Abstract

Dispersion and Bacterial Degradation of Weathered Diesel, Biodiesel and Light Crude Oil in Seawater by Sophorolipid Biosurfactant
Nayereh Saborimanesh, Ph.D.
Concordia University, 2016
The significant surface activity, low toxicity, biodegradability and stability over broad ranges of salinity, temperature, and pH make the sophorolipid biosurfactant an alternative to the toxic chemically-based dispersants for oil cleanup from marine environments. The potential application of sophorolipid biosurfactant for dispersion and bacterial degradation of weathered biodiesel (BD), diesel (D) and light crude (L) oil in artificial seawater was studied. The mixtures of artificial seawater (salinities of 0-30 ppt, pH of 6-8, temperatures of 8ºC, 22ºC and 35ºC), -weathered diesel, biodiesel, and light crude oil and various concentrations and quantities of sophorolipid were prepared, shaken (150 rpm, 20 min) and analyzed according to the swirling flask dispersant effectiveness flask method. The biodegradation experiment was conducted by incubation (100 rpm, room temperature, 28 days) of mixtures of seawater (30 ppt), weathered diesel, biodiesel and light crude oil and sophorolipid. The oil dispersion increased 1.5-fold as the sophorolipid concentration doubled. For example, nearly 16 ± 0.5%, 12 ± 0.5% and 27 ± 0.5% of the diesel, light crude oil and biodiesel were dispersed, respectively, with 80 mg/L of sophorolipid. The dispersion of diesel, biodiesel, and light crude oil increased 1.3 (BD), 1.4 (L) and 1.5-fold (D) as the quantities of the sophorolipid in the seawater doubled. The mechanisms involved in the oil dispersion at higher concentration and quantity of sophorolipid were the incorporation of oil droplets in the micelles and the formation of small oil droplets. No oil dispersion was formed at the minimum level of agitation (0% at 0 rpm), regardless of the sophorolipid concentrations, and the dispersion occurred only when the mixing increased to maximum (150 rpm). The stability of dispersed oil was influenced by the level of mixing so that the oil dispersion reduced to ˂8% due to resurfacing process. The dispersion of oil doubled as the salinity increased from 0 to 10 ppt, and gradually increased at the higher salinities (10 to 30 ppt). The effectiveness of sophorolipid at the higher salinities was due to the better surface activity of sophorolipid at solutions with a higher content of NaCl salt, the effect of “salting out”, and the lower solubilization of sophorolipid. The biodiesel, diesel, and light crude oil dispersion increased with increasing temperature from 8ºC to 22ºC and 35°C, with the exception of biodiesel that had a reduced dispersion at 35ºC. The low effectiveness of sophorolipid at temperatures of 8 and 35ºC (in the case of biodiesel) can be attributed to the significant changes in the viscosity and density of biodiesel, diesel, and light crude oil. The presence of active oil-degrading bacteria in the weathered biodiesel, diesel, and light crude oil was confirmed by a significant natural biodegradation (42%), microbial growth and characterization of bacteria by 16S rRNA pyrosequencing technique. The dominant bacteria (e.g., dominant phyla) were Firmicutes, Actinobacteria, Actinobacteria, and Proteobacteria. The bacteria seem to uptake the hydrocarbons through the changes in the cell surface hydrophobicities based on the available hydrocarbons in the system. The high level of oil biodegradation in the presence of sophorolipid (46%) was due to the increase in the solubilization and dispersion of diesel, biodiesel and light crude oil by sophorolipid biosurfactant. The present study showed that the dispersion of oil by the sophorolipid biosurfactant was influenced by the quantity and concentration of sophorolipid, mixing, temperature and salinity. However, the seawater pH had an insignificant effect on the oil dispersion by sophorolipid. This research suggests the positive effect of sophorolipid biosurfactant on tested oil dispersion under the studied conditions. However, the oil biodegradation was not significantly stimulated with the sophorolipid.