Effect of methyl substituents on the biodegradation of two-ring polycyclic aromatic hydrocarbons in crude oil identified by aerobic biodegradation simulations

Abstract

Existing studies determining the effects of substituted methyl groups on polycyclic aromatic hydrocarbon biodegradation were mostly limited to an individual compound or a class of compounds. The simultaneous transformation of the multiple series of methyl-substituted two-ring polycyclic aromatic hydrocarbons (61 compounds) in crude oil through laboratory simulations has been examined, including naphthalenes, fluorenes, biphenyls, and dibenzofurans. The biodegradation sequences were in the order of dimethylnaphthalenes > trimethylnaphthalenes > biphenyls > dibenzofurans, tetramethylnaphthalenes > pentamethylnaphthalenes > fluorenes. Polycyclic aromatic hydrocarbon biodegradation was depending on both the number and the position of methyl substituents. For naphthalenes, fluorenes, biphenyls, and dibenzofurans, the more methyl groups in the parent compound, the more resistant it was to biodegradation. Among the polycyclic aromatic hydrocarbons with the same number of methyl groups, the more ortho-methyl substitutes, the more challenging it was to be decomposed. In the case of dimethylnaphthalenes or trimethylnaphthalenes with the same number of methyl and ortho-methyl substituents, their biodegradability reduced as the number of α substituents increased. The number of ortho-methyl substituents had a greater impact than that of α/β substituents. The relative significance of these two factors has not been confirmed in previous studies.