Preferential molecular fractionation of dissolved organic matter by iron minerals with different oxidation states

The biogeochemical cycles of iron and dissolved organic matter (DOM) are strongly associated, especially at different oxidation sates. However, the role of iron minerals with different oxidation states in DOM preservation has not been clearly characterized at the molecular level. In this study, we employed electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) to investigate the molecular fractionation of DOM by adsorption onto four iron minerals with different oxidation states (ferrihydrite, hematite (α-Fe2O3), iron sulfide (FeS), and pyrite (FeS2)). The sorption capacity (normalized to per gram adsorbent) with respect to DOM was in the order of ferrihydrite > iron sulfide > hematite > pyrite. After normalization by specific surface area, the sorption capacity of ferrihydrite plummeted to the second place and was much lower than that of iron sulfide. The sorptive fractionation of DOM was significant for ferrihydrite and iron sulfide but was not obvious for pyrite. Compounds high in unsaturation and nominal oxidation state of carbon or rich in oxygen were preferentially bound to ferrihydrite and iron sulfide, leaving compounds low in unsaturation or poor in oxygenated groups in solution. For DOM containing abundant oxygen functional groups, hematite may result in stronger sorptive fractionation than iron sulfide. These findings provided new insights into the stability of aromatic and aliphatic carbon with various oxygenated groups coupled to redox iron transformation in soils.