Effective removal of anionic dye from aqueous solutions using modified carbonate material: performance and mechanism

Abstract

In this study, a carbonate material (CM) and its form heated to 900 degrees C (CM900) were used to remove Congo red (CR) from aqueous solutions. The objective was to combine the results of the characterization of the modified material, CR adsorption and spectroscopic study to propose a coherent mechanism of the CR-CM900 interaction. A bibliographical study on the adsorption of dyes by carbonates and their interactions revealed the scarcity of published works. This study will compensate somewhat for the lack of results in this field. Carbonate materials are abundant, low-cost and available in different countries around the world. The materials were characterized by X-ray diffraction and scanning electron microscopy. A solid/solution concentration of 1 g center dot L-1, pH of 6.9, equilibrium time of 2 h and temperature of 40 degrees C, were found to be the optimum conditions for a maximum amount of CR adsorbed by CM900 of 288.2 against 32.7 mg center dot g(-1) for CM. A full decarbonation of CM900 results in MgO and CaO, releases CO2 from the structure, and leads to a more porous structure. The pseudo-first-order model adequately described the kinetic data. The experimental isotherms were suitably fitted by the Redlich-Peterson model with determination coefficient and average relative error values >= 0.97% and <10.0%, respectively. Thermodynamic parameters suggested a spontaneous and endothermic process. Methanol easily desorbed the CR adsorbed by CM900 which maintained its adsorption capacity during three adsorption-desorption cycles. Fourier-transform infrared spectroscopy investigation before and after the dye adsorption showed that the CR-CM900 interaction involves a mechanism of outer-sphere complexation between the amine groups and the hydroxylated magnesium oxides. Understanding the interactions between dyes and carbonate materials is an important approach to develop the use of these materials in wastewater treatment.