Goethite equilibrium solubility

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. 

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. 

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 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. 

Well 16-17 has Fe(II) values near the detection limit of 0.05 mg/L and contains O2 at low levels, which may be the cause of the higher Pt electrode reading. The site is in a transition zone, out of redox equilibrium, and contains nitrite at ca. 0.5 mg N/L. However, the laboratory results discussed above indicate that Ehj at the WIG electrode may still provide an accurate representation of the distribution of ferric/ferrous species in the system. Thus, the pQ value (39.4) calculated from the Ehj at the WIG electrode may be reliable. This pQ indicates that a ferric oxyhydroxide with a solubility similar to that of colloid-sized goethite is present. 

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