Relationship between cyclodextrin extraction and biodegradation of phenanthrene in soil

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-p-cyclodextrin (HPCD) extractions and microbial mineralization of [C-14] phenanthrene in four soils over time. The desorption of [C-14]phenanthrene was assessed at 24-h intervals over 10 d and compared to cumulative mineralization of the [C-14] phenanthrene using an enriched pseudomonad inoculum. The cumulative total of [C-14] 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 [C-14] phenanthrene consistently approached one. Furthermore, a good linear correlation between mineralized and extracted phenanthrene was observed (single: r(2) = 0.99, gradient = 0.90, intercept = 3.09; multiple: r(2) = 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.