Reuse of iron ore tailings as an efficient adsorbent to remove dyes from aqueous solution

In this work, an iron ore tailings sample (IOT), collected from a tailings dam in Minas Gerais, Brazil, was characterized. The IOT presented point of zero charge of ∼ 6, specific surface area of 4 m² g^-1, and was mainly composed of hematite and quartz. Subsequently, experiments were performed to evaluate the adsorption of an anionic dye, Direct Red 80 (DR80), and a cationic dye, Methylene Blue (MB), by the IOT, studying the effects of its dose (dose_IOT) and the solution initial pH (pH₀). The DR80 removal increased with the decrease of the pH₀ while the opposite effect occurred in the experiments with the MB, suggesting the process is governed by the adsorption resulting from electrostatic forces. The increase in the dose_IOT increased the DR80 and MB removal, which can be attributed to the greater availability of adsorption sites. Pseudo-second order kinetic (R²> 0.9994) and the Langmuir equilibrium isotherm (R²> 0.9842) models described well the DR80 adsorption by the IOT, being the reaction rate and maximum adsorption capacity higher at lower pH₀. In a regeneration experiment, it was possible to desorb almost entirely the DR80 using a NaOH solution. Additionally, the regenerated IOT was able to adsorb the DR80, demonstrating its reusability. In a preliminary assay, the IOT decreased the colour of the textile wastewater sample at pH₀ 3. Therefore, the results indicate the potential use of IOT for removing electric-charged pollutants by adsorption, especially anionic ones under acidic conditions.