Influence of sorption mechanisms on the bioavailability of aromatic hydrocarbons in soil

The bioavailability and extractability of sorbed naphthalene and 1-naphthol was assessed in two sandy soils. Sorption conditions were varied to estimate the extent that biologically-mediated and surface-catalyzed oxidative coupling contributed to the strong binding of these compounds to the soil matrix. After a 2 d equilibration period, the extractability of the test compounds from the soils was measured by successive solvent and alkali extractions, and the nonextracted sorbed concentrations were measured by combustion of the soil. The bioavailability of naphthalene and 1-naphthol was determined by the addition of naphthalene-degrading bacteria to soil slurry reactors after the 2 d period, and to reactors which had undergone a successive water extraction procedure. Reactors were then allowed to incubate for 90 d. Biodegradation was confirmed by ¹⁴CO₂ evolution and the undegraded test compounds were measured by combustion of the soil after the 90 d period. The nonextracted sorbed concentrations were in good agreement with the biologically undegraded fractions. Biodegradation of physisorbed test compounds appeared to be controlled by the rate of desorption, and the majority of contaminants trapped in micropore sites became bioavailable over the 90 d period. Both biologically-mediated and surface-catalyzed oxidative coupling reactions were significant binding processes that greatly limited the bioavailability of 1-naphthol, and naphthalene transformation products.