From mining waste to environmetal remediation: a nanoadsorbent from Amazon bauxite tailings for the removal of erythrosine B dye

In this study, bauxite washing residues (i.e. mining tailings) from the Brazilian Amazon region were explored as low-cost precursor for the preparation of a pyroaurite-like (Mg-Fe-Al-NO3) layered double hydroxides (LDH) nanonoadsorbent. The synthesis was predicated on a simple co-precipitation approach in order to achieve a “low-end” adsorbing nanomaterial amenable to environmental remediation. The nanoadsorbent was structurally and morphologically characterized by X-ray diffraction (XRD), infrared (FTIR) and Raman spectroscopy, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and specific surface area measurements (BET method). The material was tested for erythrosine B removal from aqueous solutions. Effect of temperature, pH, contact time and initial dye concentration on the nanoadsorbent performances were also investigated. The LDH proved to be suitable for fast and efficient removal of erythrosine B. Indeed, almost complete dye adsorption on the nanostructured pyroaurite occurred in the first 20 min leading to an adsorption capacity (qe) equal to 94.25 mg/g for a specific surface area of 81 m2/g. Kinetic experiments data were fitted with a pseudo-second order model resulting into R2 equal to 0.995, whereas calculated qe values (depending on the initial dye concentration) were very close to the experimental ones. Coefficients associated to Langmuir, Freundlich and Temkin isotherm models (RL, n, and KT, respectively) revealed a strong interaction (i.e. affinity) between the dye and the nanadsorbent allowing high amounts of erythrosine B to be retained on the LDH. Non-linear fits were found to describe more accurately the adsorption process than the corresponding linear regression procedures for every and each isotherm. Based on thermodynamic parameters, the adsorption process turned out to be spontaneous and endothermic in nature. As for temperature effects, dye removal rate (percentage) increased from 89% to 93% as the temperature was raised from 35 °C to 55 °C.