Geoscience Reference
In-Depth Information
Fig. 13.4 Proposed two-step
hydrolysis mechanism
(Wolfe
1989
)
with chemical oxidants. Although molecular O
2
oxidizes sediments, which affects
their reduction capacity, this reaction occurs slowly (Macalady et al.
1986
).
The redox state also is defined by a kinetic approach, when the electron
acceptors are used predominantly by microorganisms found in the system. It is
difficult to define the terminal electron-accepting processes. Chapelle (
2005
)
considers that microbially mediated redox processes tend to become segregated
into discrete zones. At the sediment-water interface, oxic metabolism predomi-
nates. This oxic zone may comprise zones dominated by nitrate, manganese, or
ferric iron reduction. The redox zonation is a result of the ecology of aquatic
sediments. The reduction rate of sediment oxidation is explained by intrinsically
slow reaction rates of O
2
with reducing moieties in the sediment or by slow
diffusion of O
2
into the sediment. In anoxic sediments, oxidation of organics is
carried out in the food chain; fermentative microorganisms partially oxidize
organic matter with the production of fermentation products, such as acetate and
hydrogen. These fermentation products then are consumed by terminal electron-
accepting microorganisms such as Fe(III) or sulfate reducers.
Wolfe (
1989
) suggested a model to describe abiotic reduction in sediments,
where a nonreactive sorptive site and an independent reactive sorptive site are
considered. The nonreactive sorptive sink is consistent with partitioning of the
contaminant to the organic carbon matrix of the solids. The model is described by
Fig.
13.5
where P:S
0
is the compound at the reactive sorbed site; P is the com-
pound in the aqueous phase; S and S
0
are the sediments, P:S is the compound in the
non-reactive sink; k
2
,k
-2
,k
1
, and k
-1
are the sorption-desorption rate constants,
and k
c
,k
w
, and k
s
are the respective reaction rate constants. If the reaction con-
stants k
w
and k
s
are neglected, two rate-limiting situations are observed: transport
to the reactive site and reduction at the reactive site. The available kinetic data,
however, do not allow one to distinguish between the two mechanisms.
An example of a redox transformation in natural water (oxidation) and sedi-
ments (reduction) of an organ chlorinated contaminant (aldicarb insecticide) is
given in Fig.
13.6
.
Because hydrocarbon mixtures originating from petrogenic or pyrogenic resi-
dues are a major group of groundwater contaminants, which originate mainly from
human activity, these compounds are considered to illustrate transformation of
organic molecules in the subsurface environment. These hydrocarbon mixtures
generally contain a diverse group of compounds, whose behaviors, persistence,
and transformation in the subsurface and in groundwater are dissimilar. In the
partially saturated subsurface, a range of selective processes, such as solid phase
