Sorption and Abiotic Redox Transformation of Nitrobenzene at the Smectite-Water Interface.

The effect of the redox state of structural Fe on the surface reactivity of iron-bearing phyllosilicates in aqueous suspension was investigated using a molecular probe. For this purpose the structural Fe in montmorillonite and ferruginous smectite was chemically reduced by sodium dithionite in citrate-bicarbonate (CB) buffer solution under N(2) at 70 degrees C, with the excess reactants removed by washing and centrifugation. Nitrobenzene was chosen as an electron acceptor probe to react with unaltered and chemically reduced smectites. Nitrobenzene was transformed into aniline only in the presence of the reduced smectites. This abiotic reductive transformation depended on the concentration of the electron acceptor in solution and the total accessible structural Fe(II) in smectites. As much as 40% of the crystal layer structural Fe(II) of the reduced smectites was oxidized to Fe(III); these electrons migrated to the surfaces/edges and then were transported into the adjacent aqueous layer. No significant effect of the oxidation state of structural Fe on the sorption of nitrobenzene on smectites was observed, but the reduced smectites sorbed less aniline than the unaltered smectites. The electronic structure and molecular geometry of the probe were modified within the smectite-water interface. Reducing structural Fe in smectites perturbed the surface reactivity. Copyright 2001 Academic Press.

Structural Perturbations in the Solid-Water Interface of Redox Transformed Nontronite.

Redox reactions of structural Fe affect many surface and colloidal properties of Fe-containing smectites in natural environments and many industrial systems, but few studies have examined the clay-water interface under oxidizing and reducing conditions. Infrared (FTIR) spectroscopy was used to investigate the effects of structural Fe oxidation state and hydration on layer Si-O stretching vibrations in Na-nontronite. Aqueous gels of unaltered, reduced, and reoxidized smectites were equilibrated at different swelling pressures, Pi, and water contents, m(w)/m(c), using a miniature pressure-membrane apparatus. One part of each gel was used for the gravimetric determination of m(w)/m(c); the other was transferred to an attenuated total reflectance cell in the FTIR spectrometer, where the spectrum of the gel was measured. The frequencies of four component peaks of Si-O stretching, nu(Si-O), in nontronite layers and of the H-O-H bending, nu(H-O-H), in the interlayer water were determined by using a curve-fitting technique. Reduction of structural Fe shifted the Si-O vibration to lower frequency and desensitized the Si-O vibration to the hydration state. A linear relation was found between nu(Si-O) and nu(H-O-H) for nontronite in each of its various oxidation states. These observations were interpreted to mean that structural Fe oxidation state has a significant impact on interfacial processes of the aqueous colloid system of Fe-rich phyllosilicates. Copyright 2000 Academic Press.