Polycyclic aromatic hydrocarbon contamination in soils and sediments: Sustainable approaches for extraction and remediation

Polycyclic aromatic hydrocarbons (PAHs) are toxic environmental pollutants and high environmental concentrations may result from intense industrial activities. Employing greener approaches to extract PAHs for quantification and bioremediation of contaminated sites is vital for eco-sustainability. Remediation of contaminated soil and sediment may involve PAH treatment, or the promotion of in situ degradation by soil microbes. Desorption of PAHs from soil and sediment must occur either to quantify PAHs or to promote microbial degradation. Four PAHs (phenanthrene, pyrene, chrysene, and benzo[a]pyrene) were chosen to represent the varied molecular weights of PAHs, and were used to optimize the extraction conditions for PAH quantification. A novel eucalyptus oil-assisted extraction (EuAE) method for PAH extraction was compared with microwave-assisted extraction (MAE), and supercritical fluid extraction (SFE) using ethanol as a greener modifier for PAH quantification. Comparable PAH recoveries were found among SFE, EuAE and MAE methods, and over 78% of applied pyrene, chrysene, and benzo[a]pyrene were recovered in spiked soil and sediment. The optimised techniques were applied to naturally incurred samples and SFE provided the highest recoveries in more recalcitrant samples over the range of 0.5 to 10 mg/kg compared to MAE and EuAE. However, EuAE and MAE were found to have higher recoveries of phenanthrene compared to SFE. Whilst EuAE extraction times were considerably longer than SFE and MAE, it is a low technology approach using less analyst time, and ultimately costing less per sample for analysis.
Facilitation of PAH desorption from soil/sediment to allow microbial bioremediation using biosurfactants was investigated due to its potential as a greener approach. Saponin biosurfactant extracted from Eucalyptus camaldulensis leaves and sophorolipid biosurfactant were compared to rhamnolipid biosurfactant and the industrial-chemical surfactant, Tween 20. The PAH desorption capacities of saponin and sophorolipid in spiked samples under the optimized conditions were 30-50% and 30-70%, respectively, and were comparable to rhamnolipid and Tween 20. PAH removal efficiencies were in the order of sophorolipid > rhamnolipid > saponin > Tween 20 for naturally contaminated and aged soils. Based on emulsion stability, saponin biosurfactant demonstrated its applicability to contaminated matrices with diverse pH and salinity, while sophorolipid emulsion showed the highest thermal stability.
While biosurfactants showed promise for promoting PAH desorption, the use of high concentrations of these chemicals may pose a risk to the soil microbes responsible for PAH degradation, as well as the long-term fertility of a remediated soil. Consequently, the potential phytotoxic effects of biosurfactants were investigated using lettuce seed germination. Results indicated non-phytotoxic effects of saponin and sophorolipid. Additionally, the influence of biosurfactants on soil and sediment microbial respiration was studied. Although, the work showed that the inhibition of microbial respiration during 60 days of incubation was not detected in the presence of biosurfactants, biosurfactants may pose selective toxicity on specific microbial genera. Therefore, Miseq sequencing was carried out and accordingly, the relative abundance, diversity and structure of soil and sediment microbiome were not significantly affected by the saponin and sophorolipid amendment after 20 and 40 days of incubation. Even so, enhanced growth of potential PAH degraders was observed in the presence of saponin and sophorolipid. Sophorolipid and eucalyptus saponin can be successfully applied to desorb PAHs in contaminated soil and sediments, and were shown to be greener alternatives as their application may not negatively impact the overall soil/sediment fertility and growth of potential PAH degraders.