Redox of Iron in Smectites

Abstract

The reduction of structural iron inside the TOT layer of a smectite has been performed for a long time through a complex mechanism. So far one chemical method shows some efficiency: reduction with dithionite in a citrate bicarbonate buffer. An alternative reducing agent, the metallic iron, through its corrosion process, was also studied here. The introduction of a small amount of Fe (0) has a significant impact on the smectite stability. Most of the reduction is done once the metal is consumed while a 7Å phase is formed. Another anomalous redox behavior appears during the oxidation of a Fe (II) exchanged smectite. After two months in an aerated environment, the Fe (II) inside the interlayer space is only partially oxidized and its Mossbauer quadrupolar splitting is very unconventional. Both experiments show that redox in smectite cannot be determined by the chemistry of the bulk solution. Globally these experiments raise major question about i) the water states in the interlayer and ii) the pH and Eh measurements within the smectite phase. These data are important in order to perform robust modeling of swelling clay reactivity.