Gibbsite solubility

The amount of alkalinity that is necessary to leach from the soil can be estimated from the ratio of base cations to aluminum, (Ca + Mg)/Al, assuming a critical threshold value for biological effects at 1 1. (Above a ratio of (Ca -I-Mg)/Al = 1 roots are assumed to be protected from aluminum toxicity and Mg deficiency.) Then, based on gibbsite solubility and a minimum required... 

Gibbsite solubility determined after an 8-week equilibration at ambient temperatures (20 to 22"C). 

Data points above the gibbsite solubility line in Figure 5.6 represent soil solutions that are undersaturated with respect to gibbsite that is, the solutions have an ion activity product, [AP ]/[H ], that is less than 10 Data points below the line represent soil solutions that are supersaturated with respect to gibbsite that is, the solutions have an ion activity product that is greater than 10 ... 

From equation 5.21 for Al —Ca exchange and equation 5.19 for gibbsite solubility, derive an equation that relates percent Ca " saturation of exchange sites to lime potential. (Assume an ionic medium of 0.01 MCaCh and Kj = 30 for the exchange reaction in this medium.)... 

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... 

Another feature indicated by this plot is the displacement of the minimum solubility to lower pH s as temperature increases. The strong effect of temperature on Al(OH)3 solubility and aluminum solute speciation was also noted by Seip et al. (H), who pointed out that the effect could have environmental significance. Extrapolation of the gibbsite solubility relationship to pH s above 6.0 is probably not justifiable, as a different form of Al(OH)3c, bayerite, is formed in alkaline solutions (12). 

Calculate the AG change during the change from feldspar to gibbsite, soluble silica, and K+ (all components in their standard states). What is the further AG change if all the activities of soluble silica and K+ are 10-4 ... 

It follows from this equality that gibbsite solubility abruptly declines with decreasing H concentrations (with the increase in pH) and at pH = 7 reaches only 1.26T0 mole-P (9.82-10 pg-l 0 (straight line AP in Figure 2.49). 

Figure 2.49 Gibbsite solubility vs. pH of water solution (standard conditions). Solid curve is total solubility, straight lines are concentrations of individual A1 forms in solution.

Surdam et al. (1984) concluded from the results summarized above that, depending on the pH, carboxylic acids could significantly increase the solubility, and probably the mobility of aluminum. Acetic and oxalic acids increased the total amount of aluminum in solution by one and three orders of magnitude, respectively, compared to gibbsite solubility in the absence of organic acids. However, the presence of calcite, especially at low to neutral pH, could neutralize the ability of carboxylic acids to enhance the solubility of aluminosilicates. 

M.L. (1979) Gibbsite solubility and thermodynamic properties of hydroxy-aluminium ions in aqueous solution at 25 °C. Geochim. Cosmochim. Acta, 43, 861-868. 

Wesolowski, D.J. and Palmer, D.A. (1994) Aluminium speciation and equilibria in aqueous solutions. V. Gibbsite solubility at 50 °C and pH 3-9 in 0.1 molal NaCl solutions (a general model for aluminium... 

---