Search

Dissolution and epitaxial growth on dolomite [CaMg[(C[O.sub.3]).sub.2]] cleavage surfaces under well-defined aqueous solution conditions were studied in real-time by atomic force microscopy (AFM). Step-retreat speeds in undersaturated solutions were used to predict the growth speed for ordered dolomite utilizing a theoretical step-growth model. Monolayer growth on dolomite under highly supersaturated conditions was observed, but multilayer growth was inhibited due to film disorder enhanced solubility. Ordered dolomite film growth is predicted in supersaturated solutions close to equilibrium, but observed step speeds were immeasurably low under the current laboratory low-temperature conditions.

Dissolution of the (104) surface of magnetite (MgCO3) was studied as a function of bulk solution pH over the range 2.0 less than pH less than 5.0 at 60 degree using atomic force microscopy (AFM). The AFM images revealed a dramatic increase in step density associated with a large increase in dissolution flux that was attributed to the protonation of terrace-adsorbed carbonate sites.

Step growth kinetics on barium sulfate (001) is measured as a function of step length, supersaturation, and temperature. The results demonstrate that knowledge of the kink kinetics, as a function of temperature is important in testing the applicability of the classical thermodynamic model to kinetic data.

In contrast to most ionic minerals studied by SFM, the periclase (001) surface dissolves not by retreat of monolayer steps parallel to (001), but by retreat of a rough surface perpendicular to (001). At pH < 2, dissolution has an additional contribution from retreating macro-steps at the edges of nearly square pits. The macro-steps have heights up to 120 nm. In general, step direction is parallel to [110] and equivalent directions. During dissolution at low pH, a soft near-surface region is formed. Other investigators have shown that the near-surface region is protonated. Protonation is supposed to stabilize the (111) surface of periclase. Due to the structural similarities between periclase (111) and brucite (001), and similar dissolution rates of periclase and brucite at pH < 5, we conclude that during dissolution the periclase (001) surface is restructured into '(111) nano-facets' representing brucite (001)-like layers and appearing as a rough and soft surface in SFM images. The most probable reasons that the slopes of these macro-steps (up to 50 [degrees]) are lower than the slopes of perfect (111) facets are the likely poorly ordered structure of these layers, microtopography on these surface facets, and tip-surface convolution in SFM. By measuring the vertical position of the surface vs. time, we calculated the dissolution rate. At pH 1 and pH 2 we found the rates to be 17.1 [+ or -] 5.8 x [10.sup.-6] and 5.7 [+ or -] 3.7 x [10.sup.-6] mol/[m.sup.2].s, respectively. These rates are in reasonable agreement with previously reported rates of periclase and brucite (001) dissolution, and are consistent with the idea that the MgO (001) surface consists of Mg[(OH).sub.2] (001)-like layers.

One of the promising technologies expected to improve environmental chemistry research is the scanning probe microscopy, which enables researchers to gain new insights into the fundamentals of interfacial chemistry. The utility of scanning probe microscopy in environmental chemistry is in its ability to monitor key energetic parameters of reactive sites, especially between fluids and particles. The techniques ability to monitor real-time chemical reactions is also important in gaining insights into combinatorial chemistry that will enhance the effectivity of chemical treatment strategies.

Dissolution and epitaxial growth on dolomite [CaMg(CO3)2] cleavage surfaces under well-defined aqueous solution conditions were studied in real-time by atomic force microscopy (AFM). Step-retreat speeds in undersaturated solutions were used to predict the growth speed for ordered dolomite utilizing a theoretical step-growth model. Monolayer growth on dolomite under highly supersaturated conditions was observed, but multilayer growth was inhibited due to film disorder enhanced solubility. Ordered dolomite film growth is predicted in supersaturated solutions close to equilibrium, but observed step speeds were immeasurably low under the current laboratory low-temperature conditions. [ABSTRACT FROM AUTHOR]