Kinetics of co-biodegradation of n-hexane and methanol vapors in the presence of a non-aqueous phase by an isolated microbial culture from activated sludge

Volatile Organic Compounds (VOCs), despite their small share in the atmospheric pollutants, are the main cause of environmental problems and human health risks. In the biological processes, all the pollution is converted into carbon dioxide and biomass. A large number of industries discharge a mixture of pollutants such as n-hexane and methanol into the atmosphere. The different chemical and physical properties of these materials affect the rate of biological degradation and their removal efficiency by a specific microbial population. Among these properties, the solubility of each component may affect the interaction between the compounds during the biodegradation of the n-hexane/methanol mixture. To overcome the limitation of mass transfer and the low solubility of hydrophobic VOCs such as n-hexane in the water phase, it has been reported to be useful to add a non-aqueous phase (NAP) to the culture medium to absorb the hydrophobic compound. According to the previous researches, it is clear that the presence of silicone oil as an NAP has a positive effect on the simultaneous removal efficiency of methanol as a hydrophilic and n-hexane as a hydrophobic pollutant. However, its effect on the kinetic of biodegradation of VOCs is not well known. Therefore, this study aims to determine the effect of different volume fractions of silicone oil on the kinetics of simultaneous biodegradation of n-hexane and methanol.
Material and methods: n-hexane (5 g.m-3) was selected as a hydrophobic pollutant , methanol (1 g.m-3) as a hydrophilic pollutant, and silicone oil as the NAP. Microbial consortium used in this research is a mixed culture that was isolated from the adapted activated sludge from the return sludge flow of Unit No. 6 of the South Tehran Wastewater Treatment Plant. A gas chromatography (GC) device equipped with a capillary column and flame ionization detector (FID) was used to measure the concentrations of n-hexane and methanol. To measure the concentration of carbon dioxide, a thermal conductivity detector (TCD) was used. To measure the biomass concentration a spectrophotometer was used to determine the optical density (OD) of the samples.
Results and discussion: The specific degradation rate (SDR) of methanol decreased from 1.42 mgmethanol.(gbiomass.day)-1) to 1.08 mgmethanol.(gbiomass.day)-1) by increasing the volume fraction of silicone oil from 1% to 20% v/v. In contrast, when volume fraction of silicone oil reached to 10% v/v, the SDR of n-hexane increased from 17.5 (mghexane.(gbiomass.day)-1) to 21 (mghexane.(gbiomass.day)-1). Further Increasing in the volume fraction of silicone oil up to 20% v/v did not have much effect on the SDR of n-hexane. The kinetics of n-hexane biodegradation followed the Michaelis-Menten model. The addition of 1 g.m-3 of methanol to the culture medium had a negative effect on the SDR of n-hexane, and the presence of methanol increased the Ks value to 21 g.m-3 and decreased the maximum SDR of n-hexane from 141.4 to 123.1 (mghexane.(gbiomass.day)-1).
Conclusions: The presence of silicone oil in the culture medium reduced the negative effect of methanol on the biological degradation of n-hexane. Increasing in the amount of silicone oil up to 10% v/v was effective on the biodegradation rate of n-hexane, and its further increase did not have effect on the SDR of n-hexane.