For a better understanding of the preservation of organic matter in clay minerals, the adsorption of a model humic substance, the Gallic Acid (GA), onto a Na-montmorillonite (Na–Mt) was performed in batch situation for various experimental conditions (pH = 2, 5, 7) in order to mimic the natural context. The adsorption efficiency and change in the clay mineral were characterized via a set of complementary experimental techniques (Fourier transform infrared spectroscopy, X-ray diffraction, elemental analyses). Adsorption isotherms at the equilibrium were fitted with the models of Langmuir, Freundlich and Dubinin-Radushkevitch allowing one to precisely quantify the adsorption through the derived fitting parameters. From the adsorption data combined with complementary results of the modeled humic-clay complexes, different types of interactional mechanisms were inferred as a function of background acidity: (i) at pH = 2 while protonated GA was the preponderant form, anionic GA species can be adsorbed to the Na–Mt surface through electrostatic interaction, leading to the a slight covering of the clay surface favoring in a second step the GA adsorption by π-π and Van der Waals forces; XRD patterns corroborated via TGA and FT/IR results suggested the actual intercalation of the phenolic acid within the interlayer space; (ii) At pH = 5, above the pKa of phenolic acid, only 20% of the protonated form subsisted and these species were adsorbed via coordinative bonding, without however any perceptible intercalation; (iii) and in the regime with neutral environment (pH = 7), the preponderance of GA anionic species led to a poor adsorption which appeared to be only located at the external surface of the clay mineral.
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