| Microorganisms present in a solution-sorbent system can affect the mass balance of a
pollutant by degrading the pollutant in the solution. A recent study suggested that once a
biofilm is present on a sorbent then diffusion kinetics through this biofilm should be
considered. In the present study, the effect of the presence of microorganisms in a
solution-sorbent system is studied through the use of experimental results and a model
simulating the coupled processes of diffusion-limited-sorption and biodegradation. Three
types of reactors were employed, in triplicates. One reactor type (NSA) contained 8g of
sediment and 100 μg/L phenanthrene in 120 mL synthetic groundwater, the other
reactor type (SA) contained sediment, phenanthrene in synthetic ground water and
200 mg/L sodium azide as biocide, and the third reactor type (Blank) contained
only phenanthrene in synthetic groundwater and sodium azide. All solutions were
sterilized before mixed with phenanthrene and/or the sediment. The solution phase
measurements were performed under aseptic conditions frequently (every 1-10 days) up to
90 days. Batch tests with pulverized sediment and different phenanthrene initial
concentrations were performed to determine the phenanthrene sorption equilibrium
constants. Blank reactors demonstrated that no significant losses of phenanthrene
were observed when no sediment was added to the reactor. Plate counting with
bacteriological agar containing only phenanthrene as the carbon source revealed that the
numbers of microorganisms capable of degrading phenanthrene were significantly
higher in NSA reactors (thousand colonies per 100 μL) compared to SA reactors
(less than hundred colonies per 100 μL). Using the Freundlich isotherm model, the
following constants were obtained through the equilibrium batch tests: Kfr = 30
L/Kg and N=0.73. A numerical model (INTRAPAR) was used to simulate diffusion
limited non-linear sorption in order to determine the intraparticle diffusion coefficient
(Da=10-11 cm2/s) for the kinetic results from the SA reactors. This intraparticle diffusion
coefficient value is in agreement with values determined in the literature for this
kind of sediments. The SA reactor results were also simulated with the model that
couples diffusion limited sorption and biodegradation (BIO-INTRAPAR) assuming no
biodegradation. This was done in order to determine the appropriate mass transfer term
included in the model that was equal to kf = 3 10-7 cm/s. Then, all these constants
(Kfr, N, kf, Da) determined from independent experiments were used as model
inputs for the coupled model (BIO-INTRAPAR) to determine the biodegradation
constants for the results from the NSA reactors. Later data in the kinetic curve
obtained for reactors NSA (i.e. for 25-90 days) were successfully predicted by
the input constants that were determined independently and one of the different
biodegradation rate constant values tested (k1 = 0.0003 1/hr). This biodegradation rate
constant results in a half-life time of 96 days that is within the range of 28-126
days found in the literature for phenanthrene. Sensitivity analyses presented that
changing the diffusion coefficient or the biodegradation rate did not result into
better predictions for the early kinetic data (i.e. for 4.5-25 days). Only changing the
mass transfer coefficient (kf= 5 10-8 cm/s) resulted in successful fit of the early
kinetic data and the rest of the kinetic curve (i.e. for 4.5-90 days). This study can be
considered as a first indication that the presence of microorganisms not only results
in the biodegradation of pollutants in solution but can also cause a decrease of
the mass transfer of the pollutants from the solution to the sorbent phase. If this
mass transfer decrease is not taken into consideration then the model predicts lower
concentrations in the solution at early times than those concentrations actually measured. |