Cadmium extraction from hydrochloric acid solutions using Amberlite XAD-7 impregnated with Cyanex 921 (tri-octyl phosphine oxide)

Cyanex® 921 (tri-octyl phosphine oxide, TOPO) was successfully used for the impregnation of Amberlite® XAD-7 to prepare an extractant-impregnated resin (EIR). This EIR was used for the recovery of cadmium from concentrated HCl solutions. Though the maximum loading of Cyanex 921 in the EIR can reach 580 mg g−1, a lower concentration (below 400 mg g−1) enabled better use of the extractant. The Langmuir equation was used to model the sorption isotherms and the maximum sorption capacity was found to be 13 mg Cd g−1, for a EIR with 366 mg Cyanex 921 g−1 in a 3 M HCl solution (containing about 60 mg Cd L−1), in which equilibrium was reached within 8 h contact time. The extraction mechanism involved various solvating reactions with neutral CdCl2 and ion pairs (involving associations between protons and anionic cadmium chloro-species). It was difficult to determine the extracted species due to the simultaneous co-extraction of HCl on the EIR. Cadmium can be efficiently desorbed using a diverse range of eluents and the possibility of re-using the resin was demonstrated over four cycles.