Native surface structure of natural soil particles determined by combining atomic force microscopy and X-ray photoelectron spectroscopy

A procedure was developed for handling natural soil particles and probing their native surface structure by atomic force microscopy (AFM) under water. This procedure was used to investigate the nanometer scale organisation of organic matter at the surface of sand particles taken from three soil horizons. The latter were selected for the contrasted properties of their organic matter, namely Podzol E and Bh horizons and a Cambisol A–B horizon. The presence of an adsorbed layer was visualised at the surface of Podzol Bh and Cambisol particles in the form of aggregated structures that interacted with the AFM probe. Surface analysis by X-ray photoelectron spectrometry (XPS) confirmed the carbonaceous nature of this adsorbed layer. Displacement of organic matter by the scanning probe was directly evidenced for Podzol Bh sand particles. Such displacement was not observed for Cambisol particles. A dramatic effect of drying on the concentration, nanometer scale distribution and properties of the adsorbed organic matter was clearly demonstrated by combining AFM imaging and XPS analysis. The procedure developed here gives access to direct, nanoscale information of the surface structure of sand particles and offers promising prospects for the characterisation of other environmentally-relevant particles in native conditions.