Bioavailability and biodegradation of polycyclic aromatic hydrocarbons

Abstract

One of the main problems in biological soil remediation is the slow or incomplete degradation of hydrophobic organic pollutants. The principal reason for this problem is the fact that these compounds bind strongly to the soil matrix or occur as a separate non- aqueous phase in the soil. As most microbiological processes take place in the water phase, transport of the polluting compound to this phase is essential for biodegradation to occur. When this transport is the limiting factor in the biodegradation process, this is termed limiting bioavailbility.This thesis deals with the effect of bioavailability on the biodegradation of polycyclic aromatic hydrocarbons (PAHs). PAHs are hydrophobic organic pollutancs that are abundantly present in contaminated soils and give raise to ervironmental concern because of their toxicity and mutagenicity. Most PAHs are degradable by microorganisms and the important biochemical aspects of the PAH-degradation hav been revealed. PAHs are nevertheless considered persistent pollutants in soil, a fac that is attributed to their limited bioavailability.The first part of the research consisted of the isolation of bacteria capable of degrading the PAHs, naphthalene, phenanthrene, and anthracene. Subsequently a number of isolated bacterial strains were grown in batch and continuous cultures to determine the most important microbial growth parameters, such as the maximum growth rate, the Monod saturation constant, and the bacterial growth yield.The effect of bioavailability on the biodegradation of PAHs was studied in two model systems: (i) crystalline PAHs and (ii) PAHs bound to a matrix.For studying the bioavailability of crystalline PAHs the results of dissolution and biodegradation experiments were compared. In the degradation experiments it was found that two phases could be observed during batch growth: an exponential growth phase, followed by a linear growth phase, in which biomass formation was limited by the availability of the PAHs. By using a model in which Monod kinetics for bacterial growth were coupled to dissolution kinetics for substrate availability, it was shown that the observed degradation rates were matched by the rates of dissolution of the PAHs to the aqueous phase. Therefore it was concluded that in this system only aqueous phase PAHs were available for bacterial uptake and that the bioavailability of the PAHs was not directly stimulated by the presence of the microorganisms.With matrix-bound PAHs desorption and biodegradation experiments were conducted. The first matrices studied were the synthetic porous resins XAD4 and XAD-7. The desorption of naphthalene from these materials was studied in batch and continuous desorption experiments. The results from these experiments could be described using a two-compartment model in which the matrix is divided in a fraction with shallow pores and one with deep pores. In biodegradation experiments with naphthalene- loaded resins the same type of batch-growth kinetics was observed as described above for crystalline substrates: exponential growth, followed by a phase in which substrate availability limits the degradation rate. By comparing the results of the desorption experiments and the biodegradation experiments it was shown that the biodegradation proceeded faster than could be explained by desorption alone. Therefore it was concluded that the bacteria had a positive effect on the bioavailability of naphthalene that was adsorbed onto the resins. This effect was not caused by the presence of bacterial excretion products.In contrast to this it was found that the biodegradation of soil-bound naphthalene and phenanthrene could be explained by degradation of PAHs present in the aqueous bulk phase only. Thus, the bioavailability of sorbed PAHs depends on the type of matrix the PAHs are sorbed onto.The second part of this thesis deals with the most widely applied solution for the problem of limited bioavailability: the application of surface-active agents or surfactants. Surfactants are molecules that usually consist of a hydrophillic and a hydrophobic part. Due to this they have a tendency to concentrate at surfaces and interfaces and to form new interfaces. There are several different ways by which surfactants may increase the bioavailability of hydrophobic compounds in soil:- solubilization in the aqueous phase by the presence of micelles, aggregates of 20-200 surfactant molecules with a hydrophobic interior;- emulsification of liquid hydrocarbons in the waterphase;- facilitated transport, a term that covers several processes, such as mobilisation of pollutant present in soil pores or interaction pollutant with single surfactant molecules;Surfactants may also have a negative effect on pollutant bioavailability, for instance by the toxic effect or preferential degradation of the surfactant, or by interference with the natural interactions among microorganisms and pollutant.The effect of several nonionic surfactants on the bioavailability of PAHs was studied in the same model systems as described above: crystalline PAHs and PAHs sorbed onto a matrix.Dissolution experiments with crystalline naphthalene and phenanthrene showed that the presence of surfactants caused an increase in the apparent solubility and in the maximum dissolution rate of these PAHs. Both phenomena have an effect on the bioavailability of PAHs. Although it was found that micellar PAHs were not readily available for uptake by the bacteria, the transport of PAHs from the micelles is sufficiently fast to allow almost complete exponential growth on solubilized PAHs. The effect on the maximum dissolution rate is probably more important because this is the most relevant factor under bioavailability-limiting conditions. Addition of surfactant to cultures growing on PAH in the dissolution-limited phase resulted in an increase in the linear growth rate. This shows that for crystalline PAHs surfactants can be used to increase the bioavailabiltyFor sorbed naphthalene similar results were found. In desorption experiments it was shown that in the presence of surfactant, the partitioning of naphthalene to the waterphase as well as the maximum desorption rate was increased. Addition of surfactants to cultures growing on sorbed naphthalene in the desorption-limited phase resulted in an increase in the degradation rate. This shows that surfactants can be used for enhancing the bioavailability of sorbed PAHs.The first general conclusion from this thesis is that the bioavailability of hydrophobic pollutants in soil is a complex matter and therefore difficult to quantify. In model systems under laboratory conditions, however, it was possible to simulate the essential processes. This experimental work revealed the most important mechanisms that play a role in bioavailability limtations. Because of the large impact of bioavailability on both the performance of biological soil remediation and on the risks posed by soil contamination, it is essential that standard methods be developed which provide criteria for bioavailability. These criteria may be used to predict the results of biological soil remediation processes and may form a basis for soil quality limits in which the bioavailability of the pollutant is considered.Secondly, the application of surfactants can be concluded to be a promising option for enhancing the bioavailability of hydrophobic pollutants. In two model sytems it was shown that addition of surfactants speeded up the biological degradation of PAHs markedly and some explanations for this phenomenon have been found. However, to allow the use of surfactants as a standard technique in biological soil remediation, more insight into the complex interactions involved in the introduction of surfactants into soil is necessary