Detoxification of fluorene, phenanthrene, carbazole and p-cresol in columns of aquifer sand as studied by the Microtox® assay

The changing in the toxicity of mixtures of creosote-related compounds during their aerobic biodegradation was studied in columns of saturated aquifer material. The Microtox® assay was used to measure the toxicity of phenanthrene, fluorene, carbazole and p-cresol, added individually or in a mixture and to evaluate their detoxification during migration through the columns. When tested alone, phenanthrene, fluorene and p-cresol, inhibited Vibrio fischeri luminescence emission (15 min IC50=140, 750, 1500 μg/L respectively). In contrast, the toxicity of carbazole was minimal. Phenanthrene, fluorene and carbazole, added concomitantly at concentrations varying between 30 and 300 μg/L, produced between 24.4 and 49.3% inhibition. Comparison with individual toxicity responses suggested antagonistic interaction between these compounds. The toxicity of the mixture solutions decreased upon passage through the columns since the inhibition caused by the effluent solutions averaged 6%. Additionally, p-cresol (10,000 μg/L) enhanced the efficiency of detoxification of the column system; the inhibition by the compound mixture decreased from 81.4% (SD=2.0) to 15.8% (SD=1.8) after having passed through the first 3.8 cm of the column, and was undetectable at the two other downgradient ports. A comparison of the data of product disappearance and changes in toxicity indicated that detoxification was the result of the disappearance of phenanthrene and p-cresol. However, the persistence of fluorene and/or potential metabolites seemed to have caused some residual toxicity.