Low-temperature thermally enhanced bioremediation of petroleum hydrocarbons in groundwater: Performance and biogeochemical mechanisms.

This study addresses a critical knowledge gap regarding the role of biogeochemical processes in low-temperature thermally enhanced bioremediation of petroleum hydrocarbons in groundwater. A pilot-scale in situ thermally enhanced bioremediation test via hot water injection (ISTEB-HW) was conducted at a representative contaminated site to evaluate its performance and underlying mechanisms. ISTEB-HW achieved efficient removal of total petroleum hydrocarbons (TPHs), benzene and ethylbenzene within approximately four months, meeting their cleanup targets except for a residual benzene concentration of 4.7 μg/L in one monitoring well. It was found that the ISTEB-HW process significantly altered the groundwater biogeochemistry, with elevated temperature identified as the key factor for the enhanced contaminant degradation. Furthermore, microbial community analyses revealed the microbial evolution, in which the rapid enrichment of thermophilic or thermotolerant taxa, including Tepidiphilus, unclassified_p__Firmicutes, Thermotalea, Thermicanus, and Carboxydothermus, was observed during the initial heating phase (Phase 1). Post heating in Phase 2, the microbial community shifted to a new steady state dominated by functional groups such as unclassified_f__Comamonadaceae, Sulfuricurvum, norank_f__Hydrogenophilaceae, and Desulfurivibrio, contributing to the biodegradation of TPHs. Functional predictions suggested that the ISTEB-HW enhanced nitrogen fixation, denitrification, and sulfate reduction, thereby improving nutrient availability and energy metabolism conducive to petroleum hydrocarbon biodegradation. Multiple integrated lines of evidence confirmed that the contaminant removal was driven primarily by thermally enhanced biodegradation by indigenous microorganisms rather than dilution by injected hot water. Overall, this study provides the first systematic pilot-scale validation of ISTEB-HW and elucidates the biogeochemical mechanisms underlying its enhanced bioremediation, offering a scientific basis for green and effective remediation of petroleum hydrocarbon-contaminated sites.