In recent years, the halloysite (Al2Si2O5(OH)4 · 2H2O) has been highlighted owing to its naturally occurring one-dimensionalmicrostructure that enables versatile applications. Due to the demand for enhancing surface interaction, several types of research such as acid/base treatments have been conducted on the halloysite nanotubes. The objective of this study is to investigate the structural and surface properties of thermally treated halloysites under reducing atmosphere. The heat treatment is carried out in a gas-flow furnace at 400–800 °C under various atmosphere, e.g., ambient air, 4% H2-balanced Ar, and 99.99% H2. The thermal treatment of halloysites under reducing atmosphere show a similar phase transition around 500 °C as the heating under air. However, the halloysite reduced in pure hydrogen shows a significant increase of the zeta-potential, −36.7 mV for a 600 °C-treated sample, compared to the other samples. The mechanisms of the zeta-potential increase for the halloysite was also explored.

In general, the activation of lignin involves acid demineralization followed by a chemical treatment under an inert atmosphere at a high temperature. The whole process is tedious, timeconsuming, and hazardous due to the suspended acid and related pH adjustment. Instead of the acid treatment, herein, we have employed a green technique, i.e., an ultrasonic treatment of the softwood Kraft lignin (SKL) followed by its activation under an inert atmosphere, and compared it with the acid-treated activated SKL. The particle sizes, microscopic structures, and surface properties of inactivated SKL, acidtreated activated SKL (AA-SKL), and ultrasonically treated activated SKL (UA-SKL) were investigated. The ultrasonic treatment reduces the processing time (especially the time required for pH adjustment) while producing activated carbon of comparable properties to that obtained using the traditional technique. Compared to the commercial carbon black (CB), UA-SKL efficiently adsorbs the noxious volatile organic compounds (VOCs), e.g., acrolein, xylene, etc., emitted from polyurethane (PU) composite foams to be used in automobile interiors. The present technique being simple and eco-friendly can be considered for industrial upscaling toward replacing the commercial CB by the biochar of lignin-like waste materials.