Copper isotopic fractionation during adsorption on manganese oxide: Effects of pH and desorption

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Abstract

<p>We report new results for Cu<sup>2+</sup> adsorption on δ-MnO<sub>2</sub> and the resulting changes in Cu isotopic composition due to isotopic fractionation. The adsorption experiments were designed as a pH series (3.22–6.94) and an adsorption-desorption series. <sup>63</sup>Cu adsorbs preferentially on δ-MnO<sub>2</sub>, and the degree of isotopic fractionation is 0.45 ± 0.18‰ (2σ, <i>n</i> = 12). Preferential desorption of <sup>65</sup>Cu from δ-MnO<sub>2</sub> was also observed. The isotopic data are consistent with a closed isotopic equilibrium model, and the degree of fractionation indicates no systematic variation with pH or reaction time. Previously reported extended X-ray absorption fine structure study predicted the preferential adsorption of <sup>65</sup>Cu on δ-MnO<sub>2</sub>, because of the formation of Cu<sup>2+</sup> surface complexes (with 3–4 ligands) from dissolved Cu<sup>2+</sup> (with 5 ligands); however, the opposite was observed in this study. The equilibrium in isotopic fractionation is affected by the bonding environment; hence, we suggest that dissolved Cu<sup>2+</sup> species have a stronger bond than adsorbed ones.</p>

Journal

  • GEOCHEMICAL JOURNAL

    GEOCHEMICAL JOURNAL 52(2), e1-e6, 2018

    GEOCHEMICAL SOCIETY OF JAPAN

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