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Les substances humiques se fixent en surface des minéraux argileux et modifient lessites d'adsorption des polluants organiques hydrophobes (POH). Parallèlement, les substanceshumiques dissoutes complexent les POH non ioniques selon des mécanismes de liaisonréversible généralement évalués par le coefficient de partage Koc. Les solutés humiquesseraient ainsi responsables du transport des POH dans les écosystèmes aquatiques parsolubilisation de la fraction adsorbée sur le sédiment. Notre étude a pour objectif laquantification et la modélisation de ces phénomènes pour des composés modèles disponiblescommercialement : kaolinite, pyrène et acide humique Aldrich purifié.L'isotherme d'adsorption de l'acide humique (AH) sur la kaolinite à pH 6,5 et à forceionique 10-2 M suit une loi de Freundlich, dont les valeurs du modèlesont KF =0,79 et nF =1,90. Dans les mêmes conditionsexpérimentales, le pyrène, composé modèle à quatre noyaux du groupe des hydrocarburesaromatiques polycycliques (HAP), semble s'adsorber en formant des microcristaux à la surfacede l'argile. La présence d'acide humique fixé sur la kaolinite modifie l'adsorption dupyrène, qui se modélise par une isotherme de Freundlich de paramètres K'F=0,30 et n'F =2,00.Les interactions entre le pyrène et l'acide humique dissous ont été quantifiées parspectroscopie d'extinction de fluorescence et relation de Stern-Volmer. La fraction humiquedissoute non adsorbée sur le minéral argileux se caractérise par de plus fortes valeurs deKoc que l'acide humique initial (2,95.105 L.kg-1contre 1,86.105 L.kg-1). L'hypothèse de fractionnement des substanceshumiques lors de l'adsorption sur l'argile est confirmée par les variations du poidsmoléculaire moyen des AH, déterminé par chromatographie d'exclusion stérique, et desvariations des intensités de fluorescence des fractions chromatographiées.Les résultats obtenus pour la quantification du pyrène désorbé du solide argileux dansl'eau pure et dans une solution d'AH (50 mg.L-1) tendent à minimiser lerôle de la matière organique naturelle dissoute dans le transfert des polluants hydrophobesdu milieu solide au milieu liquide.Humic substances become fixed to the surface of clay minerals and modify theadsorption sites of hydrophobic organic pollutants (HOP). The dissolved humic substancescomplex the HOP according to reversible binding mechanisms normally assessed by the Kocdistribution coefficient. The humic solutes could thus be responsible of transporting HOP inaquatic ecosystems by solubilising the adsorbed fraction on sediment. Our study aims toquantify and model these phenomena using commercially available compounds:- pyrene, a polycyclic aromatic hydrocarbon (PAH) generally used as fluorescence probeand able to bind strongly to humic acid (AH); - a commercially available humic acid (Aldrich), with a high molecular weight and higharomaticity, which can easily adsorb onto colloidal matter and strongly bind PAH; - kaolinite clay, a ubiquitous mineral in aquatic systems, with a particle sizedistribution in aqueous solution similar to that observed in lake waters.We also have chosen physico-chemical conditions representative of freshwater: pH (6.5)and ionic strength (10-2 M). The adsorption isotherm of the humic acid(HA) on kaolinite at pH=6.5 and at ionic strength 10-2 M follows aFreundlich isotherm, for which model values are fixed at KF =0.79 andnF =1.90. The carboxylic groups of the dissolved HA, which represented morethan half the total acidity, were not protonated under these conditions. Despite thenegative surface charge of the particles, kaolinite was able to adsorb about 0.2mgC.g-1 of HA. A decrease in the solution pH occurred as the adsorbedHA quantity increased, suggesting a chemisorption of HA onto clay. The high-pressure sizeexclusion chromatogram (HPSEC) of the non-adsorbed HA, isolated after centrifugation of thecolloidal solution, reflects the selective adsorption of the HA. The higher the molecularweight of the HA (>70 000 Da), the better the adsorption. The hypothesis of the selectiveadsorption of the humic substances onto clay was also confirmed by the fluorescencevariations of the chromatographed fractions.Using the same experimental conditions, pyrene seemed to adsorb and generatemicro-crystals on the surface of the clay. The first part of the isotherm fits the pyrenemonomer adsorption onto the hydrophobic siloxane surface of the clay. In the second part,the dramatic increase in the adsorption isotherm could be attributed to the formation ofpyrene micro-crystals on the solid surface. For a residual pyrene concentration equal to 40nmol.L-1, the PAH quantity adsorbed onto kaolinite clay (7 nmol.g-1)was about twenty times lower than the quantity adsorbed onto montmorillonite clay (150nmol.g-1). This could be explained by the inaccessibility of theinterlayer cavity of kaolinite to any solute.Natural colloids are often coated by a humic layer. This coating can thus modify HOPadsorption onto the particles. The presence of humic acid fixed on kaolinite effectivelymodifies pyrene adsorption. This adsorption can be modelled by a Freundlich isotherm withthe parameters K'F =0.30 and n'F =2.00. The adsorbed HAmolecules seem to limit the formation of pyrene micro-crystals, even if the pyreneadsorption is not competitive with the HA fixation. Indeed, no HA desorption occurs duringpyrene fixation onto the organic coated kaolinite.The interactions between pyrene and dissolved humic acid have been quantified byextinction fluorescence spectroscopy and the Stern-Volmer relationship. The unabsorbed,dissolved humic fraction is characterised by Koc values higher than the initial humic acid(2.95 x 105 L.kg-1 compared to 1.86 x105 L.kg-1). These two values are of the samemagnitude as the published value obtained for the non-purified Aldrich HA (2.3±0.3 x105). The Koc value obtained for the non-adsorbed fraction of HA isunexpected because of the low molecular weight of these molecules. This Koc value can berelated either to a better accessibility of the HA hydrophobic cavity for pyrene, or ratherto a more efficient fluorescence quenching of pyrene by the humic solutes.The dissolved humic acid may complex the non-ionic PAH according to reversible bondingmechanisms. Therefore, HA can act as a carrier of hydrophobic contaminants if it is able todesorb HOP from the surface of the particles. The results obtained for the quantification ofthe desorbed pyrene in pure water and in a HA solution (50 mg.L-1) tendto minimise the role of dissolved natural organic matter in the transport of hydrophobicpollutants from the solid to the liquid medium. The values of pyrene concentration insolution obtained after 24 hours contact with kaolinite clay sorbed pyrene with pure wateror with the HA solution were very low (respectively 2.4 nM and 12.9 nM) and below the watersolubility of pyrene (802 nM). It has been previously demonstrated that the desorptionkinetics of pyrene from quartz material are not influenced by the organic matter content ofthe solution (SCHMITT, 1999). Our results demonstrate that the HA content of the solutiondid not greatly modify the quantity of pyrene desorbed from clay minerals

Rôle des acides humiques dans le transfert du pyrène entre les minéraux argileux et l'eau