A-FeOOH

Fig. XVII-31. (a) Nitrogen adsorption isotherms expressed as /-plots for various samples of a-FeOOH dispersed on carbon fibers, (h) Micropore size distributions as obtained by the MP method. [Reprinted with permission from K. Kaneko, Langmuir, 3, 357 (1987) (Ref. 231.) Copyright 1987, American Chemical Society.]...

Figure 6 shows transmission electron micrographs of Au particles supported by (a) monocrystalline ellipsoidal (B), (b) monocrystalline pseudocubic, and (c) monocrystalline platelet-type hematite particles (see also Figure 5 for Au particles on polycrystalline ellipsoidal (A) particles). Figure 7 shows Au particles deposited on (a) a-FeOOH, (b) P-FeOOH, (c) ZrOj (A), (d) ZrOj (B), and (e) Ti02 (anatase). 

Fig. 8.31 Left) In the Mossbauer spectrum taken in the Columbia Hills at a rock called Clovis the mineral goethite (GT) (a-FeOOH) could be identified. GT is a clear mineralogical evidence for aqueous processes on Mars. Right) The rock Clovis is made out of rather soft material as indicated by the electric drill-current when drilling the - - I cm deep hole seen in the picture. Drill fines are of brownish color. The pattern to the right of the drill hole was made by bmshing the dust off the surface by using the RAT...

Figure 5.6 Isomer shifts (relative to sodium nitroprusside) as a function of temperature in the Debye model, along with measurements on a-FeOOH (adapted from [11]).

The crystallography of iron oxides is complex because there are two oxides, FeO and Fe203, as well as a-FeOOH, which can occur in mixed crystals. Kittel [34] has given a full account of the chemistry and Feitknecht [35] has discussed the theory of the colours of these pigments. 

We will demonstrate how the surface charge of a hydrous oxide (a-FeOOH) can be calculated from an experimental titration curved (e.g., Fig. 2.2). 

In titrating a suspension of a-FeOOH (6 g e- 120 m2 g-1 2 10 4 mol g-1 surface functional groups (=FeOHTOT)) in an inert electrolyte (10 1 M NaCICU) with NaOH or HCI (Cb and Ca= concentration of base and acid, respectively, added per liter), we can write for any point on the titration curve... 

Titration of a suspension of a-FeOOH (goethite) in absence of specifically adsorbable ions. 

Sigg, L., and W. Stumm (1981), The Interaction of Anions and Weak Adds with the Hydrous Goethite (a-FeOOH) Surface", Colloids and Surfaces 2, 101-117. 

If this mechanism is consistent with the experimental relaxation data, then a plot of xp versus the expression in the brackets of Equation 35 will give a straight line with a slope of kjnt and an intercept at the origin. As shown in Figure 11, the data fit this proposed mechanism quite well. Values for i i0, reactant and product concentrations, and K nt input into Equation 35 are from the equilibrium modeling results calculated at each pH value for which kinetic runs were made. Normally a variety of different mechanisms are tested against the experimental data. Several other more complex mechanisms were tested, including those postulated for metal ion adsorption onto y-A O (7) however, only the above mechanism was consistent with the experimental data. Hence it was concluded that the bimolecular adsorption/desorption reaction was the most plausible mechanism for Pb2+ ion adsorption onto a-FeOOH. 

Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (a-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. 

To put things into perspective, we. can broadly classify these analytical methods into bulk, dry surface, and in situ interfacial techniques. This chapter focuses on the last category, illustrating two in situ techniques used to study anion binding at the goethite (a-FeOOH)-water interface titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy. In fact, CIR-FTIR could prove to be extremely powerful, since it allows direct spectroscopic observation of ions adsorbed at the mineral-water interface. 

In addition, x is generally not constant and appears to exhibit some dependency on pH and adsorption density (T). Cation coefficients are generally greater than one, although there are some marked exceptions (e.g., Cd/a-FeOOH or Na-montmorillonite). In contrast, the absolute value of net proton coefficients for anions are generally between zero and one. The negative value of x for anions indicates that anion adsorption results in an overall removal of protons from solution. 

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