Electrochemically mediated nitrate reduction on nanoconfined zerovalent iron: Properties and mechanism

Selective reduction of nitrate to N₂ is attractive but still a difficult challenge in the water treatment field. Herein, we established a flow-through electrochemical system packed with polymeric beads supported nZVI (nZVI@D201) for selective nitrate reduction. Consequently, efficient nitrate reduction in the flow mode was achieved on nZVI@D201 under electrochemical regulation with N₂ selectivity of up to 95% for at least 60 h. Otherwise, nZVI was gradually exhausted after 20 h, and the product was mainly the undesired NH₄⁺. Through a series of comparative experiments, we clarified that the enhanced nitrate reduction on nZVI under electrochemical regulation was mainly attributed to electrons (from cathode) and active hydrogen ([H]) rather than the previously speculated H₂. Combining the characterizations of nZVI during nitrate reduction by X-ray diffraction and X-ray photoelectron spectrometry, we found that nitrate reduction under electrochemical regulation was mediated by nZVI along with the resultant Fe⁰@Fe_xO_y-Fe(II) structure and was sustained by electrons (from cathode) and [H] via the in situ reduction of Fe(III) back to Fe(II). Meanwhile, the undesirable product NH₄⁺ was efficiently oxidized to N₂ by the active chlorine generated on the anode. This study not only clarifies the mechanism of enhanced nitrate reduction on nZVI via electrochemical regulation but also advances the technological coupling of nZVI reduction with electrochemistry.