Pressure induced speciation changes in the aqueous Al³⁺ system

Area of Science:

• Geochemistry
• Computational Chemistry
• Solution Chemistry

Background:

• Aqueous aluminum (Al3+) speciation is crucial in geochemical processes.
• Understanding Al3+ behavior under high pressure is vital for Earth mantle studies.
• Existing models often lack pressure and pH influence on Al3+ hydrolysis.

Purpose of the Study:

• To develop a first-principles model for Al3+ hydrolysis equilibria.
• To incorporate external pressure and solution pH effects into the model.
• To investigate Al3+ speciation in high-pressure environments.

Main Methods:

• A cluster-based first-principles approach was employed.
• The model includes the central Al3+ cation and coordinating H2O/OH- ligands.
• Solution pH was parameterized using [H3O+]; pressure effects via Planck's equation.

Main Results:

• The model accurately reproduces ambient Al3+ speciation (6-coordinated at low pH, 4-coordinated hydroxide at high pH).
• External pressure significantly alters Al3+ species solubility under high pressure.
• Moderate pressure changes (from 5 GPa) induce notable shifts in speciation.

Conclusions:

• Pressure is a critical factor influencing aqueous Al3+ solubility and speciation.
• The model provides insights into Al3+ behavior in Earth's mantle conditions.
• Findings have potential industrial and geochemical applications.