Efficient adsorption of Cd2+ from aqueous solution using metakaolin geopolymers

In this study, geopolymers were prepared using metakaolin (MK) raw material under different alkali activator moduli (SiO2/Na2O = 0.8, 1.2, 1.6, 2.0 M ratio) in order to analyze their capacity and mechanism for adsorbing cadmium (Cd2+) from solution. Instrumental analyses including X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy(XPS), Fourier transform infrared (FTIR), and Brunauer-Emmett-Teller (BET) were performed to examine the mineralogical features of the MK and geopolymers before and after Cd2+ adsorption. The effect of initial pH, temperature, contact time, and initial concentration on Cd adsorption performance was studied to obtain the equilibrium isotherm. Kinetic data of the geopolymers fitted the pseudo-second-order kinetic model well. Moreover, the adsorption equilibrium data of Cd2+ adsorbed by the geopolymers fitted the Langmuir model better than the Freundlich model. The result shows that chemisorption dominates Cd2+ adsorption by geopolymers and that the adsorption capacity differs when prepared using different alkali-activated modulus agents. The geopolymer prepared using an alkali activator modulus of 0.8 M (molar ratio) exhibited the best Cd2+ adsorption performance with a maximum adsorption capacity of 70.3 mg g−1. The removal rate of Cd2+ by geopolymer still remained above 85% after five round of recycling.