The behavior of Ni2+ on calcite surfaces

Transport of Ni2+ in the geosphere plays a role in the formation of ore dep osits as well as in the dispersion of contaminants in the environment. Some elements (Cd2+, Zn2+, Na+, K+, and Cl-) are known to diffuse in calcite at the rate of nanometers in months, so questions arose about the ability of Ni2+ to move away from adsorption sites at the surface into the bulk. Nicke l incorporation into calcite is limited by its high dehydration enthalpy an d by its ligand field hindrance to entering the distorted octahedra of calc ite, but evidence exists that calcite can tolerate several percent Ni2+ in the structure. Cleaved samples of Iceland spar were exposed for 1 minute to solutions of 10(-3) M and 10(-2) M Ni(ClO4)(2), the solution was physicall y removed and the samples were examined using the surface sensitive. techni ques: 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 Ni2+ was adsorbed while AFM confirmed that dissolution wa s taking place. The sample was stored in air and relative surface concentra tion and physical morphology were monitored for 2 years. Trends in the chem ical data suggested statistically significant loss of surface Ni2+ with tim e, but the decrease was very close to the limits for significance. AFM imag es demonstrated that surface topography of the Ni-exposed samples is modifi ed 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 Ni2+ in the bulk, explaining the very weak loss. Limited burial of Ni2+ within the near-surface could renew calci te adsorption sites, thus increasing uptake capacity. Evidence indicates th at surface recrystalization occurs even in very dry environments (<5% humid ity). This means that burial could play a role in Ni2+ mobility in unsatura ted groundwater regimes or in fractures (such as in concrete) where water f low is intermittent. An important point is, however, in comparison to incor poration rates for divalent Cd and Zn, the extent of movement of Ni2+ is ex tremely low. Thus, incorporation might have an effect on Ni2+ retardation i n flow paths extending over very long time scales (> 10,000 years) such as would be relevant for geological processes and for long-term radioactive wa ste disposal. However, incorporation by burial would have negligible effect on the amount of Ni2+ removed from groundwater by adsorption, in systems w here the transport times are short (< 100 years) such as for drinking water supplies from calcite-bearing porous media. Copyright (C) 2001 Elsevier Sc ience Ltd.