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() Assessment of Bioavailability of PAHs in Contaminated Sediments Using XAD-2 Assisted Desorption.

Tabak, H1, Lei, L2, Suidan, M2, 1 US EPA, OH, USA2 University of Cincinnati

ABSTRACT- In the bioremediation of soils/sediments contaminated with polycyclic aromatic hydrocarbons (PAHs) it is imperative to determine the fraction of the PAHs that is amenable to remediation. For example, what fraction_CR__LF_of the PAHs is available to the indigenous microorganisms, i.e._CR__LF_bioavailable?. The bioavailability of PAHs varies with the contamination_CR__LF_history and the characteristics of soils/sediments. Research was_CR__LF_undertaken to examine the feasibility of using adsorbent-assisted desorption of PAHs for assessing their bioavailability in contaminated sediments containing aged PAHs. The sediment was collected from East River, NY, near Rikersís Island and contaminated with 2-ring to 6-ring PAHs. The kinetics of desorption of PAHs from sediment were studied using abiotic sediment-seawater slurry systems, where the nonionic polymeric adsorbent XAD-2 was added in various concentrations to the sediment /water slurry to ensure that sufficient adsorption capacoty of the XAD is provided to effect the maximum rate and extent of desorption of PAHs from the sediment. By acting as an infinite PAH sink and by maintaining a constantly low PAH concentration in the aqueous phase, XAD is used to maximize the driving force for the PAHs to desorb from_CR__LF_sediment particles without changing the sediment-aqueous phase_CR__LF_partitioning coefficient.. Batch studies on the microbial activity in_CR__LF_the sediment slurry systems were also undertaken under aerobic_CR__LF_conditions to investigate the potential for PAH biodegradation by the_CR__LF_indigenous microbial consortia. The experiments were considered_CR__LF_complete when PAH concentration levels in the sediment ceased to decrease. A considerable residual level of PAHs was detected at the end of the experiments in both the biodegradation and XAD assisted desorption studies. This is attributed to the fact that a substantial fraction of PAHs is irreversibly sorbed onto the sedimemnt and thus unavailable for either biodegradation or desorption activity. All 2-ring, 3-ring and_CR__LF_4-ring PAHs exhibited significant biodegradation under aerobic_CR__LF_conditions, with their residual levels in two studies closely related to each other (the correlation coefficient was 0.94). 5-ring and 6-ring PAHs showed either marginal or no biodegradation activity at all. This trend suggests that these higher ring PAHs might become available but their recalcitrance to biodegradation by the indigenous microbiota limited the biodegradation activity under the experimental conditions used. The adsorption of the desorbed PAHs onto XAD resin was a much_CR__LF_faster process than microbial degradation, since the desorption_CR__LF_equlibrium was attainable in just 2 weeks, while it took 24 weeks for all PAHs to attain maximum biodegradation. Overall, the results indicate that XAD-assisted desorption could be used to assess the bioavailability and the biodegradation potential of most PAHs in a timely manner, as long as the recalcitrance of PAHs is not the limiting factor in bioremediation of the contaminated sediments. Because XAD-assisted desorption does not change the sediment-aqueous phase distribution coefficients of the contaminants, this process could evaluate the_CR__LF_bioavailability of aged PAHs representatively in the contaminated_CR__LF_sediments and it can also be applied to other contaminant-matrix_CR__LF_circumstances. To our knowledge, this is the first reported_CR__LF_representative chemical measurement of bioavailability which does not_CR__LF_require exhaustive search or extraction parameters for every specific_CR__LF_contaminant-matrix pair.

Key words: bioavailability , PAHs, biodegradation

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