Response Surface Methodology for Optimizing Cd(II) Adsorption onto a Novel Chemically Changed Nano Zn2Al-Layer Double Hydroxide

In this work, a novel chemically adsorbent based on Zn2Al-layered double hydroxide (LDH) that modified by indigo carmine (IC) (Zn2Al-LDH/IC) was synthesized. The chemical composition and morphology of the synthesized Zn2Al-LDH/IC were investigated using the X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy analysis. Response surface methodology (RSM) using the central composite design (CCD) is applied to optimize the adsorption process parameters for Cd(II) removal from the aqueous solution using a novel chemically modified nano Zn2Al-layered double hydroxide (Zn2Al-LDH/IC). The combined effect of adsorption parameters such as contact time, initial Cd(II) concentration, adsorbent amount and initial pH of solution were studied. The results obtained by ANOVA analysis displayed the relative significance of the process parameters in the adsorption process. The optimum conditions to remove Cd(II) from aqueous solution were at the initial Cd(II) concentration of 52 mg/L-1, pH 4.13, the adsorbent dose 0.06 g, temperature of 36.5 °C and contact time 36 min. In optimum conditions, high adsorption efficiency and maximum adsorption capacity were 47.3% and 6.11 mg/g, respectively.  Adsorption of Cd(II) by nano Zn2Al-LDH/IC could be well examined with Langmuir and Freundlich isotherms and the pseudo second-order kinetic model was fitted to the adsorption kinetic data. Furthermore, the thermodynamic data exhibited that the adsorption process of Cd(II) by nano Zn2Al-LDH/IC was spontaneously and exothermic.