Rhizosphere-enhanced biosurfactant action on slowly desorbing PAHs in contaminated soil

• Published in: The Science of the total environment 2020-06, Vol.720, p.137608-137608, Article 137608
• Main Authors: Posada-Baquero, Rosa, Jiménez-Volkerink, Sara Nienke, García, José Luis, Vila, Joaquim, Cantos, Manuel, Grifoll, Magdalena, Ortega-Calvo, Jose Julio
• Format: Article
• Language: English
• Subjects: Bacteria; Bioavailability; Biodegradation; Biodegradation, Environmental; Bioremediation; Polycyclic Aromatic Hydrocarbons; Rhizosphere; Soil; Soil Microbiology; Soil Pollutants; Sunflower
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.137608

We studied how sunflower plants affect rhamnolipid biosurfactant mobilization of slowly desorbing fractions of polycyclic aromatic hydrocarbons (PAHs) in soil from a creosote-contaminated site. Desorption kinetics of 13 individual PAHs revealed that the soil contained initially up to 50% slowly desorbing fractions. A rhamnolipid biosurfactant was applied to the soil at the completion of the sunflower cycle (75 days in greenhouse conditions). After this period, the PAHs that remained in the soil were mainly present in a slowly desorbing form as a result of the efficient biodegradation of fast-desorbing PAHs by native microbial populations. The rhamnolipid enhanced the bioavailable fraction of the remaining PAHs by up to 30%, as evidenced by a standardized desorption extraction with Tenax, but the enhancement occurred with only planted soils. The enhanced bioavailability did not decrease residual PAH concentrations under greenhouse conditions, possibly due to ecophysiological limitations in the biodegradation process that were independent of the bioavailability. However, biodegradation was enhanced during slurry treatment of greenhouse planted soils that received the biosurfactant. The addition of rhamnolipids caused a dramatic shift in the soil bacterial community structure, which was magnified in the presence of sunflower plants. The stimulated groups were identified as fast-growing and catabolically versatile bacteria. This new rhizosphere microbial biomass possibly interacted with the biosurfactant to facilitate intra-aggregate diffusion of PAHs, thus enhancing the kinetics of slow desorption. Our results show that the usually limited biosurfactant efficiency with contaminated field soils can be significantly enhanced by integrating the sunflower ontogenetic cycle into the bioremediation design. [Display omitted] •Sunflowers can enhance the action of rhamnolipids on slow desorption PAHs.•Rhamnolipids alone have no effect on bioavailability.•The enhancement does not result in lower residual PAHs levels in greenhouse conditions.•The enhancement results into faster biodegradation after slurring the soils.•Rhamnolipid-enhanced bioavailability is caused by fast-growing rhizosphere microorganisms.