Key Takeaways & Executive Findings
- •• Systematic comparison of five treatment strategies reveals that acid treatment of calcined HNTs maximizes methylene blue adsorption by increasing specific surface area while preserving tubular structure. • Optimal treatment (pre-calcination at 600°C for 3 h followed by acid treatment at 60°C for 8 h) yields a specific surface area of 443 m²·g⁻¹ and an adsorption capacity of 190 mg·g⁻¹. • Calcination temperature inversely affects MB reactivity, while moderate acid treatment enhances adsorption by expanding the lumen and reducing surface potential. • Kinetic and Arrhenius analyses confirm that chemical reactions govern acid/alkali interactions with HNTs, providing a basis for controlled structure regulation.
Abstract
To advance the precise regulation and high-value utilization of halloysite nanotubes (HNTs), this work systematically investigated five treatment strategies, including calcination, acid treatment, alkali treatment, acid treatment of calcined HNTs, and alkali treatment of calcined HNTs, to modulate their structural and application properties. The structural characteristics, surface properties, and methylene blue (MB) adsorption capacity of HNTs under multiple treatments were systematically analyzed. Calcination at varying temperatures modified the crystal structure, morphology, and surface properties of HNTs, with higher calcination temperatures reducing their reactivity towards MB. Moderate acid treatment expanded the lumen and decreased the surface potential of HNTs, significantly enhancing MB adsorption capacity. In contrast, alkali treatment dispersed the multilayered walls of HNTs and raised surface potential, reducing MB affinity. Acid treatment of calcined HNTs effectively increased their specific surface areas by leaching most of Al while maintaining the tubular structure, thereby maximizing MB adsorption. Alkali treatment of calcined HNTs destroyed the tubular structure and resulted in poor MB adsorption. HNTs pre-calcined at 600°C for 3 h and acid-treated at 60°C for 8 h exhibited an optimal specific surface area of 443 m2·g−1 and an MB adsorption capacity of 190 mg·g−1. Kinetic and Arrhenius equation fittings indicated that chemical reactions control interactions of acids and alkalis with HNTs. This study provides a comprehensive comparison and analysis of five treatment methods, offering insights into regulating the structures and surface properties of HNTs by controlling the treatment condition, thereby laying a foundation for their efficient utilization in practical applications.
1. Introduction
Recently, clay minerals such as kaolinite, diatomite, and halloysite have been widely used in various fields, including biology, environmental science, and catalysis, due to their large specific surface areas, wide availability, and excellent biocompatibility [1‒5]. Among them, halloysite nanotubes (HNTs) are dioctahedral 1:1 layered minerals composed of Al, O, and Si, with the chemical formula Al2Si2O5(OH)4·nH2O [6]. HNTs usually exhibit a nanotubular structure with tube lengths of 0.2‒2 μm and diameters of 10–50 nm [7]. The tubular structure is formed when crystalline aluminosilicate layers curl under stress caused by lattice mismatch [8]. The outer surface consists of a corner-sharing tetrahedral [SiO4] sheet, while the inner surface is composed of an edge-sharing octahedral [AlO6] sheet [9‒10]. The inner surfaces of HNTs are positively charged, and the outer surfaces are negatively charged across a wide pH range of 3‒8, based on the potential of silica and alumina surfaces in water, resulting in unique dual-surface properties [11‒12].
The unique natural tubular structure of HNTs has garnered significant attention from researchers. Compared to carbon nanotubes (CNTs), natural HNTs offer distinct advantages in various applications due to their lower cost and greater availability. The dual-surface properties of HNTs provide diverse possibilities for targeted modifications in subsequent applications. Furthermore, the larger diameters of HNTs relative to CNTs enable them to encapsulate functional guest molecules of a broader size range [8,13]. In recent years, using HNTs as drug delivery vehicles has become a prominent research area. The lumens of HNTs can be utilized to load active substances, enabling controlled release under specific conditions [14‒15]. In the field of catalysis, the surface characteristics of HNTs allow for the selective loading of active materials within the lumens, enhancing catalytic performance through the confinement effect [16‒17]. As a kind of natural clay mineral, HNTs also offer several significant advantages as adsorbents, including abundant reserves and ample adsorption sites. Appropriate treatment methods can significantly modify the surface properties and facilitate the adsorption of positively or negatively charged substances. However, the raw HNTs face several limitations, such as small specific surface areas, limited lumen volumes, and insufficient surface activity. Therefore, to achieve specific functionalities and enhance their suitability for high-value applications, appropriate structure and surface regulations are necessary.
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Xiaoyu Jiang, Sikai Zhao, Jiafang Zhang, Haiyi Lü, Jie Wang, Wenbao Liu, Baoyu Cui, Yanbai Shen (2025). Structural characteristics, surface properties and methylene blue adsorption application of halloysite nanotubes regulated with controllable treatment processes. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3057-7
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Frequently Asked Questions
What are the five treatment strategies investigated for halloysite nanotubes in this study?
The five treatment strategies are calcination, acid treatment, alkali treatment, acid treatment of calcined HNTs, and alkali treatment of calcined HNTs.
What is the optimal treatment condition for maximizing methylene blue adsorption?
The optimal condition is pre-calcination at 600°C for 3 hours followed by acid treatment at 60°C for 8 hours, yielding a specific surface area of 443 m²·g⁻¹ and an adsorption capacity of 190 mg·g⁻¹.
How does calcination temperature affect the reactivity of halloysite nanotubes towards methylene blue?
Higher calcination temperatures reduce the reactivity of HNTs towards methylene blue, as they modify the crystal structure, morphology, and surface properties.
What is the effect of acid treatment on halloysite nanotubes?
Moderate acid treatment expands the lumen and decreases the surface potential of HNTs, significantly enhancing their methylene blue adsorption capacity.
What is the effect of alkali treatment on halloysite nanotubes?
Alkali treatment disperses the multilayered walls of HNTs and raises the surface potential, reducing their affinity for methylene blue.
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