Coupling 18F Radiotracer Imaging with Synchrotron Fe Speciation to Resolve Biogeochemical Dynamics in PHC-Contaminated Soils

Abstract

In petroleum hydrocarbon (PHC)-contaminated subsoils, iron bioavailability often is believed to constrain the degradation of PHC contaminants, but spatially resolved evidence linking microbial activity with Fe transformations is scarce. Here, we bridge this gap by integrating phosphor imaging using positron-emitting radiotracer [18F]fluorobenzene─a high-fidelity surrogate for benzene, toluene, ethylbenzene, and xylene (BTEX)─with synchrotron-based X-ray spectroscopy. This multimodal approach allows for targeted mineralogical fingerprinting, guided by direct visualization of microbial activity within PHC-contaminated soils. Bulk X-ray absorption near-edge structure (XANES) showed little contrast, but microspectroscopy analysis revealed diffuse Fe at active zones and discrete Fe grains in inactive matrices, a pattern consistent with Fe(III) bioreduction to mobile Fe(II) and porewater transport before reprecipitation/reoxidation. Zones of high microbial activity were enriched in reactive phases (ferrihydrite, siderite, and green rust) comprising 27–71% of total Fe, whereas inactive matrices sequestered Fe mainly in Ca–Fe silicates with less than 25% reactive Fe. The recurring presence of green rust and vivianite aligns with phosphate-associated early-stage products under long-term tripolyphosphate biostimulation. We conclude that hydrocarbon degradation co-occurs spatially with Fe(III) bioreduction to mobile Fe(II), lateral transport in porewater, and phosphate-associated reoxidation/reprecipitation, processes that bulk methods often obscure but microscale mapping reveals. These insights support potential comanaging Fe and P amendments and targeted delivery of amendments to enhance PHC bioremediation in cold, calcareous soils.