Impacts of microbial redox conditions on the phase distribution of pyrene in soil-water systems.

Variations in the soil/sediment organic matter (SOM)-hydrophobic organic contaminant (HOC) bindings upon microbially mediated redox conditions were examined. While the extractability of pyrene associated with soil declined after its biodegradation began during aerobic incubation, its variations were almost constant (+/-3.0-4.4%) during anoxic/anaerobic incubations. The dissolved organic matter released from the soil incubated under highly reduced conditions became more humified and aromatic, had a higher average molecular weight, and was more polydispersed compared to that obtained from oxic incubation, similar to the SOM alterations in the early stage of diagenesis (humification). The concentrations of pyrene in the aqueous phase increased significantly during the soil incubations under highly reduced conditions due to its favorable interaction with the altered DOM. Our results suggest that the microbially mediated redox conditions have significant impacts on SOM and should be considered for the transport, fate, bioavailability, and exposure risk of HOCs in the geo-environments.

Effects of anaerobic incubation on the desorption of polycyclic aromatic hydrocarbons from contaminated soils.

Incubation of field-contaminated soil under anaerobic conditions can lead to increased mobilization of polycyclic aromatic hydrocarbons (PAHs) into water. In the present study, we evaluated the effects of anaerobic incubation on the rate and extent of desorption of PAH from two field-contaminated soil samples. One was a highly contaminated soil from a former wood-preserving site that had not been subject to previous treatment; the other was a soil from a former manufactured-gas plant site that had been treated in an aerobic bioreactor. A two-site desorption model was applied to quantify the fast and slowly desorbing fractions of each PAH and the corresponding first-order rate constants for each fraction. For most of the PAHs, the total amount desorbed after 18 d from anaerobically incubated samples was significantly greater than that from their counterparts not subjected to anaerobic incubation, but the overall effect was modest. The rate constant corresponding to the slowly desorbing fraction (k(2)) was much higher for the samples incubated under active anaerobic conditions than that for the controls, implying anaerobic incubation had the greatest influence on the soil compartments controlling the slow release of PAHs. Anaerobic incubation had little to no effect on the rapidly desorbing fraction.

Epifluorescence microscopy and image analysis of high-level polycyclic aromatic hydrocarbon contamination in soils.

Interactions between polycyclic aromatic hydrocarbons (PAHs) and soil are an important determinant of their chemical availability and transport. Laboratory examination of microscale PAH-soil interaction is limited by the availability of methods for particle-scale observation. Inverted epifluorescence microscopy, combined with digital photography and computer image analysis, was evaluated for specificity and linearity using dissolved PAHs. A pyrene filter (excitation wavelength, 360-400 nm; emission wavelength, 450-520 nm) gave nonspecific PAH fluorescence, and bias for fluoranthene, benzo[b]fluoranthene, benzo[g,h,i]perylene, and benz[a]anthracene was quantified in comparison to that for pyrene. Concentrations ranging from 1 to 10 mM for anthracene, fluoranthene, and pyrene and from 1 to 50 mM for naphthalene produced a linear response with low interpixel variability. Liquid-phase analyses validated use of the technique for the descriptive analysis of PAH distribution in solid samples, but liquid-phase calibration was not quantitative for spiked or field-contaminated soils. The mean luminance for three field soils was proportional to the values predicted from their chemically measured concentrations and to values from spiked, aged, uncontaminated materials. Image analysis of laboratory- and field-contaminated samples determined the area distribution of fluorescent intensity and the size of fluorescent areas exceeding a threshold luminance. These qualitative descriptions of the microscale spatial distribution of PAH contamination are presented as potential endpoints for future research on biogeochemical interactions in heavily contaminated solids.