Clay minerals as environmentally friendly materials are abundant, cheap, and easy to obtain. Their unique crystal structure and surface properties endow them superior physical adsorption and chemical reaction properties The environmental pollution treatment mainly depends on the structure and properties of the used materials. Halloysite belongs to one of Kaolin clay minerals. It has some characteristics of a hollow tubular structure, high specific surface area, rich pore structure, high mechanical properties, and non-toxicity. The crystal structure and adsorption characteristics of halloysite are used to treat environmental pollutants, such as dyes, heavy metal ions, organic pollutants, etc..

However, natural halloysite contains a lot of impurities and shows limited functionality. The performance of HNTs in environmental application can be improved after purification, surface modification, and functionalization. In recent years, researchers conducted some related research on structural modification and functionalization of halloysite, including heat treatment, chemical modification, and composite modification. High-temperature and acid-base treatments affect the specific surface area, porosity, and adsorption capacity of halloysite, enhancing the adsorption capacity for pollutants.

The surface of halloysite is rich in hydroxyl groups (i.e., Si-OH and Al-OH), which can be used as sites for surface modification or functionalization, such as changing the surface wettability or introducing functional groups, metals nanoparticles, and metal oxides, etc.. Compounding halloysite with nanomaterials or polymers can form functional nanocomposites, which expands their applications in soil remediation, air purification, and improvement of wastewater quality. Organic substances can be introduced into the structure of halloysite through various methods such as ion exchange, surface modification, covalent bonding, and intercalation modification, thereby giving them enhanced dispersibility, stability, adsorption capacity, and chemical activity. The application of these modification methods can effectively tailor the surface properties and activities of halloysite. Grafting of silane on halloysite surfaces can directlly regulate its surface hydrophilicity and charge characteristics. Electrostatic interactions and hydrogen bonds interactions occur between halloysite and metal anions and organic dyes, which can be used for treatment of wastewater.

Halloysite and modified halloysite can be used as catalyst carriers and enzyme-immobilized carriers for the degradation of pollutants. The mechanical strength and thermal stability of halloysite can be significantly improved through the deposition or coating of nanoparticles on halloysite. Since ultrafine halloysite powder is difficult to recycle and easily cause secondary pollution, the functionalization of halloysite with magnetic materials can solve the problem of separating and collecting them from the matrix after use. In addition, the performance of halloysite can be precisely controlled via changing of the size, shape, and distribution of anchored nanoparticles. The stability of the loaded nanoparticles can be improved through ligands, polymers, surfactants, or ionic liquids during the synthesis process. Halloysite can be mixed with polymer to prepare a composite for synergistic enhancement of the materials performance. In addition, the modification also gives halloysite some functions like magnetic, optical and electrical properties, providing broader possibilities for the application of halloysite in environmental fields.

These application researches of halloysite in environmental area increase the economic value of halloysite minerals. However, the regeneration of halloysite after treating pollutants is still a challenge. In addition, the adsorption effect of halloysite could change in complicated polluted environments, and the type and concentration of pollutants need to be considered in specific applications. The existing research on halloysite in environmental pollutant control mainly focuses on water pollution and air pollution. Future work should focus on its application as a soil conditioner to reduce the mobility of various pollutants in the soil environment.