Aluminum Geochemistry Eh-pH, pe-pH diagrams and Solubility

Al pe-pH Diagram: Al concentration=1E-10 ppm

The Eh-pH diagram for Aluminum in Al-O-OH system is constructed using Al activity as 1E-10. The Aluminum species can dissolve in water in both high and low pH as Aluminum can form complex ions with hydroxide ion (ano with other anions such as SO4). Eh–pH diagram is any of a class of diagrams that designates the fields of stability of mineral or chemical species in terms of the activity of hydrogen ions (pH) and the activity of electrons (Eh). All of the reactions illustrated on Eh–pH diagrams involve either proton transfer (e.g., hydrolysis) or electron transfer (oxidation or reduction) or both.

As we increase the pH of the solution, the fallowing reactions take place:

  • Al+++  + H2O  = AlOH++  + H+
  • Al+++  + 2 H2O  = Al(OH)2+  + 2 H+
  • Al+++  + 4 H2O  = Al(OH)4-  + 4 H+

Al Eh-pH - Al-O-OH System

Al Eh-pH Diagram - Al-O-OH System

Al pe-pH Diagram: Al concentration=1E-10 ppm

Al pe-pH Diagram: Al concentration=1E-10 ppm

For Al solubility, we have considered the following special available on Geochemist Workbench program

Diagram for Al+++
Al+++           Al(OH)2+        Al(OH)3         Al(OH)4-
Al13O4(OH)24(7+) Al2(OH)2++++    Al3(OH)4(5+)    AlOH++
Boehmite        Corundum        Diaspore        Gibbsite

Question to ask: If Al is so soluble according to the Eh-pH diagram, how can we have utensils made up of Al?
Aluminum metal rapidly reacts with oxygen and develops a thin layer of aluminum oxide at the surface that prevents the metal from reacting with water.

Alkalinity has no effect on Aluminum solubility. Lime application may stop the iron problem, but is incapable of solving the Aluminum solubility.

Facts about Aluminum:

  1. Aluminum is the most abundant metal in the Earth’s crust, and the third most abundant element, after oxygen and silicon.
  2. It makes up about 8% by weight of the Earth’s solid surface.
  3. Aluminium is too reactive chemically to occur in nature as a free metal. Instead, it is found combined in over 270 different minerals.
  4. Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3-2x).
  5. Because of its strong affinity to oxygen, however, it is almost never found in the elemental state; instead it is found in oxides or silicates.
  6. Feldspars, the most common group of minerals in the Earth’s crust, are aluminosilicates.
  7. Impurities in Al2O3, such as chromium or cobalt yield the gemstones ruby and sapphire, respectively.
  8. Pure Al2O3, known as corundum, is one of the hardest materials known.

Mining:
Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica but the primary mining areas for the ore are in Ghana, Indonesia, Jamaica, Russia and Surinam.Smelting of the ore mainly occurs in Australia, Brazil, Canada, Norway, Russia and the United States. Because smelting is an energy-intensive process, regions with excess natural gas supplies (such as the United Arab Emirates) are becoming aluminium refiners.

Some example Aluminum reactions forming complex ions.

Al+++  + 2 H2O  = Al(OH)2+  + 2 H+

Polynomial fit:
log K = -11.67 + .07537 × T – .0004644 × T^2 + 1.745e-6 × T^3 – 2.394e-9 × T^4

Assumptions implicit in equilibrium equation:
activity of Al+++      = 10^0

Equilibrium equation:
log K = log a[Al(OH)2+] + 2 × log a[H+] – 2 × log a[H2O]

Al+++  + 3 H2O  = Al(OH)3  + 3 H+

Polynomial fit:
log K = -18.52 + .1173 × T – .0006613 × T^2 + 2.176e-6 × T^3 – 2.64e-9 × T^4

Assumptions implicit in equilibrium equation:
activity of Al+++      = 10^0

Equilibrium equation:
log K = log a[Al(OH)3] + 3 × log a[H+] – 3 × log a[H2O]

Al+++  + 4 H2O  = Al(OH)4-  + 4 H+

Polynomial fit:
log K = -25.46 + .1596 × T – .001149 × T^2 + 4.368e-6 × T^3 – 5.834e-9 × T^4

Assumptions implicit in equilibrium equation:
activity of Al+++      = 10^0

Equilibrium equation:
log K = log a[Al(OH)4-] + 4 × log a[H+] – 4 × log a[H2O]

Al+++  + H2O  = .5 Al2(OH)2++++  + H+

Polynomial fit:
log K = -4.497 + .029 × T – .0001158 × T^2 + 1.629e-7 × T^3 + 1.085e-10 × T^4

Assumptions implicit in equilibrium equation:
activity of Al+++      = 10^0

Equilibrium equation:
log K = .5 × log a[Al2(OH)2++++] + log a[H+] – log a[H2O]

Al+++  + H2O  = AlOH++  + H+
Polynomial fit:
log K = -5.674 + .03153 × T – .0001118 × T^2 + 2.672e-7 × T^3 – 2.817e-10 × T^4

Assumptions implicit in equilibrium equation:
activity of Al+++      = 10^0

Equilibrium equation:
log K = log a[AlOH++] + log a[H+] – log a[H2O]

Al+++  + SO4–  = AlSO4+
Polynomial fit:
log K = 2.947 – .0008274 × T + .0001408 × T^2 – 5.612e-7 × T^3 + 1.156e-9 × T^4

Assumptions implicit in equilibrium equation:
activity of Al+++      = 10^0

Equilibrium equation:
log K = log a[AlSO4+] – log a[SO4–]

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