Preferential Sorption of Tannins at Aluminum Oxide Affects the Electron Exchange Capacities of Dissolved and Sorbed Humic Acid Fractions

Natural
organic matter and humic substances (HS) in soils and sediments
participate in numerous biogeochemical processes. Sorption to redox-inert
aluminum oxide (Al2O3) was recently found to
affect the redox properties of HS both in sorbed and dissolved state.
With this study, we aim to decipher the molecular basis for these observations by applying Fourier transform
ion cyclotron resonance mass spectrometry (FT-ICRMS) and mediated
electrochemical analysis to Elliott soil, Pahokee peat, and Suwannee
river humic acid (HA) samples before and after sorption to polar Al2O3 and a nonpolar sorbent (DAX-8 resin). The FT-ICRMS
data provided evidence of preferential sorption of specific HA fractions,
primarily tannin-like compounds, to Al2O3. These
oxygen-rich compounds bear a high density of redox-active functional
groups, and their adsorption leads to a depletion of electron exchange
capacity in dissolved HAs and enrichment of HAs adsorbed at Al2O3. Sorption of HAs to DAX-8 was less selective
and caused only slight changes in electron exchange capacities of
dissolved and sorbed HA fractions. By combining FT-ICRMS and electrochemical
approaches, our findings suggest that a selective sorption of oxygen-rich
compounds in HA fractions to mineral oxides is a decisive factor for
the different redox properties of dissolved and sorbed HA fractions.