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Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands

Tamrat W.Z., Rose J., Grauby O., Doelsch E., Levard C., Chaurand P., Basile-Doelsch I.. 2019. Geochimica et Cosmochimica Acta, 260 : p. 15-28.

Weathering of silicates supplies a range of cations (mainly Si, Al, Fe, but also Ca, Mg, Na, K, Mn) to the soil solution. There, cations can interact with charged functional groups of dissolved soil organic matter (OM). Unlike Al and Fe, Si does not directly bind to natural OM. However, the role of Si in the mechanisms of OM stabilization by coprecipitation with short range order mineral phases (SRO) may have been underestimated. The formation of coprecipitates was tested by titrating a biotite-weathering solution up to pH 5 in presence of 3,4-Dihydroxy-l-phenylalanine (DOPA) with initial (Fe¿+¿Al):C ratio ranging from 3 to 0.003. Size, crystallinity, chemical composition and the local structure of the coprecipitates were analyzed by TEM-EDX and Fe K-edge EXAFS. Coprecipitates are amorphous particles whatever the (Fe¿+¿Al):C ratio, but their size, composition and local structure were nevertheless seen to progressively vary with increasing C content. In low C samples (high (Fe¿+¿Al):C), coprecipitates were 2¿40¿nm in size and were dominated by Si (30¿70%). Fe represented only 20¿50% of the mineral phase and was structured in small oligomers of Fe octahedra. Around 20% of the Fe of the coprecipitates were bound to C. Conversely, in high C samples (low (Fe¿+¿Al):C), coprecipitates were 10¿90¿nm in size and Fe was the main component (45¿70%). Fe was almost exclusively linked to OM by monomeric Fe-O-C bonds. Si (5¿40%) and Al (15¿35%) were able to form oligomers occluded in the Fe-OM network. In samples with intermediate C content ((Fe¿+¿Al):C¿=¿0.3), the coprecipitates had 5¿200¿nm size particles. We suggest these coprecipitates are structured in a loose irregular 3D network of amorphous small oligmers of Fe (25¿75%), Si (15¿50%), and Al (10¿35%), forming an amorphous and open-structured mineral skeleton. Within this mineral network, we suggest the organic compounds are linked either by bonds with Fe and Al to the skeleton, by monomeric Fe O C in the porosity of the network, or by weak bonds with other OM. This conceptual model provides an alternative to the standard view that SRO-OM is formed by ferrihydrite and amorphous Al(OH)3. We suggest naming the structure ¿Nanosized Coprecipitates of inorganic oLIgomers with organiCs¿ with ¿nanoCLICs¿ as acronym. The presence of Si in the inorganic structures may have an impact not only on the amount of OM stabilized by the nanoCLICs, but in the longer term, on the persistence of the OM stabilization potential by metallic oligomers.

Thématique : Chimie et physique du sol

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