. Key Laboratory of Integrated Regulation and Resource Development of Shallow Lakes (Ministry of Education), College of Environment, Hohai University, Nanjing 210098, China . State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China . School of Environmental Engineering, Nanjing Institute of Technology, Nanjing 210008, China
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.
Na Huang,Yuanyuan Chen,Xuyin Yuan, et al. Photo-transformation of nitrate and fulvic acid driven by guest iron minerals[J]. Front. Environ. Sci. Eng.,
2025, 19(1): 7.
Fig.1 Changes in nitrogen species concentration (a) NO3–, (b) NO2–, and (c) NH4+. (d) The removal rates of TN and TOC under UV254 irradiation. Experimental conditions: c0(NO3–) = 20 mg-N/L, c0(FA) = 100 mg-C/L, α-Fe2O3 dosage = 0.4 g/L (N, F, Fe, and L are the abbreviations of NO3–, FA, α-Fe2O3, and UV254 irradiation, respectively).
Fig.2 The effects of different parameters on the NO3– reduction, including the concentration of (a) α-Fe2O3 and (b) FA. Experimental conditions: c0(NO3–) = 20 mg-N/L, c0(FA) = 100 mg-C/L for (a), α-Fe2O3 dosage = 0.4 g/L for (b).
Pristine FA
F + L
F + Fe + L
F + N + L
F + N + Fe + L
Number
5506
5133
5798
5326
5655
MWw
387.422
356.948
365.439
364.829
367.071
O/Cw
0.535
0.545
0.529
0.539
0.530
H/Cw
1.192
1.293
1.271
1.286
1.243
AImod,w
0.198
0.109
0.150
0.123
0.175
DBEw
9.403
7.798
8.171
7.919
8.347
Tab.1 Abundance weighted average molecular properties of different samples
Fig.3 The MW distributions of (a) removed and (b) produced molecules. (c) Size proportional Venn diagrams. The Van Krevelen diagrams of composition variation in (d) pristine FA, (e) F + N + L, and (f) F + N + Fe + L. (1: Lipids; 2: Aliphatic/Proteins; 3: Carbohydrates; 4: Unsaturated hydrocarbons; 5: Lignins/CRAM-like structures; 6: Tannin; 7: Aromatic structures.) The number of molecules that are removed and produced by (g) lipids, (h) aromatic structures, and (i) carbohydrate compounds.
Fig.4 (a)–(b) The counts of potential reactions classed by mass difference network analysis in the photochemistry of FA. The related transformation reactions and numbers are summarized in Table S3. (c)–(d) Classification and counting of FA photochemical precursors and products according to elements.
Fig.5 Nitrogen substance concentration: (a) NO3– and (b) NO2– in reactive species scavenging experiments. (c) The changes of TOC and TN in reactive species scavenging experiments. (d) Quantification of •OH concentration in different systems. (e) Photo-transformation of NO3– and FA driven by guest α-Fe2O3.
Fig.6 Changes in nitrogen species concentration under UV254 conditions. (a) NO3–, (b) NO2–, and (c) NH4+. (d) The removal rate of TN and TOC under UV254 conditions.
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