Micro-nanobubbles as interface regulators to enhance electron transfer on mackinawite for promoting natural attenuation.

Naphthalene (NAP), a representative polycyclic aromatic hydrocarbon (PAH), is a hazardous groundwater contaminant because of its persistence, toxicity, and potential carcinogenicity. Although naturally occurring mackinawite (FeS) contributes to pollutant attenuation through reactive oxygen species (ROS) generation during oxygen perturbation, its reactivity is limited by inefficient interfacial electron transfer. Here, micro-nanobubbles (MNBs) were introduced as a chemical-free interfacial regulation strategy to activate FeS, significantly enhancing hydroxyl radical (•OH) production and NAP degradation. Results revealed that the strong interfacial electric field of MNBs could regulate the FeS interface and generate Fe(II) active sites, leading to approximately 7.4-fold and 1.5-fold increases in the electron utilization efficiency of ion-exchangeable and structural Fe(II), respectively. Meanwhile, MNBs accelerated the initial oxidation of S(-II) in the early reaction stage, thereby preventing prolonged competition with Fe(II) for O2 and ultimately improving Fe(II) utilization efficiency and •OH production. Furthermore, DFT calculations and ECOSAR toxicity predictions indicated that the MNB-mediated pathway drove NAP toward more reactive intermediates, reducing the risk of toxicity amplification and enhancing mineralization. Overall, this study demonstrates that MNBs effectively unlock the natural attenuation potential of FeS, providing a clean and sustainable strategy for in-situ groundwater remediation.