Molecular-scale investigation on the photochemical transformation of dissolved organic matter after immobilization by iron minerals with FT-ICR MS†

The interaction between dissolved organic matter (DOM) and iron minerals has a significant effect on its stabilization and preservation in the environment. In this study, iron minerals with different crystal forms (crystalline goethite and amorphous ferrihydrite) were selected to investigate the photochemical transformation process for DOM immobilized on iron minerals under simulated sunlight irradiation at the molecular scale with the help of Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that a total of 7148 molecules were detected in alkaline-extractable sedimentary DOM, of which 38.8% and 36.2% were adsorbed by ferrihydrite and goethite, respectively, while there was no selectivity difference between the two iron minerals in terms of DOM adsorption. After simulated sunlight irradiation, the DOM adsorbed by goethite was significantly degraded (58.3%), in which the H/C ratio of the mineral-immobilized DOM increased and the O/C ratio decreased, and the photodegradation primarily involved DOM molecules with high Kendrick mass defect (KMD) values. The results confirmed that the iron mineral types play an important role in the transportation and transformation of DOM, which adds to the understanding of the fate of DOM in natural environments.