Carbon nanotubes for dye removal: A comprehensive study of batch and fixed-bed adsorption, toxicity, and functionalization approaches

Carbon nanotubes (CNTs) are increasingly being acknowledged as possible adsorbents due to their exceptional characteristics, such as increased porosity, high absorption capacity, rapid absorption rate, high specific surface area, etc. However, pristine CNTs often exhibit limitations in terms of dye adsorption efficiency due to their hydrophobic nature. Functionalized CNTs have been designed to overcome these challenges by adding other chemical groups. These modifications enhance the adsorption capacities of CNTs by producing a more favorable interface between the dye molecules and CNTs. Furthermore, in order to improve their adsorption efficiency for dye degradation, CNTs are also integrated with metals, metal oxides, polymers, MXenes, or graphene to form nanocomposite. This article discusses the characteristics and active adsorption sites of CNTs, the mechanism of dye adsorption, and the influence of significant variables. The efficacy of dye removal rises with an increase in the adsorbent concentration. Studies indicate that adsorption capacity is enhanced by chemical modification and composite synthesis. Comprehending the isotherms, kinetics, and thermodynamics of adsorption is essential for providing the fundamental knowledge needed for the construction and functioning of adsorption apparatus. The majority of dye adsorption processes, according to literature studies, follow pseudo-second order kinetics and the Langmuir isotherm model, indicating a preference for monolayer adsorption in dye degradation. The categorization, characteristics, applications, and toxicity effects of different dyes on aquatic life, animals, and humans are also covered. This review explores the use of CNTs, functionalized CNTs, and CNT-based composites in the degradation of various dyes through batch and fixed-bed adsorption studies.