Separation of rhodium(III) and iridium(IV) chlorido species by quaternary diammonium centres hosted on silica microparticles

Silica gel was functionalized with six different quaternary diammonium centres derived from ethylenediamine (EDA), tetramethylenediamine (TMDA), hexamethylenediamine (HMDA), 1,8-diaminooctane (OMDA), 1,10-diaminodecane (DMDA) and 1,12-diaminododecane (DDMDA) to produce Si-QUAT EDA, Si-QUAT TMDA, Si-QUAT HMDA, Si-QUAT OMDA, Si-QUAT DMDA and Si-QUAT DDMDA, respectively. The synthesized silica-based resins were characterized by means of FTIR, XPS, SEM, BET surface area, thermogravimetric analysis and elemental analysis. The materials were used to investigate the adsorption and separation of [RhCl5(H2O)]2− and [IrCl6]2−. Batch studies (equilibrium and kinetic studies) were conducted to study the adsorption of [RhCl5(H2O)]2− and [IrCl6]2− onto Si-QUAT EDA, Si-QUAT TMDA, Si-QUAT HMDA, Si-QUAT OMDA, Si-QUAT DMDA and Si-QUAT DDMDA using single metal aqueous solutions. The Freundlich isotherm confirmed multilayer adsorption and the Freundlich constant (kf) displayed the following ascending order; Si-QUAT EDA, Si-QUAT TMDA, Si-QUAT HMDA, Si-QUAT OMDA and Si-QUAT DMDA, and a decrease in kf for Si-QUAT DDMDA. Kinetic studies suggest a pseudo-first order kinetic model. Column studies were also conducted for a binary mixture of these metal ion chlorido species ([RhCl5(H2O)]2− and [IrCl6]2−). The iridium loading capacities increased as the carbon spacer between the diammonium centres increased in the following order; Si-QUAT EDA, Si-QUAT TMDA, Si-QUAT HMDA, Si-QUAT OMDA and Si-QUAT DMDA (4.56 mg/g, 6.88 mg/g, 14.63 mg/g, 19.01 mg/g and 29.35 mg/g, respectively). It was observed that the iridium loading capacity of Si-QUAT DDMDA decreased to 8.90 mg/g. This paper presents iridium-specific materials that could be applied in solutions of secondary PGMs sources containing rhodium and iridium as well as in feed solutions from ore processing.