Combining Silica-Loaded Iron-Catalyzed Sodium Percarbonate (SPC^SF) with <em>Bacillus</em> <em>subtilis</em> for Enhanced Remediation of Diesel-Contaminated Soil: Performance and Synergistic Mechanisms

Petroleum hydrocarbon contamination in soil is difficult to remediate due to strong adsorption and limited bioavailability. 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 re-mediation alone (31.1%). SPCSF played a dual role in the system: it generated reactive oxygen species for the oxidative breakdown of long-chain hydrocarbons into bioa-vailable intermediates, and it regulated soil pH and oxidation–reduction potential, creating favorable conditions for microbial activity. In turn, Bacillus subtilis facilitated Fe(II) reduction and stabilization, sustaining the Fe(II)/Fe(III) redox cycle and thereby enhancing hydroxyl radical generation in Fenton-like reactions. Spectroscopic analyses (three-dimensional fluorescence spectrum, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy) demonstrated that the combined treatment promoted the transformation of petroleum hydrocarbons into carboxyl-rich organic matter, providing molecular-scale evidence of effective mineralization. This synergistic interaction between chemical oxidation and microbial degradation offers an efficient and environmentally compatible approach for petroleum hydrocarbon–contaminated soil remediation.