Pyrene transfer from clay particles to water: the role of humic acid
Humic substances become fixed to the surface of clay minerals and modify the adsorption sites of hydrophobic organic pollutants (HOP). The dissolved humic substances complex the HOP according to reversible binding mechanisms normally assessed by the Koc distribution coefficient. The humic solutes could thus be responsible of transporting HOP in aquatic ecosystems by solubilising the adsorbed fraction on sediment. Our study aims to quantify and model these phenomena using commercially available compounds: - pyrene, a polycyclic aromatic hydrocarbon (PAH) generally used as fluorescence probe and able to bind strongly to humic acid (AH); - a commercially available humic acid (Aldrich), with a high molecular weight and high aromaticity, which can easily adsorb onto colloidal matter and strongly bind PAH; - kaolinite clay, a ubiquitous mineral in aquatic systems, with a particle size distribution 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 a Freundlich isotherm, for which model values are fixed at KF =0.79 and nF =1.90. The carboxylic groups of the dissolved HA, which represented more than half the total acidity, were not protonated under these conditions. Despite the negative surface charge of the particles, kaolinite was able to adsorb about 0.2 mgC.g-1 of HA. A decrease in the solution pH occurred as the adsorbed HA quantity increased, suggesting a chemisorption of HA onto clay. The high-pressure size exclusion chromatogram (HPSEC) of the non-adsorbed HA, isolated after centrifugation of the colloidal solution, reflects the selective adsorption of the HA. The higher the molecular weight of the HA (>70 000 Da), the better the adsorption. The hypothesis of the selective adsorption of the humic substances onto clay was also confirmed by the fluorescence variations of the chromatographed fractions. Using the same experimental conditions, pyrene seemed to adsorb and generate micro-crystals on the surface of the clay. The first part of the isotherm fits the pyrene monomer 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 of pyrene micro-crystals on the solid surface. For a residual pyrene concentration equal to 40 nmol.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 (150 nmol.g-1). This could be explained by the inaccessibility of the interlayer cavity of kaolinite to any solute. Natural colloids are often coated by a humic layer. This coating can thus modify HOP adsorption onto the particles. The presence of humic acid fixed on kaolinite effectively modifies pyrene adsorption. This adsorption can be modelled by a Freundlich isotherm with the parameters K'F =0.30 and n'F =2.00. The adsorbed HA molecules seem to limit the formation of pyrene micro-crystals, even if the pyrene adsorption is not competitive with the HA fixation. Indeed, no HA desorption occurs during pyrene fixation onto the organic coated kaolinite. The interactions between pyrene and dissolved humic acid have been quantified by extinction 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 x 105 L.kg-1). These two values are of the same magnitude as the published value obtained for the non-purified Aldrich HA (2.3±0.3 x 105). The Koc value obtained for the non-adsorbed fraction of HA is unexpected because of the low molecular weight of these molecules. This Koc value can be related either to a better accessibility of the HA hydrophobic cavity for pyrene, or rather to a more efficient fluorescence quenching of pyrene by the humic solutes. The dissolved humic acid may complex the non-ionic PAH according to reversible bonding mechanisms. Therefore, HA can act as a carrier of hydrophobic contaminants if it is able to desorb HOP from the surface of the particles. The results obtained for the quantification of the desorbed pyrene in pure water and in a HA solution (50 mg.L-1) tend to minimise the role of dissolved natural organic matter in the transport of hydrophobic pollutants from the solid to the liquid medium. The values of pyrene concentration in solution obtained after 24 hours contact with kaolinite clay sorbed pyrene with pure water or with the HA solution were very low (respectively 2.4 nM and 12.9 nM) and below the water solubility of pyrene (802 nM). It has been previously demonstrated that the desorption kinetics of pyrene from quartz material are not influenced by the organic matter content of the solution (SCHMITT, 1999). Our results demonstrate that the HA content of the solution did not greatly modify the quantity of pyrene desorbed from clay minerals.