Biotemplated Fabrication of 3D Hierarchically Porous MgAl-LDH/CF Composites with Effective Adsorption of Organic Dyes from Wastewater

The serious pollution of water resources has brought great challenges to human existence; it is urgent to develop adsorbents for removing toxic dyes from wastewater. In this work, we synthesized hierarchically porous MgAl-layered double hydroxide (LDH)/carbon fiber (CF) composites with excellent adsorption of Congo red (CR) by combining the biotemplating and hydrothermal method. The effects of various Mg/Al molar ratios on the X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and Fourier transform infrared (FT-IR) spectroscopy characterizations and the dye adsorption capacity of composites were investigated. Three-dimensionally stackable MgAl-LDH nanosheets with a Mg/Al molar ratio of 4 were attached uniformly on the surface of hollow carbon fibers calcined from cotton, generating higher crystallization and specific area. The maximum adsorption capacity of Mg4Al-LDH/CF was equal to 271.00 mg g(-1) (linear Langmuir model, at 200 mg g(-1) CR in 72 h), which was significantly higher than the adsorption capacity of MgAl-LDH synthesized without the use of a biotemplate (linear Langmuir model, 170.35 mg g(-1) at 200 mg g(-1) CR in 72 h). The adsorption reaction followed the linear pseudo-second-order kinetics as well as the linear Langmuir model. Anion exchange and electrostatic attraction were considered as the main adsorption mechanisms. Mg4Al-LDH/CF still maintained 85.33% of the maximum adsorption capacity after four adsorption-desorption cycles. The adsorbent synthesized by flexible and simple methods might be a competitive candidate to purify nondegradable substances from wastewater.