When assessing the potential of a contaminated site for bioremediation, it is desirable to know how much of the contaminant(s) is available for microbial degradation, thus allowing the likelihood of successful bioremediation to be predicted. The aims of this study were to investigate the degradation of PAHs in two soils by a catabolic inoculum and indigenous soil microflora and link this to the cyclodextrin extractable fraction in the presence of transformer oil (0.05, 0.01, or 0.005%). This study showed very little difference between indigenous and inoculum-derived degradation for phenanthrene in laboratory-aged soil, and strong relationships were also observed between both of the microbial degradative conditions and the amount of phenanthrene extracted by cyclodextrin. Furthermore, the indigenous degradation of PAHs in a field-contaminated soil showed significant linear correlations with the cyclodextrin extractable fraction, with gradients approximating to 1. There are several novel facets to this study. First, in aged, contaminated soils, indigenous microflora gave an equally sensitive determination of degradative availability as that measured by the catabolic inoculum. Second, this is the first time intrinsic biodegradation of PAHs has been predicted by the cyclodextrin extraction in laboratory-spiked and field-contaminated soils. The cyclodextrin extraction technique represents a powerful tool for predicting the extent of intrinsic and augmented microbial degradation and will be useful in the assessment of contaminated land prior to bioremediation.

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A number of soil extraction techniques have been proposed to determine the microbial degradability of organic contaminants in soil. Exhaustive methods using organic solvents have little relevance to the concentration of contaminants actually available to microorganisms. The present study investigated the relationship between sequential hydroxypropyl-beta-cyclodextrin (HPCD) extractions and microbial mineralization of [14C]phenanthrene in four soils over time. The desorption of [14C]phenanthrene was assessed at 24-h intervals over 10 d and compared to cumulative mineralization of the [14C]phenanthrene using an enriched pseudomonad inoculum. The cumulative total of [14C]phenanthrene extracted by HPCD exceeded the mineralization asymptote by more than 20%. The overall total extents mineralized after both single and multiple degrader inoculations, however, were statistically similar to that extracted after the first 24 h by HPCD; the ratios of extractable to mineralizable [14C]phenanthrene consistently approached one. Furthermore, a good linear correlation between mineralized and extracted phenanthrene was observed (single: r2 = 0.99, gradient = 0.90, intercept = 3.09; multiple: r2 = 0.95, gradient = 1.01, intercept = -0.48), suggesting that a single HPCD extraction accurately and reproducibly predicts the total fraction of phenanthrene available for microbial mineralization in all soils tested in the present study.

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