Evolution of Bioavailability of PAHs during Bioremediation of polluted soils

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are the best representatives of chemicals for which specific limitations in bioremediation exist due to low bioavailability. The residual concentrations of the PAHs after bioremediation are crucial because they may limit the use of the area after treatment, or land use might not even be possible if the residual concentrations do not meet the legal requirements. For this reason bioavailability should be incorporated into soil bioremediation and a very important question is how should it be measured. One of the chemical methods which has been proposed to measure bioavailability of PAHs is the desorption extraction with Tenax during 20 h (ISO 17402) (Environ. Sci. & Technol. 49, 10255-10264, 2015; Environ Toxicol Chem 20:706¿711, 2001; Integr Environ Assess Manag, 11:208¿220, 2015). We have used this method in a greenhouse experiment in which different strategies have been used (use of microorganisms, plants and (bio)surfactants) in a PAHs contaminated soil, oriented to decrease the fraction of bioavailable pollutants. These agents have been shown previously in our group to act as bioavailability-promoting agents (Environ. Technol. 21, 1099-1110, 2000; Soil Biology and Biochemistry, 57, 830-840, 2013; Environ. Sci. & Technol. 45, 3019-3026, 2011). The greenhouse experiment was carried out with a creosote polluted soil with a total concentration of PAHs of 500 mg/kg. For this study, we used sunflower plants and a rhamnolipid biosurfactant (R90). There were four different treatments: contaminated soil (used as control); contaminated soil with rhamnolipid; planted contaminated soil; and planted contaminated soil with rhamnolipid. After certain time intervals, the soil was sampled in the different treatments. The rhamnolipid was applied to the soil after 75 days of treatment. The total concentration of PAHs was determined by Soxhlet extraction and HPLC analysis; the bioavailable concentration was determined by Tenax extraction. In addition, separate biodegradation experiments were designed in soil slurries to test the disappearance of the chemicals in these greenhouse samples under laboratory conditions (using an excess of nutrients, radiorespirometry determinations with 14C-pyrene and analysis of residual concentrations of native PAHs). These analytical methodologies have been previously used in our laboratory (Environ. Sci. Technol 45, 3019-3026, 2011). The most relevant result in this study was that bioavailability increases in planted soils receiving rhamnolipids, as evidenced by Tenax extraction and it was accompanied by an increased biodegradation in soil slurries. In conclusion, Tenax extraction during 20 h has resulted a reliable and robust method to determine bioavailable concentrations in a wide set of operational conditions ranging from a different time scale to dissimilar treatments (planting, biosurfactant application, etc.).