Kinetic evidence for five-coordination in AlOH(aq)2+ ion

Area of Science:

• Environmental Chemistry
• Aqueous Geochemistry
• Solution Chemistry

Background:

• Trivalent aluminum ions (Al(III)) are prevalent in natural waters, yet their coordination structures remain poorly understood.
• Al(III) speciation varies significantly with pH, from octahedral hexacoordination in acidic conditions to tetrahedral structures in basic conditions.
• The predominant Al(III) species in the critical pH range of 4.3–7.0 are unclear, with existing models assuming hexacoordination for hydrolytic species.

Purpose of the Study:

• To elucidate the coordination structure of aqueous aluminum (Al(III)) in the environmentally relevant pH range of 4.3–7.0.
• To investigate the predominant hydrolytic species of Al(III) under ambient conditions.
• To challenge existing assumptions about Al(III) speciation and coordination in natural waters.

Main Methods:

• Kinetic studies of proton and water exchange on aqueous Al(III).
• Car-Parrinello molecular dynamics simulations.
• Analysis of Al(III) speciation across a range of pH conditions.

Main Results:

• Evidence supports a five-coordinate Al(H2O)4OH2+ ion as the predominant form of AlOH(aq)2+ in the pH range of 4.3–7.0.
• This five-coordinate structure represents a departure from the hexacoordinate structures typically observed for other trivalent metal aqua ions.
• The findings provide a new understanding of aluminum ion coordination in natural water systems.

Conclusions:

• The predominant Al(III) species in the critical pH range of 4.3–7.0 is a five-coordinate ion, Al(H2O)4OH2+.
• This discovery offers a more accurate model for aluminum speciation in natural and geochemical systems.
• The unique pentacoordinate hydrolysis product of Al(III) distinguishes it from other trivalent metal aqua ions.