Goethite strengths

The three most abundant minerals forms are Fe203 (hematite), Fe(OH)j (hydrous ferric oxide or ferrihydrite), and FeO(OH) (goethite). The chemical reactions describing their dissolution and K in slightly acidic water at 25°C, 1 atm, and the ionic strength of... 

Fig. 16.12 Pseudo-first-order rate constants, for the transformation of CFBr - 6.5pM) in suspensions of goethite (25Fe(II)j j of ImM at 25 °C ionic strength = 20mM) as a function of solution pH ( ). The contact time of Fe(II) with iron oxide before addition of CFBr3 was >24 hours. Also shown are k values for the control experiments in the absence of goethite ( ). The precipitate formed in the pH 8.9 control (arrow) was identified as a form of green rust. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalogenated methanes by surface-bound Fe(ll) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

Conditions which promote multi-domainic goethites are high ionic strength (either [KOH] or salt) and also low synthesis temperature (<40°C). In alkaline solutions, multi-domainic character decreases and domain width increases as Al substitution increases to Al/(Fe-i-Al) of 0.15, whereas at Al/( Al-nFe) >0.15 single domain crystals result (Schulze Schwertmaim, 1984 Mann et al., 1985). Multidomainic goethites can recrystallize to single domain crystals as a result of hydrothermal treatment at 125-180 °C (Fig. 4.9) (Schwertmann et al., 1985). 

Fig. 10.9 Electrophoretic mobility of synthetic Fe oxides (0.01 g L" ) in the presence of humic material from a eutrophic lake. Curve a goethite in an Na -Cl"-HCOi medium, total ionic strength 2-10" M, no humic material. Curve b ...

Some stability constants for ion pairs on Fe oxides are listed in Table 10.4. This model was applied by Davis and Leckie (1978, 1980) to adsorption of various cations and anions on ferrihydrite. The extended triple layer model of Sahai and Svenjensky (1997) incorporates recent advances in aqueous electrolyte chemistry which enable aqueous activity coefficients for electrolytes to be calculated over a wide range of ionic strengths. The model also considers the free energy of adsorption of an ion to be the sum of the contributions from an electrostatic term, a Born solvation term and a ion intrinsic term. This extended model has been applied to adsorption of Co and Cd on goethite. 

Adsorption of ferricyanide (a contaminant of sites of former gas plants) on goethite decreases with rising pH and is dependent on ionic strength it is fully reversible and is thought to involve both inner and outer sphere complexes. Adsorption of ferrocyanide on goethite involves initial formation of inner sphere complexes followed by precipitation of a Berlin Blue-like phase on the goethite surface (Rennert and Mansfeldt, 2001). 

W.A., Sparks, D.L. (1999) Multicomponent transport of sulfate in goethite-silica sand system at variable pH and ionic strength. Environ. Sci. Technol. 33 3443-3450 Meeussen, J.C.L., Scheidegger, A.M.,... 

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

Herrera Ramos A. C. and McBride M. B. (1996) Goethite dispersibility in solutions of variable ionic strength and soluble organic matter content. Clay Clays Min. 44,286-296. 

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