Nitrate removal using metal-acid light induced (MALI) cycle

The homogeneous Fe 3+ /oxalate system was examined as a Metal-Acid Light Induced (MALI) cycle for the photo-assisted reduction of nitrate (NO 3 − ). A linear correlation between NO 3 − concentration removal and C 2 O 4 2− consumption, at stoichiometric conditions, was obtained, this confirmed that CO 2 •- radicals generated through ferrioxalate photolysis are the primary reductive species, enabling complete NO 3 − conversion with no detectable accumulation of NH 4 + or gaseous NO X and only minor transient NO 2 − formation. Time-resolved kinetic experiments demonstrated that NO 3 − undergo pseudo-first order, whereas oxalate decomposition follows zero-order behavior governed exclusively by the photon flux. A study has been conducted on the influence of the different variables affecting the Fe-Oxalate-UV cycle. A photonic operational window was identified in Fe–oxalate systems, delineating the transition from reagent-controlled to photon-limited regimes. Outside this window, excess oxalate activated competing oxidative pathways that re-oxidized nitrogenated byproduct and decreased NO 3 − removal rate, thereby elucidating inconsistencies previously reported in the literature. Application to a real groundwater matrix revealed that Ca 2+ induced CaC 2 O 4 precipitation, markedly lowering UV transmittance and slowing the reaction. Mild acidification effectively suppressed precipitation restored photon utilization and produced NO 3 − reduction rates comparable to, and initially exceeding, those obtained in ultrapure water. These results close critical mechanistic and operational gaps in homogeneous photo-assisted NO 3 − reduction. The integrated kinetic, photonic and matrix-dependence framework developed here provides quantitative design guidelines for reagent dosing, light delivery and water-quality conditioning. Collectively, these insights advance the rational scale-up of the MALI cycle as a selective and practical technology for NO 3 − remediation. • Fe 3+ /oxalic acid/Light system (MALI cycle) was developed for NO 3 − removal. • Complete NO 3 − to N 2 conversion with minimal by-products was achieved. • NO 3 − followed pseudo-first order, and C 2 O 4 2− showed zero-order kinetics. • Photon flux and Fe/oxalate ratio define the NO 3 − conversion operational window. • UV lamp power drives NO 3 − removal, confirming photon flux as a key factor. • UV lamp power drives NO 3 − removal, confirming photon flux as a key factor.