Halloysite Nanotubes from Various Geological Deposits: New Insights to Acid Etching and Their Impacts on Products' Characteristics

Abstract Halloysite nanotubes (Hal) with unique nanosized tubular structures are promising nanoclay for potential applications as reinforcement fillers, anticorrosion coatings, as well as drug carriers, and adsorbents. Thus far, previous studies on acid treatment of Hal have focused only on Hal from a single source under various conditions of acid strength, temperature, and treatment duration. This study is the first to characterize and compare the effects of acid treatment on three types of Hal (Dragonite (DG), HalloPure (HP), and Camel Lake (CLA)) originated from different geological deposits, and the results considered in terms of their suitability for various applications. Analyses revealed porous tubular structure with an increased lumen diameter of up to 78% and improved surface area from 54 m2 g−1 to 160 m2 g−1 on acid-treated CLA (acid-CLA). By comparison, lumen diameters were reported to increase by only 46% and 52% for acid-DG and acid-HP, respectively. Despite that, crystallinity of acid-DG showed the most reduction, from 64.5% to 59.4%, with a high recorded surface area of 229 m2 g−1, whereas crystallinity of acid-HP showed the least reduction from 62.5% to 61.1% with a surface area of 121 m2 g−1. Therefore, the obtained results support the hypothesis that various Hal can be affected differently by acid treatment, and not all types of Hal are suitable for acid treatment, especially for those intended for use as reinforcement materials.