Interactions between aqueous metal ions and mineral surfaces and attendant isotopic fractionation are highly dependent on mineral surface properties and the local chemical environment, which may be altered by surface ordering (e.g., quartz versus amorphous silica) or by confinement in nanopores. Here we present equilibrium adsorption isotherms, X-ray absorption spectra, and isotopic measurements to illustrate the effects of nano-confinement, substrate crystallinity, ionic strength, and surface coverage on zinc (Zn) complexation and isotope partitioning. Our results demonstrate that outer-sphere Zn complexes occur on quartz in low ionic strength solutions, but not on amorphous silica (SiO2(am)), with no significant isotopic fractionation (Δ66/64Znaqueous-sorbed = -0.01 ±0.06‰) from aqueous Zn. Inner-sphere Zn complexes form on quartz and SiO2(am) surfaces, with heavier Zn sorbed preferentially and measured equilibrium isotope fractionations of Δ66/64Znaqueous-sorbed= -0.60 ±0.11‰ and -0.94 ±0.11‰, respectively. These inner-sphere Zn complexes on silica transition from octahedral to tetrahedral coordination with oxygen as surface coverage increases and nanopore size decreases. However, the change in coordination number does not alter the observed isotope signatures. The propensity for Zn to exist in tetrahedral and octahedral coordination with oxygen may help explain this observation, and we posit that the low energetic difference between the two states might be why changes in coordination number with increasing coverage and decreasing nanopore size do not manifest as unique isotope partitioning or an observable alteration to the macroscopic sorption isotherm. The drivers of substantial isotopic fractionation in the Zn-silica-aqueous solution system are (1) alteration of the bonding environment through monodentate attachment to the surface and (2) increase in the atomic-level disorder of the surface (i.e., reduced crystallinity), which increases the degree of Zn isotope fractionation.

Effects of nano-confinement and crystallinity on Zn isotope fractionation during adsorption onto silica surfaces

253rd National Meeting (2017)

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