Sustainable soil remediation: leveraging microbial biosurfactants for hydrocarbon degradation

Hydrocarbon contamination in soil poses a critical and persistent environmental challenge, demanding innovative and sustainable remediation approaches, particularly in the case of weathered pollution. Among the available remediation strategies, microbial bioremediation is widely recognized for its cost-effectiveness and environmental compatibility (Bento et al., 2005). However, the success of microbial degradation is often limited by the low bioavailability of hydrocarbons, which tend to sorb onto soil particles, resulting in limited biodegradation rate and extent. This mass transfer limitation is one of the primary barriers to effective bioremediation, either in natural or engineered systems (Ghorbannezhad et al., 2018). To overcome these challenges, surfactants have been proposed to enhance hydrocarbon solubilization and mobility. Surfactants reduce surface and interfacial tension, promoting emulsification and increasing the accessible surface area of hydrophobic contaminants. Although synthetic surfactants are effective, they often exhibit toxicity and poor biodegradability, leading to secondary pollution (Bezza, 2016). In contrast, biosurfactants (BSs) and bioemulsifiers (BEs) are surfaceactive molecules produced by microorganisms that offer several advantages, such as low toxicity, high biodegradability, and functional stability, even under extreme environmental conditions. These 570 natural compounds, including glycolipids, lipopeptides, phospholipids, fatty acids, and polymeric surfactants (Prasad et al., 2017), have been applied in various industries as emulsifiers, dispersing agents, and additives (Banat et al., 2014). Notable examples include rhamnolipids from Pseudomonas aeruginosa, trehalolipids from Rhodococcus and Mycobacterium spp., and lipopeptides such as surfactin and viscosin produced by Bacillus and Pseudomonas species. Despite the recognized potential of biosurfactants, most studies have focused on well-characterized strains and pure cultures, often under controlled laboratory conditions. This approach might not accurately reflect the metabolic potential of microbial consortia present in contaminated environments. The present study explored the stimulation of autochthonous microbial communities in weathered hydrocarbon-contaminated soils as biosurfactant producers using waste-derived substrates to enhance the mobility and bioavailability of petroleum hydrocarbons for potential application in sustainable and site-specific remediation strategies, such as soil washing, soil flushing, and in situ biostimulation, minimizing the reliance on synthetic additives while promoting waste reuse. Biosurfactant and bioemulsifier production was assessed under anaerobic conditions using weathered contaminated soil as the inoculum and a microemulsion of waste frying oil and chickpea powder as the carbon source. The role in biosynthesis of various electron acceptors (nitrate and sulfate) and mild oxidative stress was examined, aiming to develop low-toxicity formulations suitable for remediation. The performance of the produced biosurfactants was evaluated through oil displacement and emulsification index tests on a variety of hydrocarbons. This study lays the foundation for future research by providing key data on biosurfactant production, performance, and environmental compatibility in real soil systems. Its findings have the potential to drive the development of innovative, eco-friendly, and economically viable solutions for remediating hydrocarbon-contaminated soils. This exploratory study demonstrates the potential of stimulating autochthonous microbial communities in weathered hydrocarboncontaminated soils to produce effective biosurfactants and/or bioemulsifiers using waste-derived substrates. The successful 571 production of surface-active compounds under anaerobic conditions highlights a promising approach for sustainable soil remediation. The ability of either crude extracts or microfiltered extracts to achieve significant oil displacement and emulsion thickness towards various oils suggests their broad applicability. Ongoing leaching tests with weathered heating oil-contaminated soil are being conducted using selected biosurfactants to better characterize their action on contaminants. Respirometric assays are employed to assess microbial biodegradation activity following biosurfactant application, while phytotoxicity tests are used to ensure environmental safety of either concentrated biosurfactant mixtures or diluted biosurfactant solutions (5% and 15% on a weight basis), and the soil before and after the treatment. While further research is needed to fully characterize the compounds produced and optimize the production conditions, these findings lay the groundwork for developing innovative, eco-friendly, and economically viable solutions for remediating hydrocarboncontaminated sites in line with circular-economy principles. Future work will focus on identifying the active compounds, elucidating the production kinetics, and assessing the impacts on microbial communities and contaminant bioavailability.