Charge Heterogeneity and Nanostructure of 2:1 Layer Silicates by High-Resolution Transmission Electron Microscopy

Abstract--Microbial reduction of clay mineral structural Fe(III) decreases the swelling of nontronite gels, most importantly at intermediate oxidation states (40 to 80 cmoi Fe(II) kg -1 clay). The purpose of this study was to establish whether microbial reduction of structural Fe(III) decreased the swelling of other Fe-bearing smectites and to discern the influence that organic compounds of microbial origin (bacterial cells, cell fragments and/or exudates) may have on clay swelling and texture. Structural Fe(III) was reduced by incubating smectite suspensions with either a combination of Pseudomonas bacteria or a mixture of anaerobic bacteria. The influence of organics on clay swelling was estimated on smectites suspended in either organic or inorganic media in the absence of bacteria. The gravimetric water content of the reduced clay gels equilibrated at various applied pressures was recorded as a function of Fe oxidation state. Transmission electron microscopy (TEM) was employed to determine the influence of bacteria and type of media on the texture of reduced smectite gels. Reduction of structural Fe(III) by bacteria decreased the swelling pressure of all Fe-bearing smectites. Increased clay swelling, due to the presence of organics (organic medium, exudates or cell fragments), was correlated to the total Fe content, the extent of structural Fe reduction, as well as the initial swelling characteristics of the Fe-bearing smectites. High structural Fe(II) contents (>50 cmol Fe(II) kg -~) resulted in increased attractive forces between clay platelets that decreased clay swelling, even in organic medium suspensions. Microbial reduction resulted in increased face-face association of individual clay layers, forming larger and more distinct crystallite subunits than in nonreduced clay gels. But, perhaps more importantly, microbial reduction of structural Fe(III) resulted in an increased association between crystallite subunits and, thus, an overall larger particle size and pore size distribution, due to the interaction of bacteria ceils, cell fragments and organic exudates.

Section: Texture Of Biologically Reduced Ferruginous Smectitesupporting

confidence: 93%

Section: Texture Of Biologically Reduced Ferruginous Smectitesupporting

confidence: 63%

Swelling and Texture of Iron-Bearing Smectites Reduced by Bacteria

Gates

1998

“…Clay samples were embedded in LR White (Hard formula) in gelatin capsules which were filled and capped. The resin was polymerized in an oven at 60 ~ for 2 to 3 d. Sections (60-120-nm thickness) were cut with a diamond knife and collected onto carbon formvar-coated Cu or Ni grids (Malla et al 1993).…”

Section: Characterization Of the Claysmentioning

confidence: 99%

Preparation, Characterization and Electrochemistry of Synthetic Copper Clays

Xiao

1998

Abstraet--Hydrothermal treatment of aqueous mixtures of sodium hydroxide, copper chloride and excess sodium silicate (Si/Cu --> 2) at 150 ~ produced blue powders. Scanning electron microscopy/energy dispersive X-ray (SEM/EDX) analysis of the products show they all had similar chemical compositions, with Si/Cu ratios of approximately 1.33, the value expected for 2:1 trioctahedral phyllosilicates. Their Xray diffraction (XRD) patterns were consistent with that of swelling smectite-type clays. Reaction mixtures that did not contain excess Si (Si/Cu -< 1.33) did not produce smectites. They gave gray mixtures of amorphous silicates and copper oxides, with some phyllosilicates. A mixture containing a Si/Cu ratio of 5.2 heated at 250 ~ under 500 psi of Ar gave a pale blue solid containing a Si/Cu ratio of approximately 1, the value expected for chrysocolla. Transmission electron microscopy (TEM) showed this product had a well-ordered layered structure. Its XRD powder pattern was consistent with that of chrysocolla. This clay did not swell very much on exposure to glycol vapors. Peaks were observed in the cyclic voltammogram of electrodes modified with films of these synthetic Cu-clays. They were attributed to electrochemical activity of Cu(II) centers in the lattice of the clays. The presence of these redox active Cu(II) sites greatly improved charge transport in the films. Much larger voltametric waves were observed for [Os(bpy)3] 2 § ions ("bpy" = the ligand 2,2'bipyridyl) adsorbed in fihns of the synthetic Cu-clays than in films of a natural montmorillonite. The larger peak currents obtained corresponded to 10-to 15-fold increases in the fractions of the adsorbed ions that were electrochemically oxidizable in the modified electrodes.

“…Clay samples were embedded into LR White (Hard formula) in gelatin capsules that were filled and capped. The resin was polymerized in an oven at 60 ~ for 2 to 3 d. Sections (60-120 nm thickness) were cut with a diamond knife and collected onto carbon formvar-coated copper grids (Malla et al 1993).…”

Section: Characterization Of the Claysmentioning

confidence: 99%

Electrodes Modified with Synthetic Clay Minerals: Electrochemistry of Cobalt Smectites

Xiang

1996

Abstract--Hydrothermal treatment of a mixture of silicic acid, cobalt chloride, sodium dithionite and sodium hydroxide at 250 ~ under 500 psi of argon produced a pink solid. X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction data showed the product to be a well-crystallized smectite. SEM/EDX analysis gave a unit cell formula of [(Si8.05)(Co558)O20(OH)4]Na066. Heating the same mixture at 150 ~ without argon gave a less well ordered smectite of composition [(Si7.93)(COs.92)O20(OH)4]Nao.42. Two peaks were observed in the cyclic voltammograms of electrodes modified with films of these two clays recorded for the blank electrolytes in the absence of any adsorbed electroactive species. The first peak was attributed to the oxidation of a small fraction of the Co 2+ sites within the clay lattices to Co 3+. The second peak was assigned to further oxidation of these Co 3+ to Co 4+.