Preferential Sorption of Tannins at Aluminum Oxide Affects the Electron Exchange Capacities of Dissolved and Sorbed Humic Acid Fractions Edisson Subdiaga, Mourad Harir,* Silvia Orsetti, Norbert Hertkorn, Philippe Schmitt-Kopplin, and Stefan B. Haderlein* Cite This: Environ. Sci. Technol. 2020, 54, 1837-1847 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Natural organic matter and humic substances (HS) in soils and sediments participate in numerous biogeochemical processes. Sorption to redox- inert aluminum oxide (Al 2 O 3 ) 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 Al 2 O 3 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 Al 2 O 3 . 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 Al 2 O 3 . 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. ■ INTRODUCTION Humic substances (HS) are a tremendously complex mixture of countless polyfunctional organic compounds that occur ubiquitously in soils, sediments, and aquifers. They play a major role in biogeochemical processes including electron shuttling in microbial 1-4 and abiotic electron-transfer reac- tions 5,6 or stabilization of minerals and soil aggregates 7-9 among others. 10-14 The composition and structure of HS is extremely diverse and reflects ecosystem history, implying a compositional uniqueness of a HS sample. 15-18 In soils, sediments, and groundwaters, HS are predominantly present in a sorbed state as mineral coating or in a particulate state. Sorption to mineral surfaces often leads to selective fractionation of HS constituents, which depends on the type of mineral surface and other system characteristics including HS molecular diversity, pH, and ionic strength. 19-22 Occurrence of selective as well as nonselective humic acid (HA) sorption was recently reported for iron-containing clay surfaces. 23 Sorptive fractionation of fulvic acids (FAs) and HAs onto nanoparticles of α-Al 2 O 3 has also been described and was assigned to preferential removal of aromatic-rich as well as high molecular weight components of FAs and HAs from solution. 24,25 Electrospray ionization Fourier transform mass spectrometry (ESI-FTMS) in a negative ionization mode has recently been applied to characterize the fractionation on a molecular level of an aquatic FA and the water-soluble fraction of a terrestrial HA. 26,27 These studies revealed a very high affinity of certain subfractions of HS to the Al 2 O 3 surface, primarily oxygen- containing aromatic compounds, polycyclic aromatics, and aliphatic compounds with high O/C ratios. The sorption experiments, however, were conducted in the absence of background electrolytes, which are common to soils since the presence of dissolved salts negatively impacts the ESI-FTMS response. 26 Recently, this limitation has been overcome by applying solid phase extraction (SPE), which allowed reliable isolation of natural organic matter (NOM) in the presence of electrolytes with high recoveries. 28-35 Considerable efforts have been made to improve and to validate such isolation methods in different matrixes. 35-37 SPE methods using functionalized styrene-divinylbenzene polymers (PPL) 30 showed the highest recoveries (up to 89% of dissolved organic carbon (DOC)) from aquatic NOM 38,39 and advantageous Received: August 5, 2019 Revised: December 16, 2019 Accepted: January 2, 2020 Published: January 2, 2020 Article pubs.acs.org/est © 2020 American Chemical Society 1837 https://dx.doi.org/10.1021/acs.est.9b04733 Environ. Sci. Technol. 2020, 54, 1837-1847 Downloaded via HELMHOLTZ ZENTRUM MUENCHEN on March 18, 2020 at 09:00:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.