Insights into the mechanism of cationic dye adsorption on activated charcoal: The importance of π-π interactions

The mechanism and capacity of methylene green (MG5) adsorption onto commercial activated-charcoal (CAC, Norit RB4C) were investigated in batch experiments. The micro porous CAC material was found to exhibit a large specific surface area (1026 m(2)/g) and high total pore (0.502 cm(3)/g) and micropore (0.347 cm(3)/g) volumes. The point of zero charge (9.81 +/- 0.07) of CAC was determined by the "drift method" and found to be insignificantly dependent on the varying operation conditions. The dye adsorption process was low relative to the solution pH (2.0-10) and ionic strength (0-0.5 M). Kinetic studies indicated that the adsorption equilibrium was quickly reached based on low activation energy required for adsorption (Ea; 4.12 kJ/mol). CAC can remove 53-64% of the MG5 concentration from solution within 1 min. The maximum adsorption capacities determined from Langmuir model at 10 degrees C, 30 degrees C, 40 C, and 50 degrees C were 361 mg/g, 489 mg/g, 543 mg/g, and 581 mg/g, respectively. Desorption studies demonstrated that the MG5 adsorption was irreversible. The MG5 adsorption process was found to be spontaneous (-Delta G degrees), endothermic (+Delta H degrees), and increased the randomness (+Delta S degrees) in the system. Oxygenation of the CAC surface through a hydrothermal process with acrylic acid resulted in a decrease in MG5 adsorption and identified the importance of pi-pi interactions to the adsorption process. The analysis of Fourier transform infrared spectroscopy revealed that the aromatic C=C bonds decreased in intensity and upshifted after MG5 adsorption, which additionally confirms the significant contribution of pi-pi interactions. The combined results of our studies highly indicated that the primary mechanisms in MG5 adsorption were pi-pi interactions and pore filling, while hydrogen bonding and n-pi interactions were minor contributors. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.