Utilização de resíduos de café para remoção adsortiva de corantes em solução aquosa

Abstract

In this work, the adsorptive removal of dyes using coffee grounds from aqueous solutions was studied, aiming to reduce the amount of these substances in effluents. The coffee grounds were obtained from the coffee maker of the Laboratory of Applied Electrochemistry – LEAP/Ufal and was used without chemical treatment, passing only by washing with distilled water at 100 °C until the colored compounds were eliminated. Then, the resulting material was put to dry in an oven at 60°C for 24 hours. The dyes studied were: rhodamine B, safranin T, malachite green and a mixture of malachite green and rhodamine B dyes, both at the same concentration (50 ppm). The adsorbent agent was characterized by infrared spectroscopy (FTIR), X-ray diffraction (XDR), scanning electron microscopy (SEM) and zero charge pH (pHpcz) analyses. Adsorption studies were conducted to investigate the affinity between dyes and coffee residues (qualitative tests), the effects of initial concentration, pH and aqueous matrix were investigated. Kinetic and balance studies were also performed. In the adsorption tests, removal percentages above 90% were achieved for the dyes safranin T, malachite green and mixture in (617 nm) and removal of up to 84.31% for the dye rhodamine B and mixture in (558 nm). The results of the kinetic study with rhodanin B and Safranin T dyes preferentially follow pseudo-second order kinetics, obtaining, in these cases, higher values of correlation coefficient R² and lower values of lower values of mean relative error (ARE). For the mixture in (558 nm) and in (617nm) it preferably followed a pseudo-first order kinetics due to the higher value of the correlation coefficient (R2) and lower ARE. For rhodamine B, safranin T, malachite green and mixture in (558 nm) dyes, the equilibrium data were better fitted to the Freundlich model, with higher values of R2 and lower values of (ARE) for all studied temperatures (30 , 45, and 60°C). As for the mixture (617 nm), the Langmuir model is the one that best fits the data. Adsorption was tested in different aqueous matrices, higher removal percentages were obtained in distilled water, river water and mineral water. Therefore, coffee grounds were successfully used to remove dyes from water, proving to be a promising alternative biosorbent.