Relation between mass-transfer and biodegradation of hydrophobic pollutants in soil

The Dutch soil is contaminated at numerous locations with toxic organic compounds, such as polycyclic aromatic hydrocarbons (PAHs). To reduce the risks at these sites bioremediation can be applied as an alternative for the more destructive and energy intensive physicochemical soil sanitation techniques. During bioremediation microorganisms convert pollutants to less harmful compounds. Implementation of bioremediation is, however, limited because the strongly hydrophobic PAHs possess low water-solubilities and interact with soil organic matter. This results in a low mobility of PAHs in the soil as well as a low rate at which they become available for microbiological transformation. This thesis describes a study on the mutual influence of mass transfer and biodegradation processes which has been performed to gain a better insight in the mechanisms causing the persistence of PAHs in soil.To achieve this goal, well-defined experimental systems have been applied to obtain reproducible results. In these systems, PAHs were used in defined solid states, either solid phase PAHs immobilized in stainless steel cups with a specified surface area, or PAHs adsorbed on chromatographic porous spheres (Amberlite resins) of hydrophobic material.The influence of mixing on the dissolution rate and biodegradation rate of solid phase naphthalene has been investigated with the PAH immobilized in stainless steel cups in stirred fermentors (Chapter 2). Results of combined dissolution and degradation experiments have shown the necessity of quantification of the hydrodynamic flow conditions when studying the conversion of poorly water-soluble compounds. When the potential biodegradation rate of the bacterial population present exceeds the maximum dissolution rate, mass-transfer becomes limiting for naphthalene conversion. The maximum dissolution rate is strongly related to the extent of mixing and an increase of the latter results in an increase of the PAH biodegradation rate under such circumstances.During the above-mentioned experiments biofilm formation by the applied bacterial strain ( Pseudomonas 8909N) has been observed at the naphthalene-water interface. On the basis of relatively long-term experiments, in which biofilms have been grown in chemostats, it was shown that the presence of a biofilm on the solid-liquid interface resulted in a 90% reduction of the biological availability of the solid naphthalene (Chapter 3).The low solubilities of PAHs in water result in relatively low dissolution rates. The apparent solubility of hydrophobic compounds can be increased by the addition of surface active chemicals (surfactants) resulting in higher mass-transfer rates. This implies that the bioavailability can be increased by the use of surfactants. It was shown in Chapter 4 that the model system used in the foregoing chapters is very suitable for the investigation and modeling of the influence of surfactant addition on the dissolution and biodegradation of (initially) solid naphthalene. Results indicate that besides the increase in the apparent solubility due to the partition of naphthalene in micelles (aggregates of surfactant molecules), also the diffusion coefficient of micelles is a determining factor for the efficiency of surfactant addition. The flux of PAH to the bulk liquid phase is positively correlated with an the partitioning of the PAH in the micelles and the micellar diffusion coefficients.When PAHs are present as diffuse soil contaminants, they will exist in a sorbed physical state. To gain insight in the influence of sorption processes and mass transfer in porous soil aggregates on the biodegradation of PAHs, chromatographic material (Amberlite XAD4 and XAD7) is used as a model soil system (Chapter 5). A mathematical model was developed that simultaneously accounted for nonlinear sorption, for internal and external mass transfer, and for nonlinear bacterial transformation kinetics. This model was checked for its applicability on the basis of experiments with naphthalene as a model pollutant. By variation of the mixing conditions in the reactors it was shown that characterization of the external diffusion limitations is necessary in the system used. The fate of naphthalene in the porous particles could be predicted adequately by the mechanistic model. The crucial model parameters that determine the mass transfer of PAHs in soil aggregates are: the particle size, the sorption coefficient, and the effective diffusion coefficient. Nonlinear sorption results in relatively low desorption rates compared to linear sorption and, therefore, a longer clean-up period is necessary.To check whether the model developed in Chapter 5 could be used to describe the fate of PAHs in real soil aggregates, experiments were performed with Koopveen soil (Chapter 6). Aggregates with three different size fractions but with equal organic matter content (± 30%) were produced from this peat soil and proved to be stable during dynamic desorption and biodegradation experiments. These aggregates were artificially contaminated with either naphthalene or phenanthrene. The experimental results indicated that solid PAHs were probably present in the two lower size fractions due to the contamination method. However, mass transfer and biodegradation processes could be adequately modeled in the case of the largest naphthalene contaminated fraction. Results with similar soil material, that were fitted in earlier research with an empirical compartment model, could equally well be described with the current mechanistic model. The predictive capabilities of the current model are, however, superior because model parameters are related to measurable quantities.Finally, three different physical states of PAH pollutants in soil have been postulated and mass transfer models have been developed to predict the release of PAHs to an aqueous phase (Chapter 7). The effect of the different physical states of PAHs in soil on the period necessary for a certain degree of bioremediation are calculated by coupling of the mass transfer models to a biodegradation module. It was calculated that, under mass transfer limited growth conditions, micro-organisms can effectively lower the dissolved PAH concentration and maximize the driving force for mass transfer. Therefore, simple mass transfer models can be applied to calculate bioremediation periods under these conditions.The main conclusions that are formulated on the basis of the findings in this thesis and some recommendations for future research are presented in Chapter 8. The most important conclusion is that the use of model soil systems is a very powerful tool to investigate the interactions between mass transfer and biological transformation of hydrophobic compounds. These interactions could be studied in both model systems and models could be developed that make it now possible to estimate the behavior of PAHs in soil. These models can be useful to estimate ecotoxicological risks on the basis of released quantities instead of total concentrations and to estimate the feasibility of soil bioremediation or the development of new sanitition treatments.