Multiple pollutants removal by carbon sphere and layered double hydroxide composites: Adsorption behavior and mechanisms

Carbon spheres and layered double hydroxides (CSs–LDHs) composite adsorbents were used for the removal of Pb(II), Cd(II), Cu(II), Cr (VI), phenol, toluene, and acid red 1 (AR1) from water and wastewater. The CSs–LDHs composites were synthesized using a hydrothermal method, followed by calcination in the temperature range of 300–800 °C to alter their surface properties. The calcined and non-calcined CSs–LDHs adsorbents were characterized using scanning electron microscopy, Brunauer–Emmett–Teller, X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and zeta potential analyses. The results indicated that the CSs–LDHs retained the properties of the LDHs and CSs and can be used for the removal of cationic, anionic, and nonionic contaminants. The number of O-containing functional groups of the composites decreased with the increasing calcination temperature. The specific surface areas of the CSs–LDHs increased with increasing calcination temperature. The CSs–LDHs obtained via calcination at 400 °C was suitable for the adsorption of Pb(II), Cd(II), and Cu(II), and the maximum adsorption capacities (MACs) for Pb(II), Cd(II), and Cu(II) were 236, 475, and 490 mg/g, respectively. The complexation reaction between the composite and metal ions was considered to be the primary adsorption mechanism. The CSs–LDHs obtained via calcination at 800 °C was suitable for the adsorption of Cr(VI), AR1, phenol, and toluene, and the MACs for Cr(VI), AR1, phenol, and toluene were 975, 342, 237, and 108 mg/g, respectively. Complexation and ion-exchange reactions were the primary mechanisms for the adsorption of cationic contaminants, whereas ion-exchange reactions and van der Waals forces were the primary mechanisms for the adsorption of anionic and nonionic contaminants, respectively. Other contaminant adsorption mechanisms, including surface precipitation, reduction–adsorption, hydrogen bonding, and π–π interactions, were also evaluated.