Synthesis, characterization and acid-base properties of kaolinite and metal (Fe, Mn, Co) doped kaolinite

Pure kaolinite and metal ion doped kaolinite were synthesized successfully for the first time with different M/Al (M = Mn, Fe, Co) ratios (0, 0.042, 0.087, 0.19) at 220 °C by the hydrothermal method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and BET techniques (N2/BET) were used for sample characterization. XRD patterns of the products showed that kaolinite was the only crystalline phase synthesized and the lattice parameters varied systematically with metal content. The surface acid-base properties of the pure kaolinite and doped kaolinite were studied by potentiometric titration. Surface site concentration (Hs) and surface site density (Ds) increased as metal ion content in the metal doped kaolinite increased. The relevant acid-base equilibrium constants of synthesized products were calculated by applying the constant capacitance model using computer software WinSGW. The surface pKa values of synthetic kaolinite were calculated as: logKa1 = 1.48; logKa2 = −7.03. The results showed that a small number of metal ions significantly affect the surface acid-base properties of kaolinite, which may explain the origin of variability of the reported surface pKa values of kaolinite.