Hi, I am asking if anybody understand the hydrolysis of Al in acid soils. From pH 4.7, Aluminum (Al) are present in soils as [Al(H2O)6]3+, and when pH reaches 6.5-8, all Al in the soils are present as [Al(H2O)3(OH)3], or Al(OH)3. The stepwise hydrolysis of Al produces also H3O+. So, what is the fate of this hydronium ion ? and how does it affect soil pH ?
Please look at the following below links which may help you in your analysis:
https://www.esf.edu/for/briggs/FOR345/reaction.htm
http://www.scielo.br/pdf/sa/v60n3/16411.pdf
Thanks
Hydrolysis of Al generates hydrogen ion, playing a key role in soil solution hydrogen ion concentration (pH, also known as active acidity). The mechanism of hydrogen release varies with pH. Under very acid soil conditions, most aluminum is soluble in the form of Al+++ or Al(OH)++ cations, which become strongly adsorbed in preference to hydrogen. The Al+++ in solution, which is in equilibrium with exchangeable Al, tends to hydrolyze so that Al+++ and water combine to give Al(OH)++ and one hydrogen ion. Adsorbed hydrogen is a second (minor) source of hydrogen since the adsorbed hydrogen ions are in equilibrium with those in soil solution. In moderately acid soils, bases are more abundant, and so Al+++ can no longer can no longer exist as an ion, but is converted to Al(OH)++. Since hydroxyl ions are more abundant in this condition of soil acidity, more complex Al(OH)++ ions are formed, reducing the probability of their hydrolyzing to form hydrogen ions. However, those that remain in the soil solution do hydrolyze to form hydrogen ions, but their numbers are less than in the very strongly acid soil.
I think the key to understanding the fate of protons in soils is buffering. You can find an introduction to this conception here:
http://lawr.ucdavis.edu/classes/ssc102/Section8.pdf
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