The behavior of Ni~(2+) on calcite surfaces

摘要

Transport of Ni~(2+) in the geosphere plays a role in the formation of ore deposits as well as in the dispersion of contaminants in the environment. Some elements (Cd~(2+), Zn~(2+), Na~+, K~+, and Cl~-) are known to diffuse in calcite at the rate of nanometers in months, so questions arose about the ability of Ni~(2+) to move away from adsorption sites at the surface into the bulk. Nickel incorporation into calcite is limited by its high dehydration enthalpy and by its ligand field hindrance to entering the distorted octahedra of calcite, but evidence exists that calcite can tolerate several percent Ni~(2+) in the in the structure. Cleaved samples of Iceland spar were exposed for 1 minute to solutions of 10~(-3) M and 10~(-2) M Ni(ClO_4)_2, the solution was physically removed and the samples were examined using the surface sensitive techniques: X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and Atomic Force Microscopy (AFM). XPS and TOF-SIMS showed that Ni~(2+) was adsorbed while AFM confirmed that dissolution was taking place. The sample was stored in air and relative surface concentration and physical morphology were monitored for 2 years. Trends in the chemical data suggested statistically significant loss of surface Ni~(2+) with time, but the decrease was very close to the limits for significance. AFM images demonstrated that surface topography of the Ni-exposed samples is modified by spontaneous recrystalization in the water layer adsorbed from air in exactly the same way that clean calcite surfaces typically rearrange. This process could bury a small amount of Ni~(2+) in the bulk, explaining the very weak loss. Limited burial of Ni~(2+) within the near-surface could renew calcite adsorption sites, thus increasing uptake capacity. Evidence indicates that surface recrystalization occurs even in very dry environments (<5% humidity). This means that burial could play a role in Ni~(2+) mobility in unsaturated groundwater regimes or in fractures (such as in concrete) where water flow is intermittent. An important point is, however, in comparison to incorporation rates for divalent Cd and Zn, the extent of movement of Ni~(2+) is extremely low. Thus, incorporation might have an effect on Ni~(2+) retardation in flow paths extending over very long time scales (> 10,000 years) such as would be relevant for geological processes and for long-term radioactive waste disposal. However, incorporation by burial would have negligible effect on the amount of Ni~(2+) removed from groundwater by adsorption, in systems where the transport times are short (< 100 years) such as for drinking water supplies from calcite-bearing porous media.