Water Flow-Driven Abiotic Denitrification Associated with Streaming Potential Generation

Streaming potential generation through water flow in porous media represents an ancient and ubiquitous geophysical phenomenon. For centuries, this process has been regarded as merely a process of charge redistribution without involving any redox reactions. By using NO₃^- reduction as a model reaction, this study demonstrated for the first time that water flow drives abiotic denitrification associated with streaming potential generation. The nitrate (NO₃^-) reduction rate reached 10.6 μmol·L^-1·d^-1, which is comparable to that of FeS-driven chemical NO₃^- reduction (75.0-380 μmol·L^-1·d^-1) and significantly higher than that of photochemistry-driven NO₃^- reduction (0.1-1 μmol·L^-1·d^-1). Through monitoring of nitrogenous products and ¹⁵NO₃^- isotopic experiments, we showed that NO₃^- was selectively reduced to nitrogen (99%) via nitrite and nitrous oxide, confirming a denitrification process. Electron paramagnetic resonance (ESR) spectroscopy using DMPO as the probe detected the generation of hydrogen radicals (H•), which served as the reducing force for NO₃^- reduction. Using TEMPO as the electron probe, linear electron production was observed with an electron efficiency of 6.3% for NO₃^- denitrification. Moreover, H₂¹⁸O isotope experiments demonstrated that water oxidation is the ultimate electron source for NO₃^- reduction, indicating a chemical-free NO₃^- reduction process. An electric field strength of approximately 10⁶ V/cm was detected using surface-enhanced Raman scattering, providing evidence of a strong interfacial electric field (IEF)-induced electron transfer process during water flow. This work reveals ubiquitous but long-overlooked redox reactions associated with streaming potentials. Water-flow-driven denitrification also highlights a newly identified abiotic NO₃^- elimination pathway, suggesting potent chemical-free NO₃^- remediation strategies.