Morphology of Lead(II) and Chromium(III) Reaction Products on Phyllosilicate Surfaces As Determined By Atomic Force Microscopy

Abstract

Abstract--Redox and acid-base reactions play important roles in the fate of metal contaminants in soils and sediments. The presence of significant amounts of Cr, Pb and other toxic heavy metals in contaminated soils and sediments is of great environmental concern. Oxidation states and dissolution characteristics of the heavy metals can exert negative effects on the natural environment. Atomic force microscopy (AFM) was used to follow the changes in morphology and structure of reaction products of Cr and Pb formed on mineral surfaces. Nitrate salts of Cr(III) and Pb(II) were used to replace the native exchangeable cations on muscovite and smectite surfaces and the metal-mineral systems were then reacted at different pH's and redox conditions. For Pb, aggregate morphological forms were found at pH 6.1 and 12.4. At pH 6.1, the mean roughness value was 0.70 nm, and at pH 12.4 it was 5.30 rim. The fractal dimensions were 2.03 at pH 6.1 and 2.05 at pH 12.4. For Cr(III), both layered and aggregate morphological forms were found at pH 6.8 and 10.8. The mean roughness values were 0.90 nm at pH 6.8 and 4.3 nm at pH 10.8. Fractal dimensions for both were 2.00. The effect of redox conditions on morphological characteristics was studied on a smectite substrate. The reduced clays were more compacted than oxidized ones and the reduced clay could reduce Cr(VI) to Cr(III), forming new minerals on the surfaces.A geochemical equilibrium model, MINTEQA2, was used to simulate the experimental conditions and predict possible reaction products. Simulation results agreed well with data from experiments, providing evidence that modeling can provide a useful "reality check" for such studies. Together, MINTEQA2 and AFM can provide important information for evaluating the morphologies and chemical reactivities of metal reaction products formed on phyllosilicate surfaces under varying environmental conditions.

Biotite dissolution and Cr(VI) reduction at elevated pH and ionic strength

“…Fe(II) is found in many silicates, and oxidation-reduction reactions between aqueous species and structurally bound Fe on or beneath the surface of both silicates and oxides can control the redox state of associated solutes. In particular, studies have demonstrated the heterogeneous reduction of Cr(VI) by Fe(II) in biotite, vermiculite, illite, smectites, chlorite, magnetite, ilmenite, Fe(II)-hematite, Fe(II)-goethite, and sulfides (Eary and Rai, 1989;Ilton and Veblen, 1994;Gan et al, 1996;White and Peterson, 1996;Patterson et al, 1997). However, it is not known if this mechanism is active at alkaline pH, and it is not clear what role, if any, host mineral dissolution has on Cr transport and fate.…”

Chromate Removal by Dithionite-Reduced Clays: Evidence from Direct X-Ray Adsorption Near Edge Spectroscopy (XANES) of Chromate Reduction at Clay Surfaces

“…Whereas organic matter may be a primary source for Cr(VI) reduction in soils (Bartlett and Kimble, 1976), Fe(II)-containing minerals are more important for Cr(VI) reduction in subsurface formations (Eary and Rai, 1991;Anderson et al, 1994;Kent et al, 1994). Gan et al (1996) demonstrated that the reduction of structural Fe(II) increased the Cr(VI) removal efficiency of clays.…”

“…With X-ray photoelectron spectrosco- Ilton and Veblen (1994) demonstrated the coupled sorption and reduction of chromate at the biotite/water interface. Using atomic force microscopy (AFM), Gan et al (1996) showed that Cr(VI) directly reacted with an Fe(II)-containing smectite, forming precipitates on the smectite surface. Gan et al (1996) also found chromium oxide in Cr(VI)-treated smectite SWa-1, based on an infrared vibration at 540 cm -~ that is assigned to a Cr-O lattice vibration (Amonette and Rai, 1990).…”

“…Using atomic force microscopy (AFM), Gan et al (1996) showed that Cr(VI) directly reacted with an Fe(II)-containing smectite, forming precipitates on the smectite surface. Gan et al (1996) also found chromium oxide in Cr(VI)-treated smectite SWa-1, based on an infrared vibration at 540 cm -~ that is assigned to a Cr-O lattice vibration (Amonette and Rai, 1990). We are not aware of published studies reporting the direct oxidation-state measurements of Cr sorbed by clay surfaces.…”

Kinetics and Mechanism of Metal Retention/Release in Geochemical Processes in Soil - Final Report

“…Fe(II)-containing minerals are more important for Cr(VI) reduction in subsurface formations (Eary and Rai, 1991;Anderson et al, 1994;Kent, et al, 1994). The reduction of structural Fe(II) is known to increase the Cr(VI) removal efficiency of clays (Gan et al, 1996). Earlier studies of Cr(VI) reduction by Fe(II)-containing minerals suggested that the minerals must dissolve to release Fe(II) into solution before reduction could occur.…”