Effect of added complexing anions on cation exchange of Cu(II) and Zn(II) with a strong-acid resin

Ion exchange is widely used for the recovery and separation of metals from process and waste streams in chemical process industries. The effect of added complexing anions such as EDTA, NTA, and citrate on the cation exchange of Cu²⁺ and Zn²⁺ with strong-acid Purolite NRW100 resin was studied at 298 K, with the metals in solution in excess over the complexing anions. Experiments were performed as a function of solution pH (1.0-6.0), concentration of metals (4.5-22.5 mol/m³), and concentration ratio of complexing anions to metals (0-1). It was shown that the rate of metal exchange decreased when the complexing anions were present. The addition of EDTA and NTA decreased the amount of metal exchange throughout the entire pH range studied, but it was the case for the citrate system only at high pH (>4). In the absence of complexing anions, the Langmuir equation could be used to correlate the exchange isotherms at a given pH on a macroscopic basis. However, a proposed chemical model that takes into account possible solid-liquid exchange reactions and solution chemistry could be used to describe the metal exchange equilibria in the presence of complexing anions.