DFT Studies on the Water-Assisted Synergistic Proton Dissociation Mechanism for the Spontaneous Hydrolysis Reaction of Al3+ in Aqueous Solution

The kinetic mechanism of spontaneous aluminum ion (Al3+) hydrolysis reaction in aqueous solution is investigated using the density functional theory-quantum chemical cluster model method. Three typical reaction pathways for the spontaneous Al3+ hydrolysis reaction are modeled, including (1) the traditional spontaneous proton dissociation on the Al3+ inner-shell coordinated waters; (2) the conventional bulk water-assisted proton dissociation; and (3) the second-shell water-assisted synergistic dissociation of the protons on the Al3+ inner-shell waters. The results show that the electrostatic effects between Al3+ and its coordinated waters alone cannot fully account for the proton loss on an inner-shell coordinated water. It is suggested that the main reaction pathway for natural hydrolysis of aqueous Al3+ is the second-shell water-assisted synergistic proton dissociation, in which the participation of the second hydration shell is crucially important. The calculated synergistic proton dissociation rate constant, k(H)(+) = 1.14 X 10(5) s(-1), is in close agreement with the experimental results (1.09 X 10(5) s(-1) and 7.9 X 10(4) s(-1)). The first hydrolysis equilibrium constant pK(a1) of Al3+ is calculated as 5.82, also consistent with the literature value of 5.00. This work elucidates the molecular mechanism of the spontaneous Al3+ hydrolysis reaction in natural waters and has important environmental implications.