Effect of mineral surface properties (alumina, kaolinite) on the sorptive fractionation mechanisms of soil fulvic acids: Molecular-scale ESI-MS studies
Résumé
We addressed the effects of mineral surface properties (kaolinite versus Al-oxide) on the sorption-driven fractionation of a
soil fulvic acid (FA) at acidic pH, mainly by means of ESI(-)-FTMS analysis of initial and supernatant solutions of FA sorption
batch experiments. The MS data provided clear molecular-scale evidence of distinct mechanisms and molecular parameters
controlling the FA fractionation upon its sorption on clay and oxide surfaces, respectively.
Identification of sorbing and not-sorbing FA compounds in kaolinite-solution systems revealed a weak fractionation
among members of ACO2 series of aliphatics or not-condensed aromatics (NCAs) at pH 3.8, and almost no sorption of
poorly-oxygenated polycyclic aromatic compounds (PACs) and NCAs. This first molecular-scale description of a FA fractionation
in a clay-solution system suggests that H-bonding with low affinity sites (aluminol/silanol) on the basal planes of
the clay particles is the main mechanism of sorption. Due to the predominance of such weak and poorly-selective mechanism,
the sorption of aliphatic and NCA molecules bearing oxygenated functionalities was prevented at pH 5, due to dissolved Al
competing successfully for their coordination.
In contrast, a strong FA fractionation was observed onto alumina, with a preferential retention of PACs and highlyoxygenated
aliphatics and NCAs. The major part of the poorly oxygenated aliphatics was left in solution. The sorption degree
of NCAs and aliphatics was strongly correlated with molecular acidity. For PACs and poorly-oxygenated NCAs, the sorption
was driven by reactions of surface ligand exchange (for the most oxygenated compounds) or by hydrophobic interactions (for
the least oxygenated compounds).
Domaines
Chimie-Physique [physics.chem-ph]Origine | Fichiers produits par l'(les) auteur(s) |
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