High-performance, layered double hydroxides-derived mixed metal oxides from bauxite tailings as effective adsorbers for water remediation-ponceau 4R study

This paper proposes, for the first time, layered metal oxides (LDO) prepared from an industrial waste as materials for environmental remediation. Specifically, magnesium‑iron‑aluminum mixed oxides (CLDH-Xs, X = calcina tion temperature: 400 °C and 600 °C), attained by annealing an Mg-Fe-Al layered double hydroxide (LDH), were used as adsorbing materials for the removal of the anionic dye ponceau 4R (P4R) in aqueous solutions. The Mg Fe-Al LDH, in turn, was synthesized by recovering iron and aluminum from Amazon rainforest bauxite washing tailings. The materials were fully characterized by scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), specific surface area/porosity mea surements, and zeta potential analysis, before and after the adsorption experiments. The influence of the adsor bent dosage, contact time, and temperature on materials dye removal capabilities were investigated and linked to the structural, compositional and morphological features of the two adsorbents. Adsorption isotherm models were also evaluated, highlighting the strong adsorbate-adsorbent interaction (for instance a Langmuir RL value of 1.8‧10−4). Both CLDH-Xs showed excellent performances with respect to P4R adsorption. In particular, CLDH 400 °C achieved a P4R removal rate of 99 % in 30 min, with an experimental adsorption capacity equal to 94 mg/g. Conversely, a temperature of 40 °C was required in order for CLDH-600 °C to measure up to CLDH 400 °C. In both cases, the adsorption mechanism was proved to be based on chemisorption (endothermic but with a relevant favorable entropic contribution) and strictly connected to the regeneration of the original LDH by an ion-exchange effect limited to the adsorbent surface (i.e. no dye intercalation). The results suggest that mixed metal oxides, notably waste-derived, can be regarded as promising, and relatively inexpensive adsorbents, for ponceau 4R removal. Furthermore, the present approach allows for mining tailings reuse, eventually on large scale, and water remediation in light of circular economy and environmental sustainability principles.