Desorption controlled mobility and intrinsic biodegradation of anthracene in unsaturated soil

Abstract

Desorption, sorption and biodegradation control the fate of polycyclic aromatic hydrocarbons (PAH) in contaminated soils. The interaction of these processes was studied in a two-layer column experiment with uncontaminated and spiked A-horizon material (13C anthracene) at unsaturated water flow conditions. During constant irrigation (part I of the experiment) the development of a stationary effluent composition was observed. The onset of anthracene breakthrough was delayed by 350 pore volumes (pvs) indicating high affinity sorption towards soil organic matter. The coincidence of anthracene breakthrough with increasing dissolved organic carbon (DOC) concentrations pointed to a carrier-mediated transport of the contaminant. Flow interruptions of varied duration (part II of the experiment) yielded a pronounced response of solution phase parameters. Effluent DOC increased linearly with time of flow interruption, indicating a zero-order rate-limited release. In contrast, anthracene concentrations showed a sharp drop after the stopped-flow events. Thus, the effluent anthracene concentration observed in part I may have been controlled by rate-limited sorption in the uncontaminated layer. Biodegradation of anthracene was evident from 13C-labelled metabolites and the complete decline of effluent anthracene after 1400 pvs. Although PAH seepage in soils is enhanced by rate-limited sorption it may be effectively counteracted by microbial activity.