Molecular Level Description of the Sorptive Fractionation of a Fulvic Acid on Aluminum Oxide Using Electrospray Ionization Fourier Transform Mass Spectrometry

Abstract

We addressed here, by means of electrospray ionization mass spectrometry (ESI-MS) with ultrahigh resolution, the molecular level fractionation of a reference fulvic acid (SRFA) during its sorption at an alumina surface, taken as a model for surfaces of natural aluminum oxide hydrates. Examination of ESI-MS spectra of a native SRFA solution and of supernatants collected in sorption experiments at acidic pH showed that the ∼5700 compounds identified in the native solution were partitioned between the solution and alumina surface to quite varying degrees. Compounds showing the highest affinity for the surface were aromatic compounds with multiple oxygenated functionalities, polycyclic aromatic compounds depleted of hydrogen and carrying few oxygenated groups, and aliphatic compounds with very high O/C values, highlighting the fact that SRFA constituents were sorbed mainly via chemical sorption involving their oxygenated functionalities. We observed an inverse correlation between the degree of sorption of a molecule within a CH2 series and its number of CH2 groups and a positive correlation between the degree of sorption and the number of CO2 groups in a COO series, which was remarkable. These correlations provide evidence at the molecular scale that molecule acidity is the key parameter governing fulvic acid (FA) sorptive fractionation, and they are useful for predicting sorption of FA at a natural oxide surface.

Selective Adsorption of Terrestrial Dissolved Organic Matter to Inorganic Surfaces Along a Boreal Inland Water Continuum

“…This is also where WRT and the adsorption treatment diverge in their effect on DOM composition: Whereas unsaturated compounds are generally lost from solution in both processes, even fairly saturated compounds can be adsorbed as long as they have a high O/C (Figures 3c and S3). The observed adsorption of oxygen-rich compounds is in agreement with various other studies, where both highly oxidized aromatics (Coward et al, 2018;Fleury et al, 2017;Galindo & Del Nero, 2014;Lv et al, 2016), and in some cases also oxidized saturated, aliphatic compounds (Galindo & Del Nero, 2014;Avneri-Katz et al, 2017) were preferentially adsorbed. The preferential adsorption of oxygen-rich compounds indicates that the presence of carboxyl and hydroxyl functional groups may be important for adsorptive processes, as these groups allow for ligand exchange with oxide and hydroxide groups at the sorbent's surface (Chassé et al, 2015;Gu et al, 1995;Kaiser, 2003;Kaiser et al, 1997;Kothawala et al, 2012;Lv et al, 2016;Meier et al, 1999;Tipping, 1981b).…”

“…Unsaturated compounds (low H/C) are preferentially adsorbed while saturated compounds (high H/C) remain in solution. The same trend has been reported by studies using mass spectrometry to investigate the adsorption of Suwannee River fulvic acid from the International Humic Substances Society on alumina colloids (Galindo & Del Nero, 2014) and soil DOM, International Humic Substances Society Pahokee peat (Chassé et al, 2015) and peat DOM on iron oxyhydroxides (Lv et al, 2016), and soil fulvic acid on alumina colloids (Fleury et al, 2017). Accordingly, studies using other methods than mass spectrometry also found that compounds with low H/C such as polyphenols and aromatics are preferentially adsorbed (Kaiser, 2003;Kaiser et al, 1997;Kalbitz et al, 2005; Kothawala et al, Figure 6.…”

Chemodiversity of dissolved organic matter in the Amazon Basin

“…A removal of these unique high molecular weight polyaromatic DOM molecules by means of mineral adsorption or flocculation with iron (Philippe and Schaumann, 2014) or aluminum oxides (Galindo and Del Nero, 2014) after mixing with the high-sediment-load Solimões River is conceivable and corresponded to previous reports that between 4 % (Aucour et al, 2003) and 40 % of the DOC was removed at the confluence of the Rio Negro and Solimões. We were not able to sample the Solimões River directly, and hence a direct comparison between FT-ICR-MS results from the Rio Negro and Solimões were not achieved.…”

Chemodiversity of Dissolved Organic Matter in the Amazon Basin

“…A removal of these unique high molecular weight polyaromatic DOM molecules by means of mineral adsorption or flocculation with iron (Philippe and Schaumann, 2014) or aluminum oxides (Galindo and Del Nero, 2014) after mixing with the high-sediment-load Solimões River is conceivable and corresponded to previous reports that between 4 % (Aucour et al, 2003) and 40 % (Moreira-Turcq et al, 2003) of the DOC was removed at the confluence of the Rio Negro and Solimões. We were not able to sample the Solimões River directly, and hence a direct comparison between FT-ICR-MS results from the Rio Negro and Solimões were not achieved.…”