Adsorption of lead ion on amino-functionalized fly-ash-based SBA-15 mesoporous molecular sieves prepared via two-step hydrothermal method

Abstract

Using an alkali two-step hydrothermal method, SBA-15 mesoporous molecular sieves were prepared from fly ash used in a thermal power plant in Inner Mongolia (China). The effect of alkali solution (i.e., NaOH), added in different amounts at 95 °C, on the desilication rate of the ash was determined. During the 48-h hydrothermal crystallization process at 110 °C, sieves were obtained using Na2SiO3 solution as the silicon source, hydrochloric acid to adjust the pH value, and P123 (PEO-PPO-PEO) as a template. Amino-functionalized molecular sieves were prepared at 120 °C, using 3-aminopropyltriethoxysilane as a modifier and toluene as the solvent. The prepared SBA-15 and NH2-SBA-15 molecular sieves were characterized via X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and N2 adsorption. The effects of adsorption time, temperature, pH value, and the initial concentration of lead ions (Pb2+) on the efficiency of Pb2+ removal were investigated using NH2-SBA-15 as an adsorbent. The results revealed that, like its SBA-15 counterpart, NH2-SBA-15 has a typical two-dimensional hexagonal pore structure, with lower specific surface area, average pore diameter, and pore volume compared with its SBA-15 counterpart. Furthermore, the Pb2+-removal efficiency increased gradually with increasing adsorption time, adsorption temperature, and initial concentration of Pb2+. However, when the pH value was increased, the efficiency increased initially and decreased thereafter. The maximum removal rate (>98%) after 60 min at 30 °C was obtained at a pH value of 5 and a Pb2+ concentration of 100 mg/L. The adsorption process of Pb2+ and the corresponding adsorption isotherm were well described by the second-order kinetics model and the Langmuir isotherm model, respectively, and an equilibrium adsorption amount of 131 mg/g was realized. The fly-ash-based SBA-15 mesoporous molecular sieve adsorbent, developed and subsequently modified in the present study, exhibited improved selectivity and adsorption capacity, and was less expensive than existing adsorbents. This sieve can provide effective technical support for control of the industrial heavy-metal-contaminated wastewater, and provides a new approach for the efficient utilization of fly ash.