Retention behavior of transition metals on a bifunctional ion-exchange column with oxalic acid as eluent

The common eluents used with a bifunctional ion-exchange column (IonPac CS5A) for separating transition metals are pyridine-2,6-dicarboxylic acid and oxalic acid (Ox). When Ox is used, cadmium and manganese co-elute. Although much research has been done to overcome the Cd2+-Mn2+ co-elution problem, the role of lithium hydroxide in separating the transition metals has received little attention. In this study, it is found that when the Ox concentration is higher than 35 mM, Cu2+ elutes after Pb2+ and Ox plays a predominant role in the retention behavior of the seven metals. When Ox concentration is lower than 35 mM especially when its concentration (25 mM) is half of the usually used standard concentration (50 mM), Cu2+ elutes before Pb2+, and at the same time, Mn2+ and Cd2+ can also be baseline separated. Lithium hydroxide plays a predominant role in the separation of the metals separated by cation exchange. So, lithium hydroxide is used to adjust the pH of the eluent. The use of an isocratic elution (25 mM Ox/LiOH/2 mM Na2SO4, pH 3.88) allows the separation of seven metals (Cu2+, Pb2+, Co2+, Mn2+, Cd2+, Zn2+ and Ni2+) in a single run. The effects of inorganic modifiers such as NaNO3, Na2SO4 and Na4P3O7 on retention behavior of the metals are also investigated.