Multiple effects of iron oxides on the adsorption and oxidation of dissolved organic matter by manganese oxides

Adsorptive fractionation and oxidation of dissolved organic matter (DOM) on manganese (Mn) oxide surfaces alter the molecular composition and stability of DOM, but the impact of iron (Fe) oxides on the coupled adsorption-oxidation processes of DOM by Mn oxides is largely unknown. In this study, the underlying mechanisms of molecular transformation of DOM on birnessite (Bir) in the presence of ferrihydrite (Fh), with varying Fh/Bir mass ratios, were investigated at both molecular levels and microscopic scales with a suite of characterization techniques, including Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). We found that higher Fh/Bir mass ratio impeded the adsorption of DOM by birnessite and the reductive dissolution of birnessite to release dissolved Mn, and the impediment on the reductive dissolution of birnessite was proportional to Fh/Bir mass ratio. Overall, during the interactions of DOM with ferrihydrite and birnessite, phenolic compounds were preferentially adsorbed by Fe and Mn minerals, and compounds with higher oxygen contents and polymeric substances were formed. FT-ICR-MS analysis further suggested that higher Fh/Bir mass ratio inhibited the occurrence of aromatic ring-opening and carboxylation of substituted groups on aromatic rings, but promoted polymerization of phenolic compounds. Cs-STEM analysis revealed that DOM distributions on ferrihydrite, birnessite, and their mixtures were regulated by their microscopic structures and reactivity. Compared with aromatic carbon (C), carboxylic and phenolic C were more likely to associate with birnessite. Our results highlighted the significance of organo-mineral associations with the mixed minerals in regulating the distribution and reactivity of organic C. This study has provided molecular evidences for molecular transformation of DOM mediated by both Mn and Fe oxides, which contributed to advancing our understanding on coupled reactions of organic C at the mineral–water interfaces in the environment.