Geoscience Reference
In-Depth Information
In addition to the environmental pH and type of hydrolysis, contaminant
hydrolysis in sediments is controlled by the properties of the contaminant molecule
and the sediment constituents. In natural sediment, it is difficult to determine if
hydrolysis is a biologically mediated or an abiotic reaction. A simple test consists
of measuring the contaminant disappearance rate in the sediment and in distilled
water, with a pH adjusted to that of the sediment. If the two disappearance rates are
similar, the process can be assumed to be abiotic, while if the rate is greater in the
sediment system, the hydrolysis is biologically mediated.
In a sediment system, the hydrolysis rate constant of an organic contaminant is
affected by its retention and release with the solid phase. Wolfe (
1989
) proposed
the hydrolysis mechanism shown in Fig.
13.4
, where P is the organic compound, S
is the sediment, P:S is the compound in the sorbed phase, k
1
and k
-1
are the
sorption and desorption rate constants, respectively, and k
w
and k
s
are the
hydrolysis rate constants. In this proposed model, sorption of the compound to the
sediment organic carbon is by a hydrophobic mechanism, described by a partition
coefficient. The organic matrix can be a reactive or nonreactive sink, as a function
of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe
1983
,
1985
; Burkhard and Guth
1981
), using different organic compounds, show
that hydrolysis is retarded in the solid-associated phase, while alkaline and neutral
hydrolysis is unaffected and acid hydrolysis is accelerated.
Based on these results and on additional publications (e.g., Konrad and Chester
1969
; Armstrong and Konrad
1974
), Wolfe et al. (
1990
) suggested that the ten-
dency of an organic contaminant to hydrolyze in sediments is influenced by the
proximity of the molecule to the solid surface in the interstitial water and the direct
effect of solid particles on the susceptibility of adsorbed toxic organic molecules to
hydrolyze. Because abiotic hydrolysis of organic contaminants in the sediment
liquid phase, in many cases, exhibits the same degradation products as those in
clear water, it might be suggested that the solid phase in a sediment system alters
only the rate of hydrolysis, not the reaction pattern. Some constituents of the
sediment solid phase, however, may enhance hydrolysis of organic contaminants
adsorbed on their surfaces. For example, in heterogeneous hydrolysis of nonionic
pesticides, significant participation by acid groups belonging to solid phase organic
matter was observed.
13.2.2 Redox State and Reactions
Redox activity in sediments may be defined by an equilibrium approach, consid-
ering that the Eh of an aqueous system is bound in the upper boundary by the
oxidation of water and in the lower boundary by reduction of water. The term Eh
is defined as the redox potential relative to the standard hydrogen half-cel
l (see
Sect.
2.2.2
). Measurements of Eh in sediments of ponds and rivers show that the Eh
stabilizes at a depth of about 1 cm and remains stable to a depth of 5-6 cm (Wolfe
et al.
1986
). Sediments are strongly electron buffered, which is shown by titration
