001). Heavy metalloids form

two types of complexes with the soil: When water molecules are present between the soil’s surface

and the functional group in an outer-sphere complex, metalloids are directly attached to the functional

groups of the soil (Hanlie et al. 2001). Complexes within the inner sphere are typically more stable

than those outside (Bradl 2004). Natural ligands (fulvic, humic acid) and anthropogenic ligands

(nitrile tri-acetic acid and EDTA) form complexes with heavy metals (Bradl 2004). Phytoextraction

technology is used for the extraction of heavy metalloids by enhancing the mobility of heavy

metal(loid)-EDTA or NTA complexes (Meers et al. 2009). Numerous variables, including the type

of soil, affect the formation of metal(loid)-organic complexes, dominant cations, temperature, soil

solution pH, and ionic strength (Luo et al. 2010). pH governs negative charge formation on surface

functional groups (Yang et al. 2006). It has been noted that the soil’s constituents also contribute to

the formation of metalloid complexes (Bradl 2004). Fine particle soil has more active surface sites;

therefore, it has a high retention potential for the metal(loid) (Bradl 2004). Adsorption of metalloids

such as As(V) depends on the mineral surface characteristics like soil having low oxide content

adsorption property has a very negligible effect of increasing pH (Smith et al. 1999).

9.3.2 Methylation/Demethylation

Toxic metal(loid)s are removed by methylation through biological methods by transforming

(e.g., Hg) into methyl derivatives, which get removed further by the volatilization process

(Frankenberger and Losi 1995). Se, Hg and As methylated derivatives are acquired from biological

and chemical mechanisms. These mechanisms may reduce their toxicity by changing their volatility,

solubility and mobility. Both biological and chemical mechanisms can result in the methylation of

metal(loids) (Bolan et al. 2014). Biomethylation is a dominant process found in both aquatic and

soil environments. Biomethylation reduces toxicity by excreting methylated compounds from cells,

and are frequently volatile, e.g., organic arsenic. Researchers have categorized methylation into

two groups: fission-methylation and trans-methylation (Thayer and Brinckman 1982). The fission

of a chemical (methyl source) is known as fission methylation, not always having a methyl group

in order to completely get rid of a compound like formic acid (Bolan et al. 2014). Subsequently,

another substance that has been reduced to a methyl group captures the fission molecule. Unlike

trans-methylation, which includes the exchange of methyl groups between sources (donors) (methyl

acceptor). Microorganisms are active methylators in soil and sediments. For abiotic and biotic

methylation in sediment and soil, organic matter serves as a source of methyl-donor. The methylation

of Hg in sediments is influenced by organic matter and alternative electron acceptors (Martın-

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Doimeadios et al. 2004). A study has shown that methylated As species result in the breakdown of

eliminating waste arsenicals or cellular organ arsenicals complex (Li et al. 2009).

Under anaerobic and aerobic conditions, methylation of Hg occurs (Martı́n-Doimeadios et al.

2004) and in undisturbed lake sediments under anaerobic conditions, significantly higher Hg

methylation was observed (Martın-Doimeadios et al. 2004). Under these environments, Hg(II) ions

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are methylated biologically to produce monomethyl and dimethyl compounds, which are extremely