Kinetics of reductive bulk dissolution of lepidocrocite, ferrihydrite, andgoethite

The variation in Fe-oxide reactivity was investigated by studying the kinet ics of bulk reductive dissolution of a suite of synthetic Fe-oxides in 10 m M ascorbic acid at pH 3. The Fe-oxides comprised three different ferrihydri tes, five lepidocrocites, and a poorly crystalline goethite. During one of the reduction experiments, lepidocrocite crystals were subsampled and the c hange in crystal habit and size distribution was studied by transmission el ectron microscopy. The rate of complete dissolution was described by the function J/m(o) = k'( m/m(o))(gamma) where J is the overall rate of dissolution (mol/s), m(o) the initial amount of iron oxides, and m/m(o) the undissolved mineral fraction . Rate laws were derived for the different iron oxides and showed a variati on in initial rates of about two orders of magnitudes: 2-line ferrihydrites being most reactive with k' = 7.6-6.6 X 10(-4).s(-1) whereas the initial r ate for 6-line ferrihydrite is an order of magnitude lower 7.4 X 10(-5).s(- 1) and comparable to the quite homogeneous group of lepidocrocites (3.2-8.1 X 10(-5).s(-1)) with finally the initial rate of goethite being one order of magnitude lower again (5.4 X 10(-6).s(-1)). The transmission electron mi croscopy results for lepidocrocite showed strong etch-pitting of the crysta ls parallel to the c-axis resulting ultimately in disintegration of the cry stals. For the different iron oxides, the initial rate was independent of the spec ific surface area, emphasizing the importance of the crystal structure for the dissolution rate. However, among the lepidocrocites the initial rate wa s proportional to the specific surface area. The exponent, gamma was found to vary from a value near 1.0 for one of the 2-line ferrihydrites, two of the lepidocrocites and the goethite, to values close to 2.3 for the other 2-line ferrihydrite and the 6-line ferrihydrite . Thus, the largest variation in reduction rate during bulk dissolution is found for ferrihydrite. For the lepidocrocites, the preparations that predo minantly consist of single domain crystals yielded gamma -values near 1.4-1 .6, whereas the multidomainic crystal preparations yielded values of 1.0-1. 1. The parameter gamma collects the effects of factors, such as the crystal geometry, the particle size distribution and the reactive site density. Th e relative importance of these factors was evaluated and particularly the p article size distribution appears to be of importance for iron oxides. Copy right (C) 2001 Elsevier Science Ltd.