Sangchul Hwang (hwangs@uakron.edu)

and

Teresa J. Cutright, Assistant Professor (tcutright@uakron.edu)

Department of Civil Engineering,

The University of Akron,

Akron, OH 44325-3905

Biodegradation of spiked polycyclic aromatic hydrocarbons (PAHs) in the natural soils was investigated for the purpose of acquiring a better understanding of the effects of soil characteristics on PAH bioavailability. To achieve the goal, batch biodegradation studies were performed with three natural soils and phenanthrene (PHE) and pyrene (PYR). The first soil (S1) was a silty sand with 3.54% soil organic matter (SOM) and 18% clay. The second soil (S2) was a sandy lean clay comprised of 8.4% SOM and 10% clay. The last soil (S3) was a silty sand with 1.84% SOM and 9.6% clay. PHE and PYR were initially spiked with hexane to give a concentration of 100 mg/kg soil. After evaporation of hexane, two grams of soil, 20 mL of the nutrient solution, and one mL of bacteria solution (~3´ 10⁷CFU) were added to the reactors. The bacteria augmented had been acclimated to both PHE and PYR over one year, utilizing both compounds as sole carbon sources. The aerobic condition was maintained since H₂O₂ was initially added at 75 mg/L and a ~15mL air-filled headspace was provided. The control reactors in which the biocide (NaN₃) was added at 200 mg/L were run at the same time for the objective of evaluating any abiotic losses. The blank reactors containing the clean soils inoculated with the same bacteria were also run. All reactors were agitated on an orbital shaker at 125 rpm and 28± 2^oC. After the allocated contact time, the microbial growth was monitored via agar plates containing 0.3% tryptic soy broth, 1.5% agar, and the inorganic medium. The solution phase PHE and PYR was detected by fluorescence spectrometry using l _ex = 335 nm and l _em = 379 nm for PYR and l _ex = 253 nm and l _em = 365 nm for PHE.

The indigenous bacterial activity was negligible, if any. The preliminary results indicated that PYR was very recalcitrant to biodegradation in S1 soil, although the desorbed PYR was maintained over 200 ug/L during the experiment. The bacterial growth reached ~4´ 10⁷CFU/mL and slowly decreased. For S2 soil, PYR was hardly desorbed from the onset of experiment (i.e., 135 ug/L). However, since the SOM content was enough to provide carbon source, the bacterial growth achieved 3´ 10⁸CFU/mL at day 1. This was verified in the blank experiment where the clean S2 soil was augmented with the bacteria: the microbial growth reached greater than 5´ 10⁷CFU/mL at day 1. For S3 soil, PYR degradation was moderate and the bacterial growth was last at ~1.4´ 10⁷CFU/mL until 7 days. PHE degradation was substantially faster and easier than PYR degradation. These phenomena were explained because PHE has ~1,000 times higher solubility than PYR, which facilitated bacterial uptake for their growth as well as PHE is much easier to be biodegraded than PYR due to its structural characteristic. Based on the results, it was concluded that the different soil type, as to the SOM and clay contents, influenced the PAH degradation, especially for the higher molecular weight PYR. The contribution of the SOM and clay minerals is under investigation.

Authors:

Sangchul Hwang, PhD candidate (Ph: 330-972-6778; email: hwangs@uakron.edu)

Teresa J. Cutright, Assistant Professor (Ph: 330-972-4935; email: tcutright@uakron.edu)