Temporal and spatial monitoring of mobile nanoparticles in a vineyard soil: evidence of nanoaggregate formation

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Abstract
Mechanisms of formation, stabilization, liberation, transport and deposition of nanoparticles and their relationship to contaminant transport remain scarcely investigated in natural porous media. This study investigated nanoparticles mobilized in the pore space of a French vineyard soil by observing mobile soil‐derived organic matter (SOM) and minerals in pore fluids over an 8‐month monitoring period. Samples were collected in situ and investigated by transmission electron microscopy coupled to electron‐dispersive spectroscopy. The main types of nanoparticles transported within the soil were clay, bacteria, SOM and nanoaggregates. Nanometric clay particles were enriched in various metals (Fe, Zn, As and Pb) and organically‐derived constituents. Analyses of bacteria showed enrichments in Pb. SOM consisted of small carbon‐based particles (<200 nm) with slight enrichments in various metals. The fourth dominant particle type consisted of the association of particles forming organo‐mineral nanoaggregates. Based on the study of more than 22 500 individual particles, we propose a schematic interpretation of the evolution of the distribution of particles with depth in a soil profile. The increase of nanoaggregates with depth in the soil seemed to be largely controlled by the ionic strength of soil water and soil hydrodynamics. Seasonal variations in temperature also appear to affect nanoaggregation. Based on the architecture of the nanoaggregates, we propose an improvement of pre‐existing models of microaggregation by focusing on early aggregation stages suggesting the importance of bacteria and electrostatic interactions. The process of nanoaggregation can enhance the net reactivity of soil with respect to transported suspended matter, including heavy metals, and can initiate the process of C sequestration.

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