Carbon Nanotubes in Water Treatment: Progress and Challenges

Introduction

Many technologies have been developed for water treatment, however, and a majority require high capital investment for implementation especially in developing countries. Adsorption is described as a simple, techno-economical and efficient method for the removal of organic and inorganic contaminants from water (Tome et al., 2021; Shikuku and Mishra, 2021). Among the various adsorbents, those derived from lignocellulosic biomass such as activated carbons (ACs), are the widely used type of adsorbents in water treatment, because of their local availability, chemical inertness, broad-spectrum removal capability toward pollutants, and thermal stability (Sanou et al., 2016; Sanou et al., 2019; Liu et al., 2012). However, the use of ACs in water treatment is limited by several bottlenecks, such as slow adsorption kinetics and difficulty for regeneration (Liu et al., 2012). To overcome these constraints, activated carbon fibers (ACFs) were developed as the second generation of carbonaceous adsorbents and their pores are directly opening on the surface of carbon matrix, which shortens the diffusion distance of pollutants to the adsorption sites. Consequently, adsorption kinetics of ACFs is higher than those of ACs. On the other hand, carbon nanotubes (CNTs) are considered like miniaturized ACFs with one dimensional structure and all adsorption sites are located on their inner and outer layer surface. Theoretically, CNTs may be a promising third generation of carbonaceous adsorbents with a tunable surface chemistry. These new types of adsorbents have been used for adsorption of metal ions (Li et al., 2002), anionic contaminants (Li et al., 2001; Peng et al., 2005), organic compounds (Cho et al., 2011), removal of biological contaminants (Venkata et al., 2009), and for the softening of hard water (Maryam and Toraj, 2011). A great number of organic compounds and heavy metals have been studied as the target pollutants on CNTs in water treatment with various physical structures and surface chemistry. The influence of operating conditions on solution chemistry, including solution pH, ionic strength, and co-existing matter, must be investigated for application in industrial scale to clean real wastewaters. In addition, the study of kinetic and isotherm models is important in order to explain the adsorption mechanisms. The use of CNTs for water treatment is not without difficulties. When using CNT powder as an adsorbent on an industrial scale, combining the CNTs with hard water with ultrasonic agitation is neither economical nor technically feasible. Furthermore, without centrifugation process, it is impossible to entirely recover the spent and contaminant-laden CNTs powder from treated water following the adsorption process. Adsorbent recovery by filtration is also difficult since the CNTs can quickly clog the filter (Maryam and Toraj, 2011). Furthermore, CNTs have been reported to be potentially toxic and their occurrence in water is contentious from a safety standpoint (Das et al., 2018). This chapter discusses the preparation and application of CNTs and their composites for the removal of dyes, heavy metals, and emerging contaminants from water. Catalytic properties of CNTs have also been highlighted. The associated toxicity and risks of CNTs are also presented.