Mass transfer limitation in PAH-contaminated soil remediation

Batch experiments were conducted to determine the effects of Triton X-100, a nonionic phenolic ethoxylate surfactant, on the biodegradation of soil-bound naphthalene and phenanthrene. Two different types of soils, one contaminated with polynuclear aromatic hydrocarbons (PAHs) for different lengths of time, 2 days to 10 months, in the laboratory and the other, a field-contaminated soil from a manufactured gas plant (MGP) site, were used. Biodegradation of PAHs was measured by monitoring the 14CO2 production for the artificially contaminated soils and the residual PAHs in soil phase for the MGP soil. Without adding surfactant, the mineralization rate of phenanthrene was significantly smaller in the 1 0-month contaminated soil compared to that in the 2-day contaminated soil. Presumably, mineralization was mass-transfer limited in the soil with longer contamination period. Triton X-100 significantly enhanced mineralization in the 10 month-old soil but none in the 2-day old soil. The MGP soil, weathered over 2-3 decades, exhibited even greater enhancement of mineralization. Mineralization of PAHs in aged soils appears to be controlled by mass transfer rather than the rate of biodegradation. Surfactants increase the rate of release of soil-bound contaminant and thus help promote biodegradation.