Halloysite nanotubes: Novel and eco-friendly adsorbents for high-pressure CO2 capture

Naturally available halloysite nanotubes (HNTs) and HNTs/Kaolin mixtures obtained from different locations in South Australia were processed and purified to varying degrees by crushing, blunging and reblunging, sedimentation followed by filtration. The microscopic and adsorption results revealed that the samples are highly porous in nature and exhibited tubular morphology with a high specific surface area. It was found that the halloysite content in the samples obtained from different locations was quite different although the processing steps make a slight difference in the final halloysite content in the samples. Out of the several materials characterized, HNT1, halloysite from Camel Lake, has a higher quantity of halloysite content (88%) and exhibits a specific surface area of ca. 50.8 m(2)/g. It was found that the halloysite nanotube HNT1 exhibited a CO2 adsorption capacity of 3.4 mmol/g at 25 degrees C, which increased to 6.1 mmol/g at 0 degrees C. Interestingly, CO2 adsorption per unit surface area of the studied HNT1 was estimated to be 120 mu mol/m(2) which is relatively higher than that of the existing halloysites and other well-known commercial materials such as activated carbon, mesoporous silica and carbon nitride materials. The higher adsorption capacity of HNT is mainly attributed to the excellent surface and tubular morphology with small pores and free hydroxyl groups present at the inner and outside surface of HNT, which play a significant role in the CO2 adsorption process.