Utilization of carbon nanotubes in removal of heavy metals from wastewater: a review of the CNTs' potential and current challenges

Carbon nanotubes-based adsorbents have attracted substantial interest as potential adsorbents for heavy metals removal. However many aspects such as the interaction between the modified carbon nanotubes (CNTs) and the heavy metal ions, quantitative effect of the functional groups and regeneration of CNTs-based adsorbents are not fully understood yet. A critical review was performed to compile an extensive profile from several studies of using pristine and modified CNTs for heavy metals removal. CNTs demonstrated a great potential. However surface modification of CNTs is necessary as pristine CNTs may be ineffective in arsenite [As (III)] or arsenate [As (V)] removal. Isotherms and kinetic models for the removal of heavy metals are discussed in details. A particular focus has been placed on better understanding the mechanism of heavy metals removal using CNTs-based adsorbents, affecting factors, maximum adsorption capacity and regeneration. The effect of adsorbent dose, pH, initial concentration, and contact time were addressed by several researchers, which specifies the consistency and performance of CNTs-based materials potential adsorbents. To elucidate the mechanism of adsorption, FT-IR, XPS, SEM, TEM and EDX results have been reviewed and discussed. It is found that Langmuir, Freundlich and second order kinetic models are the most frequently used isotherm to describe heavy metals adsorption. CNTs-based adsorbent can be efficiently regenerated.