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 102   mg/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 Δ H 0 , entropy change Δ S 0 and Gibbs free energy change Δ G 0 , were also determined. All the Δ G 0 values are negative; the Δ H 0 and Δ S 0 values of PDOPA- f -LDH were −7.824   kJ   mol −1 and −0.01562   kJ   mol −1   K −1 , respectively. And the activation energy of system ( E a ) is calculated as 24.69   kJ   mol −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.