Distribution of microamounts of some M2+ ions during ZnSO4·7H2O crystallization

Abstract

Distribution coefficients (D) of trace amounts of Ni2+, Co2+, Mg2+, Cd2+, Cu2+, Mn2+, Ca2+, and Fe2+ have been determined during crystallization of ZnSO4·7H2O. Their dependencies on the ionic radii of M2+ ions, solubilities and structures of the corresponding sulfates as well as their abilities to form solid solutions with ZnSO4·7H2O have been analyzed. The D values are in the range <0.006 (DCa) - 1.61 (DNi) and depend mainly on similarity of the structures of the corresponding sulfates and the ability of MSO4·nH2O to form solid solutions with ZnSO4·7H2O. The order of lowering some of the determined distribution coefficients (DNi > DMg > DCo > DFe > Dcu) exactly agrees with increase of the deformation trend of octahedral coordination of corresponding ions resulting from their electronic configuration. The D coefficients do not depend on similarity of size of ionic radii of microcomponent (rM2+) to macrocomponent (rZn2+), however for ions forming monoclinic sulfate hydrates at 20°C (Co2+, Fe2+, Cd2+, Ca2+) this dependence of distribution coefficients on rM2+ is linear in a plot of log D against ionic radius. The effect of solubilities of corresponding sulfates in water on the D coefficients is complex and clouded by crystal-chemical factors.Key words: crystallization, zinc(II) sulfate heptahydrate, cocrystallization, distribution coefficients D (Henderson-Kracek, Khlopin), distribution of trace impurities.