Combining Silica-Loaded Iron-Catalyzed Sodium Percarbonate (SPCSF) with Bacillus subtilis for Enhanced Remediation of Diesel-Contaminated Soil: Performance and Synergistic Mechanisms

Petroleum hydrocarbons contamination in soil is difficult to remediate due to strong adsorption and limited bioavailability. This study investigated the coupled remediation of diesel contamination in an alkaline kaolin-based model substrate using a silica gel-loaded, iron-catalyzed sodium percarbonate composite (SPCSF) and Bacillus subtilis. The alkaline model substrate was used as a simplified representation of difficult-to-reclaim hydrocarbon- and reagent-impacted matrices that may occur at oil drilling or production sites. In this study, a combined remediation strategy integrating a silica gel-loaded, iron-catalyzed sodium percarbonate composite (SPCSF) with Bacillus subtilis ATCC 11774 was developed for diesel-contaminated soil. The remediation performance of chemical oxidation, microbial remediation, and their combined application was systematically evaluated. The simultaneous SPCSF–microbial treatment achieved the highest removal efficiency, reaching 65.1% after 31 d, which was markedly higher than that of chemical oxidation (22.5%) or microbial remediation alone (31.1%). Within the mineral model substrate used in this study, SPCSF effectively regulated pH and oxidation–reduction potential, creating conditions more favorable for microbial activity. Spectroscopic analyses (three-dimensional fluorescence spectrum, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy) indicated that SPCSF promoted the transformation of diesel hydrocarbons into bioavailable intermediates, which were further converted by microorganisms into carboxyl-rich organic matter. Bacillus subtilis was associated with a higher Fe(II) proportion in the coupled system, which may have favored maintenance of Fe redox activity and sustained Fenton-like reactivity. However, direct measurements of reactive oxygen species and Fe(II)/Fe(III) dynamics were not performed; therefore, this interpretation should be regarded as a plausible hypothesis based on indirect evidence. The specific microbial contribution to Fe redox transformation was inferred from indirect evidence and may also have been influenced by medium-derived components or microbial metabolites. This study presents a coupled supported sodium percarbonate and microbial remediation strategy providing mechanistic evidence for the compatibility of supported chemical oxidation and microbial degradation in diesel-contaminated soil.