Effective Adsorption and Photodegradation of Organic Dye Using Halloysite Nanotube Catalyst Composites and Capsules

Abstract

Halloysite nanotubes (HNTs) are naturally occurring aluminosilicate hollow materials and those prepared by modifying with a suitable catalyst can be applied to environmental purification and water treatment. In this study, we synthesized two kinds of nanomaterials (lanthanum ion-doped HNTs/TiO2 nanocomposites and alginate/HNTs/TiO2 capsules) to examine their photocatalytic effects on the degradation of organic dyes. The lanthanum ion-doped HNTs/TiO2 nanocomposites were uniformly synthesized by sol-gel and sonochemical methods and HNTs/TiO2/La nanocomposites were characterized by field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), inductively coupled plasma mass spectrometry (ICP-MS), zeta potential, Brunauer-Emmett-Teller (BET) surface area, and Barrett-Joyner-Halenda (BJH) pore analysis. It was found that the adsorption of rhodamine B is suitable for pseudo-second-order kinetics. The photodegradation experiment of rhodamine B was conducted under UV and direct sunlight, and the degradation rate constant was obtained using a pseudo-first-order kinetic model. HNTs/TiO2/La catalyst decomposed 99.36% rhodamine B for 20 minutes under UV conditions and 99.12% for 210 minutes under sunlight conditions. They also showed high catalytic activity with 95.36% degradation efficiency even after 5 recycles, implying that they can be reusable repeatedly. In addition, to improve reusability and industrial applicability, alginate/HNTs/TiO2 hybrid capsules encapsulated the HNTs/TiO2 composite were synthesized by cross-linking of alginate and Ca ions. HNTs adsorb organic pollutants and TiO2 receives light to decompose and purify water contaminated with organic pollutants and dyes. The structure of the alginate shell on the surface and the presence of HNTs and TiO2 inside the capsule were identified using SEM. The decomposition characteristics depending on temperature were confirmed by TGA analysis, and the composition ratio was confirmed through the residual mass. Alginate/HNTs/TiO2 hybrid capsule showed about 1.7 times higher adsorption of rhodamine B than empty alginate capsules and showed the highest degradation efficiency by decomposing 97.65% of rhodamine B over 2 hours. 91.53% of dye degradation efficiency was maintained even after 5-recycling of catalyst. This catalytic hybrid capsule is used in water treatment applications to purify organic pollutants such as various organic dyes. Synthesized HNT-based nanocomposites and capsules in this study can effectively be applied to environmental purification by decomposing organic dyes in wastewater.