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

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 N-2 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 < 3.0) and released large amounts of Al and Si, which can further enlarge defect pores or even make hollow spherules collapse followed by the precipitation of amorphous silica. As a result, the microporous parameters increased, reaching maxima in Allo(2.2), while the total porosity and specific surface area of products decreased with the decrease of initial pH of the leaching solutions. These changes are supposed to diminish the adsorption and storage capacities of guest species of allophane. These findings suggest that the rapid responsiveness of allophane to acid supposedly plays a crucial role in the acid buffering capacity of allophanic Andosols and that the structural destruction of allophane may significantly contribute to Al toxicity and to breaking the porous structure of acidified allophanic Andosols. For the management of allophanic Andosols, prudent measures should be taken to protect the allophane structure. And it is possible to restore the soil productivity of weakly acidified allophanic Andosols through soil remediation.