Gibbsite function

Surface protonation at the kaolinite surfaces. The excess proton density, Th.v. at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHpzc of the edge surface is about 7.5. 

The surface characteristics of kaolinite was discussed in Chapter 3.4 and in Fig. 3.9. While the siloxane layer may - to a limited extent - participate in ion exchange reactions. The functional OH-groups at the gibbsite and edge surfaces are able to surface complex heavy metal ions. (Schindler et al., 1987). 

Fig. 5 Time-resolved in-situ EDXRD data showing the intercalation of LiCl into gibbsite at 120 °C. a 3D stacked plot, b Plot of extent of reaction of the (001) reflection of gibbsite (o) and the (002) reflection of [LiAl2(OH)6]Cl- H2O (A) as a function of time. Reproduced with permission from Chem Mater (1999) 11 1771-1775...

Although gibbsite and kaolinite are important in quantity in some soils and hydrothermal deposits, they have diminishing importance in argillaceous sediments and sedimentary rocks because of their peripheral chemical position. They form the limits of any chemical framework of a clay mineral assemblage and thus rarely become functionally involved in critical clay mineral reactions. This is especially true of systems where most chemical components are inert or extensive variables of the system. More important or characteristic relations will be observed in minerals with more chemical variability which respond readily to minor changes in the thermodynamic parameters of the system in which they are found. However, as the number of chemical components which are intensive variables (perfectly mobile components) increases the aluminous phases become more important because alumina is poorly soluble in aqueous solution, and becomes the inert component and the only extensive variable. 

In order to construct the activity diagrams in a rigorous fashion, a certain amount of information must be available. Some experimental data for the mica-feldspar-kaolinite-gibbsite-montmorillonite relations are available. Data for the other minerals are often inferred from measurements of natural chemical parameters (K+, SiC, H+ concentrations in solutions) in situations where the different minerals are assumed to be stable. The relations between minerals can also be calculated as a function of K+, SiO and pH using thermochemical data for the participating phase (Hess, 1966) when they are known with precision. Frequently it is... 

Ladeira, A.C.Q., Ciminelli, V.S.T., Duarte, H.A., et al. (2001) Mechanism of anion retention from EXAFS and density functional calculations arsenic (V) adsorbed on gibbsite. Geochimica et Cosmochimica Acta, 65(8), 1211-17. 

Although numerically the two values for the solubility constants of gibbsite are different they both give the same value of Al3+ as a function of pH when used in a calculation. When the solubility of a material is expressed as a reaction with a proton the symbol Kso is used, to distinguish it from the solubility constant Kso. 

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... 

Figure 3. Variation of the zeta potential of silica and alumina (gibbsite) as a function of pH.

For consistency, pseudopotentials should be based on the same functionals as used for the valence states. This point has been illustrated by Gale and coworkers in their study of aluminum trihydroxides [47]. Table 8.3 compares their results of cell optimizations using various mixtures of psuedopotential and valence state functionals with the experimental structure of gibbsite [48]. LDA pseudopotentials in an LDA optimization of the cell gives underestimated cell parameters, consistent with the usual expectation that LDA gives over-binding in chemical bonds. 

Table 8.3 Optimized structural parameters for AI(OH)3 (gibbsite) using various functionals for the pseudopotential/valence states.

The As-HAO system presents special difficulties for IR and XAFS spectroscopic analysis. In an XAFS spectrum, the magnitude of peaks in the Fourier transformed EXAFS spectrum is a function of several variables, two of which are atomic number (z) and distance from the central As atom. With only half as many electrons as Fe, the scattering power of Al is weak, therefore peaks representing As-Al scattering in the Fourier-transformed EXAFS are smaller and more difficult to interpret. IR and Raman spectra of As(V) sorbed on gibbsite are difficult to interpret for an entirely different reason substantial overlap of peaks representing Al(V)-0/Al-OH vibrations and As(V)-0/As(V)-OH vibrations (Myneni et al, 1998). 

Figure 7.8 Solubility of (a) amorphous AKOHlj, = 10 , and (b) gibbsite [Al(OH)3], K p = 10 , as a function of pH at 25°C. Also shown are lines indicating the solubility contributions of Al and individual Al-hydroxy complexes.

Figure 7.9 Solubilities of gibbsite (AKOH),] and kaolinite [Al2Si20s(0H)4l as ZAI(aq) as a function of pH at 25 C. The solubility of kaolinite has been computed assuming SiOifaq) = 17 ppm, the average value for groundwater.

Know how to calculate the solubility of a mineral such as gibbsite, kaolinite, or FefOH) (ferrihydrite) as a function of pH, both by hand and with a geochemical code such as M1NTEQA2. 

Figure 12.29 Stability fields of phases in the system K -AP -SO -H O at 25°C and I bar pressure, as a function of pH and sulfate activity for K+ = 10" moi/kg. The following p/Tsp values are assumed 33.96 for gibbsite, 88.4 foralunite, 17.8 for jurbanite, and 116 for basaluminite. Dashed lines denote metastable equilibria. The arrows labeled Drying show that the stability field of alunite increases in size with decreasing water activity (drying) at the expense of the jurbanite and gibbsite fields.

Troi.ard, E, and Y. Tardy. 1987. The stabilitie,s of gibbsite. boehmite, aluminous goelhites and aluminous hematites in bauxites, ferricretes and laterites as a function of water activity, temperature and particle size. Geochim. Cosmochim. Acta 51 945-57. 

The function depends on the type of surface oxygens. For gibbsite type (amphoteric) groups a combination of Eq. (60) and (62) with = 0 can describe the local... 

Letters of the Greek alphabet can have two different functions. Gibbsite is also called 7-A1(0H)3, i.e. these two names are synonyms and they are separated by comma in Table 2.1. In contrast q quartz" (without comma) is one name (to be distinguished from /3 quartz, a high temperature modification). 

XAFS data, showing the formation of Ni-Al LDH phases on soil components, are shown in Figure 3.5 and Table 3.1. Radial structure functions (RSFs), collected from XAFS analyses, for Ni sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite were compared to the spectra of crystalline Ni(OH)2 and takovite. All spectra showed a peak at R 0.18 nm, which represents the first... 

Figure 10.2. Acid-base speciation of (a) citrate, (b) aluminum, and (c) iron(III) as a function of solution pH at 25°C and in 0.01 mol L NaNOs solutions. The distributions were computed using a total soluble citrate (citys) concentration of 10 mol and total soluble aluminum (Aljs) and iron(in) [Fe(ni)Ts] concentrations controlled by gibbsite (A1(OH)3(x)) and goethite [FeOOH(x)]. (Relevant thermodynamic data from May and Murray, 2000 Baes and Mesmer, 1986 Liu and Millero, 1999.)...

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... 

Phosphate ions, like F", adsorb strongly on aluminous mineral surfaces at low pH, and have been shown to accelerate gibbsite dissolution at pH <3 (Bloom and Erich, 1987). The rate of gibbsite dissolution in 1.0 X 10" M phosphate at pH 3.0 is more than 30 times the rate in 0.1 M KNO3 (Fig. 7-2). The data of Bloom and Erich (1987) show an apparent 0.88-order dependence on solution phosphate. Their data suggest that dissolution is not a simple function of the quantity of PO4 adsorbed. The reaction is also pH independent. 

The concentration of dissolved species as a function of exposure time (1-14 days) was studied in [169]. The rate of dissolution of alumina is reported in [187]. The rate of dissolution of alumina as a function of pH is reported in [188]. The rate of dissolution of corundum is reported in [189]. The kinetics of dissolution of alumina was studied in [110], and that of gibbsite in [190]. 

The concentrations of the dissolved species of Al, calculated from the reaction constants above, are plotted as a function of pH in Figure 5.3, assuming that crystalline A1(0H)3 (gibbsite) is present and controls solubility according to the dissolution reaction... 

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