Compósito hidróxido duplo lamelar/carvão ativado e potencial aplicação na adsorção de corante aniônico

Abstract

A composite (ZnCuAl-HDL/CA) based on zinc, copper and aluminium trimetallic double lamellar hydroxide (HDL) and activated carbon (AC) was synthesized by the coprecipitation method, characterized and evaluated for adsorption of the reactive red 2 molecule(RR2). The characterizations, thermal analysis, infrared molecular absorption spectroscopy with attenuated total reflectance, scanning electron microscopy, powder x-ray diffraction and N2physiosorption isotherms, corroborated each other and indicated successful deposition of HDL on the activated carbon matrix. The N2 physisorption isotherm indicated that the volume of gas adsorbedin the relative pressure range of 0.1 to 1.0 by ZnCuAl-LHD/AC is considerably larger when compared to the precursor materials. This is associated with the considerable increase in the specific surface area that was calculated by the BET method of the composite (498,7 m2 g-1) compared to pure HDL (21,70 m2 g-1). The study of RR2 adsorption kinetics showed that the maximum adsorption capacity at experimental equilibrium was relatively higher for the composite (68,05 mg g-1) when compared to AC (40,83 mg g-1) and ZnCuAl-LHD (44,38 mg g-1). The fit of the data to the empirical models pointed out a better fit to the pseudo-second-order model for the precursor materials, indicating a predominance of chemisorption. The Elovich model was also applicable to CA and ZnCuAl-LHD/AC, confirming a process dominated by chemisorption associated with surface heterogeneity. The fit of the experimental data to the intraparticle diffusion model was not adjustable for ZnCuAl-LHD, whereas for AC and ZnCuAl-LHD/AC, it was possible to observe three stages, which reflect the boundary layer diffusion, followed by diffusion through the pores and the reach of the equilibrium state. The experimental data of the adsorption isotherms of the materials indicated a better fit to Sips model for the CA and ZnCuAl-LHD/AC adsorbents, while the data of ZnCuAl-LHD/AC presented a better correlation to Temkin model but could be satisfactorily described by Sips model. In the thermodynamic study, the positive values of increasing ∆H0 for AC, ZnCuAl-LHD and ZnCuAl-LHD/AC, respectively, indicate the endothermic nature of the overall dye adsorption process. The ∆S0 values found were 164,1; 185,5 and 201,4 J(mol K)-1 for AC, ZnCuAl-LHD and ZnCuAl-LHD/AC, respectively, suggesting the occurrence of a higher randomness at the solid-liquid interface during the RR2 adsorption process by the three materials, being higher for the composite. The spontaneity of the adsorption reaction was indicated by the negative value of ∆G0 for all adsorbents studied, which decreased with increasing temperature. The reuse test of the materials through calcination showed a significant increase in the adsorption capacity of the materials when compared to the first and second cycles. After the third cycle, there was a gradual decrease in adsorption capacity, less pronounced for the composite, associated with the progressive loss of crystallinity, besides the possible incorporation of molecules of the dye and its decomposition products in the structure of the adsorbents.