Gibbsite dissolution

Energies of Activation. Low E values (<42 kJ mol-1) usually indicate diffusion-controlled processes whereas higher E values indicate chemical reaction processes (Sparks, 1985, 1986). For example, E values of 6.7-26.4 kJ mol-1 were found for pesticide sorption on soils and soil components (Haque et al., 1968 Leenheer and Ahlrichs, 1971 Khan, 1973) while gibbsite dissolution in acid solutions was characterized by E values ranging from 59 4.3 to 67 0.6 kJ mol-1 (Bloom and Erich, 1987). 

As mentioned earlier, the dissolution of oxides and hydroxides, like feldspars and ferromagnesian minerals, appears to be a surface-controlled reaction. One indication of this is the high E values found by several investigators. Bloom and Erich (1987) obtained E values ranging from 59 4.3 to 67 0.6 kj mol"1 for gibbsite dissolution in acid solutions (pH 1.5-4.0). These values are much higher than for diffusion-controlled reactions reported earlier. 

At high concentrations of NaOH, the dissolution of gibbsite is rapid and first-order with respect to NaOH activity (Packer and Dhillon, 1968 Scotford and Glastonbury, 1972). Comparison of the data for gibbsite dissolution at 41 C in 1.0 M NaOH (Scotford and Glastonbury, 1972) with estimated rates for gibbsite dissolution in 1.0 M HNO3 (Bloom and Erich,... 

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. 

Sulfate ions form weak complexes with AI but can promote the rate of gibbsite dissolution (Fig. 7-2)(Bloom and Erich, 1987). In the pH range of 1.75 to 3.0, the dissolution rate for gibbsite in 0.10 MK2SO4 is first-order (slope = 1.0) with respect to [H and greater than 40 times as fast as in 0,10 M KNO3 (Bloom and Erich, 1987). The apparent order of the reaction with respect to SO4" concentration in solution is 0.36 (Bloom and Erich, 1987). Nitrate salts also can have an effect on dissolution. Bloom and Erich (1987) found an apparent 0.56-order dependence on NO3" for concentrations of 0.08 to 1.0 Af. Similar rates were found for Ca(NO3)2, Mg(NO3)2, and KNO3 when the NOj concentration was constant. 

Bloom, P.R., and M.S. Erich, 19 QO 987. Effect of solution composition on the rate and mechanism of gibbsite dissolution inaos acid solutions. Soil Sci. Soc. Am. J, 51 1131-1136. 

Figure. 2.48 Gibbsite dissolution rate vs. solution pH (after Palandri et al.y 2004).

However, AP is capable of forming complex compoxmds. It interacts with fluoride, sulphate, organic acids and even in pure water is capable of forming compounds Al(OH) +, Al (OH) and AJ (OH). That is why gibbsite dissolution reaction at high pH values even in the absence of foreign components acquires the following format... 

Table 8.3. Acid-consuming reactions in solution, at soil-water interface, and used by microorganisms carbonate buffering (Eqs. 7-9), ion exchange (Eq. 10), silicate weathering (Eqs. 11-12), gibbsite dissolution (Eq. 13), goethite dissolution (Eq. 14), denitrification (Eq. 15) and desulfurication (Eq. 16 Stumm and Morgan 1981, MattheB 1990)...

Pseudoboehraite aluminium hydroxides are usually used as a raw material in the HAG method. We have previously Investigated the aluminium hydroxides obtained by gibbsite dissolution in the alkaly and deposition at pH 8.5-9.0 by nitric acid [5]. It was shown in [5-73 conditions of aluminium hydroxide... 

Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

There are no unequivocal weathering reactions for the siUcate minerals. Depending on the nature of parent rocks and hydrauhc regimes, various secondary minerals like gibbsite, kaolinite, smectites, and iUites are formed as reaction products. Some important dissolution processes of siUcates are given, for example, by the following reactions (19). 

The bauxite first undergoes a size reduction prior to dissolution by caustic. In the case of gibbsite, for example, the dissolution is described by the following equation ... 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

Fig. 5.13 displays the dissolution scenario of the gibbsite surface and of the edge surface. The dissolution reaction can be interpreted as a coupled release of Al and Si. The detachment of the Al center is the rate determining step. In a fast subsequent step Si is released from the same surface site. The AI(III) H+ stoichiometry of the precursor group (the group to be detached) is 1 3 at the gibbsite surface and 1 1 at the edge surface. 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

The kinetic mass transfer model developed to take into consideration the geochemical evolution of the Cigar Lake ore deposit was mainly done by simulating the evolution of the Al-Si system in the Cigar Lake ore deposit system. To this aim the system formed by kaoli-nite, gibbsite and illite as main aluminosilicate solid phases was considered and kinetics for the dissolution-precipitation processes were taken from the open scientific literature (Nagy et al. 

Nagy, K. L. Lasaga, A. C. 1992. Dissolution and precipitation kinetics of gibbsite at 80°C and pH 3. The dependence on solution saturation state. Geochimica et Cosmochimica Acta, 56, 3093-3111. 

However, Bloom and Erich (1987) found that in NO3 and SO4- solutions, the dissolution reaction for gibbsite was first-order with respect to [H+] below pH 2.5. In phosphate solutions, there was no dependence of the rate of dissolution on [H+]. They also found that dissolution rate (v) was affected by the concentration of NO3T SO4, and PO4. The order dependency of v on NO3, SO4-, and P04- concentration was 0.56, 0.36, and 0.88, respectively. 

To illustrate this the solubility constant (Kso) for the dissolution of the mineral gibbsite will be calculated. The dissolution reaction can be written as the dissociation of the mineral, where subscript (s) refers to the solid phase and (aq) to the aqueous state ... 

If the index is positive then the solution is supersaturated, whereas a negative value indicates understaturation. When modelling mineral solubility with other reactions involving protons it may be necessary to write the dissolution reaction of the solid phase as a dissolution reaction with protons. Consider the dissolution of gibbsite ... 

As an example of the use of 12 to probe a dissolution reaction, consider the soil mineral gibbsite [Al(OH)3] dissolving in an Oxisol ... 

As an example of these ideas, an activity-ratio diagram for control of Al(III) solubility by secondary minerals in an acidic soil solution will be constructed. The Jackson-Sherman weathering scenario14 indicates that when soil profiles are leached free of silica with fresh water, 2 1 layer-type clay minerals are replaced by 1 1 layer-type clay minerals, and ultimately these are replaced by metal oxyhydroxides. This sequence of clay mineral transformations can be represented by the successive dissolution reactions of smectite, kaolinite, and gibbsite ... 

Thermochemical cycles (Figure 2) involving the formation of compounds (a-quartz, gibbsite, amines, tetrapropylammonium bromide, etc.), the dissolution of these compounds and of the HFI-type samples in 25% HF, and the dilution of aqueous HF allow the calculation of the standard enthalpy of formation of the samples. 

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