Selective Dye Adsorption by pH Modulation on Amine-Functionalized Reduced Graphene Oxide-Carbon Nanotube Hybrid

Simultaneous functionalization, reduction, and hybridization of graphene oxide in graphene oxidemultiwall carbon nanotube hybrid were carried out by p-phenylenediamine (rGOCNTPPD). The adsorption properties of the hybrid for selective removal of methyl violet (MV) and methyl orange (MO) from their mixture were investigated under different experimental conditions. The adsorption was correlated with the effects of pH, contact time, and temperature. Changing the pH of the solution modulates the selectivity of adsorption behavior of the hybrid. At pH 7.0, it can remove 99.7% MV selectively from the mixture, whereas at pH 3.0, it can remove MO selectively having 98% removal efficiency. The maximum adsorption capacities (q(m)) of the hybrid for MV and MO are 298 and 294 mg/g, respectively, at 298 K; these values are higher than several previously reported qm values. The kinetics data for both dyes can be well fitted with the pseudo-second-order kinetics model, and the adsorption of both dyes followed a Langmuir adsorption isotherm. The adsorbent can be easily regenerated and reused, and its activity remains the same even after five adsorptiondesorption cycles. The rGOCNTPPD hybrid could be a promising adsorbent for selective removal of synthetic dyes from their mixture by pH modulation.