The Influence of Silicate on the Transformation of Lepidocrocite to Goethite

Abstract

Abstract--The mechanism of the transformation of lepidocrocite (7FeOOH) to goethite (aFeOOH) has previously been established and the effect of silicate on the transformation was investigated. Rather than completely inhibiting the reaction, as had been suggested, the presence of Si was found to merely retard the nucleation stage of the transformation. There was found to be no decrease in the dissolution rate of the lepidocrocite due to surface adsorption of Si.Si has no effect if introduced after the nucleation stage, and under conditions of pH and temperature where the dissolution rate of the lepidocrocite largely determines the rate of transformation, the presence of Si has a reduced effect. The results show that Si is adsorbed and incorporated into the goethite structure, and due to its retarding effect on the nucleation, larger crystals of goethite are formed, many of which are twinned.It is inferred from the results that the apparent stability of lepidocrocite occurring in soils in association with goethite cannot be attributed solely to the presence of Si in the soil system. INTRODUCTION IN AN earlier paper (Schwertmann and Taylor, 1972) the mechanism and kinetics of the transformation of lepidocrocite (TFeOOH) to its more stable polymorph, goethite (aFeOOH) have been described. It was concluded that this transformation consists of a dissolution of the lepidocrocite, a nucleation of goethite from the resultant Fe in solution and the crystalline growth of these nuclei. Any one of these three steps can be rate determining, depending on the conditions under which the transformation is being conducted.Since lepidocrocite, although less stable than goethite, exists in soils and sediments for long periods of time it is thought that soil constituents might be capable of retarding or even inhibiting the transformation. In this connection the observation of Hiller (1966) is interesting that the transformation of lepidocricite to goethite "can be completely inhibited by traces of silicate, aluminate and stannate". In our first paper (Schwertmann and Taylor, 1972) a similar observation has been mentioned. The following experiments were conducted to elucidate the influence of silicate on the various phases of the transformation.

The 5-M-NaOH Concentration Treatment for Iron Oxides in Soils

“…Adding 50% quartz caused only partial inhibition of the transformation to hematite, probably because the Si concentration in solution produced by quartz is much lower than that produced by kaolinite. This result shows that dissolved Si is possibly the blocking agent for recrystallization and is in agreement with earlier observations on such an effect on the formation of lepidocrocite (Schwertmann and Thalmann, 1976) and goethite (Schwertmann and Taylor, 1972). The addition of 30 mole % A1 also retarded the transformation to hematite but no A1 was taken up by the hematite.…”

“…In a hl mixture of goethite and lepidocrocite, the lepidocrocite was completely converted by the 5-M-NaOH treatment into a mixture of 82% goethite and 18% hematite. The addition of 50% kaolinite produced a 60:40 mixture of goethite and lepidocrocite, indicating that kaolinite retarded the transformation to goethite and inhibited the transformation to hematite, in agreement with earlier results on the influence of Si on the lepidocrocite to goethite transformation (Schwertmann and Taylor, 1972).…”

Transformation of Akaganéite Into Goethite and Hematite in Alkaline Media

“…Silicate retarded the transformation ofakagan6ite as well as that of ferrihydrite and lepidocrocite (Schwertmann and Taylor, 1972b). Whereas for ferrihydrite, stabilization against dissolution is the predominant mechanism involved , and for lepidocrocite, silicate simply interferes in nucleation of goethite (Schwertmann and Taylor, 1972b), for akagan6ite, stabilization against dissolution and interference in goethite nucleation were both important.…”

“…Seeding with goethite reduced the extent of the slow stage and thereby accelerated the overall reaction; it did not, however, overcome the effect of silicate completely. Schwertmann and Taylor (1972b) reported that although silicate strongly retarded the conversion of lepidocrocite to goethite in alkali media, the effect could be overcome entirely by addition of seed crystals ofgoethite. These authors, however, used comparatively low Si:Fe ratios (0.008) and very concentrated suspensions (12 g oxide/liter).…”

“…Silicate was chosen because it is widespread in natural systems and is believed to retard goethite formation (Carlson and Schwertmann, 1981) and because laboratory experiments have shown it to retard markedly the transformation of lepidocrocite (Schwertmann and Taylor, 1972b) and ferrihydrite to goethite.…”