Gibbsite, equilibrium solubility

Attempts to model chemical weathering of catchments have used a variety of approaches and were originally designed to understand acidification processes. The BIRKENES code (Christophersen et al., 1982) was one of the first developed to model catchment stream chemistry. It used cation-anion charge balance, a gibbsite equilibrium solubility control for aluminum concentrations, a Gapon ion exchange for metals sorption, and rates for sulfate adsorption/ desorption in a two-reservoir model. The model was calibrated by input mass fluxes and output mass fluxes for the Birkenes catchment in Norway to provide the water flux information and to fit empirical parameters. 

TABLE 10.2. Equilibrium Solubility of Gibbsite [A1(OH)3(s)] as Measured by the Total Soluble Concentration of Aluminum (Alys) in Nitrate and Citrate Background Electrolytes"... 

Figure 1. Equilibrium solubility of microcrystalline gibbsite at four different temperatures and 1 atm. pressure. Ionic strength 0.010.

Kg = A1 / H , where indicates ionic activities, and has a value given by pKg = 3pH-pAl = 8.1 for gibbsite or 9.6 for the amorphous hydroxide, at 25 C. However, this reaction is strongly temperature dependent, and pKg for the amorphous form is quoted as being 11.6-11.8 at 2°C by Lydersen et al (1990), so that the equilibrium solubility increases as the temperature decreases. The maximum Al concentraion that can be expected to occur at a given pH value in an open system at equilibrium with the hydroxide can thus be readily calculated. However, this maximum solubility is rarely attained, particularly at low pH values, for a variety of reasons, but principally because of the formation of less-soluble mineral phases with silicate, and with sulphate in areas impacted by acid deposition (Bache, 1986). 

Total concentration of aluminium in equilibrium with gibbsite. In the above calculation a total concentration may have been specified in which the solubility of a mineral phase may have been exceeded therefore, it would be appropriate to calculate the speciation and total aluminium in equilibrium with a specified mineral phase, e.g. gibbsite. 

Figure 5.4 shows [A1]T in equilibrium with gibbsite as a function of pH. The solubility of aluminium decreases as pH increases, up to about pH 7 beyond pH 7 the total concentration of A1 in solution increases as gibbsite becomes more... 

Calorimetric methods cannot distinguish between separate or linked equilibria in the same sample. For the mineral montmoril-lonite (an aluminum silicate containing Mg, Fe and other elements), under the restricted conditions where all common ions are measurable, solubility methods also cannot determine whether the two equilibria are separate or linked. However, when gibbsite, AlfOH), and hematite, Fe20o, are allowed to control Al and Fe " "in equilibrium with moncmorillonite (at low levels that cannot be measured but which can be accurately calculated), it should be possible to experimentally determine whether or not the two equilibria are linked, using solubility methods. 

Figure 10.12. (a) Predicted total soluble concentration of aluminum (Al-fs) supported by gibbsite as a function of pH and tlie imposed Al-citrate chemical speciation models. The associated distributions of Al-citrate species in the equilibrium solutions are shown for the following models (h) Motekaitis and Martell (1984) (c) Gregor and Powell (1986) (d) Lakatos et al. (2001) (e) Hanis et al. (2003) (/) Essington et al. (2005). The imposed conditions included 0.01 mol L NaNOj. 10 mol L citis. 25°C and 1 atm, and log... 

The presence of the Al-complexing organic, oxalate, has no effect on the activity of ftee Al at equilibrium as long as excess gibbsite is present, but does increase the total solubility of Al by forming soluble Al-oxalate complexes. The extent of complex formation can be determined using equations 1.64, 1.65, and 1.66 and the fact that the concentrations of the various forms of oxalate (H2L, HL, L, and ML) must sum up to the initial concentration of oxalic add ... 

Figure 6.8. Solubility of Al (in units of total molar concentration of all dissolved species) as a function of pH, assuming equilibrium with gibbsite. The shaded region denotes supersaturated conditions favorable to precipitation. (Adapted from C. F. Baes and R. E. Mesmer. 1976. The Hydrolysis of Cations. New York Wiley.)...

When the stability planes for three minerals are considered simultaneously (say, gibbsite, kaolinite, and beidellite), the three planes must intersect at a point (unless they happen to be parallel to one another). This intersection point represents the single solution composition consistent with all three minerals coexisting in equilibrium. In other words, equilibrium among these three mineral phases would fix the pH and the activities of AV and Si(OH)4 in solution at one unique invariant point. Since the gibbsite solubility expression (equation 6.33) can be written as... 

Recycle is necessary to achieve the flowrate to suspend the bed and to allow solute concentrations to build to a steady state. A blowdown of solutes is provided via sample collection. Back reactions should be prevented and experiments are designed to be far from equilibrium . Chou and Wollast (1985) found that dissolved aluminum inhibited further dissolution rates of albite even at concentrations below the solubility limit for gibbsite. With the fluidized-bed apparatus (Fig. 6), they could control the aluminum concentrations to a low level at steady state by withdrawing sample at a high-rate. 

The value of this constant comes from the free energies of formation of the reactants and products of Eq. 7.10. The equilibrium constant defines the soluble silica activity at which gibbsite and kaolinite are in equilibrium ... 

The conclusion that polynuclear hydroxide complex ions do not constitute a significant part of the solute species of aluminum at equilibrium in the solutions studied here is in accord with the conclusions of Frink and Sawhney (4). The microcrystalline gibbsite itself is metastable in the sense that as crystals grow in size with longer aging time their solubility decreases. Although at equilibrium metastable species can be ignored, it can be so diflBcult to attain an equilibrium condition that the concept has little practical usefulness. 

---