NATO/CCMS Pilot Study Evaluation of Demonstrated and ... - CLU-IN

NATO/CCMS Pilot Project on Contaminated Land and Groundwater (Phase III) January 2001
With focus on the applicability of the preparation of water soluble fractions in slow stirring batch system
the results can be summarized as follows: Once equilibrium is reached in the system a fraction of a
compound will be transferred from the NAPL phase into the aqueous phase leading to a lower
concentration in the NAPL phase. Equilibrium concentrations in the aqueous phase therefore will be
lower compared to calculations based on initial concentrations in the NAPL phase. This effect is only
relevant for relatively soluble substances like benzene and in the presence of small NAPL volumes and is
independent of the NAPLs viscosity. The relative diffusivities of the NAPL compounds govern the
dissolution kinetics in terms of mass transfer limitations within the NAPL phase. Thus, in low viscosity
NAPLs, the depletion process is controlled by diffusion within the NAPL layer of relatively soluble
substances like benzene, whereas in high viscosity NAPLs, even the dissolution of relatively insoluble
substances like Naphthalene may be diffusion-limited. With the theoretical framework presented the
mechanisms affecting the dissolution of NAPLs into the aqueous phase in slow stirring batch systems can
be quantified. The models allow us to predict the errors in equilibrium concentrations and the time frame
to reach saturation.
Dissolution of NAPL compounds in a flow through system
The objective of the second study was twofold: First the dynamic changes of NAPL-water equilibria as
the soluble compounds deplete from a complex NAPL mixture was studied. Second an easy to use model
based on Raoult’s law to predict such dissolution patterns with respect to time varying NAPL mass and
composition was developed [5].
The experimental setup consisted of a flow through vessel containing deionized water and diesel fuel
(Figure 8, section A). The resulting concentrations in the water were measured in the effluent of the
vessel. The results were compared with the calculated aqueous concentrations based on Raoult's law for
supercooled liquid solubilities. The model considers the dynamic changes of the diesel fuel / water
equilibrium due to continuous depletion of the soluble compounds from diesel fuel.
It could be shown that Raoult's law is valid during dynamic dissolution of aromatic compounds from
complex NAPL mixtures (e.g., diesel fuel) in non-disperse liquid/liquid systems (in this case the SSM).
This is true as long as a significant depletion of substances is observable. At low concentrations in the
NAPL phase non-equilibrium effects probably play a major role in the dissolution behavior, resulting in
underestimation of the aqueous concentration. However deviations at these concentration levels are not
important from a risk point of view. The quality of predictions was improved by considering time varying
NAPL mass. Although the model could be confirmed in an idealized laboratory system, it can not be
applied to complex field situations with the same accuracy. However this study provides a simple method
to assess contaminated sites on an "initial action" basis and supports the planning of long term remedial
strategies at such sites.
Biodegradation of dissolved NAPL compounds
The effluent of the flow through vessel was fed into two columns filled with quartz sand which were
operated in series [6]. The first column was operated under enhanced denitrifying conditions whereas the
second column was operated under aerobic conditions (Figure 8, section B and C). The two columns
represent two degradation zones downstream of a contamination plume under different redox conditions
as it is commonly found in contaminated aquifers. As an example of the measured BTEX and PAH
compounds observed benzene and ethylbenzene concentration curves in the effluent of the flow through
reactor (section A), the denitrifying column (section B) and the aerobic column (section C) respectively
are drawn in Figure 9. Degradation under denitrifying conditions only occurred in the case of
ethylbenzene, whereas benzene seems to be persistent to denitrification. The slight decrease of benzene
concentrations in the effluent of the denitrifying column is attributed to small amounts of oxygen intruded
into the system at the beginning of the experiment. Under aerobic conditions benzene and ethylbenzene
were rapidly degraded. Based on these results a mass balance was performed for each compound as well
as for the total amount of diesel constituents after each section of the experimental setup (Figure 8) and
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