Naphthenic acids (NAs) are carboxylic acids found in crude oil and petroleum products. The objectives of the research presented here were to: a) assess the occurrence and fate of NAs in crude oil and refinery wastewater streams; b) evaluate the biotransformation potential and inhibitory effects of NAs under nitrifying, denitrifying and methanogenic/fermentative conditions; c) investigate the factors affecting NA biotransformation under aerobic conditions and the microbes involved; and d) assess the toxicity of individual model NAs using quantitative structure-activity relationships (QSAR) and examine the effect of structure on NA biotransformation potential. NAs are ubiquitous in refinery wastewater streams and the desalter brine was found to be the main source of NAs in refinery wastewater. A commercial NA mixture was not biodegraded under nitrate-reducing or methanogenic/fermentative conditions. NAs were degraded under aerobic conditions by an NA-enriched culture; however, a residual fraction was not degraded under all conditions studied. The results indicated that NAs are not inherently recalcitrant and the residual fraction was due to the individual NA concentrations being below the minimum substrate concentrations at which they are no longer degraded. A fraction of the NA mixture was completely mineralized to carbon dioxide, with the remaining portion biotransformed to more oxidized intermediates. Overall, the results indicated that NAs were degraded under aerobic conditions; however, biological treatment of NA-bearing wastewater will not completely remove NA concentrations and thus, biological treatment must be combined with physical/chemical treatment to achieve complete NA removal.
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Fate and effect of naphthenic acids in biological systems
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