Hydrolyzed Aluminium Ions

Vermiculite is particularly susceptible to decomposition under acid conditions. This susceptibility is one explanation for the relatively low vermiculite contents of slightly acid surface soils from arid and semiarid regions, when compared with corresponding subsoils. Another explanation for low vermiculite contents of surface soils is K. cycling by plants, with the subsequent conversion of vermiculite particles to their collapsed (micaceous) equivalents. 

The chemical behavior of acid soils and minerals is intimately linked to the aqueous solution chemistry of aluminium. Aluminium hydrolyzes to monomeric and polymeric hydroxyaluminium complexes made up of Al(OH)2+ and Al(OH) J. Ultimately A1 precipitates as solid-phase gibbsite (Al(OH)3) when the solubility product of this mineral is exceeded. The hydrolysis reactions of the monomers are 

Each reaction is driven to the right by the consumption of H+ (hydronium, H30+) ions by reacting with hydroxyl ions. Successive hydrolysis reactions are associated with increasing pH, The distribution of A1 ions with pH is shown in Pig. 10.2. For convenience, the H20 ligands have been omitted from the formulas. Such diagrams 

FIGURE 10.2. Relative distribution and average charge of the soluble aluminum species as a function of pH, ionicstrength = 0.1 M. (From G. Marion et al. 1976. SoilSci. 121 76-82.) 

The hydrolysis reactions of Eq. 10.2 liberate H+ and lower the solution pH unless OH- is present. This stepwise production of hydrogen ions is similar to that which occurs during the dissociation of polyprotonated acids. It is the primary reason why early workers attributed weak-acid properties to acid soils and clays. 

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The strong interaction of polyvalent cations with polyions is well known to strongly alter the rheological properties of hydrolyzed polyacrylamide used in the tertiairy oil recovery process (1-4). The influence of divalent cations have already been studied(5-7) but the role played by the presence of small quantities of aluminium ions has never been investigated. 

Matijevic, E., Janauer, G. E. and Kerker, M. J. Coll. Sci. 19 (1964) 333. Reversal of charge of hydro-phobic colloids by hydrolyzed metal ions, I. Aluminium nitrate. 

Soil pH measurements can be ambiguous. Two factors that affect soil pH measurements are the soil-solution ratio and the salt concentration. Increasing either factor normally decreases the measured soil pH because H and A1 cations on or near soil colloid surfaces can be displaced by exchange with soluble cations. Once displaced into solution, the A1 ions can hydrolyze (Eq. 10.2) and further lower the pH. Preferential retention of hydroxy aluminium polymers by soil colloids drives the hydrolysis reactions further toward completion and leads to lower pH. Increasing the neutral salt concentration to 0.1 or 1 M can lower the measured soil pH as much as 0.5 to 1.5 units, compared to soil pH measured in distilled water suspensions. 

Sinibaldi [ref. 99] has investigated the hydrolysis polymerization of six metal ions, i.e., iron(III), zirconium(IV), aluminium(III), lanthanum(III), thorium(lV) and titanium(IV), by thin-layer method with dextran and polyacrylamide gels. An acetate buffer (0.5 M) was used as eluent, because the solution was found to eliminate the adsorption or precipitation of hydrolyzed sample solutions which were prepared by adding various amounts of NaHC03 to the solution of a metal ion. The hydrolyzed solution of iron(III) yielded two spots on highly crosslinked gels (Bio-Gel P-2 and Sephadex G-25), of which the faster moved with the speed of the completely excluded marker Blue Dextran, and also... 

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