Carboxylic acid sorption on synthetic clays in marine water: in vitro experiments and implications for organo-clay behaviour under marine conditions
Résumé
In order to investigate the role of clay minerals in organic matter preservation, the fixation of pure organic compounds on two synthetic low- and high-charge saponites was investigated in laboratory experiments simulating marine water conditions. The clays were exposed to four carboxylic acids: pentadecanoic, docosanoic, 5β-cholanic acid and ursolic, dissolved in treated natural sea water. Characterization of the resulting organo-clay association indicates that, under marine water column conditions, the organic fixation is only a sorption process, no intercalation being observed. The surface coverage, similar for the two clays (ca. 0.04 mg organic carbon m-2), demonstrates that the sorption is controlled by the surface properties of clays rather than their cation exchange capacity. The weaker sorption of docosanoic acid underlines the major role of the molecular properties, but the lack of selectivity among the three others does not corroborate the influence of molecular size on the sorption process. The general failure of a chemolysis treatment performed on the organo-clay associations demonstrates the high stability of these complexes. Results suggest that the bonding mechanisms are dominated by ligand exchange and not by hydrophobic effects, cation bridges or cation exchange. The minor extractible organic fraction consists of acid molecules connected to clay surfaces by van der Waals interactions. The high stability of the bonds formed in this environment, close to those observed from previous studies of marine sediments, could explain in part the organic matter preservation during transfer across marine water columns, especially metabolisable materials.
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