A comparative study of Cd(ii) adsorption on calcined raw attapulgite and calcined aluminium hydroxide-modified attapulgites in aqueous solution

In this study, raw attapulgite and two aluminium hydroxide-modified attapulgites prepared using different aluminium salts were calcined at 600 °C to successfully prepare three novel adsorbents (C-ATP, C-ATP-SO(4)(2−) and C-ATP-Cl(−)). The three adsorbents were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis and X-ray photoelectron spectroscopy (XPS). Batch experiments revealed that the Cd(ii) adsorption capacity of the three adsorbents increased with increasing pH, increasing the initial concentration of Cd(ii) in solution, and with longer adsorption times. The order of adsorption capacity was always C-ATP > C-ATP-Cl(−) > C-ATP-SO(4)(2−). C-ATP and C-ATP-Cl(−) were better described by the Langmuir model, while C-ATP-SO(4)(2−) was better described by the Freundlich model. The three adsorbents reached adsorption equilibrium within 2 h, and all followed pseudo-second order kinetics. The adsorption of Cd(ii) onto the three adsorbents was physisorption, as suggested by the calculated thermodynamic parameters. Although the adsorption of Cd(ii) on C-ATP and C-ATP-Cl(−) was exothermic, the adsorption on C-ATP-SO(4)(2−) was endothermic. Ion exchange and cadmium precipitation were the primary mechanisms of cadmium adsorption on the three adsorbents analysed by XPS. The presence of SO(4)(2−) in C-ATP-SO(4)(2−) may result in weaker binding of Cd(ii) by the adsorbent than C-ATP-Cl(−).