Comparative study of Pb2+, Ni2+, and methylene blue adsorption on spherical waste solid-based geopolymer adsorbents enhanced with carbon nanotubes

Spherical porous carbon nanotube-reinforced geopolymer (CNTs/GP) adsorbents were spheroidized in poly-ethylene glycol 400 solvent by combing carbon nanotubes (CNTs) with a fly ash and slag-based geopolymer slurry. The effects of CNTs on the phase evolution and pore structure of the CNTs/GP spheres and their adsorption behavior for methylene blue (MB) and heavy metals (Pb2+ and Ni2+) were evaluated. The CNTs/GP spheres had a perfect spherical shape and mesoporous structure. The spheres were composed of an amorphous phase and had a high BET surface area of 73.06 m(2)/g with 3 wt% CNTs. The addition of CNTs into the spheres enhanced the removal efficiency for the pollutants (Pb2+, Ni2+ and MB). The calculated q(m) of the sphere with 3 wt% CNTs was 25.549 mg/g for MB, 52.743 mg/g for Pb2+; and 8.915 mg/g for Ni2+. The adsorption data of the spheres were well obeyed the Langmuir isotherm model and pseudo-second-order kinetics model, suggesting that multiple mechanisms such as physisorption, chemisorption and ion exchange were involved. The CNTs/GP spheres also presented good cyclic regeneration properties; hence, they can be applied as potential adsorbents for the treatment of hazardous wastewater.