Goethite healing

Fig. 4.9 Crystal healing of goethite polydomainic serrated goethite crystals formed at 4 °C transformed to monodomainic smooth crystals after hydrothermal treatment at 180°C (Schwert-mann et al., 1985, with permission).

Fig. 12.19 Dissolution-time curves of a goethite synthesized at4°Cin0.3MKOHbefore and after hydrothermal crystal healing at 125,140,160 and 180°C for 16 h. The solid lines correspond to the Avrami-Ero-fejev model (Schwertmann et al. 1985, modified, with permission).

When we come to describe sorption of anions such as phosphate, we might expect that the maximum surface cover could be calculated from the known configuration of the adsorbed ions on the surface. However, this only works for goethite which has been healed [37]. For other samples of goethite, the observed maximum exceeded the theoretical even for the short reaction time of one hour. Subsequent dissolution studies strongly indicated that the difference was due to penetration of the phosphate into pores. 

Most of the published studies on the reaction of ions with iron oxides have used samples of oxides which have been exposed to various extents to carbon dioxide. Models fitted to data from such studies do not reflect fundamental characteristics of the surface but are affected by the degree of exposure to carbon dioxide. In the case of goethite, most of the samples have consisted of multi-domainal crystals. It is only when such crystals have been healed by hydrothermal treatment that the true adsorption reactions of the surface can... 

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