Goethite equilibria

Lengweiler et al. (1961) found that the solubility of goethite, like that of ferrihydrite, increased as the pH rose above 12. For ferrihydrite, equilibrium between the solid and Fe(OH)4 was reached quite rapidly, whereas for goethite, equilibrium was not reached even after 40 days (25 °C). A value of 1.40 + 0.1 for of goethite (surface area ca. 100 m g" ) was only reached after 3 years (Fig. 9.5) (Bigham et al., 1996). As expected on thermodynamic grounds, the solubility of goethite was 10 to 10 times less than that of ferrihydrite. 

Zhou, Q. Maurice, P.A. (2001) Size fractionation upon adsorption of fulvic add on goethite equilibrium and kinetic studies. Geochim. Cosmochim. Acta 65 803—812... 

The quantity g allows for nucleation of G. If F.X > Q, Qn being the critical value of the goethite equilibrium constant Q for nucleation, then g is a small number go whereas g vanishes other-... 

This discrepancy might be explained if after about an hour the reaction approached equilibrium and slowed due to a diminishing thermodynamic drive. If the Fe+++ produced did not precipitate on the hematite surface, and did not form either hematite or goethite (FeOOH), it would accumulate in solution and weaken the drive for uranyl reduction. As the saturation index for hematite reached about 1.7, or about 1.25 for goethite, reaction would cease. 

Mechanisms of Sorption Processes. Kinetic studies are valuable for hypothesizing mechanisms of reactions in homogeneous solution, but the interpretation of kinetic data for sorption processes is more difficult. Recently it has been shown that the mechanisms of very fast adsorption reactions may be interpreted from the results of chemical relaxation studies (25-27). Yasunaga and Ikeda (Chapter 12) summarize recent studies that have utilized relaxation techniques to examine the adsorption of cations and anions on hydrous oxide and aluminosilicate surfaces. Hayes and Leckie (Chapter 7) present new interpretations for the mechanism of lead ion adsorption by goethite. In both papers it is concluded that the kinetic and equilibrium adsorption data are consistent with the rate relationships derived from an interfacial model in which metal ions are located nearer to the surface than adsorbed counterions. 

It may prove possible to apply titration calorimetry data in one further direction. If AG can be estimated for SAL-goethite complexation and reaction enthalpies can be obtained under equilibrium conditions, then an entropy change for this reaction can also be derived. This can only be done, however, if the adsorption reaction can be shown to be reversible. Since this has not been proven as yet in our systems, such thermodynamic extensions of titration calorimetry can only be speculative at this time. 

Figure 8.2 depicts the stability fields of goethite and hematite as a function of temperature and water pressure using data from several sources. The graph shows clearly that as the temperature increases, the stability field for hematite widens (see also Chap. 14). The goethite stability field broadens as Pnp increases. At PH2O = 0, the equilibrium temperature is 100 °C and rises to 300 °C at PH2O = 2 MPa. 

So far the discussion of the goethite/hematite equilibrium refers to aqueous systems in which the water activitY (i. e. relative humidity), anp, is unity. In many cases, however, the water activity may be <1. This applies to soils and sediments where can be lowered by the binding of water in pores. When considering the dehy-droxylation reaction. 

The extent to which a sparingly soluble solid dissolves is expressed by the solubility product. This describes the equilibrium established between the solid and the concentration of its ions in a saturated solution. Consider, for example, the dissolution of goethite in water ... 

The comparison of the ion activity product (lAP) of the dissolved constituent ions (e.g. for goethite, Fe " and OH ) with JQo of a Fe oxide provides an indication of whether the oxide will precipitate or dissolve in a particular solution. If the lAP exceeds Kso> the solution is supersaturated with respect to the oxide and precipitation takes place. If lAP = K o, the system is in equilibrium and if lAP < K o, the oxide will dissolve until equilibrium is reached. Interference with nudeation may retard or even inhibit predpitation in a supersaturated solution and prevent true equilibrium from being attained. 

Fig. 9.1 Activities of single ion species and total Fe activity (Feji heavy line) in equilibrium with goethite as a function of pH.

The sum of all the soluble Fe " species, i. e. Fer, in equilibrium with goethite as a function of pH, is the heavy line in the solubility diagram in Figure 9.1, whereas the activities of the single species are shown by the weak lines. Inclusion of the hydrolysis species results in a much higher solubility than would be observed by consideration of the solubility product i. e. Fe ", alone. For example, at pH 6, is <10 M, whereas Upe. = 10 M. Only at very low and very high pH is Up. essentially equal to UpeJt and ape(OH)j. respectively. 

Researchers in the aluminium industry have investigated the solubility of goethite in sodium aluminate and NaOH solutions. Basu (1983) found, using samples of natural goethite, that the equilibrium solubility of goethite in sodium aluminate solution was close to zero at room temperature and increased exponentially as the temperature rose above 100 °C. She also found that the isothermal solubility was greater in 5 M NaOH than in 5 M sodium aluminate solution at 150 °C, for example, [Fej] was 20 and 50 mgL , respectively. 

Fig. n.9 a) Retention of Cr " by goethite Tcr (unitless) vs. Cr " equilibrium concentration showing the transition from adsorption to surface precipitation, b) Structural model of incorporated and adsorbed Cr " by goethite (Charlet. Manceau, 1992, with permission). 

Equilibrium data on the adsorption of Al3 by goethite (a-FeOOH) can be described by the reaction... 

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