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Zinc Isotopic Fractionation: Why Organic Matters

Abstract : Zinc isotopic fractionation during adsorption onto a purified humic acid (PHA), an analogue of organic matter (OM), has been investigated experimentally as a function of pH. The Donnan Membrane device (DM) was used to separate Zn bound to the PHA from free Zn2+ ions in solution and allowed us to measure the isotopic ratios of free Zn2+. Below pH 6, adsorption of Zn on the PHA resulted in no measurable isotopic fractionation, while at higher pH, Zn bound to PHA was heavier than free Zn2+ (Delta Zn-66(PHA-FreeZn2+) = +0.24 parts per thousand +/- 0.06 2 sigma, n = 4). This can be explained by changes in Zn speciation with pH, with higher complexation constants and shorter bond lengths for Zn-PHA complex compared to the free Zn2+. Complexation of Zn with PHA occurred mostly through binding to high affinity sites (HAS) and low affinity sites (LAS). Fractionation factors for Zn-66/Zn-64 ratios determined by mass balance calculations were equal to 1.0004 for HAS (alpha(HAS-solution)) and 1.0000 for LAS (alpha(LAS-solution)). Delta 66Zn(OM-FreeZn2+) should then vary according to the heterogeneous nature of the OM, because of the variable relative proportions of these two types of sites. The NICA-Donnan model, along with these fractionation factors and measured delta Zn-66(TotalDissolved), was used to simulate the corresponding isotopic composition of free Zn 21 in the Seine River, France: Delta Zn-66(TotaIDissolved-FreeZn2+) varied from +0.02 parts per thousand to +0.18 parts per thousand, depending on the HAS/LAS ratio assumed for the OM. This study allows a better understanding of Zn isotope fractionation mechanisms associated with organic matter binding.
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Submitted on : Wednesday, July 1, 2020 - 10:21:28 AM
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Delphine Jouvin, Pascale Louvat, Farid Juillot, Chloé Maréchal, Marc Benedetti. Zinc Isotopic Fractionation: Why Organic Matters. Environmental Science and Technology, American Chemical Society, 2009, 43 (15), pp.5747-5754. ⟨10.1021/es803012e⟩. ⟨insu-02885955⟩

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