Structural perturbations in the solid-water interface of redox transformednontronite

Redox reactions of structural Fe affect many surface and colloidal properti es of Fe-containing smectites in natural environments and many industrial s ystems, but few studies have examined the clay-water interface under oxidiz ing and reducing conditions. Infrared (FTIR) spectroscopy was used to inves tigate the effects of structural Fe oxidation state and hydration on layer Si-O stretching vibrations in Na-nontronite. Aqueous gels of unaltered, red uced, and reoxidized smectites were equilibrated at different swelling pres sures, Pi, and water contents, m(w)/m(c), using a miniature pressure-membra ne apparatus. One part of each gel was used for the gravimetric determinati on of m(w)/m(c); the other was transferred to an attenuated total reflectan ce cell in the FTIR spectrometer, where the spectrum of the gel was measure d. The frequencies of four component peaks of Si-O stretching, nu(Si-O) in nontronite layers and of the H-O-H bending, vad-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(S i-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 st ate has a significant impact on interfacial processes of the aqueous colloi d system of Fe-rich phyllosilicates. (C) 2000 Academic Press.