Amino-terminated SiO2-Al2O3 composite aerogels from fly ash for improved removal of Cu2+ and Pb2+ ions in wastewater: one-pot synthesis, excellent adsorption capacity and mechanism

In this study, by using a sol-gel grafting-atmospheric drying method, amino-terminated SiO2-Al2O3 composite aerogels, namely 3-aminopropyltriethoxysilane (APTES) or 3-(2-amino-ethoxy) propylmethyldimethoxysilane (AEAPMDS) modified SiO2-Al2O3 aerogels (AMSAAs), were synthesized from the fly ash and characterized by field-emission scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy etc.. And the AMSAAs were verified as excellent adsorbents for removing heavy metal ions (Cu2+ and Pb2+ ions) from wastewater. The effects of modification conditions and testing parameters including pH value, adsorbent dose, initial ions concentration, adsorption time and temperature were systematically investigated. Results demonstrated that 0.2 mol/L APTES modified aerogels (0.2APTES-SAAs) possessed the best adsorption properties. Under the optimal pH value of 4.0-6.0 and the adsorbent dose of 0.4-0.6 g/L, the equilibrium adsorption capacities of Cu2+ and Pb2+ ions were as high as 195 mg/g and 500 mg/g within 20-30 min, respectively. The adsorption processes were agreed fairly well with Freundlich isotherm adsorption model and the pseudo-second-order kinetic model, which indicated that the adsorption processes were heterogeneous multilayer adsorption and controlled by the chemical reaction between AMSAAs and heavy metal ions. The obtained adsorption thermodynamic parameters (Delta H degrees, Delta S degrees and Delta G degrees) revealed that the adsorption processes were exothermic and spontaneous with decreased randomness at the solid-liquid interface. The excellent recyclability of as-prepared AMSAAs proved as economically promising adsorbents for practical applications.