Polyethyleneimine (PEI) incorporated Cu-BTC composites: Extended applications in ultra-high efficient removal of congo red

Recently, metal-organic frameworks (MOFs) have been applied in the efficient removal of persistent organic dyes owing to their high porosity, large surface area and tunable pore size. In this paper, polyethyleneimine (PEI) incorporated Cu-BTC composites have been applied in the efficient removal of congo red for the first time. Some means such as Field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy(XPS), Fourier-transform infrared spectroscopy (FT-IR), Thermal gravity analysis (TG) and Zeta potential measurement have been employed to character these Cu-BTC composites. The modification of PEI with a certain loading amount can alter the morphology and also change the surface charge of Cu-BTC evidently, as a result the PEI-modified Cu-BTC composites present the positive zeta potential within a wide pH range, that is beneficial for the effective adsorption toward anionic dyes. Four persistent anionic dyes including congo red (CR), acid blue 1(AB I), methyl orange(MO) and direct blue 80 (DB80) are used to evaluate the adsorption activity of PEI-modified Cu-BTC, and among them CR displays the best adsorption selectivity, which capacity is about 20 times of the values of other three dyes. And the adsorption capacity of CR is also higher than the most reported removal rate by other MOF-based adsorbents. The adsorption kinetics analyses demonstrate that a combined physical and chemical interaction have worked in the CR adsorption at room temperature, and the adsorption thermodynamics indicates a endothermic, increase of entropy and spontaneous process with a negative Delta G(circle minus). The adsorption mechanism is also discussed, and the hydrogen bond formed between PEI-modified Cu-BTC and CR might be responsible for the ultra-high adsorption capacity of CR. The better reusability indicates that these PEI-modified Cu-BTC can be applied in the efficient removal of CR from practical waste water.