Study of Competitive Behavior of Cd(II) and Pb(II) Ions Through a Bulk Liquid Membrane

Aims: To investigate the competitive behavior of Cd(II) and Pb(II) ions within their equimolar mixtures, during transportation through a bulk liquid membrane, using macrocyclic carriers.

Study Design: Study was based on transport experiments, using homemade transport cell.

Place and Duration of Study: Department of Analytical Chemistry, Faculty of Technology, between September 2017 and February 2018.

Methodology: The bulk liquid membrane systems consisted of homemade transport cell constructed for transport of cations. Cell contained three separated phases: two aqueous and non-aqueous membrane phase between them. 1,2-dichloroethane (1,2-DCE) and dichloromethane (DCM) were used as liquid membranes. Macrocyclic ligands: dibenzo-18-crown-6 (DB18C6) and 18-crown-6 (18C6) were used as ligands for cations within the membranes. Both aqueous phases were buffered at pH = 5. Source phase contained an equimolar mixture of investigated metal ions and picrates as counter ions. Receiving phase contained thiosulphate as stripping agent. Duration of transport experiments were 3 hours and concentration of transported cations was measured using flame atomic absorption spectrometry.

Results: Higher ligand selectivity for Pb(II) ions resulted with higher transport rates compared to Cd(II) ions: 63.25% of Pb(II) > 51.80% of Cd(II) using 18C6 in 1,2-DCE; 38.90% of Pb(II) > 30.50% of Cd(II) using DB18C6 in 1,2-DCE; 43.15 % of Pb(II) > 35.40 % of Cd(II) using 18C6 in DCM and 26.75% of Pb(II) > 8.90% of Cd(II) using DB18C6 in DCM. Higher selectivity of 18C6 as ionophore is also evident here. 1,2-DCE showed higher efficiency compared to DCM in competitive experiments (unlike individual experiments). Overall transport of Pb(II) in competitive experiments is lower compared to individual ones in DCM membrane: 40.30%<70.40% (with 18C6) and 26.75%<36.05% (with DB18C6). Overall transport of Cd(II) in competitive experiments is lower compared to individual ones in DCM membrane: 35.40%<48.10% (with 18C6) and 8.90<38.25% (with DB18C6).

Conclusion: The results showed that higher ligand selectivity for Pb(II) ions lead to higher transport rates compared to Cd(II) ions from their equimolar mixtures. 18C6 was more selective for both cations as ionophore compared to DB18C6. 1,2-DCE showed higher efficiency as membrane solvent in competitive experiments compared to DCM, unlike the individual experiments. Competition between cations decreased their overall transport in DCM, but increased in 1,2-DCE membrane.