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

Cyanexo (R) 921 (tri-octyl phosphine oxide, TOPO) was successfully used for the impregnation of Amberlite (R) 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. (C) 2007 Elsevier Ltd. All rights reserved.