Modeling soil ped formation: properties of aggregates formed by montmorillonitic clay, Al or Fe poorly-ordered oxides and polyphenol in acidic milieu

A modeling approach was applied to understand and clarify the composite phenomena of the organo-mineral complexing and the ped formation in soil. The properties of conglomerates formed at pH 5.5 by montmorillonitic clay (C), synthetic Al- or Fe-oxyhydroxides, and polyphenol (pp) as humic material were studied. Investigational methods included SEM, FT-IR spectroscopy, X-ray diffraction, particle-size distribution, mechanical stability against ultrasonic dispersion and chemical stability to mild and strong reactants and to oxidative treatments. The models showed distinct morphological, mineralogical, mechanical, chemical and physico-chemical features. By SEM studies, the C-Al-pp model exhibited an irregularly rugged surface, while the C-Fe-pp showed evidence of spheroidal outgrowths. FT-IR spectroscopy provided clues for the effective interaction among the organic and inorganic components of conglomerates. X-ray diffraction patterns showed that Al- and, by a lesser extent, Fe- hydrolytic products partly entered the clay layers, thus expanding the basal period. The presence of broad and weak diffusion bands also suggested the formation of poorly-ordered Al- and Fe-oxy-hydroxides. The presence of Al- led to the prevailingly formation of small massive packets (<20 μm) and microaggregates (20-50 μm), quite stable to mechanical and oxidative breakdown; on the contrary, in the presence of Fe-, larger micro- and mesoaggregates (50-130 μm), well resistant to acidic etching, were formed. The experimental data highlighted the different role of Al- and Fe-oxy-hydroxides in determining the peculiar properties of the organo-mineral complexes, also validating the suitability and reliability of the adopted modeling approach to improve the understanding of factor and processes of soil ped formation.A modeling approach was applied to understand and clarify the composite phenomena of the organo-mineral complexing and the ped formation in soil. The properties of conglomerates formed at pH 5.5 by montmorillonitic clay (C), synthetic Al- or Fe-oxyhydroxides, and polyphenol (pp) as humic material were studied. Investigational methods included SEM, FT-IR spectroscopy, X-ray diffraction, particle-size distribution, mechanical stability against ultrasonic dispersion and chemical stability to mild and strong reactants and to oxidative treatments. The models showed distinct morphological, mineralogical, mechanical, chemical and physico-chemical features. By SEM studies, the C-Al-pp model exhibited an irregularly rugged surface, while the C-Fe-pp showed evidence of spheroidal outgrowths. FT-IR spectroscopy provided clues for the effective interaction among the organic and inorganic components of conglomerates. X-ray diffraction patterns showed that Al- and, by a lesser extent, Fe- hydrolytic products partly entered the clay layers, thus expanding the basal period. The presence of broad and weak diffusion bands also suggested the formation of poorly-ordered Al- and Fe-oxy-hydroxides. The presence of Al- led to the prevailingly formation of small massive packets (