Nonexhaustive cyclodextrin-based extraction technique for the evaluation of PAH bioavailability

Traditionally, soil extraction techniques have been concerned with the dete rmination of "total" organic contaminant concentrations, following an "exha ustive" extraction. However, in light of the increasing body of knowledge r elating to organic contaminant availability and aging, such methods have li ttle relevance to the amount of contaminant that may pose an ecological ris k i.e., the "bioavailable" portion. Less exhaustive techniques have therefo re been the subject of more recent approaches in the hope that they may acc ess the "labile" or bioavailable pool. The use of an aqueous-based extracti on technique utilizing hydroxypropyl-beta-cyclodextrin (HPCD) is presented here for the extraction of PAHs from soil. The optimization of the method i s described in terms of HPCD concentration, extraction time, and solution b uffering. The procedure is then tested and validated for a range of C-14-la beled PAHs (phenanthrene, pyrene, and benzo[a]pyrene) added at a range of c oncentrations to a range of soil types. The amounts of soil-associated phen anthrene mineralized by catabolically active microorganisms were correlated with total residual phenanthrene concentrations (r(2) = 0.889; slope of be st fit line = 0.763; intercept = -5.662; n = 24), dichloromethane (DCM)-ext ractable phenanthrene concentrations (r2 = 0.986; slope of best fit line = 0.648; intercept = 0.340; n = 24), butan-1-ol (BuOH)-extractable phenanthre ne concentrations (r(2) = 0.957; slope of best fit line = 0.614; intercept = 0.544; n = 24), and HPCD-extractable phenanthrene concentrations (r(2) = 0.964; slope of best fit line = 0.997; intercept = 0.162; n = 24). Th us, i n this study, the microbially bioavailable concentrations of soil-associate d phenanthrene were best predicted using the optimized HPCD extraction tech nique. In contrast, the DCM Soxhlet extraction and the BuOH shake extractio n both overestimated phenanthrene bioavailability by, on average, >60%.