A critical review of the occurrence of perfluoroalkyl acids in aqueous environments and their removal by adsorption onto carbon nanotubes

The presence of perfluoroalkyl acids (PFAAs) in aquatic environments is a cause of concern, due to their toxicity, possible ecological impact and adverse effects in man. The release of these pollutants into receiving water bodies occurs primarily through the discharge of untreated wastewater and industrial effluents. Consequently, there is a need to remediate wastewater containing these compounds before its discharge. In this review, the occurrence of PFAAs in water streams is reviewed, with the aim of providing in-depth information on the harmful effects arising through exposure to these pollutants by both man and the environment. One viable strategy for the removal of PFAAs from wastewaters is adsorption. This technique is discussed in relation to a number of conventional adsorbents and they are compared with the behaviour of a more effective adsorbent, namely, carbon nanotubes (CNTs). In particular, various functionalization strategies can increase the efficiency of CNTs for the removal of PFAAs. Sorption of PFAAs onto CNTs demonstrates good removal efficiencies and equilibrium is attained faster than with conventional adsorbents. This is attributed to the inherent properties of CNTs, such as large surface area/porosity, and the ease with which new functional groups are introduced onto the walls of the tubes. The adsorption mechanism of PFAAs is primarily enhanced through electrostatic interactions; however, other intermolecular forces, such as hydrogen bonding, hydrophobic interactions and ion-exchange, also play a role. This review aims at providing information on the occurrence and fate of PFAAs and the interactions involved in their removal from aqueous solutions by CNTs.