Despite the popularity of bioremediation for treating hydrocarbon contaminated soils, it has been observed that a residual hydrocarbon fraction remains in the soil even when optimal biodegradation conditions have been provided.  As shown in Figure 1 below, total petroleum hydrocarbon (TPH) concentrations always decline rapidly initially but reach asymptotic levels towards the end of bioremediation treatment.  If residual TPH concentrations remain above regulated TPH levels (TPH_reg) it will be difficult to obtain site closure (as, for example, for Soil B in Figure 1).  It is postulated that the observed incomplete biodegradation is due to the limited availability of these residual hydrocarbons to hydrocarbon-degrading microbes.  Bioavailability limitations are thought to be especially significant in aged soils where the lipophilic contaminant molecules have had sufficient time to diffuse into soil micropores, thereby becoming inaccessible to petroleum degrading microorganisms.

Figure 1.  Typical TPH biodegradation kinetics showing relative TPH concentration over time for two hypothetical soils.

The biodegradation rate and extent of hydrophobic contaminants in aged soils is greatly affected by the complex interactions between the contaminant molecules, the soil particles, the surrounding water, and the contaminant-degrading bacteria.  As shown in Figure 2, the hydrophobic contaminant may be:  a) sorbed to soil mineral surfaces and organic matter within soil pores, b) dissolved in a non-aqueous phase liquid (NAPL) that may coat soil surfaces as a thin oily film, or c) dissolved in the water film that surrounds the soil particles.  The contaminant-degrading microorganisms may be either attached to soil or NAPL surfaces or may be freely dispersed in the soil water phase.

Figure 2.  Conceptual model of the species and processes involved in biodegradation.

As indicated above, hydrocarbon biodegradation in aged soils generally occurs in two stages.  Initially, when many hydrocarbons are freely available, the relatively high rate of biodegradation is controlled by the rate of uptake and metabolism by soil bacteria, i.e., biodegradation is reaction-rate limited.  As bioremediation proceeds and all readily available hydrocarbons have been metabolized, the rate of biodegradation slows down as it becomes controlled by the rate of hydrocarbon desorption from soils or dissolution from non-aqueous phase liquids (NAPLs), i.e., biodegradation is mass-transfer rate limited.  Limitations in contaminant bioavailability during bioremediation are generally believed to be related to the very slow rates of hydrocarbon desorption or dissolution.

Researchers at PNNL are currently addressing the following research questions: 

1. How do the properties of soil solids and the molecular structure of hydrocarbons affect the bioavailability and toxicity in aged soils?
2. How does aging affect bioavailability?
3. Is it possible to develop a simple test or assay that can be used to determine whether the biodegradation process in complex contaminant mixtures is mass-transfer or reaction-rate limited?
4. How is the shift from reaction-rate to mass-transfer rate limitation during the bioremediation process affected by the properties of the soil (e.g., particle size, soil organic matter content, pore size distribution, surface area, etc.) and the characteristics of the contaminant mixture (K_ow, total contaminant concentration, etc.)?
5. Does bioremediation reduce the toxicity (e.g., Microtox^TM) of aged soils? What is the residual toxicity of fully bioremediated soils?
6. To what extent are hydrocarbons in contaminated soils available to earth-dwelling species?  Is it possible to estimate the bioaccumulation potential of hydrophobic organic compounds with novel semipermeable membrane devices that simulate bioconcentration mechanisms in exposed organisms?
7. To what extent does bioremediation reduce the leachability of volatile and soluble hydrocarbons such as benzene, toluene, ethyl-benzene, and xylenes?
8. How does the contaminant distribution change on soil particle surfaces during biodegradation?  How "clean" are the surfaces of fully bioremediated soil particles?