Unlocking Metal Sequestration in Soil Nanoparticles
Résumé
The natural speciation of Mn (0.19g/kg), Ni (46mg/kg) and Zn (42mg/kg) in the clayey horizon (120cm depth, pH=5.6) of a paddy soil in northern Taiwan was studied by X-ray microfluorescence, microdiffraction, and micro and polarized EXAFS spectroscopy. In Mn-rich aggregates and mottles, Ni and Zn are taken up by lithiophorite, but by a different structural mechanism: Ni (r=0.70 .ANGS.) substitutes for Mn (r(Mn'4+')=0.54 .ANGS., r(Mn'3+')=0.65 .ANGS.) in the manganese layer, whereas Zn (r=0.74 .ANGS.) fills vacant sites in the gibbsitic layer, similarly to lithium (r=0.74 .ANGS.). In contrast with nickel, which is detected only where there is lithiophorite, the Zn-lithiophorite association observed in Mn-rich areas of the soil matrix is not representative of the bulk soil. Powder and polarized EXAFS spectroscopy, complemented by X-ray diffraction, revealed that Zn is predominantly bound to hydroxy-Al interlayers sandwiched between 2:1 vermiculite layers in the fine soil matrix. The incorporation of Zn in the gibbsitic layer of both lithiophorite and vermiculite increases the stability of these minerals by providing positive charge to balance the negative charge from the 2:1 phyllosilicate layer and the manganese layer of lithiophorite. This binding environment for zinc is probably the dominant mechanism by which zinc is sequestered in acidic to near-neutral aluminium-rich clayey soils. (author abst.)