Adsorption of heavy metals on a complex Al-Si-O bearing mineral system: Insights from theory and experiments

The adsorption behaviour of heavy metallic ions such as Co (II), Pb (II) and Zn (II) on an Al-Si-O mineral system mostly comprising of pyrophyllite and diaspore was explored via experimentation and first principle calculations. The characterization studies involving X-ray Diffraction (XRD), Electron Probe Micro-Analysis (EPMA), Scanning Electron Microscope attached with Energy Dispersive Spectroscopy (SEM-EDS) in addition to the quantitative mineralogical analysis under reflected light microscope revealed the presence of pyrophyllite {Al2Si4O10(OH)2} as the major mineral followed by the phases like diaspore {α-AlO(OH}, corundum (Al2O3), kaolinite {Al2Si2O5(OH)4} and muscovite {KAl2(AlSi3O10)(F,OH)2} in the decreasing order of abundance. The adsorption data fitted well to the Langmuir isotherm with Co (II) having the maximum monolayer capacity of 9.23 mg/g while the pseudo second order model could explain the sorption behaviour for all the metallic ions. The adsorption capacities of the metallic salts followed the order Co (II) > Pb (II) > Zn (II). Density Functional Theory (DFT) based calculations validated the experimental trend of adsorption capacities indicating that Co atom is able to coordinate on the constituent mineral surfaces in a much stronger manner compared to the other two metal atoms. While corundum (0 0 1) plane was found to the best adsorbing plane among all the mineral planes considered over here, the interaction of the metal atoms on the edge surface of pyrophyllite (0 1 0) was stronger compared to that on its basal plane (0 0 1).