Hollow Spherical Capsules From Geopolymerized Gel Beads With Halloysite Nanotubes for Pollutants Removal and CO ₂ Capture

A scalable protocol is proposed for the fabrication of hollow spherical capsules by geopolymerization of halloysite clay nanotubes (HNTs) incorporated within alginate gel beads. Alginate/HNTs composite beads have been geopolymerized by alkaline treatment through their immersion in highly concentrated NaOH solution. The influence of the immersion time on the structure and mesoscopic properties of the beads has been studied to optimize the geopolymerization efficacy as well as the mechanical resistance and adsorption performances of the capsules. SEM images demonstrate that the alkaline treatment is efficient in the conversion of compact beads to hollow spherical capsules, which possess improved water uptake capacity and enhance flexibility under compressive forces as evidenced by the relevant increase (in the range ca. 55-75%) of the elastic modulus. Due to their internal cavity and surface porosities, geopolymerized capsules exhibit better adsorption performances toward hydrocarbons with respect to the untreated alginate/HNTs beads. Moreover, geopolymerization of alginate/HNTs beads significantly improves the CO₂ capture efficiency. The CO₂ amount adsorbed by the composites bead has been doubled after 5 seconds of alkaline treatment. This study highlights that the geopolymerization of halloysite loaded in biopolymeric beads can be exploited to obtain sustainable materials suitable for CO₂ storage and removal of contaminants.