Unrolling the tubes of halloysite to form dickite and its application in heavy metal ions removal

Abstract

Aluminol groups are effective for removing toxic metal ions. However, their use is significantly limited because they are usually on the inner surface of halloysite tubes. Herein, we realign and expose the abundant aluminol groups using a hydrothermal process. We found that dickite was gradually formed with increased sodium metasilicate proportion. Also, a uniform mushroom-like structure was formed, extending from the orifice to the middle of the hollow tubes and eventually forming a sphere. Further, part of the halloysite tube gradually broke, expanding into lamellar, eventually forming a small spheroid. The Al-O-Si groups were fractured by sodium metasilicate and aluminol groups on the inner surface of halloysite tubes, forming many aluminol groups. Moreover, the average pore size of the halloysite increased, indicating that numerous halloysite mesopores converted to dickite macropores. Thus, we inferred that the tubes gradually unfolded when the metasilicate first binds with the aluminol groups on the inner of the halloysite tubes. The modification enhanced the material's adsorption capacity for Pb2+, Cd2+, and Cr3+ nearly 20, 12, and 10 times, respectively, stronger than the original halloysite mineral. Consequently, exposing and realigning aluminol groups of halloysite significantly improved toxic metal ions' removal.