This study aims at determining zinc isotopes fractionation during their sorption onto the most abundant clay in reactive systems, kaolinite, and under various conditions of pH, ionic strength and initial Zn concentrations. Results show a systematic enrichment in heavy Zn on kaolinite surface with a ∆sorbed-solution ranging from 0.11 ‰ at low pH and low ionic strength to 0.49 ‰ at high pH and high ionic strength. These results are correctly described (for Zn concentration and isotopic ratio) using a thermodynamic sorption model with two Zn-binding sites: interlayer basal surfaces and edges sites of the clay. Distinct Zn isotopic fractionation factors are deducted for ionic exchange onto basal sites at low pH and low ionic strength (∆sorbed-solution = 0.18 ± 0.04 ‰) and for specific complexation at high pH onto edge sites (∆sorbed-solution = 0.49 ± 0.04 ‰). Crystallographic evidences explain the processes involved in each case: modification of hydrated Zn complex geometry during clay interlayers sorption and shortening of Zn-O bond lengths linked to aluminol groups during monodendate inner-sphere complexation onto edge sites.

Zn isotope fractionation during sorption onto kaolinite

251st National Meeting (2016)

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