Iron minerals as catalysts in photo-assisted NO3– reduction in aqueous phase

Presence of nitrogenous compounds in drinking water cannot exceed 50 mg/L for NO3−, 0.1 for NO2−, and 0.5 mg/L for NH4+. In order to address this issue, this study tackles the use of iron-rich minerals (hematite, ilmenite, goethite and magnetite) as catalysts in photo-assisted nitrate reduction employing oxalate as reducing agent. Firstly, the catalytic activity of the different minerals was tested at different pH levels, finding an outstanding performance of magnetite at pH0 5, with a 50 % NO3− reduction in 60 min. After selecting magnetite as catalyst, a parametric study was performed evaluating initial pollutant concentration, catalyst load and reducing agent dosage. Best operating conditions were found to be [NO3−]: 75 mg/L, [Fe3O4]: 100 mg/L, [C2O42−]: 370 mg/L and pH0 5, for which complete NO3− reduction and selectivity towards N2(g) was reached after 150 min. Furthermore, magnetite maintained its activity in 5 consecutive cycles without any treatment between runs, showing its great stability. Reaction mechanism follows two pathways: i) direct electron transfer and ii) CO2• − mediated reduction. This mechanism was validated using methyl viologen as CO2• −scavenger showing that both pathways contribute equally in NO3− reduction. Finally, process feasibility was tested employing real groundwater samples naturally containing NO3−, showing promising results towards the implementation of photo-assisted catalytic reduction for drinking water treatment. Thus, a highly efficient technology has been developed, overcoming the limitations of photo-assisted nitrate removal processes by utilizing a cost-effective, stable, and highly active catalyst. This approach achieves the legislation for drinking water while generating negligible [NH4+]