Cd 2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES)

Abstract

Cadmium uptake by calcite from aqueous solution was studied using techniques sensitive to the near-surface: X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). These techniques allowed direct observations of structure and bonding environments at the calcite surface. The results indicate that the main processes involved in cadmium uptake by calcite are adsorption and solid-state diffusion into the crystal, which leads eventually to the formation of solid-solution. Pure calcite crystals were cleaved from precleaned Iceland spar and were briefly exposed to aqueous solutions containing various concentrations of Cd 2+, CO 3 2−, ClO 4 −, and/or Cl −. Some Cd 2+ was radiolabelled. LEED results demonstrate that the calcite surface is atomically ordered, even after hydration and cadmium uptake. γ-scintillation data from crystals exposed briefly to solutions of 109Cd 2+ indicate that surface uptake ranged from the equivalent of about 1 to 4 monolayers. XPS analyses in the first 2 hours after exposure detected Cd within the top 30 Å, but crystals stored in air or in ultra-high vacuum showed a decrease in Cd surface concentration with time such that after two days, Cd was barely detectable in the near-surface region. In other experiments, LEED verified the crystallinity of otavite (CdCO 3) grown epitaxially over the {101} cleavage faces of calcite, and XPS showed almost no Ca in the near-surface on scans taken immediately after precipitation; but after storage for a month in ultrahigh vacuum, binding energy shifts and the presence of a Ca peak strongly suggested the development of solid-solution by diffusion through the solid. No Cd enrichment was observed at sites of surface defects using AES, indicating that solid-state diffusion into the mineral surface was not accomplished simply by migration along microfractures alone. This work suggests that solid-state diffusion may play a role in the rate and extent of uptake of certain trace metals from solution and probably leads to the formation of solid-solution in calcite and other carbonate minerals. It also suggests that the process of diffusion into the solid mineral host should be considered in hydrogeochemical models that intend to simulate and predict trace-metal mobility in carbonate terrains.