Weathered Railroad Diesel Removed from a Loamy Sand Soil by Means of Mono-rhamnolipids

ABSTRACT

The extensive use of diesel as fuel has contributed to the current problem of soil contamination, and the aged diesel is a highly toxic pollutant for the soil microbial flora. The objective of the present work was to determine the ability of mono-rhamnolipid surfactants to solubilize weathered diesel absorbed for decades in a loamy sand soil. The mono-rhamnolipids were produced by P. aeruginosa ATCC 9027, using glycerol as the carbon source. The mono-rhamnolipid characterization revealed that 90% of congeners contained saturated fatty acids of short chains. At its CMC (70.3 mg/L), the mixture of mono-rhamnolipids decreased the surface tension of deionized water (30.3 mNm⁻¹) and emulsified complex mixtures of petroleum hydrocarbons (73%). A 0.5% mono-rhamnolipid solution allowed solubilizing the following hydrocarbon fractions identified in the weathered diesel: aliphatics (66 ± 3%), polycyclic aromatic hydrocarbons (PAHs) (23.16 ± 11%), and asphaltenes (41.4 ± 6%). Within the group of PAHs, four PAHs derivatives, well known to be highly carcinogenic and mutagenic to humans, were identified: neutral, methylated, nitrated, and oxydated. The PAHs and the first three derivatives were removed in an average of 20%. However, regarding the oxy-PAHs, the increment of their concentration allowed assuming the biodegradation of the PAHs during the desorption process. Results show that hydrocarbon desorption with mono-rhamnolipid solutions of high content of saturated fatty acids offers an effective and viable alternative for soil bioremediation because of the low concentration of surfactants required (0.2–0.5%), the short treatment times (≈72 h) and above all, because mono-rhamnolipids are biodegradable compounds friendly to the environment.

KEYWORDS:

• Aged diesel
• biosurfactant characterization
• methylated-PAH
• nitrated-PAH
• oxy-PAH

Acknowledgments

The authors would like to thank B. María de Lourdes Rojas Morales from the LANSE, and Lorena Ramirez Reyes from the Central Laboratories, both for its technical and steadfast help.

Disclosure statement

We have no conflicts of interest to disclose.

Additional information

Funding

This work was supported by the Consejo Nacional de Ciencia y Tecnología under Grant INFRA-2012-01-188339.