Bioavailability of Polycyclic Aromatic Hydrocarbons in Soils and Sediments

Polycyclic aromatic hydrocarbons (PAHs) constitute a group of priority pollutants which are of increasing environmental concern because of their adverse effects on humans, animals, and plants. Soils and sediments generally serve as a sink for PAHs, which leads to the accumulation of PAHs at contaminated sites. In the last decade, bioremediation has been frequently used for the clean-up of such contaminated sites. However, despite the common use and cost-effectiveness of bioremediation, it is generally observed that a residual fraction remains undegraded even when optimal biodegradation conditions have been provided. In many cases the recalcitrance of this residual fraction is caused by a limited bioavailability of PAHs. The present thesis focuses on the development of simple and rapid laboratory methods for the prediction of PAH bioavailability. As an integrated part of this study, it was aimed to expand the current knowledge on the structure of amorphous and condensed soil/sediment organic matter (SOM) domains. It is believed that PAHs sorbed in amorphous domains are readily bioavailable, while PAHs sorbed in condensed domains are poorly bioavailable. Three different methods were investigated for the prediction of PAH bioavailability: persulfate oxidation, cyclodextrin extraction, and surfactant extraction. Persulfate oxidation appeared to be a good and rapid method for the prediction of PAH bioavailability. It was demonstrated that a 3 hour oxidation at 70 ^o C was sufficient for the removal of all bioavailable PAHs. The oxidation method was successfully validated in a study with 14 historically contaminated soil and sediment samples. Cyclodextrin extraction and surfactant extraction were investigated in a study with two sediment samples, using hydroxypropyl-b-cyclodextrin and Triton X-100 (surfactant) as model compounds. It was demonstrated that hydroxypropyl-β-cyclodextrin extracted primarily readily bioavailable PAHs, while Triton X-100 extracted both readily and poorly bioavailable PAHs. Moreover, hydroxypropyl-β-cyclodextrin did not affect the biodegradation of PAHs, while Triton X-100 enhanced the degradation of low molecular weight PAHs. Altogether, it may be concluded that persulfate oxidation currently provides the most rapid validated method for the prediction of PAH bioavailability in soils and sediments. To study the composition of amorphous and condensed SOM domains, two different approaches were followed: (i) samples were subjected to persulfate oxidation to remove amorphous SOM, before and after which the composition of SOM was studied by thermogravimetric analysis, pyrolysis-GC/MS, and CPMAS 13C-NMR; (ii) samples were split in two parts, one part was bioremediated to remove bioavailable PAHs, and both the bioremediated and non-bioremediated part were subjected to 9 different chemical treatments with a known effect on SOM structure. Before and after chemical treatment PAH concentrations and PAH bioavailability were measured. The two approaches led to the following general conclusions on the structure of amorphous and condensed SOM domains: (1) Condensed SOM is less polar than amorphous SOM. (2) Labile components like carbohydrates, peptides, fatty acids, and free alkanes are likely to be associated with the amorphous SOM domain. (3) There is no clear relationship between aromaticity and the degree of SOM condensation. (4) Condensed SOM is situated in the humin fraction, indicating that it has a relatively high C content, a relatively low O content, and a relatively high degree of polymerization. (5) Humic and fulvic acids are primarily associated with amorphous SOM. (6) Coal and soot are specific condensed facies with a high affinity for PAHs. Apart from these general characteristics it appeared that the composition of the SOM domains was highly sample specific