In thiswork, it was observed that the method used for the preparation of the magneticpowder in which the Fe3O4 nanoparticles adhered to thesurface of the nanotubes of the HNT, was effective, as evidenced by thecharacterization tests.Theresults of the adsorption tests revealed that all the experimental parametersevaluated influenced the removal of the MG dye by the HNT-Fe3O4magnetic powder with pH as one of the most important parameters in this processsince it changes the chemical structure of the dye. The values used for thestirring rate of the system (200 rpm), adsorbent mass (0.15g) and initial pH (5.
5) were those where the highest percentage of MG dyeremoval occurred. However, since the mass of adsorbent has a direct relationshipwith the initial concentration of dye, it is necessary to define in advance theremoval percentage required. In situations in which the removal percentageshould be close to 100%, a greater amount of adsorbent should be used.Concerningthe kinetics and adsorption equilibrium, the experimental data were betterfitted to the pseudo-second-order model and Freundlich isotherm, indicatingthat the interaction between the MG and the HNT-Fe3O4 magneticpowder occurs in multiple layers.
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The results for the kinetic constant k2 are in the range of 3.9 ×10-2 to 7.09× 10-3 g mg-1 min-1, showing that if a moreconcentrated dye solution is used, then a longer time is required to reachequilibrium. The value of the Freundlichconstant (kF) and theconstant n were 1.86 mg g-1 (mg L-1)-1/n and 1.37, respectively, indicatingthat the adsorption process is favorable and occurs at heterogeneous sites.
Thermodynamicsrevealed that the adsorption process is spontaneous (?G°ads = – 4.61 kJ mol-1)and exothermic (?H°ads = –28.13 kJ mol-1), which is characteristic of physisorption andsuggests that there is a decrease in the randomness of the system (?S°ads= – 77.60 J mol-1 K-1).Finally,the regeneration process of the adsorbent used through calcination waseffective, despite the loss of 33% in the adsorption capacity of the magneticpowder in the first cycle.
In subsequent cycles, the HNT-Fe3O4maintained its near-constant removal capacity (approximately 49%), indicatingthat it could be reused at least four more times.