Insights into rhamnolipid-based soil remediation technologies by safe microorganisms: A critical review

Petroleum-contaminated soil is a global problem that must be faced with novel and green technologies in the near future. Considering the toxicity of fossil-based surfactants, microbial surface-active compounds (SACs) have shown various advantages, including high remobilization of hydrocarbons, low toxicity, biodegradability, sustainability, and others. Their main criticisms are the downstream costs (i.e., extraction, purification), the pathogenic character of some biosurfactant producers, the low yield obtained, and heterogeneity (i.e., the number and abundance of congeners). The use of surfactants at low concentrations is of great interest for efficient and controllable bioremoval of pollutants from porous soil media without harming the autochthonous microbial communities and pollutants' degraders. In the absence of valid benchmarking studies among numerous biosurfactants’ classes, novel rhamnolipids (RLs) by non-Pseudomonas aeruginosa producers also deserve in-depth investigations in the future. Herein, the main RL producers with a nonpathogenic character, with lower yields and safe capacities, were highlighted, which would be very advantageous for applying the circular economy across soil remediation sites, even at a large-scale. This review emphasizes the main current knowledge regarding the use of these glycolipids within the emerging soil remediation technologies, including soil washing, bioremediation, phytoremediators, bio-electrochemical systems, nanoparticle technologies, and others. The main prospects that should be addressed towards tackling RL costs by including safe producers and their broths moving towards replacing chemical surfactants, at least partly, were discussed. This review enhances the understanding of the novel RL-soil treatments and provides critical information to accelerate the transition to a circular economy within biosurfactant-based soil remediation technologies.