Functionalization of halloysite nanotubes as remediation agents for organic and inorganic contaminants: Insights into synthesis routes, removal techniques, and eco-friendly perspectives

Abstract

Halloysite is considered a promising natural clay mineral due to its unique multi-rolled tubular structure (nanotubes), tunable surface properties, high aspect ratio, and natural abundance. Halloysite nanotubes (HNTs) have attracted significant interest for being used as support material for other active chemical agents. The resultant functional materials can be effectively used in wastewater treatment. This review focuses on the utilization of HNTs to enhance the efficacy of functional materials in the remediation of environmental pollutants from wastewater. HNTs are readily modifiable with various functional agents, such as nanoparticles, polymers, surfactants, enzymes, oxidants and reductants, and nano‑carbon materials. This is due to their large surface area, distinctive cylindrical shape, mesoporous interior, surface and sub-surface active sites, and exceptional compatibility with living organisms. As a result, HNTs-supported material can improve its adsorption capacity, catalytic activity, and selectivity toward persistent and non-persistent contaminants. This review discusses different nanomaterials (e.g., metal and metal oxide nanoparticles, nanoclusters) and their HNTs-based nanohybrids, emphasizing their multifunctional performances and eco-friendly aspects in remediation technology. However, most of the chemical modifications of HNTs rely on using synthetic surfactants, polymers, and ligands. While HNT itself is biocompatible, the risk of leaching of these chemicals and their direct exposure to the whole composite may pose secondary risk to the natural environment during in-situ application. However, functionalization of HNTs in a more eco-friendly manner could minimize that risk, resulting in achieving sustainable water remediation. The functionalization of HNTs is discussed in detail to provide a comprehensive understanding of sustainable wastewater management. Additionally, this review addresses the issue of non-specific secondary pollution caused by modified HNTs with an emphasis on minimizing environmental impact. Further, the limitations of HNTs-based nanocomposites in real-world wastewater treatment are identified, and recommendations are provided.

Keywords: Degradation; Halloysite nanotubes; Heavy metals; Organic pollutants; Selective adsorption; Wastewater.