Biodegradation of PAHs in aggregates of a low permeability soil

In the bioremediation of low-permeability soils, pollutant and, especially, the oxygen bioavailability are often the rate limiting steps. In cases whe n a biopile treatment can represent an applicable technique, pretreatment o f the excavated soil is often necessary to attain an adequate air-filled po rosity in the soil and to avoid the presence of large soil aggregates. The present work was performed to evaluate the influence of soil aggregate size in the bioremediation of a silt-clay type soil contaminated by PAHs. M icrocosms were arranged with spherical soil aggregates of different diamete r in near water-saturation conditions, concentration of two and three aroma tic ring PAHs, total biomass, and respiration rates were monitored. PAH con centration profiles inside the particles were also obtained. A simple and q uick way to estimate the critical dimension of the soil aggregates was deve loped based on the evaluation of an oxygen penetration depth, that is, the distance from the external surface to the aggregate core beyond which oxyge n concentration is practically zero. A very different time course of PAHs consumption was found in the external layer and the inner core of the aggregates as well as in aggregates of diff erent dimensions. The results suggest that only the 3 mm external layer of the sphere is not limited by oxygen diffusion.