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

Abstract

Presence of nitrogenous compounds in drinking water cannot exceed 50 mg/L for NO_3⁻, 0.1 for NO_2⁻, and 0.5 mg/L for NH_4⁺. 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 % NO_3⁻ 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 N_2(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 NO_3⁻, 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 [NH_4⁺].