The composition of interlayered clays affects the retention and transformations of organic compounds

Abstract

This study was conducted to compare the stability of Al-, Fe- and mixed Fe and Al-hydroxide interlayers in montmorillonite (Al-Mt, Fe-Mt and Fe,Al-Mt, respectively) and their ability to act as catalysts in the oxidative polymerization of hydroquinone and catechol. Hydroxy-Al polymers formed well-crystalline interlayers and showed the highest stability, whereas, hydroxy-Fe montmorillonite was found to be a poorly organized material with low stability to heat and chemical dissolution treatments. Mixed Fe- and Al-interlayered clay showed intermediate properties. Soil minerals affect the abiotic transformation of diphenol to humic substances. In the absence of clay minerals, very little formation of humic substances occurred, in comparison to humification in the presence of montmorillonite alone and where interlayered clays were present. The amounts of humic acids extracted follows the order; Na-montmorillonite $>$ Fe-Mt = Fe, Al-Mt $>$ Al-Mt in clay-hydroquinone systems. The quantity and quality of the humic substances formed from two common diphenolic precursors were compared. More humic substances were extracted from supernatants of clay-hydroquinone complexes than in clay-catechol systems. In contrast, higher amounts of humic substances were found in the solid phase of clay-catechol systems than in clay-hydroquinone, indicating the higher reactivity of catechols at the clay surfaces. The humic substances that were associated with the solid phase resemble natural humic and fulvic acids. The associated changes in mineralogical properties of hydroxy-interlayered montmorillonite due to reaction with soluble, low-molecular-weight organic compounds was studied at an adjusted pH of 6, aging for 30 d. Hydroquinone and catechol did not cause any significant structural changes in hydroxy-interlayered or non-interlayered montmorillonites, whereas citric acid dissolved the interlayer hydroxy-polymers. Very high third-range buffer capacities were observed in hydroxy-interlayered-hydroquinone and -c techol complexes. The sorption of atrazine was dependent on the amounts of organic carbon, degree of humification and the chelating ability of organic compounds present in soil. These factors were in turn due to the unequal effects of the different types of interlayers in montmorillonite.