Polycyclic aromatic hydrocarbons extraction and removal from wastewater by carbon nanotubes: A review of the current technologies, challenges and prospects

Polycyclic aromatic hydrocarbons (PAHs) that increasingly emanating from natural and human activity sources are invisible pollutants and the presence of even minute amounts of these substances makes them undesirable due to their negative attributes. The PAHs extraction and removal from effluents are major steps for analysis and removal. Chemical, biological and physical approaches are used for the removal of PAHs from wastewater. However, some of these techniques have toxic by-products. Currently, physical approaches such as liquid-liquid solvent extraction, filtration, and adsorption appear to be the best methods due to their safety, affordability, universal nature and ease of operation. However, some of the materials used in the currently available physical methods face the challenges of fouling, low recovery, their relatively long extraction time and high usage of toxic solvent. To overcome these limitations, and for the cleaner production of water, the incorporation of carbon nanotubes (CNTs) into existing physical procedures without distorting their existing structural architectures is proposed. CNTs are considered because hydrophobic hollow pores of CNTs absorb most aromatic pollutants while allowing the friction-free passage of water without consuming much energy. The functionalization of CNTs with various receptors and groups further permit the selective trapping of pollutants of interest. The ability of the proposed methods to cheaply and sustainably remove PAHs from contaminated water will make hitherto carcinogenic polluted water cleaner for public consumption. This comprehensive article presents an extensive literature review of CNTs and their use for various physical techniques. It further presents CNT as a central focus, with an in-depth critical analysis of the state-of-the-art trends of PAH removal using CNTs, current hurdles and future challenges, so that it could be used as a reference manual adding scientific value for developing any future methods for water purification and the removal of any aromatic pollutants.