Photo-transformation of nitrate and fulvic acid driven by guest iron minerals

The photochemical interactions between nitrate (NO₃⁻) and natural organic matter (NOM) are vital for aquatic chemistry. However, the effects of guest iron minerals, which may enter the aquatic environments due to both human and natural activities, on those interactions are widely ignored. This work evaluated the effects of hematite (α-Fe₂O₃) on the photochemical conversion products and pathways of NO₃⁻, fulvic acid (FA) under 12 h of ultraviolet irradiation. The addition of 0.4 g/L of guest α-Fe₂O₃ accelerated the reduction of NO₃⁻ by 24.3%, with NH₄⁺ as the primary reduction product, and hampered the mineralization of FA. These effects were dependent on the dosage amount of α-Fe₂O₃ and FA concentrations. The studies on the molecule-level changes of FA revealed that the complete oxidation to CO₂ and the partial oxidation pathways that alter the molecular composition of FA were suppressed, and the mineralization rate decreased by 27.8%. Particularly, the conversion rates of CHON and CHONS were reduced by 21.0% and 20.3%, respectively, increasing the unsaturated products. The scavenging experiments and quantitative measurements of hydroxyl radicals (•OH) proposed that the photogenerated electrons and holes from α-Fe₂O₃ were the key for the altered transformation of NO₃⁻ and FA. This work revealed the guest effects of iron mineral particles on the photochemical interactions between NO₃⁻ and NOM in the natural surface waters.