Insight into the adsorption of acid fuchsin using CoAl layered double hydroxide: Central composite design, kinetics, isotherms and thermodynamic studies

Massive amounts of wastewater contaminated by hazardous and synthetic dyes are produced by the textile industry. Therefore, it is crucial to develop innovative techniques for removing these pollutants. In the present study, CoAl- LDH with excellent adsorption ability was synthesized via coprecipitation method at ambient temperature. The surface morphology and composition of the resulted compound were evaluated using several methods including X-ray diffraction (XRD), Fourier Transform Infrared spectrometry (FTIR), scanning electron microscopy (SEM), Thermogravimetric analysis (TGA), differential thermogravimetry (DTG), and Differential scanning calorimetry (DSC). The results given by Brunauer, Emmett and Teller theory (BET) showed that the material has a specific surface area of 48 m2.g-1 and pore volume of 0.42 cm3.g-1. The as-synthesized material was tested for the acid fuchsin removal from an aqueous medium and the impact of several parameters was investigated. The experimental parameters such as the pH, the initial concentration, the adsorbent dose and the time were optimized using the central composite response surface methodology. The optimal adsorption capacity is attained at pH = 4.5, Initial concentration C0 = 96.71 ppm, adsorbent dose d=0.40 g.L-1 and time t = 180 min. The adsorption process fitted well with the pseudo-second order reaction, and followed Langmuir adsorption isotherm providing a considerable Langmuir adsorption capacity of 384.6 mg. g-1. The effect of different temperatures reveals the thermodynamic parameters, i.e., the free Gibbs energy ΔG° (-4.86 to -1.16 kJ.mol-1), enthalpy change (ΔH°=79.69kJ.mol-1) and entropy (ΔS°=137.61 JK-1.mol-1), the negative ΔG° and positive ΔH° values signify that the fuchsin acid adsorption is spontaneous and endothermic.