Graphene Oxide/Carbon Nanotube Membranes for Highly Efficient Removal of Metal Ions from Water

Graphene oxide (GO) has an excellent adsorption capacity toward metal ions. Therefore, it is widely recognized as an auspicious material for fabrication of membranes applied in metal ion separation. However, GO membranes are not stable in aqueous solution because of electrostatic repulsion between GO nanosheets which are negatively charged. This paper shows that stable GO membranes can be easily obtained by the noncovalent interaction of GO with oxidized carbon nanotubes (CNTs). The experiment also shows that the GO/CNTs membranes can be used for the effective adsorption of metal ions. The kinetic data, adsorption isotherms, competitive adsorption experiment, and X-ray photoelectron spectroscopy indicate that the adsorption of metal ions is based on chemisorption. The membranes are remarkably durable in acidic, neutral, and basic solutions. Although the significant stabilization of the membranes by CNTs is observed, they strongly influence the adsorption process. Our study reveals that even a small amount of CNTs (GO/CNTs in the ratio 8:1) significantly reduces adsorption capacities of the membranes which were as follows: 37, 40, 50, 42, 48, and 98 mg g-1 for Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II), respectively. The reduction of the membrane adsorption capacities results from the creation of micro- and nanochannels formed by entangled CNTs. Durability and adsorptive properties of studied membranes indicate their potential use for the removal of metals from water.