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Abstract
Atomic force microscopy (AFM) is one of the major techniques for investigating mineralsurface heterogeneity and heterogeneous systems in situ. To date, little attention has been paid to the use of AFM to analyze on a nanometer scale the in situ surface features of naturally occurring soil clays under ambient conditions. In this study, the in situ surface features of clay particles of an Ultisol from Hengshan, China were investigated on a nanometer scale under ambient conditions by using AFM coupled with X-ray diffraction (XRD) before and after sequential selective-dissolution treatments. The results showed that the soil clay after different selective-dissolution treatments displayed distinct AFM surface features. The noncrystalline materials present as surface coatings on phyllo-shaped (predominantly kaolinite) and spheroidal particles (predominantly goethite and hematite) were clearly observed by AFM. The treatment with acid ammonium oxalate in the dark (AOD) virtually removed all of the film-shaped noncrystalline materials and some small spheroid-like particles. The dithionite-citrate-bicarbonate (DCB) treatment removed spheroid-like particles present outside pores but not those inside pores. The AOD and DCB treatments also extracted poorly crystalline aluminosilicates. Citrate extraction following the DCB treatment removed not only the interlayer materials of vermiculite but also the spheroid-like particles present inside pores and abraded the corners and edges of kaolinite particles. The results showed that AFM can be used as a major technique to investigate the in situ surface features of naturally occurring soil mineral colloids under ambient conditions, especially noncrystalline materials which cannot be easily studied by conventional microscopic methods such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) due to the aggregation caused by high vacuum.
