Bioavailability and biodegradation of polycyclic aromatic hydrocarbons

Contaminant bioavailability plays an influential role in the efficacy of polycyclic aromatic hydrocarbon biodegradation.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants arising from the incomplete combustion of organic material. Both anthropogenic (e.g. processing and combustion of fossil fuels, waste incineration) and geogenic processes (e.g. fires, volcanoes) contribute to the burden of PAHs in environmental matrices¹. The concern regarding the presence of PAHs in the environment and their potential to exert toxic, mutagenic and carcinogenic effects² has led to the development of a number of physical, chemical and biological techniques for the remediation of PAH-contaminated soil. Bioremediation is considered a ‘green’ technology for the remediation of PAH-contaminated soil; however, its efficacy is dependent on a number of variables including the presence and activity of PAH degrading microorganisms (e.g. genera of Burkholderia, Mycobacterium, Pseudomonas, Sphingomonas, Stenotrophomonas), physico-chemical properties of the PAHs (which will influence their biodegradability) and environmental parameters including the availability of essential nutrients and oxygen, soil pH, moisture and temperature³. Another parameter that is influential for bioremediation success is PAH bioavailability. In the context of PAH biodegradation, bioavailability refers to the fraction of the total soil-bound PAH concentration that is desorbable from the soil matrix and is therefore potentially available for biodegradation⁴.

Following their entry into the soil environment, PAHs may diffuse and be occluded in soil micropores⁴, which limits their propensity for desorption and therefore their bioavailability for biodegradation. However, the predominant mechanism for PAH retention within the soil matrix, and the reduction in PAH bioavailability, is via sorption to organic carbon (OC)⁵. PAH retention is influenced by the nature of the OC (i.e. amorphous or rubbery versus condensed or glassy) in addition to the octanol-water partitioning co-efficient (K_ow) of the PAH. The interaction between PAHs and amorphous domains constitutes a rapid sorption phase but is susceptible to rapid desorption while association with condensed OC results in slow sorption-desorption behaviour⁵. As suggested by Semple et al.⁴, PAH bioavailability may be estimated using methodologies that quantify the rapidly and slowly desorbable domains (e.g. non-exhaustive extraction methods) as these fractions represent PAHs that have the potential to partition into soil solution and be available for biodegradation (Figure 1).

Figure 1. Schematic diagram detailing the relationship between the rates of PAH desorption and biodegradation. Phase A represents rapid PAH biodegradation where PAH removal is limited by microbial degradation kinetics and not the desorption of PAHs from the soil matrix (i.e. the rapidly desorbable PAH fraction from amorphous organic carbon). In phase, B, the rate of PAH biodegradation is limited by the slow desorption of PAHs from the soil matrix (i.e. PAH desorption from condensed organic carbon) while Phase C represents the sequestered or unavailable PAH fraction.

A non-exhaustive extraction method, utilising hydroxypropyl-β-cyclodextrin (HP-β-CD), has been shown to provide an estimate of the desorbable PAH fraction from contaminated soil that may therefore be available for biodegradation^6–9. The methodology, albeit simplistic, involves extraction of PAH-contaminated soil with HP-β-CD (40 mM) at a soil:solution ratio of 1:20 for 20 h. Following extraction, soils are retrieved via centrifugation, dried and the residual PAH concentration determined following ‘exhaustive’ extraction and GC / HPLC analysis. The desorbable fraction is calculated by the difference between the initial PAH concentration and the residual PAH concentration following HP-β-CD extraction. A number of studies have shown the correlation between PAH bioavailability estimates, utilising HP-β-CD extraction, and PAH biodegradation/mineralisation, utilising soil microcosms^6–9. This suggests that non-exhaustive extraction methods, such as HP-β-CD, may be utilised to predict the endpoints of PAH biodegradation based on bioavailability.

The slope of the bioavailability-biodegradability relationship may, however, vary for individual PAHs due to differences in their physico-chemical properties which will influence desorption and biodegradability (Figure 2). For example, a number of researchers have determined that the slope of the bioavailability-biodegradability relationship for three-ring PAHs is close to 1 (0.85–1.01), indicating the similarity of HP-β-CD extraction to remove low molecular weight PAHs to the same extent as biodegradation. However, for five-ring compounds, the slope of the bioavailability-biodegradability relationship may range up to 1.60 indicating that larger residual PAH fractions remain in the soil following biodegradation compared with HP-β-CD extraction. Although a disparity exists between PAH bioavailability and biodegradability, as a consequence high molecular weight (HMW) PAH biodegradation limitations (i.e. high activation energy, unfavourable Gibbs free energy and slow transport across cell membranes), the linearity of the bioavailability-biodegradability relationship suggests that it may be utilised for predicting the extent of HMW PAH biodegradation. As a consequence, the assessment of PAH bioavailability (using HP-β-CD extraction) and the prediction of PAH bioremediation efficacy, has the potential to be used to screen the appropriateness of bioremediation strategies prior to on-site implementation. This would benefit industry by providing a simple, rapid and inexpensive assay for determining the endpoints of PAH bioremediation.

Figure 2. Relationship between the residual phenanthrene (■) and benzo[a]pyrene (□) concentration following 3 months of enhanced natural attenuation of PAH-contaminated soil (n = 15) and the residual concentration following HP-β-CD extraction.