Studies on the mechanism of hydrolysis and polymerization of aluminum salts in aqueous solution: Correlations between the "Core-links" model and "Cage-like" Keggin-Al₁₃ model

Two conflicting models describe the mechanism of aluminum (Al) hydrolysis and polymerization in aqueous solution, namely, the "Core-links" model and the "Cage-like" Keggin-Al₁₃ model. For the sake of simplicity, the expressions of Al₁₃ in the "Core-links" model and the "Cage-like" model are termed as C-Al₁₃ and K-Al₁₃, respectively. The two models have co-existed for almost 50 years, but describe differences in the transformation of polymeric Al speciation in aqueous solution, such as, hydrolysis, polymerization, flocculation, precipitation and crystallization. Many of the polynuclear Al species presented in the literature cannot be adequately described with either the "Core-links" model or the "Cage-like" Keggin-Al₁₃ model. This paper introduces new considerations for the mechanism of Al hydrolysis and polymerization in aqueous solution, a "Continuous" model representing a unification of the "Core-links" model and "Cage-like" model, based on the systematic summary and analysis of numerous published experimental results. The general viewpoints are:(1) The "Core-links" model can only describe the transient state process for speciation changes of Al in hydrolysis and polymerization. Under a moderate titration rate of Al solution using alkaline solution, the transformation of polynuclear Al species in forced hydrolyzed Al solutions has gone through the continuous speciation change process: from small polymer (linear shape) → middle polymer (plane shape) → large polymer (stereoscopic conformation). This is a continuous transient course and can be described by the "Core-links" model.(2) The "Cage-like" Keggin-Al ₁₃ model may only depict the metastable (sub-steady state) speciation of polynuclear Al, which is formed through the structural re-organization (self-assembly) during the aging of transient species. Aging is the prerequisite condition for the formation of K-Al₁₃. After aging the transient species of polymeric Al produced in titration, the concentration of polymeric Al determined by Ferron timed-spectrometry is equal to that of Al₁₃ concentration measured by ²⁷Al-NMR, namely Al_b(photometry) ≈ K-Al₁₃ (OH^-/Al = 1.0-2.8). So that the existence of K-Al₁₃ is universally approved. However, the "Cage-like" K-Al₁₃ structural model cannot explain the whole transformation process of polynuclear Al in aqueous solution, since many polymeric Al species produced in titration processes (transient state) cannot be detected by ²⁷Al-NMR. In addition, the slow base-neutralization and the moderate reaction atmosphere favors the polymeric Al, facilitating the formation of K-Al₁₃ by self-assembly.(3) The two models can be unified. They actually reflect the different stages of the Al speciation in hydrolysis and polymerization: Al³⁺ → "Core-links" species (transient state) → K-Al₁₃ (metastable state) → Al(OH)₃(s, steady state) → Al(OH)₄^-. There is an inevitable intrinsic connection between the interesting polymeric Al₁₃ species C-Al₁₃ and K-Al₁₃. This connection can be summarized as C-Al₁₃⁹⁺ → K-Al₁₃⁷⁺, which is an irreversible self-assembly course.(4) The reason for the many seemingly inconsistent and even paradoxical literature reports is due to assessment at the different stages of the Al species chain in addition to differences in experimental conditions. Therefore, a combined "Continuous" model is presented to describe the linkage of "Core-links" model and "Cage-like" model. It is based on our newly introduced concept of experimental condition comprehensive parameter - "the flux of alkali neutralization Φ".