A review on halloysite-based adsorbents to remove pollutants in water and wastewater

Adsorption is a very promising and efficient technology for the removal of hazardous pollutants from water. In this regard, clay minerals have gained prime importance because of their natural origin, widespread existence and their unique features. Halloysite is a 1:1 clay mineral which is highly abundant in nature and has proven to be a bio-compatible material. Recently, halloysite has emerged as an innovative and efficient adsorbent for water treatment however no comprehensive review appeared on this topic. Its promising role in water treatment is evident from the large number of adsorption studies related to the halloysite-based materials, including this mineral in raw and modified forms and its hybrid composites with other reactive materials, which are compiled here. After a brief description of the general structure and main characteristics of halloysite in this review article, we discuss the research data related to the use of halloysite-based materials to remove heavy metals, dyes and other miscellaneous pollutants from water. Properties of the adsorbent can be tuned by surface modification for an optimized performance. Moreover, efficiency of adsorption process depends on various parameters like solution pH, pollutant concentration, contact time, temperature, nature and dose of adsorbent, which are also discussed in detail. Moreover, we critically examine the isotherm, kinetic and thermodynamic approaches, which are basic requirements to interpret an adsorption study but they have not been reviewed so far for halloysite-based adsorbents.