Metalloids transformations

PHYSICOCHEMICAL-BIOLOGICAL INTERACTIONS ON METAL AND METALLOID TRANSFORMATIONS IN SOILS AN OVERVIEW... 

FIGURE 12.2 Soil redox (mV) condition inflnencing metal and metalloid transformations. 

A cardinal issne is the species of the metal or metalloid that is examined. Metals snch as mercury or tin are methylated from cationic Hg + or Sn", whereas the metalloids are transformed from the oxyanions of As, Sb, Se, or Te. The classical Challenger mechanism that involves seqnential reductions and methylations is well established, at least for fungal methylation of the oxyanions of As (Bentley and Chasteen 2002), and Se—and is assumed to be—for Te (Chasteen and Bentley 2003). Methylation may take place under aerobic conditions for fungi or anaerobic conditions for bacteria. 

Metalls and metalloids are characterized by special ecochemical features. They are not biodegradable, but undergo a biochemical cycle during which transformations into more or less toxic species occur. They are accumulated by organisms and cause increased toxic effects in mammals and man after long term exposure [55]. 

A heterogeneous natural system such as the subsurface contains a variety of solid surfaces and dissolved constituents that can catalyze transformation reactions of contaminants. In addition to catalytically induced oxidation of synthetic organic pollutants, which are enhanced mainly by the presence of clay minerals, transformation of metals and metalloids occurs with the presence of catalysts such as Mn-oxides and Fe-containing minerals. These species can alter transformation pathways and rates through phase partitioning and acid-base and metal catalysis. 

In 2006, the speciation of metals and metalloids (As, Bi, Hg, Pb, Sb, Se and Sn) associated with alkyl groups and biomacromolecules in the environment was critically reviewed by Hirner.85 More than 60 species of alkylated metals and metalloids have been found in different ecosystems and terrestrial locations all over the world.85-87 These alkylated metals or metalloids are of interest due to their toxicological properties (e.g. monomethyl mercury, MMHg, which gained worldwide attention during the Minamata tragedy, and are not only known to be produced by microbial methylation within most anaerobic compartments of the environment, but also in the course of enzymatic transformation during human metabolism.85... 

The displacement of metalloid groups from naphthalene, however, is unusually slow. The rate differential between the 1- and 2-positions is little more than a factor of three for protodesilylation or protodegermyla-tion. Even more important is the failure to observe a steric acceleration for these reactions. Benkeser and Krysiak (1954) showed that the rate of protodetrimethylsilylation reaction was increased by o-methyl substituents to an extent greater than the anticipated electronic contribution. Presumably, steric strains are relieved in the transformation from trigonal to tetrahedral geometry in the transition state (de la Mare, 1958). The failure to observe this acceleration and a greater lf-N/2f-N ratio for these reactions is puzzling. 

The Michaelis-Menten equation is often employed in soil-water systems to describe kinetics of ion uptake by plant roots and microbial cells, as well as microbial degradation-transformation of organics (e.g., pesticides, industrial organics, nitrogen, sulfur, and natural organics) and oxidation or reduction of metals or metalloids. Derivation of the Michaelis-Menten equation(s) is demonstrated below. 

Phytovolatilization involves the use of plants and plant-associated soil microbes to take up contaminants from the soil, transform them into volatile forms, and release them into the atmosphere (Lin, 2008). Phytovolatilization occurs as growing trees and other plants take up water and the organic and inorganic contaminants. Metalloids, such as selenium. As, and tin, can be methylated to volatile compounds or mercury that can be biologically transformed to elemental Hg. Phytovolatilization has been primarily used for the removal of mercury and selenium. 

Gadd, G. M. Sayer, J. A. (2000). Fungal transformations of metals and metalloids. In Environmental Microbe-Metal Interactions, ed. D. R. Lovley. Washington, DC American Society for Microbiology, pp. 237-56. 

Although removal of organic and microbiological pollutants from waters has been thoroughly studied, less attention has been paid to the transformation of metal or metalloid ions in species of lower toxicity or more easily isolated. Metals in their various oxidation states have infinite lifetimes, and chemical or biological treatments present severe restrictions or are economically prohibitive. Removal of these species is carried out, generally, by precipitation, electrolysis, chemical oxidation, adsorption, or chelation, all of them presenting drawbacks. 

In mechanism (c), oxidative transformation of the metal species takes place by holes or hydroxyl radicals (or other reactive oxygen species, ROS) attack (Figure 3). This occurs according to reaction (12) when the oxidation of the metal or metalloid to a higher oxidation state is thermodynamically possible (cases of Pb(II), Mn(II), T1(I), and As(III)). 

A cross-coupling reaction can be partially defined by equation (1), where Nu is a carbon (or heteroatom) nucleophile see Nucleophile), R X is an electrophilic substrate, X is a halogen or other appropriate leaving group, and M is a metal or metalloid. At first glance, it would appear that simple nucleophihc substitution reactions should fall under this definition. However, what makes the cross-coupling chemistry special is its ability to perform transformations that cannot be accomplished with simple substitution chemistry. 

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