Synthesis of functionalized MgAl-layered double hydroxides via modified mussel inspired chemistry and their application in organic dye adsorption.

In this paper, a novel strategy for the preparation of poly(levodopa) functionalized MgAl-layered double hydroxide (PDOPA-f-LDH) was developed based on the modified mussel inspired chemistry. The utilization of PDOPA-f-LDH for the removal of methylene blue (MB) from aqueous solution was also examined. Taken advantage of the self-polymerization of levodopa (DOPA) in alkaline solution and the strong affinity of catechol groups to the substrate surface, the LDH was covered homogeneously by a layer of polymer coating of DOPA, leading to the functionalization toward LDH. The structure, surface morphology, thermostability and elemental composition of as-prepared PDOPA-f-LDH were investigated by the transmission electron microscope, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. Besides, the surface charge of the PDOPA-f-LDH was also investigated using zeta potential. The effects of various parameters, including contact time, initial MB concentration, solution pH and temperature, on the adsorption of MB onto PDOPA-f-LDH were systematically investigated. Results show that the adsorption capacity of functionalized LDH at 25°C could reach up to 102mg/g, which is much higher than that of pure LDH in the same experimental conditions. The adsorption kinetics and isotherm of MB adsorption were studied in batch experiments. The pseudo-second-order model is found to be the best to describe the adsorption kinetics. The isotherm result shows that the Freundlich isotherm is the better-fit-isotherm model to represent the equilibrium data. The values of thermodynamic parameters, including enthalpy change ΔH0, entropy change ΔS0 and Gibbs free energy change ΔG0, were also determined. All the ΔG0 values are negative; the ΔH0 and ΔS0 values of PDOPA-f-LDH were -7.824kJmol-1 and -0.01562kJmol-1K-1, respectively. And the activation energy of system (Ea) is calculated as 24.69kJmol-1. The fact suggests that the MB adsorption on PDOPA-f-LDH is a spontaneous and exothermic process. These obtained results indicate that the prepared PDOPA-f-LDH could be used as an interesting adsorbent with great potential to adsorb the cationic dyeing pollutants from aqueous media.