Influence of Activated Charcoal on Desorption Kinetics and Biodegradation of Phenanthrene in Soil

The observed strong sorption of polycyclic aromatic hydrocarbons (PAHs) to black carbon (BC) presents potential implications for PAH bioaccessibility in soils. The effects of BC on the desorption kinetics and mineralization of phenanthrene in four soils was investigated after 1, 25, 50, and 100 d soil PAT contact time, using sequential hydroxypropyl-beta-cyclodextrin (HPCD) extractions in soils amended with 0, 0.1, 1, and 5% (dry wt. soil) activated charcoal (AC, a form of BC). The rapidly (%F-rap) and slowly (%F-slow) desorbing phenanthrene fractions and their rate constants were determined using a first-order two-compartment (biphasic) desorption model. A minimum 7.8-fold decrease in %F-rap occurred when AC was increased from 0 to 5%, with a corresponding increase in %F-slow. Desorption rate constants followed the progression k(rap) (% h(-1)) > k(slow) (% h(-1)) and were in the order of 10(-1) to 10(-2) and 10(-3) to 10(-4), respectively. Linear regressions between %F-rap and the fractions degraded by a phenanthrene-degrading inoculum (%F-min) indicated that slopes did not approximate I at concentrations greater than 0% AC; % F-min often exceeded %F-rap, indicating a fraction of sorbed phenanthrene (%F-slow) remained microbially accessible. Therefore, HPCD-desorption kinetics alone may not be an adequate basis for the prediction of the bioaccessibility of PAHs to microorganisms or bioremediation potential in AC-amended soils.