REDUCTION OF STRUCTURAL IRON IN FERRUGINOUS SMECTITE BY FREE-RADICALS

The oxidation state of structural iron greatly influences the physical -chemical properties of clay minerals, a phenomenon that may have sign ificant implications for pollutant fate in the environment, for agricu ltural productivity, and for industrial uses of clays. Knowledge of re dox mechanisms is fundamental to understanding the underlying basis fo r iron's effects on clays. Past studies revealed that the extent of Fe reduction varied depending on the reducing agent used, but this varia tion may not have been a simple function of the reduction potential of the reducing agent. The objective of this study was to identify the r elationship between the Fe reduction mechanism and free radical activi ty in the reducing agent. Several reducing agents and their mixtures w ith the Na-saturated, 0.5 to 2 mum size fraction of ferruginous smecti te (SWa-1) were analyzed by electron spin resonance (ESR) spectroscopy to determine the presence of unpaired electrons or free radicals. Onl y Na2S2O4 exhibited paramagnetic free-radical behavior with a signal a t about g = 2.01 1, which was attributed to the sulphoxylate (SO2-.) f ree radical. The free radical was labile in aqueous solution, and the ability of Na2S2O4 solution to reduce structural Fe in the smectite de creased with age of the solution and paralleled the disappearance of t he free radical signal in the ESR spectrum. The paramagnetic species w as preserved and enhanced if Na2S2O4 was added to the clay suspension, indicating that either the clay surface stabilized the SO2- .radical or the additional unpaired electrons were produced in the clay structu re.