Preparação, caracterização e aplicação de um compósito ferromagnético na remoção do corante verde de malaquita em meio aquoso

Abstract

Textile industries have directly contributed to the contamination of the environment, mainly due to the disposal of effluents, which are generated in the dyeing process. Frequently, these effluents have different colors and variable compositions. Therefore, there is great concern in performing the appropriate treatment of them. Among the techniques used for this purpose, adsorption has received special attention because it is highly efficient and presents low cost. In this context, this study aimed to evaluate the adsorbent capacity of a composite based on nanotubular halloysite (HNT) and magnetite nanoparticles (Fe3O4) for the removal of cationic dye malachite green (MG) in aqueous medium. Firstly, the HNT-Fe3O4 composite was prepared by the chemical precipitation method using a solution of iron salts (II) and (III), and HNT. Then, it was characterized by means of several techniques including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), specific surface area and pore size, using the Brunauer, Emmett and Teller method (BET), and determination of pH at the point of zero charge (pHPZC). After the characterization, the adsorption capacity of the HNT-Fe3O4 composite was investigated in a batch system, under controlled temperature (25 °C). The remaining concentration of MG in adsorption experiments was estimated using the molecular absorption spectroscopy in the visible region with an external calibration curve. Furthermore, the influence of experimental parameters, such as system stirring rate, adsorbent mass, initial pH, initial dye concentration and contact time were also evaluated. The experimental results showed that the adsorption kinetics was better described using a pseudo-second-order model. Regarding the adsorption equilibrium, Freundlich isotherm showed a better fit (R2 = 0.9578) to adsorption data, with values for the Freundlich constant (kF) and maximum adsorption capacity (qm) equal to 1.86 L g-1 and 44.25 mg g-1. On the other hand, the thermodynamic parameters revealed a physical and spontaneous adsorption process, with values of ΔG°ads, ΔH°ads, and ΔS°ads equal to -4.61 kJ mol-1, -28.13 kJ mol-1, and -77.60 J mol-1 K-1, respectively. Finally, the composite HNT-Fe3O4 proved to be a good adsorbent for the removal of MG cationic dye in aqueous medium and can be reused for several adsorption cycles without losing significant properties after the regeneration process by calcination.