Structural alterations of synthetic allophane under acidic conditions: Implications for understanding the acidification of allophanic Andosols

Abstract Acidification of allophanic Andosols has significant effects on their physicochemical properties and productivity. Understanding of the acidification and related effects is limited by the structural changes of allophane – the major mineral composition of allophanic Andosols. In this work, we systematically investigated the structural stability of allophane and the mechanisms of structural changes under different acidic conditions (initial pH 4.0–2.0) by using chemical analysis, transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, solid-state magic-angle-spinning nuclear magnetic resonance, and N2 physisorption analysis. The results indicated that the structure of allophane was readily altered in acidic conditions. Acid leaching in conditions with initial pH > 3.0 only dissolved some polymerized silicates from allophane, enlarging the defect pores. By contrast, the dissolution of the imogolite-like local structure was observed in strongly acidic conditions (initial pH