The effect of surface heterogeneity between α-quartz and α-cristobalite on adsorption behaviors toward Cu2+ solution

Mineral surface microstructure and properties not only exert an important influence on the surface dissolution and growth of minerals, but also control the reaction rate and model between mineral and heavy metal ions in environment. The surface difference of silica polymorphs had been verified in some previous studies. To further study the effect of surface heterogeneity between α-quartz and α-cristobalite on adsorption behaviors toward heavy metal ions, the adsorption characteristics of Cu2+ ions were investigated by adsorption experiments. Cu2+ ions were predominantly adsorbed on α-quartz because of its larger surface site density. The reaction between α-quartz/α-cristobalite and Cu2+ ions was an endothermic process. The increase of pH could prompt the formation of Cu precipitation on the surface, corresponding to the declined intensity of EPR signal of g=2.08/2.05 at a higher pH. Cu2+ ions were bonded with surface oxygen atoms to form an inner-sphere structure at lower pH and initial concentration solutions. The intensity loss of g=2.08 in α-quartz in a higher initial concentration demonstrated the formation of Cu(II) dimeric/polynuclear species, resulting from the larger surface site density. While, from the increased EPR signal of g=2.08, mononuclear adsorption was still the predominant form on α-cristobalite under the same reaction conditions.