To enhance the adsorption capacity of sepiolite (Sep) on Cd2+ in solution, an acid-base Sep (ABsep) was obtained using the method of combined acid-base modification. The structural properties of Sep and ABsep were analyzed by adsorption-desorption isotherms of N2, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray powder diffraction (XRD). Static adsorption experiments were carried out to evaluate the effects of time, mass ratio of ABsep/Cd2+, temperature, adsorbent dose, pH, and co-existing ions on the adsorption of Cd2+ by ABsep. The results showed that the pore structure of the ABsep was more developed than that of Sep. In comparison to Sep, the specific surface area, average pore diameter, and pore volume of ABsep increased by 66.1%, 15.7%, and 34.8%, respectively, and the exchangeable ion contents also increased. The main components of the ABsep were SiO2 and Mg(OH)2. The adsorption process of Cd2+ by Sep and ABsep could be well fitted with a pseudo-second-order kinetic equation and Langmuir isotherm, and both were spontaneous endothermic reactions, which were mainly chemical adsorption along with physical adsorption. The best mass ratio of ABsep:Cd2+ was 3:1, and the maximum saturated adsorption capacity fitted by the Sips model of the ABsep on Cd2+ at 298 K was 142.43 mg·g-1, which was 3.55 times that of Sep. As the adsorbent dose increased, the adsorption amounts of Cd2+ first increased and then decreased, with the optimum dose being 0.3 g·L-1. The amount of Cd2+ adsorption by the ABsep increased with the initial pH of the solution, whereby the best pH was 7. We also found that the presence of K+, Na+, Mg2+, and Ca2+ at different concentrations could inhibit the adsorption of Cd2+, whereby the inhibition of Mg2+ was the highest.
Keywords: acidic and basic modification; adsorption; cadmium; sepiolite; substructure.