Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 380

and hydroxide portions bound 44% of the total chromium in the soil. Meanwhile, 10% and
12% of the chromium was associated with the organic and residual fractions. Rhamnolipid
was able to remove most of the exchangeable (96%) and carbonate (90%) portions and some
of the oxide and hydroxide portion (22%) but from the other fractions. This information is
important in designing the appropriate conditions for soil washing and for potential aspects
of natural attenuation in the presence of biosurfactant producing microorganisms.
Mercury can be found as Hg(II), volatile elemental mercury (Hg(0)), methyl, and dimethyl
forms. Metabolism occurs through aerobic and anaerobic mechanisms through uptake,
conversion of Hg(II) to Hg(0), methyl and dimethylmercury or to insoluble Hg(II) sulide
precipitates. Although volatilization or reduction during natural attenuation would still
render mercury mobile, Hg(II) sulides are immobile if suficient levels of sulfate and elec-
tron donors are available.
Arsenic can be found as the valence states As(0), As(II), As(III), and As(V). Forms in
the environment include As 2 S 3 , elemental As, arsenate ( AsO 3− ), arsenite ( AsO 2 − ), and other
organic forms such as trimethyl arsine and methylated arsenates. The anionic forms are
mobile and highly toxic. Microbial transformation under aerobic conditions produces
energy through oxidation of arsenite. Other mechanisms include methylation, oxidation,
and reduction under anaerobic or aerobic conditions.
Selenium, which is a micronutrient for animals, humans, plants, and some microorgan-
isms can be found naturally in four major species, selenite ( SeO 2− , IV), selenate ( SeO 2− , VI),
elemental selenium (Se(0)), and selenide (-II) (Frankenberger and Losi, 1995; Ehrlich, 1996).
Oxidation of selenium can occur under aerobic conditions, whereas selenate can be trans-
formed anaerobically to selenide or elemental selenium. Methylation of selenium detoxi-
ies selenium for the bacteria by removing the selenium from the bacteria. Immobilization
of selenate and selinite is accomplished via conversion to insoluble selenium. Due to the
many forms of selenium, selenium decontamination by microorganisms is not promising.

10.6.2.6 Oxidation-Reduction Reactions

It is useful to recall that (a) the chemical reaction process deined as oxidation refers to a
removal of electrons from the subject of interest and (b) reduction refers to the process where
the “subject (electron acceptor or oxidant )” gains electrons from an electron donor ( reduc-
tant ). By gaining electrons, a loss in positive valence by the subject of interest results and
the process is called a reduction. Oxidation-reduction (redox) reactions have been briely
discussed in Chapter 9. Biological transformation of organic chemical compounds results
from biologically mediated redox reactions. Bacteria in the soil utilize redox reactions as
a means to extract the energy required for growth. They are the catalysts for reactions
involving molecular oxygen and organic chemicals (and also soil organic matter) in the
ground. Redox reactions involve the transfer of electrons between the reactants. The activ-
ity of the electron e − in the chemical system plays a signiicant role. Reactions are directed
toward establishing a greater stability of the outermost electrons of the reactants, i.e., elec-
trons in the outermost shell of the substances involved. The link between redox reactions
and acid-base reactions is evidenced by the proton transfer that accompanies the transfer
of electrons in a redox reaction. Manahan (1990) gives the example of the loss of three
hydrogen ions that accompanies the loss of an electron by Fe(II) at pH 7 resulting in the
formation of a highly insoluble ferric hydroxide, as indicated by the following:

2+ 2+ +

Fe (H O) → Fe (OH)(H O) +H (10.2)

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