Search

In situ atomic force microscopy (AFM) was used to follow the far-from-equilibrium dissolution of the {001} cleavage surface of natural gibbsite in nitric acid. The main dissolution mechanism was the retreat of straight monolayer steps, the edges of which are paraallel to the , , and directions. The stability of these steps can be expressed as > > . The results are explained in terms of the positions of the terminal O atoms and their associated A1 atoms at the steps. New steps were formed at etch pits that opened where screw dislocations emerged on the surface. The dissolution rates were calculated from the change in size of pits and islands. The values obtained were 9.5 x [10.sup.-9] - 2.3 x [10.sup.-8] mol/[m.sup.2].s, normalized to the total surface area, and 1.8 - 3.6 x [10.sup.-7] mol/[m.sup.2].s, normalized to the surface area of the step fronts. The rates of dissolution calculated using only the surface area of the step fronts are similar to literature values obtained by other methods.

Rapid and accurate determination of the surface area of three kaolinite clay standards, taking into account the complex microtopography of the particles, was achieved using atomic force microscopy images and computerized image analysis. All surface areas were determined to within 3%. Edge surface area is 18.2-30.0% of the total surface area depending on the particular kaolinite standard. Specific surface areas agree to within 4% of published values determined by the BET method. The approach can be applied to clay and nanoparticle samples too small in quantity for BET analysis, since it requires ~11 orders of magnitude less material.

A combination of atomic force microscopy, scanning election microscopy, transmission electron microscopy, energy dispersive spectroscopy, electron diffraction, and X-ray diffraction were used to study reactions of 0.5-500 mg/L aqueous Pb with [Ca.sub.5][([PO.sub.4]).sub.3]OH, hydroxylapatite (HAP), at pH 6 and 22 [degrees] C. Following 2 h reaction time, concentrations of [Pb.sub.aq] ([[Pb.sub.aq]]) decreased from 500 mg/L to [is less than] 100 mg/L, and from 0.5- 100 mg/L to [is less than] 15 [micro]g/L. This loss of [Pb.sub.aq] from solution (i.e.. sorption) resulted partially from simultaneous dissolution of HAP and precipitation of [Pb.sub.5][([PO.sub.4]).sub.3]OH, hydroxypyromorphite (HPY), or another solid Pb phase. The initial saturation state with respect to HPY (defined as the ratio of the ion activity product to equilibrium solubility product) influenced strongly precipitation processes. At a high degree of saturation (initial [[Pb.sub.aq]] [is greater than] 100 mg/L), small nuclei or aggregates of poorly crystalline HPY precipitated homogeneously in solution. At intermediate saturation (initial [[Pb.sub.aq]] ~10-100 mg/L), large, euhedral needles of HPY precipitated homogeneously in solution. At a low degree of saturation (initial [[Pb.sub.aq]] [is less than] 10 mg/L), a needle-like Pb-containing phase grew heterogeneously on HAP These results agree well with concepts derived from nucleation and growth theories and demonstrate that initial saturation state influences strongly the sorption process.

The dissolution of individual hectorite (a trioctahedral smectite) particles has been observed at molecular scales in acidic aqueous solution with atomic force microscopy (AFM). A new sample preparation technique was used to attach nanometer-sized hectorite particles to a mica substrate. The reactive surface area of individual hectorite particles was identified and its change during a dissolution experiment was quantified. The dissolution of hectorite under pH 2 conditions takes place exclusively at the edge surfaces. In contrast, the basal surface is completely unreactive within the investigated time scale of several hours. The short edges of the hectorite laths were found to react somewhat more quickly than the long edges. The edge surface area represents 1.5-3.3% of the total surface area. The total surface area has been determined from the actual particle dimensions derived from AFM data to be 730 [m.sup.2]/g. The dissolution rate normalized to the reactive edge surface area (ESA) has been determined to be 7.3 x [10.sup.-9] mol hectorite/([m.sup.2]-s), which represents a total surface area (TSA) normalized dissolution rate of 1.9 x [10.sup.-10] mol hectorite/([m.sup.2]-s). The ESA/TSA ratio increases by about 15% within 1 h exposure to a pH 2 aqueous solution at 22 [degrees] C.