A comparative study of the behavior of four types of smectite is reported: two with a low Fe content, SHCa-1 (hectorite from San Bernadino, California), SWy-1 (montmorillonite fromWyoming) and two nontronites NG-1 (from Hohen Hagen, Germany) and SWa-1 (Grant County, Washington). Cation exchange was performed with a freshly prepared 1 M Fe nitrate aqueous solution. Intercalation with the same solution partially neutralized with an anhydrous carbonate solution, giving a molar ratio of OH−/Fe = 2 was also studied. The modified clays were characterized by X-ray diffraction, N2 adsorption-desorption, Mössbauer spectroscopy, thermogravimetric analysis, atomic absorption spectroscopy, X-ray fluorescence and inductively coupled plasma analysis.
In the cation exchange process, the goethite impurities grew with the Fe-rich clays but not with the Fe- poor clays. This exchange was also found to have no effect on the thermal stability and structure of the clay minerals with low Fe content, whereas it had a slight effect on the structure of the Fe-rich clays and on their thermal stability. The extent of the intercalation, however, appears to depend on the expandability of the clay layers to accommodate the Fe(III) polycations, and increases from the nontronites to montmorillonite and to hectorite. This intercalation treatment has only a slight effect on both the structure and the thermal stability of hectorite and montmorillonite. In contrast, the nontronites undergo a dramatic change in both their structure and thermal stability. The formation of Fe oxyhydroxide and oxide phases in both procedures becomes very important when the initial structural Fe content of the clay minerals is high, increasing from SHCa-1, SWy-1, SWa-1 to NG-1. The contribution of the structural Fe of the clays to the formation of the Fe oxyhydroxide is not negligible, because of partial leaching of Fe from the octahedral sheet of Fe-rich clays due to the low pH of the solutions.