Hydrolysis of Al13 and its coagulation mechanism: Role of speciation stability and transformation

Abstract

Al₁₃ ([AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺) is widely recognized as one of the advantageous species of Al-based coagulants, and its transformation process and micro-interface interaction with pollutants in water treatment continue to attract the attention of researchers. Based on relevant literatures in the past decades, this review summarizes and discusses the characterization methods, stability and aggregation, coagulation performance and mechanism of Al₁₃. The technique development and method establishment such as ²⁷Al nuclear magnetic resonance, electrospray ionization mass spectrometry, and Al-Ferron complexation timed spectrophotometry provide technical support for qualitative and quantitative detection of the species transformation of Al₁₃. Al₁₃ pre-formed by forced hydrolysis is of high structural stability, and solution pH, high Al concentration, and high temperature are important factors affecting its further hydrolysis, aggregation, dissociation or polymerization. Under circumneutral pH conditions in practical water treatment processes, unlike traditional Al salts which undergo extensive hydrolysis to generate Al(OH)₃, Al₁₃ transforms into Al₁₃ aggregates. This is the key for Al₁₃ to exert its superior coagulation performance and makes it shows significantly higher efficiency than traditional Al salts in removing particulate matter and organic substances. Specifically, Al₁₃ shows a broader effective dosage range and pH range compared with AlCl₃ through the coagulation mechanisms such as electrostatic patch, in-situ aggregation bridging, and complexation adsorption. Furthermore, the flocs formed by Al₁₃ coagulation exhibit a more compact crystalline structure and higher strength, which helps reduce residual Al concentrations. A comprehensive investigation into the hydrolysis characteristics of Al₁₃ and its role in coagulation is crucial for optimizing coagulation processes, while also providing a theoretical foundation for developing novel high-efficiency composite coagulants.