Separation and recovery of heavy metals using a cation-exchange resin in the presence of organic macro-cations

Commercial polymer cation-exchange resins have been investigated for the separation and recovery of Cu(II) and Ag(I) from synthetic solutions. They contain various ionic species after modification of the cation-exchange resin properties by fixation of organic macro-cations under different experimental conditions. The modification was carried out by impregnation of the resin under different conditions (time of immersion, concentration, pH) into solutions containing tetramethylammonium, tetrabutylammonium and polyethyleneimine. The selectivity coefficient and the capacity of the resin for the ions Li(I), K(I), Fe(III), Cu(II), Ag(I) and Cd(II) in batch operations were determined and the kinetic of metal adsorption by both the modified and the unmodified resin was also studied. After the adsorption of the organic macro-cation, ion-exchange capacity was varied and the rate of the ion-exchange reaction of monovalent ions became slow, though exchange capacity for copper ions increased when the cation-exchange resin was modified in polyethyleneimine solution. Several stripping solutions, i.e., SCN- 0.01 M, HNO3 0.5 M, HNO3 1 M and thiourea 0.1 M in HNO3 0.5M were tested in batch experiments for the recovery of metals loaded in the resin adsorption operation. When such solutions were tested in a mixture of ions adsorbed in the resin, the following result were obtained : elution of Cu(II) fixed on PEI-modified resin with acids was inferior by 30% compared to 65% in the case of unmodified resin. On the other hand, thiourea 0.1 M in HNO3 0.5 M was shown to be the best eluting solution for the simultaneous recovery of Ag(I) and Cu(II), yielding a value of 80% for each metal.

This study shows that the modification of the cation-exchange resin by adsorption of a polyelectrolyte or organic macro-cation permits control of the selectivity of resin towards a specific metal, which can be preferentially extracted from resin using a chemical reactive. Modified resins were used in successive retentions and allowed the separation of transition metals from alkaline one, or the separation of monovalent ions from multivalent ones or separation of less from more hydrated ions. This separation depended on the order of the arrangement of resins in the system and the experimental conditions. Conventional elution using acid or chelating compounds led to the extraction of specific ions. The extraction increased when the affinity of the resin towards the metallic ion decreased. The extraction of Ag(I) from unmodified and TMAHmodified resin was higher by 90% when a thiourea + HNO3 0.1 M mixture was used.