Enhanced bioremediation of oil-contaminated soil in a slurry bioreactor by H2O2-stimulation of oil-degrading/biosurfactant-generating bacteria: performance optimization and bacterial metagenomics

Oil-contaminated soil is the main challenge for oil-rich countries, and this study aimed to investigate the performance of the H₂O₂-stimulated slurry bioreactor for the bioremediation of real oil-contaminated soil. The effect of biomass concentration, soil to water (S/W) ratio, slurry temperature, pH, and H₂O₂ concentration were optimized for the removal of total petroleum hydrocarbons (TPH) from oil-contaminated soil. TPH removal efficiency, biosurfactants production, and peroxidase and dehydrogenase activities were measured. The optimum conditions for the complete biodegradation of 32 \({\text{g}}_{{{\text{TPH}}}} /{\text{kg}}_{{{\text{soil}}}}\) in the slurry bioreactor during 6 days were biomass of 2250 mg/L, S/W ratio of 20%, the temperature of 30 °C, pH of 7, and an H₂O₂ concentration of 120 mg/L. The highest peroxidase, dehydrogenase, surfactin, and rhamnolipid formation were also obtained under optimum conditions. The results pointed out that complete biodegradation of 32 g/kg of TPH in oil-contaminated soil at a short reaction time of 6 days is achievable in the developed process operated under optimum conditions. The GC/FID analysis of solid and liquid phases showed that the bioprocess completely biodegraded the different TPH fractions. H₂O₂ efficiently stimulated the biosurfactant-generating bacteria to produce peroxidase and thereby accelerating the bioremediation rate. Accordingly, an H₂O₂-mediated slurry bioreactor inoculated with biosurfactant/peroxidase-generating bacteria is a promising technique for cleaning up oil-contaminated soils.