Removal of toxic dyes from aqueous solution by adsorption onto highly recyclable xGnP® graphite nanoplatelets

The adsorptive capacity of xGnP (R) graphite nanoplatelets (GNP) was evaluated for the removal of Drimaren Dark Blue CL-R (DDB), Direct Red 80 (DR80), Chrysophenine (CH) and Eosin Yellowish (EY). Kinetic pseudo-second-order (PSO) model best fitted the experimental data for all evaluated dyes. Moreover, sorption capacity at equilibrium state for DDB, DR80, CH and EY were 22.14, 38.36, 28.68 and 156.77 mg g(-1), respectively. Equilibrium data for the DDB dye were best fitted to the BET type IV model, while DR80, CH and EY dyes were adjusted by BET Type II. From these models, it was achieved the following maximum monolayer adsorption capacities: 20.51, 26.41, 39.41 and 66.00 mg. g(-1) for DDB, DR80, CH and EY, respectively. Furthermore, investigations over the effect of pH on adsorption revealed that its behavior is governed by strong electrostatic and pi-pi stacking interactions. Thermodynamic studies exhibited negative values, of enthalpy and Gibbs free energy, for all dyes, indicating the spontaneous and exothermic nature of adsorption. Hence, adsorption tends to be more favorable as the temperature decreases. GNP also showed high regeneration capacity, keeping its sorption efficiency after six adsorption-desorption cycles. Dyes solutions also showed low phytotoxicity after their adsorption on GNP.