Reactions of Polynuclear Aromatic Hydrocarbons on Soil

Abstract

The interactions of anthracene with three soils and montmorillonite clay were studied as a function of time and moisture content. Selected experiments also used phenanthrene and pyrene. The soils contained a range of organic matter, from 0.6 to 4.4%, and all had a high clay content. Anthracene was loaded onto the soils by dissolving in methylene chloride and then soaking the soil from 30 min to 17 d. After removing the methylene chloride, the soil was extracted to see how much contaminant was recoverable. Up to 100% of anthracene and pyrene were not extractable from the soil, and up to 25% of the phenanthrene was not extractable. Experiments with [14C]anthracene and analysis of extracts by UV, TLC, and GC/MS showed that anthracene was oligomerized to higher molecular weight aromatic products. The lowest member of the series, bianthracene, was detected by GC/MS. This reaction was consistent with prior studies of benzene polymerization on clays that were substituted by ion exchange with transition metals. Addition of water inhibited the reactions, and a kinetic model based on competitive inhibition between water and anthracene for access to active sites was consistent with the data. The reacted anthracene was not available to an active bacterial culture for degradation. These results suggest that analysis of air-dried or oven-dried soil samples may be subject to error and that thermal desorption processes for treating soil may cause polymerization on the soil surface.