Geochemistry and Geomicrobiology at Mill Tailings Sites

Mill tailings sites represent long-term environmental legacies of uranium mining, where radionuclide migration is governed by complex mineral–microbe interactions at the nanoscale. Our research began with laboratory studies on the biomineralization of rare earth elements (REEs) and f-block elements, demonstrating how microbial cells induce nanocrystallization and phosphate mineral formation. These early studies established mechanistic frameworks for understanding radionuclide immobilization.

We then investigated interactions between extracellular polymeric substances (EPS) and nanoparticles, revealing how microbial polymers influence nanoparticle aggregation, colloidal stability, and metal adsorption. These findings provided key insights into radionuclide transport under environmentally relevant conditions.

Building on these laboratory studies, we conducted field-based research at the Ningyo-toge uranium mine tailings site in Japan. We demonstrated that nanoscale ferrihydrite forms in association with amorphous silica colloids and plays a central role in the sequestration of uranium and arsenic. We also isolated manganese-oxidizing fungi from the site and showed that biogenic manganese oxides form through enzymatic and superoxide-mediated pathways. Using molecular pH probes and confocal microscopy, we characterized microscale pH environments controlling Mn-oxide formation.

Furthermore, we examined radium sequestration mechanisms by biogenic Mn oxides using barium as a surrogate element. Spectroscopic analyses revealed inner-sphere complexation, providing a molecular-scale structural model for Ra adsorption.

Together, this work integrates nanoscale mineralogy, environmental geochemistry, and geomicrobiology to reveal natural attenuation processes that govern contaminant fate in uranium mill tailings systems.