Gallic acid removal using fresh and calcined Ni-Al layered double hydroxides: Kinetics, equilibrium and response surface methodology (RSM) optimisation

Abstract

ABSTRACT Emerging contaminants have recently aroused attention because of their harmful impact on ecosystem and human health. Among those contaminants, standout gallic acid which is known for its high toxicity even at low concentration. In this work, we investigated the application of Ni-Al layered double hydroxides (LDHs) as adsorbents in the removal of gallic acid from synthetic aqueous solutions. The solids were synthesised by co-precipitation method with a cationic ratio of 3. X-ray diffraction (XRD), thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), surface area measurement (N2 adsorption/desorption) and scanning electron microscopy (SEM) methods were used to characterise the adsorbents. The influence of pH, initial concentration of gallic acid and mass of adsorbent parameters were evaluated. The Brunauer–Emmett–Teller (BET) surface area increased from 107.91 to 141.33 m2/g when the sample was calcined. The adsorption experiments results showed that the pseudo-second-order model was the better kinetic model to describe the gallic acid remove. The equilibrium adsorption data were analysed using the Freundlich, Langmuir, Sips and Toth isotherm equation models. For both fresh and calcined Ni-Al-LDH, the adsorption equilibrium followed the Freundlich model. The process was endothermic, being the maximum adsorption capacities of 137.9 and 82.97 mg/g for fresh and calcined Ni-Al-LDH, respectively. The findings showed more than 94% of the gallic acid could be removed by Ni-Al-LDH calcined at 500°C. This study report also the result of response surface methodology with central composite design. This methodology was employed to optimise the effect of process variables such as pH, initial concentration of gallic acid and mass of adsorbent (m) on adsorption efficiency. Results showed that the optimised data of pH 5, initial concentration 20 mg/L and amount of 20 mg were achieved. It can be concluded that Ni-Al-LDHs are attractive materials for the treating of waters and wastewater containing emerging molecules.