Reduced Mineralization of Low Concentrations of Phenanthrene Because of Sequestering in Nonaqueous-Phase Liquids

A nonaqueous-phase liquid (NAPL) may sequester a large fraction of a hydrophobic pollutant away from the aqueous phase. A study was conducted to determine whether the low aqueous concentrations of the compound may be associated with the absence of biodegradation. A phenanthrene-degrading mixed culture did not mineralize phenanthrene when initially dissolved in di-2-ethylhexyl phthalate (DEHP) at concentrations of 0.6-20 µg/mL. Under these conditions, the concentration of phenanthrene in water at equilibrium was less than 1 ng/mL. Such a threshold was not observed when a strain of Pseudomonas or a sample of subsoil was used as the inoculum or when the NAPL added was 2,2,4,4,6,8,8-heptamethyl-nonane. However, the biodegradation rates by all three populations at the low concentrations of phenanthrene in the NAPLs were slow and far less than expected from the rates at higher concentrations. At high concentrations, the rates of mineralization were higher than the rates of partitioning of phenanthrene to water, whereas mineralization was much slower than partitioning at low concentrations. We suggest that some NAPLs may sequester hydrophobic compounds away from the aqueous phase to an extent that the concentration falls below the threshold for biodegradation or to a level that results in unexpectedly slow biodegradation.