Study of organic-matter intercalated vermiculitic phases in the hydromorphic red earth sediments

Abstract

Al-hydroxy intercalated vermiculitic phases have been widely observed in pedogenic environment, but soil organic intercalations have rarely been reported. In the present study, mixed-layer illite–vermiculite (I/V) with organic matter (OM) intercalation under hydromorphic conditions in the Xuancheng red earth sediments was investigated in order to better understand the interlayer components occurring in mixed-layered clay minerals and the formation of this unusual OM interlayered mixed-layer clay species. Selective extraction of organic fillings by ammonium ethylene diamine tetraacetic acid (EDTA) and acid ammonium oxalate (NH4OX) showed that the components associated with organic complexes in the interlayer space of the 2:1 clays are mainly Fe- and A1-(oxy)hydroxides, which block substantial swelling after ethylene glycol solvation and complete collapse after heating. The OM interlayer filling contains CH3(CH2), CO, COO-, and NH functional groups and are resistant to H2O2 extraction although the binding intensity to the interlayer surface may change. Upon heating to ~300 °C the OM filling escapes from the interlayer space which causes the collapse of the 2:1 clay species. Alteration of primary illite during pedogenesis causes release of K and intercalation of hydrated cations in the interlayer space, and also leaching of Al and Fe. There is accumulation of exchangeable cations and Al and Fe oligomers in the interlayer region, which affects the behavior of I/V clays on glycolation and heating. Hydromorphic conditions favor the transformation of organic polymers into small size organic acids, and thus, favor the intercalation of interlayer OM filling into the 2:1 clays. Pedogenesis processes involves both the weathering of clay minerals and the decomposition of OM, leading to formation of interlayer clay-organic complexes in a hydromorphic soil.