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Halloysite nanotubes (HNT) have received widespread interest due to their unique microstructure and properties. They have been widely used as nano-containers or supporting materials to prepare advanced functional materials in many fields, spanning from life science and medical uses to energy and environmental applications. It is well-known that the performance of HNT is highly influenced by their surface area and pore volume, depending on the detailed structure properties, such as lumen space, wall thickness and length of HNT. Particularly, the loading ability of HNT is related to the detailed microstructures, such as morphology parameters (the inner/outer diameter and length), wall structure, active sites, interlayer space, as well as surface modification by organic groups and inorganic nanoparticles. As a special one-dimension nanoporous clay mineral with tubular morphology, the fundamental knowledge on the microstructure of HNT is of key importance to their specific applications. This review summarized several key factors related to the microstructures of halloysite, such as the morphology and origin of HNT, the coexisting impurities and the dispersion degrees of HNT, then rationalize the relations between the microstructures and physicochemical properties of halloysite, followed by the recent progress on microstructures evolution of HNT under various modification methods such as calcination, acid/alkali treatment, pseudomorphic transformations, and surface modifications as well as functional loading, physical expansion and intercalation. [Display omitted] • Several key factors related to the microstructures of halloysite are summarized. • The relations between microstructures and physicochemical properties of HNTs. • Recent progress on microstructures and properties of HNTs under various modification methods. [ABSTRACT FROM AUTHOR]

The high concentrations of heavy metal ions in industrial effluents are one of the most challenging wastewaters to deal with. In this study and in order to change the inner diameter of the halloysite nanotubes (HNT), their internal surface was coated by different polymers (polydopamine, polyaniline and polystyrene) and then, the modified nanotubes were incorporated in the polyamide thin film NF membrane for the treatment of metal ions solutions. Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) studies indicated that the internal coating was successful. Furthermore, the neat and modified nanofiltration (NF) membranes were characterized in terms of morphology, contact angle, water flux and metal ion rejection. The modified membranes displayed lower contact angle values (29.4% reduction) that can be linked to the hydrophilic groups in the structure of the coated polymers and the smoother surface of the thin film nanocomposite (TFN) membrane. In addition, the permeation fluxes of the modified membranes were improved significantly (33.4% improvement) without any major reduction in their ion rejection; that shows HNTs act as the channels to transfer water through the membrane structure while the coating of the inner surface of HNT reduces the size of the channel and makes more repulsion and steric hindrance for the ion to pass through the nanotubes. Among the fabricated membranes, the NF membrane with 0.05 wt% polystyrene-coated HNT showed the lowest contact angle (55.30o) and the highest water permeation flux (27.51 L m-2 hr-1), compared to the neat TFC membrane (15.26 L m-2 hr-1) and the NF membrane with 0.05 wt% unmodified HNT (20.62 L m-2 hr-1). [Display omitted] • The internal surface of HNTs was coated with different polymers through a novel polymerization method. • Polydopamine, polyaniline and polystyrene were used as the polymer for coating. • The modified HNTs were used to improve the performance of TFN NF membrane. • HNTs act as the channels to transfer water through the membrane structure. • The permeations of the TFN membranes with modified HNTs were improved significantly without any reduction in the rejection. [ABSTRACT FROM AUTHOR]

Halloysite nanotubes (HNTs) were functionalized using 3-aminopropyltriethoxysilane (APTES) and incorporated into polyethersulfone (PES) membranes to improve the hydrophilicity of the membranes as well as the interfacial interaction between HNTs and the polymer matrix. The intrinsic properties, permeability, and selectivity of the prepared membranes were analyzed to evaluate the membrane performance. In addition, humic acid (HA) fouling experiments were conducted to measure the antifouling properties of the fabricated membranes. As HNTs and functionalized HNTs (f-HNTs) contents are increased, hydrophilicity and mechanical strength were enhanced, and membranes with f-HNTs showed further improved performance. The pure water flux of membranes with 2% HNTs and f-HNT was 7.5 times higher than that of a pristine PES membrane without a trade-off relation between the water flux and HA rejection. The membranes with f-HNTs showed the highest antifouling properties compared to membranes with HNTs because enhanced hydrophilicity played a key role in preventing accumulation of HA. [ABSTRACT FROM AUTHOR]

Halloysite is a natural nanosized tubular clay mineral that has many potentially important uses in different industrial fields. In this paper, the key structural characteristics and properties of halloysite and their related applications are comprehensively reviewed. Research advances on halloysite, especially those from the past 20 years, are summarized with some critical comments. Attention is mainly paid to the structure and morphology of halloysite and their changes, the formation of tubular structures, the physicochemical properties, the surface chemical modifications, and the halloysite-based advanced materials and some related applications. Additionally, future prospects and key problems to be solved in halloysite studies are discussed. This review shed new light on both fundamental and applied studies that focused on halloysite. [ABSTRACT FROM AUTHOR]