Geoscience Reference
In-Depth Information
P in the upper layer of the subsurface. These groups contain glycerol, fatty acids,
and phosphate (Sims and Pierzjinski
2005
). The P in the structure is a diester,
which is more susceptible to degradation in soils than monoesters.
Additional transformable organic P compounds found in the subsurface envi-
ronment are nucleic acids and phosphonates. Nucleic acids constitute a minor
portion of identifiable organic P in the subsurface. They are readily degraded, and
their presence in the subsurface environment is due to continuous microorganism
production rather than persistence. Phosphonates are organic P compounds with
direct C-P bonding, as opposed to the C-O-P bonding typical of most other
organo-P molecules commonly found in soils; the main compound reported to date
is 2-aminoethyl phosphonic (Newman and Tate
1980
).
Sims and Pierzjinski (
2005
) note that the balance between mineralization (due
to a biological process where plants produce enzymes and microorganisms
hydrolyze organic compounds, releasing inorganic P into solution) and immobi-
lization (conversion of inorganic P to organic P in biomass) ultimately controls P
concentration in the aqueous solution. This concentration, however, is affected by
the solution pH as well as by the properties of the solution surrounding the solid
phase. The P products produced in the various stages of microbially mediated
transformation have different properties, and their transport in the subsurface being
affected by the type of speciation involved.
Metal transformation includes two main processes: oxidation-reduction of
inorganic forms and conversion of metals to organic complex species (and the
reverse conversion of organic to inorganic forms). Microbially mediated oxida-
tions and reductions are the most typical pathway for metal transformation. Under
acidic conditions, metallic iron (Fe
0
) readily oxidizes to the ferrous state (Fe
2+
),
but at a pH greater than 5, it is oxidized to Fe
3+
. Under acidic conditions, Fe
3+
is
readily reduced. Thibacillus ferroxidant mediates this reaction in an acid envi-
ronment and derives both energy and reducing power from the reaction.
Paul and Clark (
1989
) showed that before Fe
3+
is microbiologically reduced, it
is chelated by organic compounds. During oxidation, electrons are moved through
an electron transport chain, with cytochrome c being the point of entrance into the
transport chain. Oxidation can be caused by direct involvement of enzymes or by
microorganisms that raise the redox potential or the pH. Iron reduction occurs
when ferric iron (Fe
3+
) serves as a respiratory electron acceptor or by reaction with
microbial end products such as formaldehyde or H
2
S. Microbiologically mediated
transformation, as affected by pH, also is observed in the oxidation of Mn
2+
, when
both bacteria and fungi can oxidize manganese ions only in neutral and acid
environments. Dissimilatory metal reduction bacteria can couple organic matter
oxidation to metal contaminant reduction (Lovley
1993
). Rates of these reactions
depend on the reduction potential of the solid or solution phase metal, the surface
area, and the presence of competing terminal electron acceptors.
An additional environmental factor that may affect metal contaminant trans-
formation in the subsurface is the air-water ratio. A toxic metal like mercury does
not remain in a metallic form in an anaerobic environment. Microorganisms
transform metallic mercury to methylmercury (CH
3
-Hg
+
) and dimethylmercury
