Hydrothermal treatment of aqueous mixtures of sodium hydroxide, copper chloride and excess sodium silicate (Si/Cu ≥ 2) at 150 °C 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 X-ray 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 °C 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 voltam-mogram 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 films 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.

Hydrothermal treatment of a mixture of silicic acid, cobalt chloride, sodium dithionite and sodium hydroxide at 250 °C 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)(Co5.58)O20(OH)4]Na066. Heating the same mixture at 150 °C without argon gave a less well ordered smectite of composition [(Si7.93)(CO5.92)O20(OH)4]Na0.42. 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 Co2+ sites within the clay lattices to Co3+. The second peak was assigned to further oxidation of these Co3+ to Co4+.

Much attention has been paid to the adsorption of Fe(II) onto mineral surfaces as it is a crucial step in enhancing the reductive activity of Fe(II) species. The present study elucidates the role of Fe(II) adsorbed on Fe (oxyhydr)oxides (γ-FeOOH, α-FeOOH, and α-Fe2O3) for the reductive transformation of 2-nitrophenol (2-NP), using cyclic voltammetry (CV). Studies of Fe(II) adsorption and 2-NP reduction kinetics showed that an increase in pH gave rise to an elevated density of adsorbed Fe(II) on mineral surfaces, which further resulted in an enhanced reaction rate of 2-NP reduction. In addition, CV tests showed that the enhanced activity of Fe(II) species is attributed to the negative shift of peak oxidation potential (EP) of the Fe(III)/Fe(II) couple. The dependence of adsorbed Fe(II) reactivity on pH values was proven by the three linear correlations obtained (ln k vs. pH, EPvs. pH, and ln k vs. EP). The present study demonstrated that the reductive activity of adsorbed Fe(II) species can be indicated by the EP value of active Fe(II) species. Moreover, the electrochemical approach can be used as an effective tool to study the reductive activity of adsorbed Fe(II) species in subsurface environments.