Methods for handling redox-sensitive smectite dispersions

Abstract

AbstractRedox activation (reduction of structural Fe) of smectites greatly alters their chemical reactivity and physical properties, which may be exploited for various environmental, agricultural or industrial purposes. Their re-oxidation during preparation, characterization, and use is, however, a significant risk to their utility. In this study, methods and apparatus were developed and described which mitigated reoxidation. Ferruginous smectite (sample SWa-1, Na saturated) was used as the model smectite. It was reduced with sodium dithionite in a citrate-bicarbonate buffer solution at 70°C for 4 h, which achieved a maximum Fe(II)/total Fe ratio of 0.9113 ± 0.0048. The first step in rendering reduced samples useful is to remove from them the reducing agents and other solutes present during reduction. This was accomplished in the present study by reducing the sample in an inert-atmosphere reaction tube (IRT) (a 50 mL centrifuge tube equipped with a removable septum cap), then removing solutes from the suspension by centrifuge washing. The washing steps were performed with the aid of a controlled-atmosphere liquid exchanger (CALE) which provided connections between the sample suspension and deoxygenated solutions. The reduced state was measured by 1, 10-phenanthroline or by Mössbauer spectroscopy at 77 K to give Fe(II)/total Fe ratios. Some samples were freeze dried after washing. Results revealed that if reduced smectites are washed without protection from atmospheric O2, the extent of reoxidation is on the order of 40 to 60%. If the sample is subsequently dried, reoxidation increases to more than 76%. If the sample is protected using the IRT and the CALE, however, reoxidation is decreased to less than 2%. Freeze drying in a glove box allowed reoxidaton to increase to slightly more than 10%. These results indicate that more reoxidation occurred during the drying stage than during the washing stage. These observations lead to the conclusions that (1) protection of reduced samples from atmospheric O2 is essential if extensive reoxidation is to be prevented, and (2) the methods and apparatus described herein are effective for accomplishing that purpose in abiotically reduced smectites. They may also be effective if applied to microbially reduced smectites.