Separate and Combined Environmental Behaviour of Surfactants and Polycyclic Aromatic Hydrocarbons (PAHs)

Surfactants and their effects on the behaviour and toxicity of polycyclic aromatic hydrocarbons (PAHs) were studied. Six surfactants covering a wide range of different types and characteristics were examined: anionic LAS, cationic TDTMA, nonionic Tween-80, Brij30, 10LE, and Brij35. PAHs studied include pyrene and benzo[a]. Adsorption of LAS on soils underwent two stages: linear and exponential adsorption. Its mechanism was mainly specific site surface interactions and adsorption capacity depended on soil physical clay content. LAS significantly influenced the adsorption of phenanthrene on soil. The velocity of LAS migration in soil was dependent on water flow flux and independent of its concentration. The migration of selected PAHs in soil was slow and followed the order of phenanthrene > pyrene > benzo[a]pyrene. The presence of LAS or Tween-80 could enhance their migration. Phenanthrene and all test surfactants were toxic to Vibrio fischeri in a descendent order of TDTMA > phenanthrene > Brij30 > 10LE > LAS > Brij35 > Tween-80. The impact of nonionic surfactants and phenanthrene was reversible. The combination of phenanthrene and surfactants was more toxic to the bacteria in an order of TDTMA > Brij30 > LAS > Brij35 > 10LE > Tween-80. The combined effects of phenanthrene with the later four surfactants were underadditive or antagonistic while overadditive or synergistic with TDTMA and Brij30. Mycobacterium sp. could grow on Tween-80 and Brij surfactants as sole carbon and energy source but not on LAS and TDTMA. The toxicity of Brij surfactants increased with the increase of ethoxylate group in their molecules, which was contrary to the responses of Vibrio fischeri. TDTMA was the most toxic, followed by LAS and Brij35. The combined effects of phenanthrene with Tween-80 or Brij surfactants were positive and additive while negative with LAS or TDTMA. LAS and Tween-80 had similar effects on soil microorganisms as on Mycobacterium sp. TDTMA was less toxic to soil microorganisms than LAS though the former was much more toxic to Mycobacterium sp. and Vibrio fischeri than the later. Phenanthrene exhibited completely contrary effects on soil microbial activities in different soils. The coexistence of any test surfactants could generally increase soil microbial activities. The combined effects of PAHs with TDTMA were overadditive while with Tween-80 was slightly overadditive at low respiration levels and underadditive at high levels. The presence of surfactants inhibited the biodegradation of phenanthrene by Mycobacterium sp. in a descent order of TDTMA > LAS > Tween-80. The biodegradation of pyrene in soil was much lower than that of phenanthrene. LAS and TDTMA at low concentrations and Tween-80 enhanced the biodegradation of both PAHs. Surfactant concentrations, at which the biodegradation of PAHs was enhanced or inhibited, varied significantly among soils. The relationship between enhancement of phenanthrene biodegradation and microbial activities was also remarkably different among soils and among surfactants. Mycobacterium sp. strain KR2 could grow on solid phenanthrene through direct contact. It was inhibited by LAS, TDTMA, Brij30 and 10LE, while enhanced by Tween-80 and Brij35. The mineralization and metabolism of phenanthrene were quick in wheat-solution-lava microcosm. The presence of Tween-80 significantly increased the metabolism of phenanthrene.